An intimate knowledge of vertebral anatomy and anatomic variations is essential for surgeons performing instrumented lumbar spinal fusion procedures. With the majority of current constructs relying mainly on transpedicular screw fixation, particular attention must be paid to pedicle anatomy. Biomechanical studies have supported the trend toward pedicle screw constructs showing pedicle screw pullout strength is superior to sublaminar titanium cables and sublaminar hooks .
In general, instrumented spinal fusion procedures have shown a steady trend of increasing volume over the last decade. One review looked at over 200,000 admissions for spinal surgery in high volume hospitals from 2005 to 2014 to identify trends in lumbar spinal fusion. These hospitals performed 765.5 lumbar procedures per year on average. Over the 10-year period, the number of fusion procedures in all hospitals studied increased 55% (from 9,685 to 15,002 per year) while non-fusion procedures decreased 39% (from 12,344 to 7,574) . Minimizing failure and complication rates is imperative for such a high volume and expanding surgery.
The importance of precision when inserting pedicle screws is best appreciated when analyzing the possible failures and complications associated with inaccuracy. Particularly, and the basis of this study, erroneous screw selection and/or placement could lead to intraoperative and postoperative injury. Neurologic, meningeal, bony, and/or vascular injuries can all occur secondary to misguided or improperly sized pedicle screws. Lonstein et al retrospectively reviewed imaging of 915 spinal fusions involving 4,790 pedicle screws (76.3% lumbosacral procedures). They found 2.4% of patients had complications attributed to the use of pedicle screws. These complications included nerve root irritation, screw breakage, pedicle fracture, and dural tear . In a survey analysis of 617 spinal fusion surgeries where pedicle screws were utilized, Esses et al showed an incidence of screw misplacement in 5.2% and pedicle fracture in 2.3% of patients. In terms of postoperative complications, this same study showed 2.9% screw breakage as well as lower incidence of nerve root injury, screw loosening, and screw cutout or back out .
Additionally, insufficient pedicle screw fixation theoretically contributes to failure of spinal fusion and postoperative pseudoarthrosis, although this is heavily debated in the literature . One retrospective review showed an incidence of pseudoarthrosis of 14% and iatrogenic instability of 5%; complications collectively referred to as Failed Back Surgery Syndrome (FBSS) . The consequences of FBSS include patient dissatisfaction, continued back pain, and possible need for revision surgery.
Biomechanical studies have examined pedicle screw size and its relation to construct stability. One such study examined pullout strength of various pedicle screw diameters utilizing finite element analysis for 720 trials. The authors identify the expected finding that pedicle screws of larger diameter increase pull out strength as well as vertebral fixation strength while decreasing the stress around the screws .
Misenhimer et al studied pedicle diameter (PD) and maximum safe pedicle screw diameter (MSPSD) in cadaver models. After progressive and sequential loading of pedicles with increasingly larger screw diameter they found that plastic deformity of the pedicles preceded fracture or cutout when the screw was larger than the endosteal diameter or within 80% of the outer cortical diameter as measured by Computed Tomography (CT) scan. After fracture or screw cutout, there was no cortical purchase within the pedicles . Another study by Yongjung and Lenke showed that with appropriate tapping of the pedicle, safe pedicle expansion up to 200% the internal pedicle diameter can be appreciated .
The surgical goal, therefore, is to insert the safest maximum sized pedicle screw for each pedicle. This customized approach theoretically increases the stability of the instrumentation construct while minimizing the risk of iatrogenic injury from pedicle fracture. Larger pedicle screws within the safe range for a given pedicle also translate to a decrease in screw pullout and screw breakage. The accompanying reduction in motion at the fusion site from building stronger constructs theoretically lead to less incidence of pseudoarthrosis as well.
While pre-operative planning with CT scan measurements may be the safest way to judge trajectory and maximal screw size, it is not standard practice for many spine surgeons. Commonly, spine surgeons utilize a preoperative radiographic assessment and/or intraoperative fluoroscopic assessment method for screw selection. This practice involves heavy reliance on surgeon experience and ability to account for variations in image magnification. It also necessitates quality imaging, which can be highly variable based on patient body habitus, radiology technician skill, and patient positioning. Of note, navigated spinal instrumentation also allows direct measurement of PD and is one of the reasons this technique is increasing in popularity.
The goals of the present study were to investigate how height and weight correlated with pedicle diameter. We hypothesized that these routinely obtained, non-invasive measurements would provide an easily referenced data point that can aid in perioperative estimation of MSPSD. The analysis was two-tailed. First, we investigated whether height, weight, and BMI significantly correlated with PD in our diverse population. Previous studies have demonstrated positive correlation between some of these variables. One such study using digital caliper measurement of PD in cadavers showed that taller and heavier subjects had statistically significant larger PD . A similar CT based study on a Turkish population demonstrated that transverse PD values are directly proportional to subject height . In the second part of the analysis, we subdivided the study population to investigate both ends of the height spectrum. We evaluated the theoretical safety profiles of a range of pedicle screw sizes in taller and shorter subjects. To our knowledge, this is the first study to assess theoretical pedicle screw safety in this manner.
We previously reported on the significant difference in PD between different races and its utility in perioperative planning for posterior lumbar spinal fusion surgery . Our large CT based anatomic study will further supplement the available anatomic data on pedicles and thus the preoperative radiographic and intraoperative fluoroscopic assessment methods for estimating MSPSD.