The challenge of diagnosing lumbar segmental instability

doi:10.21203/rs.3.rs-3659900/v1

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Research Article

The challenge of diagnosing lumbar segmental instability

https://doi.org/10.21203/rs.3.rs-3659900/v1

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Background

Lumbar spinal instability is very commonly discussed in research studies and is routinely used in clinical practice to make treatment decisions. That practice must be reconciled with expert consensus in the peer-reviewed literature: there is currently no validated diagnostic test for spinal instability. Some treatments for instability can have serious complications, so correct diagnosis is important. Biomechanically rational and clinically effective diagnostic tests for instability are needed, where instability is defined as incompetence of the intervertebral motion restraints forming the passive part of the motion control system.

Methods

This study critically examines and identifies deficiencies in previously employed metrics and criteria for diagnosing spinal instability. New metrics are described that account for the deficiencies. The new metrics were retrospectively applied to 7621 lumbar spine flexion-extension studies to document the prevalence of abnormalities in different patient populations.

Results

Traditional measurements, such as intervertebral rotation or translation, may fail to find abnormalities in intervertebral motion due to factors such as inconsistent patient effort and radiographic magnification. The proposed biomechanically grounded metrics for lumbar spine sagittal plane shear and vertical instability appear more adept at finding abnormalities in patient populations where abnormalities might be expected and not in patients where instability would not be expected.

Discussion

New approaches to detecting abnormal sagittal plane intervertebral motion may lead to enhanced and standardized diagnosis of lumbar spine instability. Further clinical research is imperative to validate the efficacy of these metrics in diagnosis and treatment algorithms.

Biomedical Engineering

Orthopedic Surgery

Nuclear Medicine & Medical Imaging

Neurosurgery

lumbar

spinal

instability

diagnosis

intervertebral motion

segmental

radiographic

History

Since the early 1900s, the concept of spinal instability has been popularized and assumed to be clinically relevant. Instability is frequently referenced as an indication for surgery in papers and payer guidelines. It is also referenced as a contraindication for some treatments (e.g., total disc replacement). Despite these facts, there is no expert consensus on how to define instability, nor is there any routine diagnostic test for spinal instability supported by high-quality evidence. (1–8) As Bogduk stated, “Instability is readily abused as a diagnostic rubric. It is easy to say a patient has instability; it is much harder to satisfy any criteria that justify the use of this term.” (9) Clinicians may individually have their own subjective “sense” of what constitutes “instability”, yet the clinical efficacy of this “sense” is not validated, objective, or universally communicable. Herein lies a problem that fuels controversies such as whether fusion is needed to treat back pain attributed to motion at a specific segment of the spine(2) or if fusion in addition to decompression is required for surgical treatment of lumbar spinal stenosis(10).

From 1944 to 2023, numerous studies have addressed lumbar spinal instability, with conflicting findings regarding its correlation with symptoms.(11–19) The lack of a universally accepted diagnostic test may contribute to these inconsistencies. Inadequate diagnostic methods hinder our understanding of the relationship between spinal instability and clinical symptoms and compromise research to establish optimal treatment. This review aims to evaluate the criteria for instability currently used in research and practice and to propose improved alternative diagnostic techniques for spinal instability applicable in both research and clinical practice.

Definitions

First and foremost, it is critical to align on a definition for the topic of discussion (despite the fact that the topic’s definition is what is being critiqued). This paper focuses on mechanical instability in the lumbar spine as a potentially treatable source of symptoms in patients with degenerative disorders (11, 18, 20) and not “spinal instability” as used in the setting of severe spinal trauma or metastatic disease.

There are many noninvasive physical examination tests and clinical assessments used to detect “spinal instability”, such as the passive lumbar extension test.(21–24) These clinical assessments are not directly addressed in this paper, although a well-validated, imaging-based diagnostic test might facilitate a better understanding of the significance of provocative maneuvers used during physical exam intended to help diagnose abnormal spinal motion. The diagnosis of spinal instability requires an appreciation of the passive (vertebrae, discs, and ligaments), active (muscles and tendons), and neural components that work together to achieve spinal stability. (25, 26) Most attempts at imaging-based diagnosis of spinal instability focus on measuring the magnitude or quality of intervertebral motion and classification of this motion as normal or abnormal, with the goal of diagnosing abnormal passive motion control. However, in clinical practice, measured motion is dependent on passive, active and neural control, rendering the diagnosis of abnormal passive control challenging.(18) It can be argued that current surgical treatments primarily target the passive control of spinal stability, although they may iatrogenically alter active (muscular) and neural control. In addition, the active and neural control systems may at least transiently compensate for damage to the passive control system (e.g., by muscle spasming), and this can complicate diagnosis and optimization of treatment.

This paper is focused on the objective and quantitative diagnosis of incompetent, segmental, intervertebral motion that is controlled by the sum of the passive motion constraints: these include the vertebrae, intervertebral discs, ligaments (i.e., anterior longitudinal, posterior longitudinal, ligamentum flavum, interspinous and supraspinous), and facet joints (including both bony elements and their associated capsular soft tissue restraints) that work together to maintain intervertebral motion within normal limits. Incompetence of this passive control system includes injury to (traumatic or iatrogenic) or functional degeneration of the normal segmental restraints to intervertebral motion.(27)

For the remainder of this paper, it will be assumed that the goal is to diagnose abnormalities of the passive elements of the spine stability system. The label “instability_PCS” will be used in the following discussion to maintain a focus on abnormalities of the passive elements of intervertebral motion control. Instability_PCS is assumed to be a structural disorder that can potentially be addressed by physical therapy, surgical, biologic, or regenerative treatments. Variability in the exact etiology of instability_PCS in each patient is assumed; in some cases, it could stem from an injury (e.g., disc avulsion), while in most cases, it is likely the result of degenerative changes of the disc and facets. Instability_PCS may occur at one or more levels. In all cases, intervertebral motion is not maintained within normal limits by a healthy passive motion control system. The challenge is to reliably and objectively diagnose instability_PCS.

When assessed from medical images, studies that address instability_PCS are most commonly focused on sagittal plane motion. This focus is justified given the importance of forward and backward bending in activities of daily living as well as the relative ease at which sagittal plane motion is reliably imaged and analyzed in clinical practice compared to axial twisting or lateral bending. To that end, this paper will focus on sagittal plane motion tests for instability_PCS, although the authors acknowledge that it may also prove important to diagnose axial and coronal plane motion abnormalities, as well as abnormal coupling between these motions. (1, 9, 28–32)

Biomechanics

Numerous ex vivo and cadaveric studies, as well as computer models, have been used to help understand how damage to or degeneration of the intervertebral disc, facet joints, and/or ligaments can result in instability_PCS.(33–43) These studies provide evidence that such instability_PCS can result in abnormal intervertebral motion. There is also evidence that the mechanical properties of individual intervertebral motion restraints may change with age, which further complicates the diagnosis of instability_PCS.(44) The results of these basic science studies have yet to be incorporated into validated diagnostic tests for instability_PCS. Other authors have identified imaging findings such as osteophytes and annular tears that may be associated with instability. (45–47), however, it could be insightful to repeat these investigations once a validated test for instability is available.

It will be assumed that during the normal activities of daily living, segmental intervertebral motion is controlled in a healthy spine so that motion between vertebrae will not result in damage or irritation to the nerves or other tissues in close proximity to the vertebrae. It will be further assumed that when the intervertebral motion restraints are incompetent, abnormal motion can occur that may (or may not) irritate nerves and other tissues and that may (or may not) result in pain and other symptoms. Even if the abnormal motion does not cause symptoms at a specific moment in time, in the presence of inflammation or continued mechanical irritation, it can be hypothesized that it can subsequently result in clinically relevant symptoms.(48, 49) It is understood that many factors may influence patient outcomes, and a diagnostic test for instability_PCS may be just one of several important factors. However, a validated diagnostic test for instability_PCS can reasonably be expected to improve patient selection for procedures designed to treat abnormal spinal motion.

It is also important to appreciate that stiff spines may be associated with symptoms (50–52), and it may be as important to diagnose hypomobile spine segments as it is to diagnose hypermobile segments. Hypomobility may be driven by the presence of osteophytes, facet hypertrophy, disc height loss, and scar tissue (which may be natural biological mechanisms for restabilizing a spine). Hypomobility may also stem from involuntary muscle spasms or motor control issues. In the context of diagnosing instability_PCS, muscle spasms or involuntary motor control may result in a false-negative test (e.g., the patient appears to have normal intervertebral motion as a result of guarding even though abnormal intervertebral motion occurs at other times in the patient’s life). (14, 53, 54) This is another hypothesis that has been inadequately explored.

Lack of a “gold standard”

A level 1 study of a diagnostic test for spinal instability requires an existing “gold standard” test.(55, 56) Such a test should have clinically efficacious sensitivity and specificity and must be supported by clearly developed rationale and high-quality evidence. Unfortunately, no such test currently exists for instability_PCS.(1–8) Interim approaches are therefore needed in efforts to establish a “gold standard” test.

Since there are many possible causes for patient symptoms and instability_PCS may not always result in specific symptoms at a specific point in time, patient symptoms may be sub-optimal for validation of an instability_PCS test.(57) Low-quality approaches such as documenting associations between a novel diagnostic test and an unvalidated test (even if the unvalidated test was used in many prior studies) are also of limited value.(58, 59) Cadaver studies can be used to find associations between damage to intervertebral motion restraints and intervertebral motion; however, there is always uncertainty about how well these ex vivo tests represent the active muscle, gravitational, and environmental conditions found in a live patient. (34–40) Imaging the spine in multiple positions with high-resolution CT or MRI is another possibility to achieve greater insight into the nature of intervertebral motion. However, it is unknown whether patient positioning can be reliably achieved within a CT or MRI that stresses the spine enough to provoke intervertebral motion characteristic of instability_PCS.

Another approach to demonstrating the efficacy of a new test for abnormal motion would be to use intraoperative measurements of intervertebral motion, though that would require agreement on what specific measurement (e.g., the sagittal plane translation component of spinous process displacements?) is capable of detecting instability_PCS along with validation studies of the intraoperative measurements. (60–64) Based on the hypothesis that the clinical consequence of instability_PCS is irritation of perivertebral tissues, another approach would be to document an association between a diagnostic test for instability_PCS and tissue inflammation. That would require a validated test for tissue inflammation, and methods to account for the possibility of abnormal motion that has not (yet) resulted in inflammation. Computer models validated to represent all important elements of intervertebral motion control found in vivo (passive, active and neural control) and validated to represent the biomechanical properties of all natural intervertebral motion restraints may be a promising approach to validating a test as the “gold standard”; however, these models also have limitations.

The final option is to accept tests of sensitivity, specificity, etc. will not be possible at this stage and focus on a metric that, based on rational arguments, is likely to be abnormal in the presence of instability_PCS and contribute to symptoms. It would be important to validate that the metric can be reliably measured and then test the clinical efficacy of the metric in large sample clinical trials.

Measurement quality

Measurement errors (both systematic and random) associated with the production of an instability_PCS test must be minimized, well documented, and easily referenced by ordering physicians. Unfortunately, clinicians today lack tools for accurately and reliably measuring intervertebral motion (current methods mostly rely on manual line drawings) and consistently achieve an acceptable level of accuracy. (53, 65, 66) In addition to the difficulties of measuring what is often a very small (but potentially meaningful) displacement of a rigid body between two positions, intervertebral translation measurements must also account for radiographic magnification when measured in millimeters. The literature suggests that X-ray magnification can vary between 9 and 63%(67), and most peer-reviewed publications reporting radiographic measurements of translation do not explicitly describe whether validated corrections were made for radiographic magnification. To put this into perspective consider the following example: If the actual translation in a patient was 2 mm and the radiographic magnification was 50%, then the direct uncorrected translation measurement from the X-ray would be 3 mm. If 3 mm of translation was the threshold used to classify a level as unstable, then the level could be incorrectly classified as unstable. Aside from these technical hallmarks, it is important to acknowledge the lack of personalization associated with millimeter-based measurements and diagnostic thresholds if the measure is not considered relative to the patient’s individual anatomy. A 3 mm translation when the anterior-posterior endplate width (EPW) is 42 mm may not be as significant as a 3 mm translation when EPW is 25 mm.

Insufficiently stressed spine

In a laboratory, mechanical testing has a somewhat gross but effective basis – put the system in question under stress and measure deformation to a predefined end-point. Extrapolate the concept to a functional spinal unit in vivo and consider the question at hand: How can we test the spine for deficient intervertebral motion restraints? The principals of biomechanics are clear – stress the system sufficiently and observe the response.

The ability or inability of the disc or ligaments to control motion within normal limits can only be assessed if that motion segment is stressed to the point where the elements of the passive intervertebral motion control system would restrain motion if they were functioning normally. A simple analogy is a dog secured by an elastic leash to a tree. The leash is intended to keep the dog within a controlled area. Whether the leash will prevent the dog from going out of bounds will not be known until the dog tries to stretch the leash to its fully tensioned length. Similarly, a diagnostic test for instability_PCS requires that the motion segment be stressed to where the disc and ligaments would be tensioned and restrain motion if they are functioning normally. Only then can it be determined if the passive motion control system is functioning normally. Multiple investigators have recognized that the spine must be stressed adequately to obtain diagnostic quality data (11, 68–70), yet this is not explicitly addressed in most published studies addressing instability_PCS.

Although validated criteria for verifying that the spine has been sufficiently stressed do not exist, it is possible to rationalize reasonable criteria by careful review of existing evidence. As with any well-researched mechanical system, a foundational understanding of intervertebral mechanics was achieved through ex vivo experiments that resulted in publication of force-displacement curves that characterize the mechanics of the disc and intervertebral ligaments. (27, 71–76) A representative curve is shown in Fig. 1 (Reproduced from Crawford et al(77)) and illustrates a toe-region (neutral zone) where the spinal ligament is displacing but resisting little load, and this leads up to the point where displacement increases relatively linearly with load. It is in this linear region (elastic zone) where the structure is substantially providing intervertebral motion control. Therefore, it may be rationalized that unless the spine is loaded to the point where a healthy disc or ligament would enter the elastic zone, a reliable assessment of motion control integrity may not be rendered. Hypothetically, if a ligament is completely damaged (e.g., avulsed or ruptured), then excess displacement will occur in an adequately stressed motion segment. If the ligament is partially degenerated or incompetent, then the neutral zone may be abnormally wide, which may also be detected if the motion segment is adequately stressed. It is also possible, due to redundancy in the intervertebral motion control system, that damage to a single ligament would not measurably affect intervertebral displacements (in which case the damage may not yet be clinically important).

Studies that have addressed the concepts of neutral and lax zones that exist in motion segments may help establish criteria for whether the spine has been sufficiently stressed to reliably diagnose instability_PCS. (77–90) Within the neutral and lax zones, intervertebral translations and rotations can occur with little applied force or moments. This has also been described as a region of minimal passive stiffness.(79) Measurements of intervertebral translation made while segmental intervertebral rotation is within the normal neutral zone may not be reliable for the diagnosis of instability_PCS since the ability of the disc and ligaments to restrain intervertebral motion to within normal limits is not being fully exercised. A review of data from existing neutral/lax zone research suggests that at least 5 deg of rotation must occur between flexion and extension to have confidence that the level has been sufficiently stressed to detect instability_PCS.(77, 81, 88, 91–94) Although five degrees between flexion and extension may serve as interim criteria, guidelines to assure that a spine has been adequately stressed need to be formally validated. One problem with the use of the aforementioned criteria involves asymmetry of motion with respect to a neutral position. When only a flexion and an extension image are available, it is possible that 5 deg may be almost all in flexion or mostly in extension. If it is mostly in flexion, then the test may not detect abnormalities in the anterior longitudinal ligament and anterior aspects of the disc. Conversely, if the motion is predominately in extension, then the test may fail to detect incompetence of the posterior ligaments, posterior aspects of the disc, and the facet joints. By this logic, it may be worth considering separate criteria for flexion and extension with respect to a neutral position (e.g., sufficient flexion is qualified by a difference of 3 degrees from neutral, while sufficient extension is qualified by a difference of 2 degrees from neutral). This two-component test may also provide additional insights with regard to the location (anterior or posterior) of the instability_PCS. Last, it has also been shown that compressive forces on the spine can “stabilize” a motion segment(95, 96), and it therefore may be necessary to stress the spine to the point where compressive forces are overcome and the elements of the passive motion control system would be in tension (and restraining motion) if they are functioning normally.

Patient positioning protocols

Flexion-extension

In clinical practice, patient flexion-extension positioning protocols are used to obtain radiographs of the spine in two (hopefully mechanically stressed) positions. These are then used to measure sagittal plane intervertebral motion. A myriad of patient positioning protocols have been tested or deployed (11, 58, 70, 97–105) and to some extent, compared, though generally without well-validated success criteria. (47, 98, 106–108) (109, 110) Knutsson used specialized positioning and aggressive coaching to adequately stress the spine.(11) Dvorak et al used examiner-assisted positioning.(68) Axelsson et al. found that the best method (out of those they studied) to provoke translation is sitting in a special chair.(98) Morita et al. reported significantly greater translation when patients were led by a technician’s hand (versus unassisted standing flexion-extension). Cheng et al. demonstrated that recumbent flexion-extension with a mechanically controlled device provokes more translation than the “standard of care” flexion-extension they analyzed. The list goes on, and the research continues while the “standard of care” remains unstandardized. This tells us that 1) the studies were not appropriately powered to influence wide adoption, 2) some protocols (methods, equipment, instructions, etc.) are not easily implemented or practical in routine clinical workflows, 3) acquiring X-rays of the lumbar spine in sufficiently stressed positions is not widely accepted as clinically important, and/or 4) most ordering clinicians do not realize how often a patient fails to adequately stress the spine in a standard of care flexion-extension exam.

With upright standing flexion-extension radiographs, it is known that more patient effort leads to more intervertebral motion – this is a fairly simple and intuitive concept. (68, 69, 111, 112) Unfortunately, achieving maximum patient effort is not a simple task. For starters, it may be argued that symptomatic patients cannot be expected to exert much effort because it is painful or uncomfortable.(70, 97) This argument is supplemented by evidence that fear avoidance can limit motion (113–115) and that intervertebral motion is substantially increased after analgesic injections.(116) In addition, muscle spasms may increase pain or directly limit motion.(117, 118) Aside from patient factors, a successful stress test will require appropriate training of the radiology technologist that is responsible for positioning the patient during X-ray acquisition. Merrill’s Atlas of Radiographic Positioning & Procedures states the following with regards to Patient Instructions: “The radiographer must be sure that the patient understands not only what to do but also why it must be done. A patient is more likely to follow instructions correctly if the reason for the instructions is clear.”(119) This text also provides instructions for acquiring recumbent hyperflexion and hyperextension radiographs of the lumbar spine (it does not provide instructions for standing flexion-extension X-rays). Positioning instructions for hyperflexion say to “…lean forward and draw the thighs up to forcibly flex the spine as much as possible” and instructions for hyperextension say to “…lean the thorax backward and posteriorly extend the thighs and limbs as much as possible”.

Despite arguments and evidence that may be used against the potential of lumbar flexion-extension radiographs for the diagnosis of instability_PCS, data document that good patient effort can be obtained in symptomatic patients. Figure 2 shows data from a large multisite study of lumbar stenosis patients.(120) Data were available for at least 10 subjects from each site. The patient inclusion/exclusion criteria were the same at all sites, so no differences would be expected between sites. Figure 2 documents that some of the sites averaged far greater intervertebral rotation than did others. High-performing sites consistently achieved good patient effort and thereby diagnostic quality flexion-extension studies. The most likely explanation for differences between high- and low-performing sites is the flexion-extension protocol used at the site. It is also possible that at high-performing sites, the physician and/or radiology technologist can overcome a patient’s fear of motion by explaining to the patient that maximal voluntary flexion and extension will not injure their back and is necessary for reliable diagnosis.

At this point, it is important to acknowledge the following: flexion-extension X-rays should be ordered for a specific diagnostic purpose, which typically is to test for the presence/absence of motion abnormalities. If these radiographs do not capture the patient’s full range of motion related to symptoms, then the test may be invalid, and the patient may have been unnecessarily exposed to ionizing radiation. Thus, the importance of acquiring high-quality flexion-extension radiographs lies not only in enabling accurate diagnosis of instability_PCS but also in maximizing patient safety.

It is currently unknown how many false negative tests for spinal instability occur due to insufficiently stressed spines. It is also unknown how many studies of lumbar flexion-extension radiographs would have diagnosed instability_PCS if a flexion-extension protocol, validated to sufficiently stress the spine, had been used. It is not unreasonable to assume that adoption of a good flexion-extension protocol will allow diagnosis of instability_PCS that may otherwise be missed. The data in Fig. 2 may also serve as a benchmark that sites can use to assess the quality of their flexion-extension exams. A simple flexion-extension protocol that has been used to collect several hundred exams of asymptomatic volunteers and used in a clinical study is described in a YouTube video: https://youtu.be/YDcMMZdc7dc

Supine-Standing

Many studies have investigated the potential of measuring intervertebral motion by comparing images of the patient supine with images of the patient standing or flexed. (102, 121–123) Some of those studies found that the greatest intervertebral translation was between upright and supine positions, while in other patients, the greatest translation was between upright flexion and extension. Again, it is unknown whether rigorous, quality-controlled protocols would have changed the results. One potential advantage of comparing supine versus upright images is less dependency on patient effort, although a careful analysis is needed to determine how each element of the passive motion control system is stressed using a supine vs upright protocol. Will spinal loads supine compared to spinal loads standing result in stresses that would reliably reveal incompetent anterior and/or passive motion restraints at all levels in each patient? In some patients in the supine position, thick adipose tissue posterior to the spine could influence vertebral body displacement via forces exerted on the spine by the tissues that are compressed between the table and the spine. Little is known about the change in motion segment loading between the standing and supine positions and what factors may influence the change in loading. Disc pressure changes have been studied (124), but those measurements do not help with understanding sagittal plane shear loading. The standing-to-supine protocol may yield a false negative result for instability_PCS if the change in sagittal plane shear forces between standing and supine does not provoke abnormal translations. Although it may be possible to obtain some answers using computer models, a study comparing supine radiographs or even MRIs to upright and high-quality bending views may answer these questions. If the false negative rate is acceptable, then flexion-extension views could be obtained less frequently, and less radiation could be applied to the patient. A fulcrum bending protocol has shown some potential, although this would require careful positioning of a bolster as well as the ability to routinely obtain a cross-table lateral X-ray.(125) It is also unknown whether the same normative reference data and instability criteria can be used for both flexion-extension and upright-supine imaging.

Confounding variables in the interpretation of intervertebral motion data

All patients are unique, and each comes with multiple factors that may influence both their symptoms and their response to treatment. Any given intervertebral motion segment may exhibit features that impact the influence that motion has on symptoms. For example, intervertebral rotation and translation may be inherently reduced in patients with naturally narrow discs. (50) A sagittal plane translation of 8% endplate width may have greater clinical consequences when the disc height is 2 mm compared to when the disc height is 12 mm. Similarly, a sagittal plane translation of 8% may have greater significance when there is Meyerding grade 2 spondylolisthesis compared to a level with no spondylolisthesis. It is also possible that a sagittal plane translation of 8% is better tolerated when the spaces through which nerves run (central canal, lateral recesses, and foramen) are genetically large compared to when they are genetically small. These hypotheses have yet to be critically investigated.

Psychosocial factors, lifestyle factors, and the complex perception of a multitude of possible pain generators may contribute to symptoms and treatment response and thereby confound interpretation of diagnostic tests for instability_PCS. (126, 127) For example, a sedentary lightweight individual may be able to tolerate greater instability_PCS than a heavier patient with a high physical demand lifestyle. Since there are many possible pain generators in the spine (128–130), it may be difficult to find strong associations between abnormal intervertebral motion and pain, since pain may also come from sources other than an unstable level in some patients. Instability_PCS should be expected to be just one of multiple issues that influence patient symptoms.

Is spondylolisthesis diagnostic for instability?

The presence of spondylolisthesis is sometimes equated with the presence of instability, although this is not well supported by scientific evidence. (9) Spondylolisthesis can be classified as static if there is no more than normal sagittal plane translation between flexion and extension or dynamic if there is abnormal translation.(131, 132) Differentiating between static and dynamic spondylolisthesis requires determining the presence and extent of spondylolisthesis as well as whether the intervertebral motion is normal or abnormal.(7, 133–135) It is not known whether translation measured in radiographically normal asymptomatic volunteers is the best reference for interpreting translation in the presence of spondylolisthesis, as it is possible that even a little motion in a highly stenotic segment with spondylolisthesis is too much motion (e.g., is enough to cause symptoms). Ultimately, the binary distinction (static or dynamic) commonly used to classify spondylolisthesis may be an oversimplification of a complicated problem.

Some sagittal plane offset (SPO) of one vertebra relative to an adjacent vertebra can be normal, (136) and the term spondylolisthesis is best reserved for abnormal SPO. Diagnosis of spondylolisthesis when the amount of spondylolisthesis is close to the normal range of SPO can be challenging because the amount of SPO that can occur in a healthy normal spine is dependent on both level (e.g., normal range for L1-L2 is not the same as for L5-S1), disc heights and disc angle.(136) A spondylolisthesis index can be used to easily determine if the measured SPO is normal or abnormal (e.g., spondylolisthesis) for a specific level and for specific disc heights and disc angles. (136)

In the presence of spondylolisthesis, some translation can occur between flexion and extension or between supine and standing.(99, 106, 134, 135, 137) Determining whether the measured translation is abnormal is incompletely understood; however, in line with the rationale previously presented, it is important to assure that the spine is sufficiently stressed to detect dynamic spondylolisthesis if it can occur. Many studies have found that specific patient positioning protocols can provoke spondylolisthesis that would not be appreciated from other protocols.(47, 98, 99, 106–110, 134, 135, 137) In some patients, at some levels, measuring intervertebral motion between supine and standing positions may best provoke translation diagnostic of dynamic spondylolisthesis, whereas in others, flexion vs extension may be the most provocative. Additional research is needed.

Choice of data used to define normal

An objective diagnostic test for instability_PCS must have reference data that can be used to classify a test result as normal vs abnormal and inform how far a patient test result is from normal limits. The reference data should ideally have been collected using the same measurement technology and the same image acquisition/patient positioning protocol, unless it can be proven that an alternative measurement technology or protocol provides equivalent results. Ideally, reference data should also be validated to represent the characteristics of the patient being tested, e.g., comorbidities, sex, age, and other demographic variables, that can be proven to substantially effect the results. It is unknown whether the optimal reference data should only represent non-degenerated discs in asymptomatic volunteers or should also include degenerated levels if they are verifiably asymptomatic levels. That unknown can be tested using properly designed clinical trials. If artificial intelligence/machine learning technology (AI/ML) is used to produce the test result, that technology should be generalizable to other imaging equipment, acquisition protocols, and patient characteristics associated with the inputs to the AI/ML solution(s).(138)

Multiple peer-reviewed publications provide intervertebral rotation, translation and other data intended to help define “normal” (Table 1). The data document a wide range of “normal” rotation and translation. The results are not consistent between studies. The results are likely dependent on how the motion was provoked, the effort made to assure that the spines were adequately stressed, and how the motion was measured. No previously published intervertebral motion data represent the entire population of people who might benefit from a diagnostic test for instability_PCS. It is also not well-known whether adjustments are needed for age, sex, and other variables.

The statistical methods used to define “normal” intervertebral motion from data for an asymptomatic population must also be considered as alternatives to the 95% confidence interval approach that is commonly used. (52) Compromises must be made as the field works toward a standardized reference data set. In the interim, the limitations of reference data used for an instability_PCS test result should be documented and understood by the clinician that uses the test result.

Table 1

Summary of published intervertebral rotation data that could be used to help define “normal” rotation. Average rotations at each level are provided if available in the paper. The numbers in parentheses are the upper limits of normal if provided or estimated from the mean and std dev if the std dev was provided. Sd = standard deviation
Study	Year	N	Ages	Position	Pelvis	L1-L2	L2-L3	L3-L4	L4-L5	L5-S1
Tanz(139)	1953	39		Supine	free	5.6	7.6	8.6	12.2	8.2
Allbrook(140)	1957	20		Stand	free	5.7 (13.7)	8.3 (14.5)	13.2 (19.9)	18.9 (27.2)	17.6 (26.6)
Clayson(141)	1962	26				12.6	15.8	15.9	17.7	18.7
Froning(142)	1968	30	20–69	Kneel/Stand	free	17	16	13	11	9
Hayes(143)	1989	59		Sit/Stand	free	7 (14)	9 (16)	10 (18)	13 (20)	14 (27)
Pearcy(144)	1984	11	25–36	Stand	fixed	13 (22.8)	14 (17.9)	13 (18.9)	16 (23.8)	14 (23.8)
Boden(145)	1990	40	19–43	Stand/Sit	free	8.2 (15.3)	7.7 (15.3)	7.7 (17.5)	9.4 (22.1)	9.4 (21.4)
Dvorak(68)	1991	41	18–50	Stand	fixed	11.9 (16.3)	14.5 (19)	15.3 (19.3)	18.2 (24.1)	17 (25.5)
Miyasaka(69) (Max)	2000	90	20–39	Stand	free	12.1 (20.1)	15.1 (20.4)	15.7 (21.4)	18.2 (24.3)	17.7 (30.6)
Miyasaka(69) (Mod)	2000	90	20–39	Stand	free	9.3 (22)	9.5 (20.3)	9.1 (22.8)	7 (16.8)	7.5 (19.1)
Lee(146) (max)	2002	30	20–29	Stand	free	12.7 (19.1)	12.1 (17.3)	10 (15.2)	7.2 (12.4)	5.2 (9.9)
Wong (147) Male	2004	50	20–76	Stand	free	12.4 (16.5)	12.9(16)	11 (13.4)	9.3 (13)	7 (10.3)
Wong(147) Female	2004	50	20–76	Stand	free	14.4 (19.1)	11 (14.3)	14.4 (17.5)	10.7 (13.8)	7.6 (10.5)
Li(148)	2009	11	50–60	Stand			5.4 (12.8)	4.3 (11)	1.9 (4.1)
Mellor(149)	2014	40	21–50	Passive, supine	fixed		8.7 (12.5)	9.6 (13)	11.8 (16.5)
Staub (66)	2014	160	18–82	Seated	free	11 (16.3)	12.4 (17.4)	13 (18.2)	14.5 (21.6)	12.8 (23.4)
Cheng (109)	2016	57	48 sd 11	Standing	free	7.6 (16)	11.2 (20.6)	10.1 (20.1)	11.9 (25.8)	8.1 (20.3)
Cheng (109)	2016	57	48 sd 11	VMAStanding	guided	8.6	10.1 (17.4)	9.2 (17.2)	8.3 (16.9)	10.3 (19.3)
Cheng (109)	2016	57	48 sd 11	VMALying	guided	5.9	6.8 (11.5)	7.7 (12.8)	9.1 (15.6)	8.5 (16.7)
Breen(150)	2021	127	21–80	VMAstanding	guided		9.5 (17.1)	10.6 (16.4)	10.4 (18.1)	5.7 (16.7)

End range vs mid-range controversy

Numerous studies may support the hypothesis that the range of motion between vertebrae measured from conventional flexion and extension X-rays is not as valuable as analysis of the pattern of motion between the end-ranges of the flexion-extension cycle. (109, 111, 132, 151–159) These studies support that the greatest intervertebral rotation and translation can occur as the patient moves from flexion to extension and not at the end-range of intervertebral motion. These studies also document that irregularities and discontinuities in intervertebral motion can occur during the flexion-extension cycle that cannot be detected from a two-frame flexion-extension study. Conversely, other studies document continuous rather than discontinuous motion.(13, 132, 156, 159–162) The proportion of patients where mid-cycle peaks and irregularities in motion occur is not well documented.

The mechanisms that may cause mid-cycle irregularities are poorly understood. Muscle spasms, voluntary or involuntary responses to actual or anticipated spikes in pain intensity, uncertainty about how they are supposed to move, bone-on-bone catching, breathing cycle, and other phenomena are possible explanations. It is not known whether these discontinuities are repeatable or diagnostically useful. It is also not known if the patient (and X-ray technician) can be taught a flexion-extension protocol that would consistently allow diagnosis of instability_PCS from end-range flexion-extension X-rays and thereby eliminate the need for continuous imaging and the additional diagnostic complexity. It is not known if there are diagnosis and treatment algorithms for mid-range instability_PCS that might differ from those for end-range instability_PCS.

Continuous motion can be assessed from a sequence of fluoroscopic or fast digital radiographic images that capture motion throughout the flexion-extension cycle. Not all sites that currently perform flexion-extension exams have that capability, and the additional expense and exposure to radiation would need to be justified. No study that has compared analysis of continuous imaging to analysis of a two-frame flexion-extension study documented whether the two-frame flexion-extension exams were obtained using a validated protocol that assures that the spine was sufficiently stressed. It is also possible that the positions between vertebrae that occur when a patient moves to and holds a flexed or extended position are not the same as when a patient is performing continuous flexion-extension motion and not stopping at end-range positions. Thus, research to definitively compare continuous imaging with end-range imaging would need to separately collect both continuous and end-range imaging using validated protocols.

Perhaps the greatest challenge is the selection of the optimal metric(s) for the diagnosis of instability_PCS. The remainder of this document will address this challenge. Multiple sagittal plane instability_PCS metrics and interpretation criteria have been used in clinical diagnosis and research studies. Due to the lack of a validated “gold standard” test for instability_PCS, all prior research studies were “validated” against unvalidated criteria. Elmose et al recently cataloged the use of various spinal instability definitions.(163) Other review papers have also discussed spinal instability criteria.(5, 6, 13, 26) (7, 164) The following is an attempt to summarize and build upon that work, with the goal of working toward a standardized and practical diagnostic test for instability_PCS.

Intervertebral rotation and/or translation above a limit

The most common approach to classifying a level as stable or unstable is to use a threshold level of intervertebral rotation and/or translation.(165) The threshold levels of rotation and translation detailed on page 352 of the text book by White & Panjabi (166) have been used in many studies. These thresholds for intervertebral rotation and translation were originally intended to be used within a point system that includes other factors.(127) They were not intended to be used in isolation, though they commonly are. The White & Panjabi motion thresholds are sagittal plane translation of > 4.5 mm or 15% of the sagittal width of the vertebral body and sagittal plane rotation greater than 15° at L1-2, L2-3, and L3-4, 20° at L4-5, and 25° at L5-S1 on flexion and extension radiography. As White & Panjabi noted, these criteria are based on a study by Posner et al(34) (with Dr. White as a co-author) using cadaver spines and are based on simulation of traumatic and not degenerative instability. Limitations of these criteria include the following:

May not be appropriate for use in assessing for instability associated with degenerative changes, since they were based on simulation of traumatic injuries. (167) The diagnostic performance of these thresholds is unknown.
They are very dependent on patient effort when asked to flex and extend. A patient may exceed the White & Panjabi radiographic criteria when motivated to maximally flex and extend during activities of daily living, but this would not be apparent if they do not maximally flex and extend when radiographs are obtained.
As previously discussed, unless radiographic magnification is known, errors will exist when translation is measured in millimeters. The magnitude of the error could result in misdiagnosis.
They were adapted from a study of 7 cadaver spines using methodology that would be considered low-tech by today’s standards, and the study has never been repeated. Spinal segments were tested with combinations of compressive and shear forces that may not be the best representation of common physiologic forces.

As noted, the White & Panjabi criteria were adapted from the Posner et al study. Yone et al. tested a more direct interpretation of the Posner et al. study. (168) Yone et al. used the Posner et al. study to classify a level as unstable using Table 2. Note that Yone et al. used measurements of sagittal plane offset and disc angle from any available lateral radiograph and not the measured translation and rotation that occurs between flexion and extension as was reported by Posner et al.

Table 2

Criteria used by Yone et al(168) to classify levels as unstable.
Level	Sagittal Plane Offset (% Endplate Width)		Intervertebral Disc Angle (degrees)
Level	Anterior	Posterior	Intervertebral Disc Angle (degrees)
L1-L2 to L4-L5	8	9	-9
L5-S1	6	9	1

Yone et al found that outcomes were worse when lumbar stenosis patients were treated by decompression alone if the level was unstable by their interpretation of the Posner et al criteria. This interpretation is not an assessment of translation between flexion and extension but an assessment of the maximum amount of spondylolisthesis measured from any single X-ray. The results are also dependent on how much stress the patient applied to the spine when asked to flex or extend. The Yone et al study may best be used in support of the observation that lumbar stenosis surgery outcomes are worse in the presence of spondylolisthesis, for the type of stenosis surgery they used. The Posner et al criteria have also been applied to analysis of the apparent reduction of spondylolisthesis measured by comparing a standing lateral radiograph to a supine MRI exam. (169) In that study, patients were included if they were clinically suspected of having instability. The Posner criteria applied to flexion-extension X-rays were used as the “gold standard”. A total of 45/75 (60%) patients were found to be “unstable” per the Posner criteria applied to flexion-extension X-rays versus 32/75 patients (42.6%) using spontaneous reduction seen on magnetic resonance imaging.

In a paper discussing indications for lumbar spine fusion, Hanley described instability criteria with supporting evidence: “Most surgeons define segmental instability as either 10° of angular motion or 4 mm of translation on controlled flexion-extension radiographs.”(170) No other supporting evidence was given in this paper that is commonly cited to justify the use of the > 10 deg rotation instability criteria. In another paper, Hanley et al. cited Spratt et al. for the translation criteria and elaborated that 5 mm translation is the threshold that should be used at L5-S1.(171, 172) As documented in multiple reviews, studies of intervertebral rotation in asymptomatic volunteers have found that 10 deg of angular rotation is well within normal limits. (66, 173, 174) Thus, using the 10° of rotation criteria, many asymptomatic people would be diagnosed as having unstable levels. The logic for using a 10° rotation threshold may be that symptomatic patients may be reluctant to flex and extend, and thus, 10 deg rotation would be high in symptomatic patients. However, as previously noted (Fig. 2), symptomatic patients can average more than 10 deg of rotation with good flexion-extension protocols. Thus, the 10° rotation criterion is not supported by scientific evidence.

A threshold level of > 3 mm translation has been used as an indicator of instability.(175, 176) This threshold is referenced in papers by Boden et al, Iguchi et al, and Kanemura et al.(17, 19, 145) None of these papers stated if corrections were made for variable radiographic magnification. Boden and Weisel measured translation from pre-employment X-rays of 40 male volunteers(145) using a method attributed to Quinnell and Stockdale(177) and concluded that “Normal lumbar vertebral levels should have less than 3.0 mm of dynamic antero-posterior (AP) translation (< 8% of vertebral body width).” These criteria are specific to their implementation of the Quinell and Stockdale method (which does not appear to be commonly used and was intended to be only an approximation of true displacement). (177) The Boden and Weisel criteria are not validated to be applicable to other translation measurement protocols. With other methods of measuring translation, translation of 8% endplate width is well within the normal range of translation. (66)

Despite the limited scientific justification, a review by Simmonds et al. cited many studies where radiographic instability is defined as “a disc angle change > 10° or change in translation > 3 mm, from standing or supine radiographs to dynamic radiographs” (7) More recent studies also use these criteria. (178) (176). Even with these low thresholds for rotation and translation, it must be appreciated that a patient may have > 10 deg of rotation or > 3 mm of translation during activities of daily living, but that may not be detected due to insufficient patient effort when asked to flex and extend. Collecting high-quality flexion-extension X-rays or making an adjustment for patient effort is needed.

Several authors have investigated the potential of reporting rotation as a percentage of total rotation through the lumbar spine (e.g., L1 to S1 rotation).(52, 149, 150, 174, 179–181) Several published studies provide data documenting that different levels contribute unevenly to total motion or that different levels in the spine are sequentially recruited as motion proceeds between flexion and extension.(82, 156, 182, 183) Thus, the proportion of motion that each level contributes could depend on what proportion of the flexion-extension cycle was captured by the flexion-extension X-rays used to measure rotation. A standardized flexion-extension protocol with validated quality control criteria might help to avoid that limitation. Robust reference data may also help to account for uneven contributions to overall motion.

Enlargement of the Neutral Zone

Panjabi et al and others have described the concept of neutral and lax zones in the relative motions between vertebrae.(77, 79–85, 88, 90) The neutral and lax zones are where intervertebral rotation and translation can occur with little or low force. Motion within the normal neutral/lax zones is poorly controlled. The motion is insufficient to reliably assess the ability of the annulus and ligaments to restrain motion to within normal limits since the annulus and ligaments are not stressed when motion is within the neutral zone. Once motion is outside of the normal neutral and lax zones, higher forces are required to achieve intervertebral motion in a healthy spine, and it becomes possible to detect if the intervertebral motion restraints are functioning normally. A reliable test for instability_PCS requires that the spine is loaded to the point where motion would be outside the neutral/lax zones in a healthy spine, and the intervertebral motion restrains would be restraining motion if they are functioning normally.

When intervertebral motion restraints are incompetent, the NZ can become larger, and that phenomenon may serve as the foundation for a diagnostic test for instability_PCS.(79, 80, 184) The NZ can be measured in the laboratory using cadaver spines where both the applied load and resulting displacements can be accurately measured. Direct clinical use of the NZ to diagnose instability_PCS would require measurement and analysis of intervertebral motion versus load/moment curves for individual motion segments. No clinically practical methods currently exist to directly measure intervertebral loads or moments in a living person. It may be possible to estimate spinal loading using models(185–187) and combine that with noninvasively measured motion and thereby estimate the NZ in patients. However, that hypothesis and the required methodology have yet to be fully developed and tested. It is also not known if this would provide additional, actionable and clinically efficacious value beyond a simpler analysis of non-invasively measured intervertebral motion. Attempts have been made to measure the neutral zone intraoperatively, but this would have limited clinical utility. (86, 87) Nevertheless, if a spine is stressed sufficiently, by a valid flexion-extension or other protocol, it may be possible to reliably detect that rotations or translations are greater than would be expected if the intervertebral motion restraints were functioning normally and the neutral/lax zones were normal.

Abnormal COR

In two-dimensional images, the center-of-rotation (COR) describes a point about which one vertebra rotates with respect to an adjacent vertebra.(188) The COR can be measured from just two images (e.g., flexion and extension) or between any two frames from a series of images that capture a full flexion-extension cycle. A series of images allows for analysis of movement of the COR during the flexion-extension cycle and thereby assessment of the instantaneous center of rotation (ICR). (189)

The COR is typically reported as anterior-posterior and cranial-caudal coordinates relative to a frame of reference defined by the inferior vertebra. In clinical practice, a clinician would need to know if the coordinates of the COR are normal or abnormal. Reference data for the COR between flexion and extension in a population of asymptomatic volunteers are available.(66, 188, 190–192) There are some consistencies and some differences between studies. The external validity or clinical efficacy of currently available data is unknown.

When measured from a series of radiographic images obtained over the entire flexion-extension cycle, the variability of the COR can be reported, with the hypothesis that during the flexion-extension cycle, the COR will move substantially more in the presence of instability_PCS.(151, 152, 193, 194) Evidence supports that the 2D coordinates of the COR shift with instability, and motion of the COR during the flexion-extension cycle is wider in degenerated cadaveric spines and in patients suspected to have lumbar spine instability.(89, 151, 156, 190, 192) The movement of the COR as the spine flexes may also depend on exactly how the spine is loaded(195), supporting the need for a standardized loading protocol. The location of the COR is partly determined by facet joint forces,(196) so how the spine is loaded when flexion and extension X-rays are obtained may influence COR data. Thus, patient positioning protocols should be standardized to the extent possible if COR is used to diagnose instability_PCS. Quantifying the continuous movement pattern of the COR between flexion and extension is challenging. The anterior-posterior width or cranial-caudal height of the COR movement pattern is one possibility. The area that includes all COR points is another option. Determining whether the COR movement pattern throughout a flexion-extension cycle has greater diagnostic efficacy than the coordinates of a single COR point measured from end-range flexion and extension is unknown. The coordinates of the COR are also correlated with other intervertebral motion metrics. (197) This will be discussed later in this paper. It is not known if COR has advantages over other metrics that may be easier to interpret.

Facet fluid sign or vacuum sign

Numerous investigators have observed and studied what appears in an MRI exam to be an abnormally large amount of fluid in the facet joints and have suggested or investigated an association between this facet fluid sign and instability. (59, 163, 198–208) A recent review concluded that dynamic spondylolisthesis is 8 times more likely in the presence of a facet fluid sign.(209) Other authors have noted or studied the vacuum sign that can appear in the facet joints on a CT exam or even radiographs.(11, 210–214) The hypothesis is that with instability_PCS, an abnormally large gap can occur between articular processes that comprise a facet joint. That gap can fill with either fluid or gas, and little is known about how to optimize the diagnostic utility of these phenomena.

There are potential limitations to the use of the fluid sign to diagnose instability_PCS. First, fluid exists in a healthy joint, and criteria must be validated to determine whether the amount of fluid is normal or abnormal. The gap between facet joints can be uneven when viewed in coronal or sagittal plane images, particularly in full flexion or extension or with collapsed disc height. Thus, axial slices in the wide part of the gap may show a thick fluid layer, whereas slices in the narrow part may not. No study has rigorously validated strict interpretation criteria; for example, a fluid gap > 1.5 mm must be detected in at least two slices through the facet joint. Since the orientation of the slice plane relative to the facet joint is highly variable, it may be difficult to obtain two good slices through the facet joint, especially with thick slices and volume averaging. Analysis of the gap from thin slice CT (relative to a normative database) or high-resolution isotropic MRI are potential options. Thus, abnormal facet widening may exist but be undetected in some exams, or normal facet gaps may be diagnosed as abnormal. Without strict criteria, substantial intraobserver error can be expected. Although it may be possible to obtain reasonable observer agreement in controlled research studies(203, 204, 215), assessment reliability in routine clinical practice may be more difficult to achieve(216). In addition, even if good fluid sign agreement can be obtained, the sensitivity and specificity of a gold standard test for instability need to be determined.

Second, a vacuum sign in the facet joints, as observed in a CT exam, is also considered an indicator of instability (although this is not validated against a gold standard).(3, 6, 212, 217–219) However, a vacuum would not appear as a bright fluid sign in an MRI exam. This supports that an abnormally wide gap in a facet joint must first be filled with fluid prior to the MRI exam. If not, the MRI exam could yield a false-negative instability_PCS diagnosis. It is not known under what conditions and how long it takes for an abnormally wide facet joint gap to fill with fluid.

Third, it is not known whether supine positioning will always correctly stress the spine to provoke facet joint widening in the presence of instability_PCS. Presumably, there must be sufficient forces between vertebrae to cause abnormal facet gapping in a supine patient with instability_PCS. Upon review of midsagittal slices from CT exams, it is not clear how this could be a reasonable expectation at all levels from L1-L2 to L5-S1, given the wide variability between patients in supine lordosis and variability in thickness and composition of soft tissues posterior to the spine. Comprehensive validation of the reliability of the facet fluid sign is needed. Despite all of the potential limitations, it has been suggested that the facet fluid sign is the best currently available test for lumbar spine instability.(208) Evidence for the ability of the facet fluid sign to predict clinical outcomes is beginning to emerge.(220) The sensitivity and specificity will not be definitively known until a gold standard test for instability_PCS is available.

Rotation Dependent Translation (RDT)

Between the flexed and extended positions, the amount of sagittal plane translation between vertebrae, corrected for the amount of rotation (to help control for variability in patient effort), may serve as the basis for a diagnostic test for lumbar instability_PCS.(13, 59) Healthy facet joints have a very strong capsule that, together with the geometry of the facet joints, allows for only small sagittal plane translations. (42, 161, 221, 222) A healthy intervertebral disc will also limit translation. Since there is no reason why sagittal plane translation would be desirable independent of rotation, it is likely that normal intervertebral translations are limited to what is required to achieve the intervertebral rotations required for activities of daily living. A diagnostic test for abnormal RDT has the potential for detecting abnormally high translations that can occur with instability_PCS. (13, 59, 66) This is somewhat supported by data documenting that abnormal RDT is associated with the facet fluid sign.(59) Sagittal plane instability is believed to require both laxity of the facet joint and disc degeneration.(223) Some support exists for an association between abnormal translation and facet degeneration (206, 224) Disc degeneration alone may not result in abnormal sagittal plane shear translation.(225, 226)

There is currently only limited evidence of an association between abnormal RDT and symptoms.(13) This is understandable given the lack of a validated test for abnormal RDTs. Although no strong evidence currently exists, abnormally high translation may irritate nerve roots or facet joint nociceptors, potentially causing inflammation and thus forming an indirect association between abnormally high translation and symptoms. As Kirkaldy-Willis and Farfan hypothesized 40 years ago, “size reduction of the lateral nerve root canal may of itself produce minor symptomatology, but with the increased motion it may become a severe clinical problem.”(1) The association between the degree of stenosis and symptoms is only moderate.(227–230) Lumbar spinal stenosis can be found in asymptomatic people.(231) It is largely unknown why stenosis results in symptoms in only some people. In addition to the pressure on nerve roots that may be caused by stenosis, inflammation or prior irritation contributes to symptoms.(49, 232, 233) It is possible that abnormal RDT results in greater symptoms when the nerve roots are inflamed and that abnormal RDT can be found in asymptomatic people. Thus, analogous to disc degeneration observed on X-ray or MRI, the diagnosis of abnormalities in RDT may be helpful in symptomatic patient management even if abnormal RDT (or disc degeneration) can be asymptomatic. The role of abnormally high (or possibly abnormally low) RDT in patient symptoms can be studied once a diagnostic test for abnormal RDT is validated.

Abnormal RDT can only be diagnosed if normal RDT is documented. Data to help define normal RDT have been published where a specific definition of rotation and translation was used (Fig. 3).(66) RDT is level dependent, requiring level-specific look-up tables to interpret measurements. However, RDT can also be reported as the number of standard deviations from average.(59) This metric can be referred to as the sagittal plane shear index (SPSI). SPSI simplifies the interpretation of RDT. A value of 0 would mean that RDT is exactly average (for the specific level) for asymptomatic and radiographically normal levels. A value between − 2 and 2 is within the 95% confidence interval for asymptomatic volunteers. A value of 3 would indicate RDT is 3 Std Dev above average normal, and this would be objectively abnormal.

Retrospective reanalysis of flexion-extension radiographs was performed to help better understand RDT. The reanalysis was performed using a fully automated emulation of the previously validated Quantitative Motion Analysis (QMA) method (SpineCAMP™, Medical Metrics, Inc., Houston, TX). (234–236) This method uses a pipeline of neural networks and coded logic to produce four anatomic landmarks for each vertebra(237) and determine transformation matrices to move landmarks from the flexion to the extension image. The registered landmarks are then used to calculate the intervertebral motion metrics. Many researchers have developed neural networks to place anatomic landmarks on vertebral bodies in spine radiographs.(238–243) It is expected that the results described below can be reproduced using any method validated to reliably place standardized anatomic landmarks on vertebral bodies. Standard placement of lumbar vertebral landmarks has been previously described.(237)

Based on a reanalysis of flexion-extension radiographs for 162 asymptomatic volunteers (66), the R² was 0.61 between a normalized expression of RDT (SPSI) and the cranial-caudal coordinate of the COR. This relationship with the cranial-caudal coordinate of the center of rotation (COR) is as expected.(189) The strength of the relationship between RDT and COR helps to appreciate that the accumulation of knowledge regarding COR has relevance to the diagnostic tests based on RDT.

The potential for RDT to serve as the basis for a diagnostic test for instability_PCS would be strengthened if sagittal plane intervertebral translation is linearly related to rotation. Figures showing the relationship between intervertebral translation and rotation support that the relationship between translation and rotation can be approximately linear.(13, 132, 156, 160–162) However, there is also ample evidence to support that translation is not always linearly related to rotation. (111, 154, 180, 244, 245) It may be that in a healthy spine (or in a cadaver spine tested ex vivo), with no confounding effects from the active and neural control elements of spinal stability, translation is approximately linearly related to rotation when the motion segment is outside the neutral zone and moving in the elastic zone. However, if the motion segment is in the neutral zone, particularly if the neutral zone is abnormally large due to instability_PCS, translation cannot be assumed to change linearly with rotation. It is also likely that muscle spasms, spikes in pain during flexion or extension, or neural control issues (such as uncertainty about how to flex and extend for the test) could cause non-linearity in the relationship between translation and rotation. Thus, although translation can be linearly related to rotation in some cases, that linear relationship cannot be assumed true for all levels in all patients. The linearity of the translation as a function of rotation may also depend on the extent of spondylolisthesis.(159) Finally, the SPSI metric requires dividing translation by the amount of rotation, and this becomes unstable when rotation approaches zero.(13, 66)

Alternatively, a diagnostic test for abnormal RDT can be based on data documenting that translation is linearly related to rotation (outside the neutral zone) across a population of normal healthy spines. With that interpretation, normal translation for any amount of rotation can be determined from a linear regression equation fit to translation versus rotation data for a population of healthy motion segments. The upper and lower limits of the 95% confidence interval for this linear regression can be used to determine if the translation is within or outside of normal for the specific amount of rotation that was measured. That interpretation can be reported as a standardized metric, where a value of zero indicates that translation was exactly the average found in healthy motion segments. A value of 3 would indicate that the translation 3 standard error of the forecast was above the average normal in healthy motion segments. The standard error of the forecast provides a point estimate of the translational variability that exists at a specific amount of rotation and at a specific level.

Using previously published data from radiographs of 162 normal and asymptomatic volunteers (66), the amount of sagittal plane translation that occurs for different amounts of rotation was observed to have a relatively linear relationship between translation and rotation. Figure 4 shows the data for the L4-L5 level. The relationship between translation and rotation was approximately linear for all levels (L1-L2 to L5-S1), with an R² of > 0.55 for L1-L2 to L4-L5 and R² = 0.17 for L5-S1. The observed linear relationship between translation and rotation across a population allows the use of these data to estimate average normal translation and 95% confidence intervals for a specific amount of rotation. Until proven otherwise, it can be assumed that use of this RDT test for instability_PCS requires that the spine be stressed so it would be outside of the neutral/lax zones in the absence of instability_PCS. Until better quality control criteria are validated, it can also be argued, based on a review of existing neutral-zone data (77, 79–85, 88, 90), that 5 deg of intervertebral rotation between flexion and extension is sufficient to assure that the motion segment is adequately stressed.

Several different types of sagittal plane intervertebral translation can be measured, such as translation of the posterior-inferior corner of the superior vertebra in the direction defined by the superior endplate of the inferior vertebra, translation of the superior vertebral centroid relative to the inferior vertebra, or translations in the cranial-caudal direction. It is valuable to contemplate the potential physiologic implications of an abnormality for a specific type of translation measurement.

When sagittal plane translation is measured as the translation of the posterior inferior corner of the superior vertebra, in the direction defined by the superior endplate of the inferior vertebra (Fig. 3), it can be described as motion that could particularly affect tissues in the foraminal and spinal canal regions. If the foraminal region translation is reported as an index relative to the average and 95% confidence intervals for an asymptomatic population, then this can be referred to as the Foraminal Sagittal Translation (FST) index. If the FST-index is zero, then translation is exactly average relative to the asymptomatic population at the level being assessed for the amount of rotation that occurred. An abnormally high (e.g., 3) FST index would inform clinicians that the posterior-inferior edge of the superior vertebra is translating in the sagittal plane much more than it should, with respect to the inferior vertebra. The FST-Index may help in assessing the Kirkaldy-Willis and Farfan hypothesis: “size reduction of the lateral nerve root canal may of itself produce minor symptomatology, but with the increased motion it may become a severe clinical problem.”(1) Examples of levels with normal and abnormal FST-Index can be viewed at:

https://www.dropbox.com/sh/7z3vu3i977ip530/AAD8Oc-Ref_PAJd0tEPopkdXa?dl=0

In each online example, the inferior vertebra for the level (e.g., L5 if L4-L5 is the target) will remain in a constant position on the display as the flexion and extension images are alternately displayed. This is referred to as “stabilization” and facilitates interpretation of the relative motion between vertebrae.

One additional advantage of a standardized and normalized metric such as the FST-index is that there is no need to calibrate X-rays to obtain accurate measurements in units of millimeters.

A diagnostic test for instability_PCS should detect abnormalities in populations of patients where abnormalities might be expected (e.g., lumbar spinal stenosis) but not detect abnormalities where instability_PCS would not be expected (e.g., subjects in disc arthroplasty trials where instability was an exclusion criterion). It is yet unknown how high the FST index (or alternative metric) must be before it becomes clinically significant. An FST-index > 2 can be just outside of normal limits and may be within the test error. A FST-index > 3 is well outside of normal limits and may prove to be a more efficacious diagnostic threshold.

In its role as an imaging core laboratory, Medical Metrics, Inc. (MMI) has analyzed thousands of flexion-extension exams from studies of treatments for spinal stenosis as well as studies of disc arthroplasty and biologic treatments for disc degeneration. MMI pools data from multiple studies to develop benchmark data that can be used to help identify problems with incoming flexion-extension studies. These flexion-extension exams were retrospectively analyzed using the previously described SpineCAMP, and the resulting intervertebral motion data were used to help understand the prevalence of an abnormal FST index in different populations of patients. In calculating the following prevalence data, only pretreatment data and only levels with > 5 deg rotation are included. The pooled analysis included 7,621 pretreatment flexion-extension studies. Table 3 has the proportion of treatment and adjacent levels where the FST-Index was > 2 (includes borderline abnormalities) and where the FST-index was > 3 (more substantially abnormal). In stenosis, fusion, and dynamic stabilization patients, the high FST-Index is generally at the treatment level, while in disc arthroplasty and biologic treatment patients, the high FST-Index is generally at an adjacent level. Instability_PCS might be expected in a proportion of stenosis and fusion patients. Instability adjacent to disc arthroplasty levels may affect treatment outcomes.

The proportion of levels that would be classified as abnormal using previously described instability criteria was more variable. Including all pretreatment data (not just those with rotation > 5 deg as with the FST-Index) and defining instability as > 10 deg rotation(170), 9–12% of treatment levels in spinal stenosis and fusion studies would be classified as unstable, and 25–42% of treatment levels in studies of disc arthroplasty or biologic treatments for disc degeneration would be classified as unstable. This difference between study types may be in part due to how different symptoms affect patient willingness to flex and extend but suggest that the > 10 deg rotation criteria will misclassify many levels in patient populations where instability would not be expected. Of note, in the asymptomatic population previously discussed, 72% of levels have > 10 deg rotation.

Using the White & Panjabi criteria, depending on the study type, between 0.1 and 2.1% of treatment levels would be classified as unstable if unstable is defined as rotation > 15 deg at L1-L2 to L3-L4, > 20 deg at L4-L5, or > 25 deg at L5-S1. If instability was defined as intervertebral translation > 8% endplate width, then 4–9% of treatment levels would be classified as unstable (all study types combined). The scale factor was not known for all studies, so intervertebral translation instability criteria that are in units of millimeters could not be assessed in the pooled data. These data support that the White and Panjabi-based criteria may fail to diagnose a proportion of levels with abnormal motion.

Table 3

Prevalence of FST-Index abnormalities in pooled data from different study types.
Study Type	Index Levels		Adjacent Levels
Study Type	% with FST-Index > 2	% with FST-Index > 3	% with FST-Index > 2	% with FST-Index > 3
Treatment for Lumbar Stenosis	11	6	5.1	1.9
Selected for Fusion Surgery	15	7	4.1	1.2
Selected for Dynamic Stabilization	16	7	6.3	2.4
Disc Arthroplasty	2.5	0.7	5.8	2.7
Biologic for disc treatment	3.4	0.9	6.1	2.3

A relationship between intervertebral disc degeneration and segmental stability has been previously hypothesized and studied.(1, 246, 247) This relationship is apparent in the previously described asymptomatic volunteers, as shown in Fig. 5. Disc degeneration was graded by an experienced musculoskeletal radiologist. Although a significant relationship is evident in Fig. 5, this is only a trend, and there is wide variation in the FST index within each radiographic grade of disc degeneration, even within KL grade 0 (Fig. 6). This is also evidence of limitations in the Kellgren-Lawrence radiographic grading system that may inadequately detect early stages of degeneration.

Abnormalities in disc height changes with loading

In the early stages of degeneration, the intervertebral disc can become more flexible.(62, 160, 226, 248, 249) This may also be the state where biologic treatments to halt or reverse degeneration may be most effective since nutrient supply is less impaired, which is required for cell viability.(250, 251) Abnormally high cranial-caudal or “vertical” translations between vertebrae might be diagnostic of loss of pressure in the nucleus, softening of the annulus, incompetence of the longitudinal ligaments, annular avulsions, or other causes. (252–256) Such excessive vertical translations may be associated with the abnormal intervertebral loading patterns found with degeneration that may activate pain-sensing nerves in the facets or endplates. (257) Degeneration has been shown to alter loading across the disc space.(258, 259) Discs can become stiffer with advanced degeneration (Figs. 5 and 6).

Disc “softness” could be measured by comparing disc height in loaded versus unloaded positions and identifying abnormally high disc height changes with loading/unloading. An adjustment may be required to account for the phenomena of diurnal change in disc height.(260) Disc height changes with loading have been previously reported.(261) (262)

The diagnosis of abnormal vertical translations would also be dependent on whether the spine is adequately stressed. One option is to compare disc space between a loaded (eg upright standing) versus a minimally loaded (eg supine) position. Detecting disc height compressibility may require a prolonged period of standing in some patients to compress the disc to its lowest possible height. (263) Similarly, prolonged unloading in a supine position may be required for the disc to achieve its maximum possible height, although that has not been adequately studied. Analysis of compressibility with increased loads on the spine has been used to study the effects of backpacks(264) but may be impractical as a routine diagnostic test.

Alternatively, the change in anterior and posterior disc heights between flexion and extension may be diagnostic for abnormal disc compressibility.(265) Using a specific definition of change in disc heights (Fig. 7), the change in anterior and posterior disc heights was found to be linearly related to intervertebral rotation across the population of asymptomatic volunteers that was previously described. This may be a powerful phenomenological foundation for a diagnostic test: a change in anterior or posterior disc height greater than what occurs in healthy discs may be diagnostic for hypercompressible (and also hypo-compressible) discs.

Similar to the FST-index, an anterior disc widening index (ADW-Index) and a posterior disc widening index (PDW-Index) can be calculated based on predicting the average normal disc widening for the amount of rotation and the 95% confidence intervals for the standard error of the forecast. These indices may help identify discs where the change in the anterior or posterior disc heights is abnormally high (or abnormally low) for the amount of rotation that was measured. The clinical value of these metrics remains to be determined. The importance of applying quality-control criteria to assure that the spine is adequately stressed is unknown with respect to vertical translations. A similar approach might be possible when comparing disc height changes between the supine and standing positions, but that would require valid normative data.

Applied to the pooled, pretreatment lumbar spine flexion-extension studies described above, Table 4 provides the prevalence of ADW abnormalities at the treatment and adjacent levels (for levels where the rotation was > 5 deg). PDW-Index abnormalities were rare at any level (treatment or adjacent) in any study type: < 1% in stenosis, fusion, or dynamic stabilization studies; 5% in disc arthroplasty and disc biologics studies.

Table 4

Prevalence of ADW-Index abnormalities in pooled data from different study types.
Study Type	Index Levels		Adjacent Levels
Study Type	% with ADW-Index > 2	% with ADW-Index > 3	% with ADW-Index > 2	% with ADW-Index > 3
Treatment for Lumbar Stenosis	27	15	7.6	3.4
Selected for Fusion Surgery	48	26	6.9	2.8
Selected for Dynamic Stabilization	40	22	7.8	3.0
Disc Arthroplasty	11	3.9	4.6	1.3
Biologic for disc treatment	6.3	1.6	2.1	0.6

Composite Metrics

From the perspective of nerve roots and tissues within the spinal canal, lateral recesses, and foramen, some level of mechanical tissue “agitation” may occur from the translational component of motion, and some “agitation” may occur from the rotational component. The foraminal area is known to change with flexion-extension and is smaller in the presence of degeneration.(266) Foraminal “agitation” could be quantified using a metric such as the foraminal agitation area (FAA - Fig. 9). The FAA is dependent on intervertebral rotation and translation but also on disc height. Since intervertebral rotation is dependent on effort exerted by the patient when asked to flex or extend, it may be helpful in clinical practice to correct for that source of variability. In radiographically normal levels in asymptomatic volunteers, the FAA is linearly related to rotation (Fig. 10), and this may be captured by expressing FAA as a Foraminal Agitation Index in units of the standard error of the estimate from the average FAA found in healthy discs. The standard error of the estimate provides the expected variability in translation for a specific amount of rotation.

It is reasonable to hypothesize that instability_PCS may best be diagnosed using a composite score composed of disc morphometry, disc health, and intervertebral motion metrics. Integration with standardized symptom assessments and specific MRI findings may also improve clinical efficacy.(267, 268) It must be appreciated that there can be multiple sources of symptoms in a patient from multiple different levels. It may thus be unreasonable to expect a definitive association between a metric that may diagnose instability_PCS at a specific level and symptoms. It is possible that patients may have severe symptoms from something other than instability_PCS and that patients may have instability_PCS but not be symptomatic. This is analogous to the issue of stenosis, where people can have significant stenosis yet remain asymptomatic.(269) Stenosis is not a definitive indication for treatment, and instability_PCS will not be a definitive indication for treatment. Nevertheless, it is reasonable to test for an association between pretreatment instability_PCS and treatment outcomes, since instability_PCS may prove to be one true indication for the optimal treatment in appropriately symptomatic patients. However, a treatment that generally works for patients with instability_PCS may also appear to fail due to a different source of symptoms in a particular patient. Development and validation of such a metric to diagnose instability_PCS will require a large sample size to identify and sort through all the important factors. Multilayer perceptron models or alternative methods may help to learn the clinical scenarios where instability_PCS is important and useful in diagnosis and treatment planning. Multiple large-scale spine registries are currently enrolling patients and collecting potentially valuable standardized patient-reported outcomes.(270, 271) Many of the knowledge deficits noted in this paper could be addressed if these registries would collect high-quality flexion-extension studies for cohorts of enrolled patients, and then obtain validated intervertebral morphometry and motion metrics. This small incremental effort may lead to efficacious new strategies for optimizing patient outcomes.

The effects of translation and rotation on nerve roots and other tissues may be exacerbated by congenitally narrow foramen or spinal canals, loss of disc height, spondylolisthesis, stenosis, and other factors. This hypothesis can be appreciated by viewing examples of intervertebral motion in symptomatic patients:

https://www.dropbox.com/sh/7z3vu3i977ip530/AAD8Oc-Ref_PAJd0tEPopkdXa?dl=0

Based on the definition of requirements for validating a diagnostic test(55), there are no valid tests for abnormalities in the ability of the spine to provide the asymptomatic intervertebral motion control that is critical to activities of daily living. This is despite hundreds of research studies between 1944 and 2022. Over these decades, incredible scientific and technological achievements have occurred in many other areas of life (that seem immensely more challenging than a diagnostic test for instability_PCS). Appreciating that spinal disorders are one of the most disabling and costly of all medical conditions(126) and hypothesizing that treatable instability_PCS may play a substantial role in optimizing outcomes for a proportion of patients, it seems justifiable to commit sufficient resources to developing and validating diagnostic tests for instability_PCS. There is far more unknown than known about the diagnosis and treatment of instability_PCS. A wealth of new knowledge could be facilitated by a validated test for instability_PCS. Several potential tests are described in this paper. Given that spine registries generally have a goal of improving outcomes, large spine registries may be the ideal source of evidence for determining if and how intervertebral motion measurements can be used to help improve outcomes. A systematic approach to addressing the unknowns associated with the diagnosis of instability_PCS does not seem very complicated compared to many other human achievements (e.g., men on the moon, vehicles on Mars, rapid development of COVID-19 vaccines). The first step toward progress would be to reach expert consensus on validation requirements for an instability_PCS test. One option would be validation of clinical efficacy (e.g., ability to help predict which lumbar stenosis patients will benefit from fusion in addition to decompression).

Ethics Statement:

New metrics are described that were developed from a retrospective analysis of images previously collected under an IRB-approved protocol (Baylor College of Medicine IRB H-12858). (66) These metrics were retrospectively applied to pooled flexion-extension exams. No images were collected specifically for the present study. The only data used in the present study were calculated from the coordinates of anatomical landmarks obtained using fully automated methods. No one involved in the study had access to subject identifiers. No demographics or private health information was available or used in the present study. Competing Interest Statement: John Hipp and Trevor Grieco are employees of Medical Metrics, Inc., which developed automated methods for producing anatomic landmarks and calculations derived from the landmarks. John Hipp owns stock in Medical Metrics, Inc. The research was entirely supported by Medical Metrics, Inc. and no external funding was received.

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The challenge of diagnosing lumbar segmental instability

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Abstract

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Introduction

General challenges to the diagnosis of instabilityPCS

Best metrics to detect instabilityPCS

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