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 joint 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 joint motion (type C) comprised 46.3% of the total joints.
Interestingly 5.7% of all joints displayed anti-directional end-range joint motion (type A).
Type A joints were found predominantly in the upper cervical region, with only a few in the mid and lower cervical regions, suggesting that the anatomical structure of the vertebrae may influence the prevalence of this motion [
9,
18]. The finding that joints can complete their motion in opposition to the direction of head motion is not unexpected. Previous documentation that large proportions of anti-directional cervical flexion and extension motions were normal in healthy subjects, gave some indication of this possibility [
10,
13].
Surplus motion
This study suggests that it may be possible to use the average pro-directional surplus joint motion as a percentage of end-range ROM as an indicator for the reliability of end-range motion to predict the maximum joint motion. Analysis of the quartiles of surplus motion demonstrated a clear pattern for both flexion and extension. Surplus joint motion as a percentage of end-range joint 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 should be done with caution. Conversely, it could be argued that large end-ranges can be more readily utilised as a predictor for maximum joint 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 upper cervical joint appeared to flex in the beginning of the flexion motion excursion, but to move anti-directionally later in the motion, towards a lesser degree of flexion. The small average motion of C0/C1 does not reflect the maximum motion capacity of C0/C1 during flexion. This is illustrated by the maximum joint motion of type S for C0/C1 (Fig.
3) and the large range for pro-directional surplus motion for C0/C1, shown in Table
3. 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, but we consider this unlikely.
The average cervical ROM measured between upright and end-range in this study was similar to previous reports despite differences in the methodology [
3,
8,
9].
Cervical joint motion between upright and end-range positions has previously been assessed 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. The current study showed that end-range flexion and end-range extension joint motion were significantly different for C0/C1, C5/C6 and C6/C7. By assessing ROM in 10% epochs, this study aimed to give a more detailed picture of the joint motion pattern. The C0/C1 joint was also included in this study as we know it to be important in its contribution to cervical spine motion [
15].
The cervical flexion motion of C0/C1 (2.3°) demonstrated the smallest average joint motion found in this 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 [
8].
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 joint motion percentages. In contrast, cervical joints with large end-range motion were associated with small percentages of surplus joint motion, consequently offering a more reliable prediction of the maximum motion of a joint. It is a reasonable consideration that in order for the joints of the cervical spine to produce multiplane motion, a joint’s motion capacity cannot be expended by motion in a single plane. However, it is clear that in most clinical interpretations of neck motion the concept of surplus motion is not applied.
Orthopedic surgeons use the terminology compensation for additional joint motion found in joints adjacent to a surgical fusion. Several biomechanical studies have documented a mechanism by which adjacent unfused levels compensate for the loss of cervical range of motion (ROM) in fused levels [
19]. 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 joint motion. The complexity of joint motion has been demonstrated in recent research [
10,
13,
17,
20‐
22].
Study limitations
Quantification and analysis of video-fluoroscopy has some limitations. The largest confounder is the measurement error; however, the experimental procedures and reproducibility of image analysis have previously been published [
16]. High reliability of the vertebral marking procedure has been established and high ICCs have been documented in previous studies [
10,
16].
Likewise, repeatability of the joint motion angle has previously been published [
13]. Although Wang et al. [
13] demonstrated that cervical joints accurately repeat their motion; it must be acknowledged that they were not investigating surplus motion, but joint motion angles.
It may be considered a limitation of the study that data was taken from a single motion excursion, rather than taking an average of multiple excursions, however this decision was made in order to reduce radiation exposure to subjects.
The study group was primarily younger adult males and females, which raises the question: are the results applicable to an older population? 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, potentially limiting their application.
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 ROM in this study was similar to the results of previous studies [
3,
8,
9].
It is recognised in this study that surplus joint 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 C joints in this study do not demonstrate pro-directional surplus motion, a proportion of these joints is very likely to produce anti-direction surplus motion. Likewise, a proportion of type S joints will likely produce anti-directional surplus motion. It is also of note that the variability in joint motion will influence how joints are grouped (type C, S and A) 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.
Future studies
Future studies may look at the effect of variations in the cervical lordosis and age, among other demographic variations, on the prevalence and distribution of surplus joint motion in healthy adults.
The quantification of surplus motion will provide reference values against which symptomatic patient data can be compared. Future investigations into the effect of pain on surplus motion would be beneficial, in order to establish the diagnostic utility of surplus joint motion. Studies of pain effects on joint motion have documented that both experimental and recurrent neck pain altered anti-directional motion patterns in the cervical spine [
20‐
22]. Lastly a more detailed classification of joint types may be of interest, addressing the prevalence of anti-directional surplus motion.