J Manipulative Physiol Ther 2006 (Feb); 29 (2): 100–106 ~ FULL TEXT
Per J. Palmgren, DC, Peter J. Sandström, DC, Fredrik J. Lundqvist, DC,
Hannu Heikkilä, MD, PhD
Department of Research,
Scandinavian College of Chiropractic,
OBJECTIVE: The objective of this study was to examine alteration in head repositioning accuracy (HRA), range of motion, and pain intensity in patients with chronic cervical pain syndrome without a history of cervical trauma.
METHODS: The study was a prospective, randomized, controlled trial. Forty-one patients with chronic cervical pain were randomly assigned to either a control group or a chiropractic treatment group. All patients were clinically examined, given general information on cervical pain, and provided with training instructions based on the clinical evaluation. The treatment included sessions with high-velocity and low-amplitude manipulation, proprioceptive neuromuscular facilitation, ischemic compression of myofascial trigger points, and spinal rehabilitation exercises aiming to normalize cervical range of motion (CROM) and HRA. Subjective pain intensity, cervical kinesthetic sensibility, and CROM were recorded before and after the study period.
RESULTS: There was no difference between the treatment patients and the control subjects at the beginning with regard to age, sex, subjective pain intensity, range of motion, and HRA. At the 5-week follow-up, the treatment patients showed significant reductions in pain and improvement of all HRA aspects measured whereas the control subjects did not show any reduction in pain and improvement in only one HRA aspect. No significant difference was detected in CROM.
CONCLUSION: The results of this study suggest that chiropractic care can be effective in influencing the complex process of proprioceptive sensibility and pain of cervical origin. Short, specific chiropractic treatment programs with proper patient information may alter the course of chronic cervical pain.
From the Full-Text Article:
Neuromusculoskeletal disorders have become one of the foremost medical problems in the industrialized world. Despite the focus of interest on low back pain, disorders of the cervical spine receive an escalating amount of attention. Neck pain is a very cost-demanding health problem. In 1995, the costs for back pain totaled 29.4 billion Swedish kronor (SEK; - $3.9 billion), of which 2.4 billion SEK ($0.3 billion) constituted direct costs of investigation, treatment, and hospital care. This area of cost has escalated by 35% between 1987 and 1995.  In 2001, costs for back pain represented 11% of the total costs for short-term sick leaves in Sweden and approximately 13% of all early retirement pensions were granted for back problems. 
Neck-related problems are more common among women and are treated to a larger extent than are other spinal problems.  Statistically, men and women show the same amount of sick leaves regarding neck and back pain. However, the average cost for treatment of neck-related disorders for the society is higher for men than for women, primarily because it is more common for women to be on partial sick leave and, therefore, women receive fewer compensation days (net = 86 [women] vs 91 [men]). The average cost for men is 86,300 SEK ($11,500) as compared with that for women at 68,000 SEK ($9070). 
Once neck pain has become chronic (duration of >3 months), on a yearly basis, a slight minority (44%) of patients seek help from their general practitioners (GPs). In spite of the fact that the patients' conditions are nonspecific and chronic, GPs still find indications for further diagnostic imaging in two thirds of patients.  Available studies on neck problems are insufficient to estimate incidence and prevalence, and there is often a problem of definition between traumatic and nontraumatic induced neck and shoulder pain.  Prevalence rates between 1% and 5% were found by Hagberg and Wegman,  with a higher incidence among white collar workers than among blue collar workers. In Finland, chronic neck pain syndromes were identified in 9.5% of men and 13.5% of women in a questionnaire study among 8000 adults.  In 2004, a large study performed at the Karolinska Institute with 516 men and 697 women showed a statistically significant difference in the prevalence of neck and shoulder pain between men and women; 18% of men and 29% of women had to find care for a new episode of neck and shoulder pain during a period of 4 to 6 years.  Enthoven et al  showed in a prospective 5-year follow-up study that 50% of patients with neck pain will relapse within a 5-year period. In Norway, 34% of responders in a random sample of 10,000 had experienced neck pain during the previous year and 13.8% reported pain that lasted for more than 6 months. 
Despite its high prevalence, not much is known about the risk factors for neck pain. Neck and shoulder/arm diseases constitute 58% of reported occupational diseases in the musculoskeletal system.  Both psychological and physical stress in occupational environments are predisposing factors for chronic cervical pain syndromes. 
Head orientation in space and relative to the trunk uses visual, vestibular, and cervical proprioceptive cues. The neck muscle proprioceptive system influences the oculomotor and vestibular systems, and proprioception is involved primarily in postural control and oculomotor control. Cervical kinesthetic performance in healthy subjects is often not reported in the literature. Cervicocephalic kinesthesia has to do with the sensitivity for changes in the angles of joints, a function dependent on mechanoreceptor input deriving from the extensive muscular and articular proprioceptive systems. [13, 14] In the last decade, the focus of interest has centered more on the potential role of cervical mechanoreceptive dysfunction in patients with chronic neck pain.
A method for evaluation of cervicocephalic kinesthesia was introduced by Revel et al.  The test assesses the ability to perceive both the movement and the position of the head relative to the trunk. It involves information from the cervical proprioceptive apparatus and from the vestibular system, but a number of experimental data suggest a primarily cervical proprioceptive role.  Disturbed kinesthetic sensitivity has been implicated in the functional instability of joints and their susceptibility to reinjury, chronic pain, and even degenerative joint disease.  There is also evidence suggesting that removal of deleterious or abnormal afferent input at the site of articulation alone may result in improved proprioception and motor response.  Decreased head relocation accuracy has been found in whiplash patients in comparison with asymptomatic subjects. [18-20] Among subjects with nontraumatic neck pain, the results have been less obvious concerning the presence of kinesthetic deficits. [15, 21, 22] Reduced head relocation accuracy and increased cervical joint position errors have been shown in patients with idiopathic neck pain and with both acute and persisting whiplash-associated disorders. [15, 18, 20, 23, 24]
The hypothesis of this study was that chiropractic intervention, consisting of high-velocity and low-amplitude spinal manipulative treatment, soft tissue techniques, and spine-stabilizing exercises, would provide more effective pain relief and improvement of cervical kinesthetic sensibility than noninvasive chiropractic intervention alone.
This study is relatively small (n = 20 in each group) but has an appropriate control and randomization procedure. Furthermore, only a few measurements have been done (subjective pain intensity, range of motion, and HRA). The results still indicate a positive outcome for patients receiving chiropractic treatment, with improvement of cervical kinesthetic functions and reduction of cervical pain. The treatment group significantly decreased their pain, measured with VAS, by the order of 29 mm, a noteworthy improved pain situation. No significant difference was observed in the active, biomechanically based CROM. However, in the proprioception-based HRA test, the treatment group showed significantly improved results in all assessed aspects whereas the control group showed improvement in only 1 of 6 aspects.
The present study does not explain by which mechanisms chiropractic treatment exerts its effects but gives an indication that it could be related to proprioceptive functions rather than by direct changes of biomechanical factors such as range of motion. Speculations can be made on which aspects of the chiropractic treatment could be responsible for the effect. Ischemic compression treatment of trigger points and proprioceptive neuromuscular facilitation mainly affect the more superficial muscles that proportionally have less-developed proprioceptive afferents.  The data from this study cannot reveal whether any significant effect was experienced at the level of deep interarticular muscles or whether trigger-point treatment may conceivably isolate these muscles in all of their intricate planes. Chiropractic manipulation, on the other hand, is believed to have its effect on the joints and the adjacent tissues.  A high-velocity and low-amplitude manipulative thrust in the plane of main movement of a joint is likely to affect also the profound interarticular muscles. Because the interventions in this study were concentrated to the muscles and articulations of the neck, it is likely that the observed effects were related to changes in mechanoreceptor afferent input rather than to changes in the vestibular system. Earlier studies have shown the presence of mechanoreceptive and nociceptive nerve endings in cervical facet capsules. Thus, these tissues can be monitored by the central nervous system and neural input from the facets is likely to be significant for proprioception along with pain sensation in the cervical spine.  The deep muscles of the vertebral column, predominantly those of the upper cervical complex, are arranged in a diversity of orientations. Deep muscles of the cervical spine have higher concentrations of mechanoreceptors in areas flanking the articulations than in the more superficial areas.  Signals from muscle receptors in individual neck muscles or muscle subsections may have a substantial potential to provide a detailed representation of head position and head movement.
A theory presented by Johansson and Sojka  suggests which mechanisms may be implicated in the source and dissemination of muscular tension. Increased muscle spindle sensitivity may be mediated by the sympathetic nervous system acting on the intrafusal fibers of the muscle spindles as a feedback loop. The correlation between interneurons and motor neurons in the spinal cord may also contribute to increased muscle tension. Assuming increased muscle tension and sensitized muscle spindles, the latter condition may give rise to erroneous proprioceptive signaling, particularly if spindles in dissimilar neck muscles or on different sides of the neck are disproportionately sensitized. Erroneous cervical proprioceptive information converges in the central nervous system with vestibular and visual signals, which could affect the mental perception of body orientation and lead to a misinterpretation of relation to the surrounding.
The method used for evaluating and measuring cervicocephalic kinesthetic sensibility is relatively simple to perform and reproducible and can therefore be used in scientific investigations aiming to identify certain subgroups with neck and shoulder pain. Kristjansson et al  compared 5 cervicocephalic relocation tests in 3 different subject groups in a case-control study and found that the HRA method introduced by Revel et al15 was the most reliable one for disclosing relocation inaccuracy among patients with neck pain. Subgroups categorized and classified objectively according to proprioceptive or nonproprioceptive etiology could thus be the focus of further studies. Further studies could also reveal if the techniques used here could be a valuable tool in daily chiropractic care.
The results of this study support that chiropractic care can be effective in influencing the complex process of proprioceptive sensibility and pain of cervical origin. Relocation dysfunction could be improved in subjects with nontraumatic neck pain. Similar conclusions were drawn by Karlberg et al  and Heikkilä and Wenngren ; this suggests that changes in the quality of proprioceptive information from the cervical spine region may affect postural control as well as reduction of cervical pain.