This study changes our understanding of cervical motion by demonstrating that a little under half of the cervical joints (48.1%) produced pro-directional surplus motion with an average of approximately 2°. Surplus motion should not be considered abnormal as 113 out of 219 joints in flexion and 109 out of 217 joints for extension demonstrated motion surplus to end-range.
Approximately 1/5 of all joints demonstrated both pro-directional and anti-directional surplus motion, passing upright and end-range positions with similar frequency. Those joints that did not produce pro-directional surplus motion (Type classic) comprised 46.3% of the total joints.
Surprisingly 5.7% of all joints produced anti-directional end-range (type Anti-directional)
Type anti-directional joints were found predominantly in the upper cervical region, with only a few in the mid cervical region. The finding that joints can complete their motion in opposition to the direction of head motion is unexpected. Arguably previous documentation that large proportions of anti-directional cervical flexion and extension motions were normal in healthy subjects, gave some indication of this possibility (12, 18-21, 26).
Cervical ROM is dependent on method, sex, age, joint degeneration and lifestyle factors (11). The average cervical ROM measured between upright and end-range in this study was similar to previous reports despite differences in the methodology. Cervical ROM in this study was measured between C0 and C7, and a control was applied for motion in the upper thoracic region (5, 10, 11, 23-25). Hole et al. reported that for healthy subjects between the ages of 20 and 69, cervical ROM ranged between 50.0° and 64.0° for flexion in males and between 51.6° and 63.3° for flexion in females. Extension ranges were between 46.5° and 82.4° for males and 60.9° and 80.1° for females (28).
Cervical joint motion between upright and end-range positions has previously been assess by Wu et al using video fluoroscopy. In this case motion was assessed in ranges of one third and the C0/C1 joint was omitted from the study. Wu et al. reported flexion joint motion as C2/C3 5,5°, C3/C4 7.8°, C4/C5 10.0°, C5/C6 9.8°, C6/C7 9.2° and for extension joint motion for C2/C3 7,6°, C3/C4 9.8°, C4/C5 12.3°, C5/C6 9.4°, C6/C7 8.1°(5). The current study showed that end-range flexion and end-range extension motion were significantly different for C0/C1, C5/C6 and C6/C7. By assessing range of motion in 10% epochs, this study aimed to give a more detailed picture of the motion pattern. The C0/C1 joint was also included as we know it to be important in its contribution to cervical spine motion.
The cervical flexion motion of C0/C1 (2.3°) demonstrated the smallest average joint motion found in the study. No previous studies have reported the amount of motion found between upright and end-range flexion for C0/C1. One study reported end-range flexion to end-range extension motion for C0/C1, and the combined flexion and extension motion of that study was comparable to the findings of this study (10). Joint ROM from end-range flexion to end-range extension, assessed from stationary flexion and extension X-rays, for females and males has been reported to be as follows: 14.5° and 12.6° for C0/C1, 11.6° and 10.9° for C1/C2, 7.8° and 8.4° for C2/C3, 11.6° and 15.2° for C3/C4, 14.4° and 17.0° for C4/C5, 12.2° and 17.9° for C5/C6, 9.8° and 11.4° for C6/C7, respectively (10).
Surplus motion
It may be possible to use the average pro-directional surplus motion as a percentage of end-range ROM as an indicator for the reliability of end-range motion to predict the maximum motion. Analysis of the quartiles of surplus motion demonstrated a clear pattern for both flexion and extension. Surplus motion as a percentage of end-range motion decreased with an increase in end-range motion.
As small end-ranges are associated with large percentages of surplus motion, using end-range in these situations to predict a joint’s maximum motion may be unreliable. Conversely, it could be argued that large end-ranges can be more readily utilised as a predictor for maximum motion due to their association with small percentages of surplus motion. This does however question the reliability of flexion-extension X-rays as an accurate indicator for a joints maximum motion.
Maximum demonstrated joint motion.
The data suggests that the end-range motion does not reflect the maximum possible motion for an individual joint. This is especially clear for C0/C1 during flexion, where the average ROM was 2.33° and the average pro-directional surplus motion was 2.36° with a range up to 14.23°. The maximum possible motion of healthy cervical joints is therefore unknown. It is not clear if the maximum measured motion found for all single joints in this study reflects the maximum possible motion capacity of healthy cervical joints.
Study limitations
Quantification and analysis of video-fluoroscopy has some limitations. The largest confounder is the measurement error. The experimental procedures and reproducibility of image analysis have previously been published (27). High reliability of the vertebral marking procedures has been established and high ICCs have been documented in previous studies (12, 17, 27).
Neck pain is a recurrent disease and absence of neck pain does not ensure that the subject has a normal neck without any characteristics of neck pain.
The study group was primarily younger adult males and females. Other demographic or anatomical stratification for sex, age, height, weight, posture, and type of neck: long, thin, short and amount of adipose tissue, may also influence the cervical ROM and the study results.
Variations in the curvature of the neck, were not considered central to the investigation as all patients were deemed healthy and screened for previous trauma, disease processes or episodes of previous cervical pain. Additionally cervical range of motion in this study agrees with the results of previous studies. Future studies may look at the effect of variations in the cervical lordosis in healthy adults on the prevalence and distribution of surplus motion.
It is recognised in this study that surplus motion can be both pro-directional and anti-directional and that some joints produce surplus motions in both directions. For the purpose of clarity, and because the focus of this paper is maximum pro-directional joint motion, joint classification in this study is based on end-range. While type Classic joints in this study do not demonstrate pro-directional surplus motion, they may, in some cases, produce anti-direction surplus motion.
Likewise, type Surplus may also, in some cases, produce anti-directional surplus motion. It is also of note that the variability in joint motion will influence how joints are grouped (Type, Classic, Surplus and Anti-directional) from motion to motion.
It could also be argued that the study is limited by the choice only to include flexion and extension, as this does not allow us to investigate the full dynamic capability of the joints in multiple planes. However, there must be consideration given to the level of radiation exposure healthy subjects are subjected to.
Clinical implications
The results indicate that the end-range motion seen on flexion-extension X-rays may not be reliable for the diagnosis of reduced joint motion, as joints with small end-range motion were associated with large surplus motion percentages. In contrast, cervical joints with large end-range motion were associated with small percentages of surplus motion, consequently offering a more reliable prediction of the maximum motion of a joint. It is reasonable to consider that the entirety of a cervical joint’s motion capacity is not expended at end-range, as multiplane motion would be difficult if joint structures were fully stretched by motion in just one plane.
However, it is clear that in most clinical interpretations of neck motion the concept of surplus motion is not applied, and yet clinical experience indicates the presence of surplus motion.
Orthopedic surgeons use the terminology compensation for additional joint motion found in joints adjacent to a surgical fusion. The compensation is perceived as a new ability for further cervical single joint motion; however, the compensation may be pre-existing surplus motion of the adjacent joints. This clinical implication may raise the question; is the success of surgical fusion dependent upon pre-surgical surplus motion in the adjacent joints?
Chiropractors have previously used the term para-physiological space to explain the motion which allows an adjustment to occur when a cervical joint is brought to tension.
However, it is possible that the para-physiological space may simply be the surplus motion of the cervical joints. It would seem that we cannot fully understand cervical motion during a physical examination, the fixation or the manipulation without first having a better understanding of surplus motion. The complexity of joint motion has been demonstrated in recent research (12, 18-21, 26).