Methodological considerations
Lack of statistically significant differences in five of six HRA tests and nearly all CSP tests may be due to the small number of study participants. Following the study, analysis indicated that the power was only about 30% (range, 30% to 60%) for HRA and about 3% to 40% for CSP. Compared to a more desirable 80%, these values indicate that groups were not large enough to ensure that differences would be detected in our study, if present. Thus, we cannot be sure that so few differences exist between groups in HRA and CSP.
Another limitation was that we did not assess functional performance. Therefore, the experimental group may not have had a sufficient degree of pain and functional impairment to result in a detectable difference between groups.
Comparison with Findings of Others
Although our sample was small, and HRA and CSP results varied among participants, one of six test movements for HRA showed significant differences, suggesting a possible interaction of some or several underlying mechanisms.
Although chronic neck pain can be defined in clinical terms, underlying pathophysiological mechanisms are still primarily unidentified. As with chronic low back pain, investigations have failed to demonstrate a consistent relation linking structural pathology and neck-related pain [
32‐
37]. There are no clear criteria for how chronic neck pain should be diagnosed and classified [
1]. Furthermore, large inherent variations in functional proprioceptive impairment and pain within one group of patients with nontraumatic neck pain might contribute to the variety of results.
Our findings are consistent with studies reporting no significant impairment of kinesthesia in patients with nontraumatic neck pain, or whiplash-associated disorders with mild disability [
5,
18,
38]. However, our results contrast with some findings involving chronic cervical pain in which the cause was not controlled [
7] or involved trauma [
12,
17,
39‐
41]. In a group of 30 patients with chronic neck pain, Revel et al. [
7] noted error scores almost double in magnitude (compared with an age-matched group of healthy individuals), indicative of significant impairments. Heikkilä and Åström [
12] and Heikkilä and Wenngren [
39] found significantly larger HRA errors in whiplash groups than in healthy controls. Overall, differences observed were not as great as those reported originally by Revel and colleagues [
7]. Using a different measuring device, Loudon et al. [
40] examined a small whiplash group with chronic symptoms, and reported that they had larger mean position-sense errors than did healthy individuals. In a study of patients with idiopathic or traumatic neck pain by Sjölander et al. [
17], larger repositioning errors were found in patients with chronic neck pain than among asymptomatic subjects. These effects were more pronounced for patients with trauma than for those with insidious neck pain. The authors did not find any systematic over- or under-estimation among patients. They suggested that increased repositioning errors observed in chronic neck pain are a result of poor position sense due to disturbed proprioceptive input, rather than of systematic bias in motor control systems at central levels.
In contrast, and in concordance with our findings, Rix and Bagust [
5] observed no significant differences in repositioning accuracy between groups with chronic, nontraumatic neck pain, when compared with control groups, except for mean global error scores following flexion. Also, Teng et al. [
18], who investigated 20 patients with chronic neck pain, reported that history of chronic neck pain did not correlate with cervicocephalic kinesthetic sensibility in middle-aged adults. Edmondston et al. [
42] investigated 21 subjects with postural neck pain, and 22 who were asymptomatic. They assessed subjects' ability to replicate self-selected 'good' posture. No significant differences in posture repositioning errors between groups were observed. The authors concluded that individuals with postural neck pain may have a different perception of "good" posture, but no significant difference in kinesthetic sensibility compared with matched asymptomatic subjects. Armstrong et al. [
38] investigated 23 subjects with whiplash, and compared them with a matched control group. They found no differences in head and neck position sense between individuals with chronic whiplash-associated disorders and the controls. Woodhouse and Vasseljen [
19] investigated 116 patients with traumatic or nontraumatic chronic neck pain. Cervical movements in the associated planes relative to the primary movement plane were reduced among the two groups with neck pain, in comparison with 57 asymptomatic controls. The authors postulated that changes were probably not related to a history of neck trauma, or to current pain, but more likely due to a history of long-lasting pain. They found no differences between groups in cervicocephalic kinesthetic sensibility.
In our study, we did find a statistically significant altered global HRA in the neck pain group for one of the test movements: flexion. However, due to the lack of homogeneity and variations in only one-sixth of the test movements, this might have limited clinical meaning and generalizability.
The relationship between head repositioning acuity and functional performance is clinically important. Investigators have observed larger repositioning errors in persons reporting greater problems with function (higher Neck Disability Index) [
14,
43] than in those with milder problems [
14,
38,
43]. Larger repositioning errors in patients with chronic whiplash-associated disorders have also been observed, with dizziness and unsteadiness [
14]. More recently, Owens et al. [
44], using normal student volunteers, showed that a recent history of cervical extensor muscle contraction could produce HRA errors similar to those reported in patients with whiplash. The authors suggested that this supports the role of paraspinal muscles in sensorimotor dysfunction not necessarily related to trauma.
In patients with chronic neck pain, and under various testing conditions, investigators have observed considerable abnormalities in standing vertical posture [
21‐
23,
45,
46]. There are, however, conflicting reports on characteristics of postural balance during quiet standing in these patients [
24]. Others have pointed out large variations in postural performance among patients [
21], or have recommended dynamic posturography on a sway-referenced force plate, for better quantification of postural problems [
47]. In terms of postural stability and balance, considerable research is still needed to provide sound diagnostic tests appropriate for use in a routine, clinical setting.
Clinical and Research Implications
Because functional and structural cervical pathology underlying chronic neck pain remain largely unclear, continued research is crucial. However, it has been suggested that deficits in proprioception and motor control, rather than chronic pain itself, might be prime factors limiting function and quality of life in affected patients [
17,
21].
Subgroups classified objectively, according to proprioceptive or nonproprioceptive etiology, could be the focus of further research. Moreover, future work also might consider whether methods used in our study could contribute to daily clinical care. We would like to see further investigations of measurements of functional proprioceptive impairment, and its association with pain. Future research should combine measures used in the present study with measures of disability (e.g., the Neck Disability Index). This is important, because kinesthetic deficits in the neck have been linked to severity of pain and disability. Furthermore, to support comparison of results among studies, we recommend standardization of hardware and protocols in studies using HRA, force platforms, and CSP. Lastly, we recommend investigation of effects of different treatment modalities on chronic neck pain, as measured by sensorimotor function tests, such as HRA and CSP.