J Manipulative Physiol Ther. 2009 (Feb); 32 (2 Sup): S87–S96 ~ FULL TEXT
Republished from: Spine (Phila Pa 1976). 2008 (Feb 15); 33 (4 Suppl): S75–82
Linda J. Carroll, PhD, Sheilah Hogg-Johnson, PhD, Gabrielle van der Velde, DC,
Scott Haldeman, DC, MD, PhD, Lena W. Holm, DrMedSc, Eugene J. Carragee, MD, FACS,
Eric L. Hurwitz, DC, PhD, Pierre Côté, DC, PhD, Margareta Nordin, PT, DrMedSc,
Paul M. Peloso, MD, MSc, FRCP(C), Jaime Guzman, MD, MSc, FRCP(C),
J. David Cassidy, DC, PhD, DrMedSc
Department of Public Health Sciences,
School of Public Health,
University of Alberta, Canada.
STUDY DESIGN: Best evidence synthesis.
OBJECTIVE: To perform a best evidence synthesis on the course and prognostic factors for neck pain and its associated disorders in Grades I-III whiplash-associated disorders (WAD).
SUMMARY OF BACKGROUND DATA: Knowledge of the course of recovery of WAD guides expectations for recovery. Identifying prognostic factors assists in planning management and intervention strategies and effective compensation policies to decrease the burden of WAD.
METHODS: The Bone and Joint Decade 2000-2010 Task Force on Neck Pain and its Associated Disorders (Neck Pain Task Force) conducted a critical review of the literature published between 1980 and 2006 to assemble the best evidence on neck pain and its associated disorders. Studies meeting criteria for scientific validity were included in a best evidence synthesis.
RESULTS: We found 226 articles related to course and prognostic factors in neck pain and its associated disorders. After a critical review, 70 (31%) were accepted on scientific merit; 47 of these studies related to course and prognostic factors in WAD. The evidence suggests that approximately 50% of those with WAD will report neck pain symptoms 1 year after their injuries. Greater initial pain, more symptoms, and greater initial disability predicted slower recovery. Few factors related to the collision itself (for example, direction of the collision, headrest type) were prognostic; however, postinjury psychological factors such as passive coping style, depressed mood, and fear of movement were prognostic for slower or less complete recovery. There is also preliminary evidence that the prevailing compensation system is prognostic for recovery in WAD.
CONCLUSION: The Neck Pain Task Force undertook a best evidence synthesis to establish a baseline of the current best evidence on the course and prognosis for WAD. Recovery of WAD seems to be multifactorial.
From the Full-Text Article:
Neck pain is common in workers and, as would be expected, the frequency of neck pain varies widely across different occupations, with the highest frequency seen among those working in hospitals and offices, and lowest frequency among workers in industrial/service and forestry sectors. 
The question of whether workers experience resolution of their neck pain, and, if so, how long this takes, is of particular concern to workers themselves, to employers, and to health care providers. Knowledge about the course of neck pain in workers is useful for 2 reasons. First, this knowledge provides information that can be used to guide expectations. Second, it can help determine the effectiveness of neck pain interventions by establishing whether a particular intervention improves (or worsens) the usual course of recovery.
Knowing the determinants of neck pain course (i.e., prognostic factors) is also important to those who wish to plan effective workplace policies, to formulate effective workplace and nonworkplace interventions, and to help workers make effective lifestyle changes that will decrease the frequency and burden of neck pain. Identifying factors associated with poor prognosis also provides a target for intervention studies, both inside and outside of the workplace, and also indicates which workers are most likely to benefit from specific interventions.
It is important to consider study design in reviewing the literature on course and prognostic factors for neck pain. Cohort studies of prognostic factors necessarily involve longitudinal research designs that permit tracking of study participants over time. Studies of prognostic factors in workers must begin with workers who have neck pain at the onset of the study. These individuals are then tracked over time to identify any factors and characteristics that distinguish those who recover from those who do not. In other words, prognostic factors are those personal characteristics or circumstances which predict the course of recover — or the failure to recover — from neck pain. Because cross-sectional studies provide a “snapshot” in time, factors found to be associated with neck pain in such studies could be risk factors, prognostic factors, or consequences of neck pain. Findings from cross-sectional studies on neck pain in workers are reported elsewhere. 
In examining findings provided by longitudinal studies, the strength of the evidence produced by these studies should also be considered. One paradigm classifies cohort studies into a 3-level hierarchy of knowledge. This model has been used to interpret evidence obtained in prognostic studies of breast cancer, whiplash injuries, and mild traumatic brain injuries. [2–5]
Phase I studies are descriptive and hypothesis generating, exploring crude associations between potential prognostic factors and health outcomes.
Phase II studies are also exploratory, but use stratified or multivariable analyses to identify sets of prognostic factors.
A Phase III study is hypothesis driven and confirmatory. The goal is to confirm or refute the independence of any apparent relationship between a particular prognostic factor and the outcome of interest, after adjusting for confounding.
In the current article, we have used this hierarchy to help us interpret findings from prognostic studies in workers. The course of neck pain and prognostic factors for recovery in the general population and in whiplash-associated disorders (WAD) are presented elsewhere. [6, 7] Although there may be many similarities across these populations, we believe this way of organizing our findings will be most useful to audiences. Within the working population, we further separated the tables reporting the course of neck pain from those reporting prognostic factors for recovery from neck pain.
Materials and Methods
Design and Data Collection
The literature search and critical review strategy are outlined in detail elsewhere.  In brief, we systematically searched the electronic library database Medline for literature published from 1980 through 2005 on neck pain and its associated disorders; we also systematically checked the reference lists of relevant articles and updated the search to include key articles for 2006 and early 2007.  We screened each citation for relevance to the Neck Pain Task Force mandate, using a priori inclusion and exclusion criteria. We made no attempt to assess the scientific quality of each study when establishing its relevance to the Neck Pain Task Force mandate. Studies were considered relevant if:
they pertained to the assessment, incidence, prevalence, determinants or risk factors, prevention, course, prognosis, treatment and rehabilitation, and/or economic costs of neck pain;
they contained data and findings specific to neck pain and/or disorders associated with neck pain, or described a systematic review of the literature on neck pain;
they included at least 20 persons with neck pain or at risk for neck pain.
We excluded studies on neck pain that was associated with serious local pathology or systemic disease, such as neck pain from infections; fractures or dislocations (except where such studies inform differential diagnosis in neck pain); myelopathy; rheumatoid arthritis and other inflammatory joint diseases; or tumors.
Rotating pairs of Scientific Secretariat members performed independent, in-depth critical reviews of each article, identifying methodologic strengths and weaknesses. The criteria used in the methodologic appraisal of the studies can be seen online through doi:10.1016/j.jmpt.2008.11.015. Our methodologic appraisal focused on sources of potential selection bias, information bias, and confounding. We also considered whether or not these biases would likely result in erroneous or misleading conclusions. After discussions of each article, decisions were made about the article's scientific merit. Studies judged to have adequate internal validity and to be methodologically rigorous, such that the results could be accepted with reasonable confidence, were considered to be scientifically admissible and were summarized in evidence tables. These evidence tables were used to formulate the best evidence synthesis.
We classified the studies identifying prognostic factors into Phase I, II, or III studies (described earlier). We used this framework in our synthesis of the studies and in our development of summary statements of the evidence. Where the evidence from different studies varied, more emphasis was given to evidence from well-conducted Phase III studies, and secondarily, to well-conducted Phase II studies. The best evidence synthesis consists of a qualitative integration of the studies judged to be scientifically admissible, and links all summary statements and conclusions to the evidence tables, so that the evidence that formed the basis of any statements is made clear. [9–11]
In accordance with our conceptual framework on the course and care of neck pain,12 and similar to the organization of risk factors for new onset (incidence) of neck pain, [1, 13, 14] we further classified prognostic factors into the following categories:
Demographic and socioeconomic factors: These are usually either nonmodifiable (e.g., age and gender) or not easily modifiable (e.g., socioeconomic status).
Health factors and pain history: These can be classified as “impairments” according to the WHO's ICF framework. 
Workplace factors: These would include job tasks and ergonomic design of the workplace. Many workplace factors are potentially modifiable.
Psychological and social factors: These would include depression, anxiety, and coping strategies, as well as interpersonal factors (e.g., relationships with friends). Again, many of these factors are potentially modifiable.
Societal factors: This would include the prevailing compensation systems and laws. Such factors are potentially modifiable, although not on an individual basis.
Genetic factors: These are potentially important prognostic factors, although not considered modifiable.
Health behaviors: This would include health lifestyle factors such as physical exercise. Such factors are also potentially modifiable.
This article reports our findings from the scientifically admissible articles on course and prognostic factors for recovery from neck pain in the working population. We identified 1203 articles; after critically appraising these studies, we judged 552 to be scientifically admissible for our best evidence synthesis. Seventy scientifically admissible studies pertained to course and prognostic factors for neck pain; of these, 14 studies pertained neck pain in workers. These 2 groups of studies are summarized in Tables 1 and 2 (available online through doi:10.1016/j.jmpt.2008.11.015), respectively.
The Course of Neck Pain
The preponderance of evidence indicates that, in workers with neck pain, over 50% will report neck pain 1 year later. However, because the course of neck pain was generally determined using reports of neck pain at discrete follow-up points, we were not able to distinguish whether this reflected persistent (continuous) or recurrent neck pain.
Our findings on course of neck pain are based on 10 studies (of 8 distinct cohorts) related to the course of neck pain in the working population (Table 1, available online through doi:10.1016/j.jmpt.2008.11.015). These studies examined the course of neck pain in:
Swedish dentists with prevalent neck pain. 
Finnish female bank tellers with prevalent neck pain. 
Workers in nursing homes in The Netherlands with prevalent neck pain. [18, 19]
Hospital, office, warehouse, or airport workers with prevalent neck pain who were covered by occupational medical departments in France. 
General population of workers with prevalent neck pain in France. 
General population of workers Norway with “chronic neck pain,” defined by the authors as neck pain lasting for 6 months or longer, and accompanied by functional limitations. 
American military personnel who underwent surgery for cervical disc herniation (no information was provided on pain presence, duration, or intensity). 
Subjects in all cohorts (except the Canadian one) had prevalent neck pain at baseline. Measures of neck pain at baseline ranged from 3-month period prevalence (neck pain within the past 3 months) to 12-month period prevalence. The study of American military personnel provided information on medical fitness to return to duty, but did not provide information on the presence of neck pain at follow-up.
In the 6 distinct cohorts of workers with prevalent neck pain (i.e., neck pain that lasted for some unspecified period of time), findings consistently indicated that more than 60% of subjects went on to report neck pain at follow-up. This is similar to findings of course of neck pain in the general population6 and in WAD (Grades I–III). 
In those with neck pain at an index point, the 3-month period prevalence of neck pain was 61% and 44% after 1 and 2 years, respectively ; 6-month period prevalence was 72.2% after 1 year ; the 1-year period prevalence was 78% after 30 months,  and was 80.8% at 4 years.  Those subjects reporting more severe neck pain at baseline had less neck pain recovery,  and 62% of those with moderate or severe neck pain at baseline reported at least moderate neck pain 30 months later.  A study that tracked changes in neck pain frequency among bank tellers reported that 9 months later, 23.7% had experienced a decrease in symptom frequency, with the remainder showing no change (59.5%), a fluctuating course (11.2%) or an increase in symptom frequency (5.6%). 
The Canadian workers' compensation cohort studied new episodes of compensated work absence for neck pain (although this may not represent new episodes of neck pain). In this cohort of workers, the average work absence among claimants was 74.5 days. The study found that when compared with low back pain, neck pain resulted in similar levels of long-term work absence. In addition, neck pain claims were frequently recurrent, with 38.5% of claimants having at least 1 additional compensated work absence due to neck pain in the 3 years subsequent to the index claim. [23, 24]
In a study of workers with chronic pain at baseline (defined as long-standing pain with functional limitations), 64.8% of men and 53.2% of women had experienced some decrease in neck pain severity 5 years later and no longer met the case definition for chronic neck pain (using the same definition).  However, the authors of this study did not report on what degree of recovery was experienced. Finally, the study of US military personnel found that after surgical treatment for cervical disc herniation, 86% were considered medically fit for duty within 10 to 48 months.  Again, no information was provided about either the presence or severity of neck pain at follow-up.
Most of the differences in findings could be explained by study design issues. Studies with (a) shorter follow-up periods, (b) longer period prevalence measures (e.g., pain in the past 12 months vs. pain in the past 3 months), and/or (c) a less restrictive case definition for pain (e.g., any pain vs. pain accompanied by limitations or pain requiring time off work) reported a higher prevalence of pain at follow-up.
Prognostic Factors for Neck Pain
Our best evidence synthesis includes 8 studies reporting prognostic factors for neck pain in workers (Table 2, available online through doi:10.1016/j.jmpt.2008.11.015). Four of these studies also reported information on course of neck pain (above). Working populations in the studies of prognostic factors included:
male construction workers, 
workers in nursing homes in The Netherlands, 
female sewing machine operators in Denmark, 
female electronics assembly workers in Sweden, 
forest industry workers in Finland with a new episodes of neck pain, 
military personnel in the United States who had undergone surgical intervention for cervical disc herniation, 
general populations of working persons (work type unspecified) in France, 
general populations of working persons (work type unspecified) in Norway. 
All studies (except the 1 involving forest industry workers) consisted of workers with prevalent neck pain of unknown duration at baseline. The heterogeneity of the study populations, prognostic factors studied, follow-up periods, outcomes and analyses did not support statistical pooling of results. Therefore, individual study results are presented in evidence tables, which form the framework for our summary and conclusions. These tables provide extensive information about the size of the estimated associations, and the precision of these estimates where these were provided. We refer the reader to these tables for this specific information.
Demographic and Socioeconomic Factors
Evidence varies in the 4 studies (all Phase II) assessing gender as a prognosis factor in workers' neck pain. Where a study did find a positive gender association, the size of the effect was modest. In the 2 general (occupation unspecified) working populations, women were slightly more likely to have persistent or recurrent pain [odds ratios (OR) < 2]. [21, 22] However, gender did not predict recurrent pain in nursing home workers or length of sick leave in forest workers with new episodes of neck pain. [18, 29]
The evidence indicates that age is not a prognostic factor for outcome in workers with neck pain: 6 of the 7 studies assessing age in the prognosis of neck pain reported no association. [18, 22, 25–27, 29] Of these, 1 was Phase I study and 5 were Phase II studies. One Phase II study did find an association between age and prognosis among female workers in France with chronic neck pain at the index point. The case definition in this study was longstanding pain, with functional limitations. The study found that workers under age 50 years were less likely to continue to meet the case definition for chronic neck pain 5 years later, compared with workers over age 55. The effect of age was modest (OR = 0.6; for measures of precision for this and other estimates, see Table 2, available online through doi:10.1016/j.jmpt.2008.11.015).  This study demonstrated no age effect for recovery in male workers with chronic neck pain after adjusting for other factors.
Prior Health, Prior Pain, and Comorbidities
We accepted 5 studies (all Phase II), all of which presented evidence that prior pain (self-reported) and/or prior sick leave predicted a less favorable outcome in workers with neck pain. Prior musculoskeletal pain was a moderately strong predictor (OR = 1.7–2.6) of persistent and/or recurrent pain. [18, 21, 22] One study found that severe current neck pain combined with a history of sick leave for a prior episode of neck pain was a strong predictor (OR > 5) of longer sick leave for current neck pain.  In addition, prior sick leave for reasons other than neck pain predicted worsening of cervicobrachial symptoms over a 1-year period.  However, general health status was not associated with neck pain recurrence.  We found no scientifically admissible study or studies that assessed the role of degenerative changes in prognosis for neck pain among workers.
Occupation Type and Physical/Ergonomic Job Characteristics
Eight studies addressed the prognostic role of occupation type/classification and physical job demands/ergonomic factors [18, 21, 22, 25–29]; all except 125 were Phase II studies (with multivariable analyses to explore sets of predictors).
Occupation Type and Classification
All the studies investigating this issue (1 Phase I study and 4 Phase II studies) found evidence that job type/classification had an influence on prognosis.
Metal workers were twice as likely as welders to have neck-pain related sick leave of more than 3 days,26 and blue-collar workers were at least 6 times more likely to have taken longer sick leave (>3 days) compared with white-collar workers.  However, length of sick leave does not necessarily reflect pain duration, and the course of neck pain in these groups was not reported. A Phase I study of military service personnel who had undergone cervical disc herniation surgery found that enlisted personnel were almost 3 times more likely than officers to be judged as medically unfit for duty after surgery.  A study of sewing machine operators in Denmark found that those who changed employment were over 4 times more likely to recover from long-lasting pain than those who stayed in the same job.  Another study also found that changing employment predicted improvement of cervicobrachial symptoms in female electronics assembly workers; previous heavy work and high productivity were prognostic of worsening symptoms, although no information is provided in this study about the strength of these associations. 
Physical, Ergonomic, and Job Demand Factors
The evidence from 6 studies (1 Phase I study and 5 Phase II studies) indicates that these factors do not predict outcome of neck pain in workers. In sewing machine operators with neck pain, the type of machine operated, the workload and pace of tasks, and the presence of ergonomic adjustments in the workplace were not associated with workers' pain status 6 years later.  In women with neck pain who worked in the electronic manufacturing industry, work type within the assembly line, duration of employment, working hours, strenuous work operations and work posture did not predict symptom changes.  Moreover, in a general (occupation unspecified) working population, job characteristics such as hours at work, heavy lifting, overhead work, posture and pace of work did not predict persistent or recurrent pain.  Duration of employment or work-related physical loads were not prognostic factors for recurrence of neck pain in a study involving nursing home workers. 
The 2 exceptions to these negative findings about physical, ergonomic, and job demand factors were in different samples of persons with neck pain. One was a study of US military personnel who had undergone surgery for cervical disc herniation. It found that a shorter duration of service was associated with greater likelihood of being judged medically unfit for duty after cervical surgery, although this increase in odds was less than 20%.  The other was a study of workers (unspecified occupations) who had chronic neck pain at baseline. It found that high job demands (defined in the study as having to hurry, to performing several tasks at the same time, or being interrupted often) predicted continued chronic pain at 5 years, as did repetitive work (but only in women). Again, the associations for these prognostic factors were modest (<30% increased odds of a poor outcome). 
We found no scientifically admissible study or studies examining the impact of vibration or long hours of driving on prognosis of neck pain.
Psychological and Social Factors (Work and Nonwork-Related)
The evidence from 4 Phase II studies examining this issue indicates that most of the psychosocial factors studied did not predict the course of neck pain in workers. Psychosocial factors under scrutiny were: perceived psycho-logical and work stress, job satisfaction, depressive symptoms, psychological load, satisfaction with colleagues, and social support. [18, 21, 22, 28] However, 1 of these studies did find that workers with neck pain who perceived themselves as having little influence over their work were more likely to again report neck pain 4 years later. This association was of moderate strength (OR = 2.54).  We found no scientifically admissible study or studies that looked at coping strategies, anger and frustration in workers as prognostic factors in neck pain. We believe these factors deserve to be evaluated, because they were prognostic of outcome in the general population and in persons with WAD. [6, 7]
Compensation, Legal, and Societal Factors
Only 1 Phase I study examined compensation factors as predictors of outcome in a specialized group of workers (US military personnel who had undergone surgical treatment for cervical disc herniation). In this study, compensation factors did not predict whether these personnel were rated as medically unfit for duty after surgical treatment. 
We found no scientifically admissible study or studies looking at prognostic factors for recovery in workers' compensation claimants.
Evidence is consistent in the 2 studies on this topic (both Phase II) that workers who exercised had a better prognosis for neck pain. After adjustment for a variety of work-related factors and job changes, physical training outside of work predicted improvement in cervicobrachial symptoms among Swedish female electronics assembly workers (no effect sizes reported).  Men in the general working population in France who engaged in sporting activities were 50% more likely to experience an improvement in chronic neck pain21 compared with male workers who did not take part in sports.
We found no scientifically admissible study or studies that examined the potential prognostic role of body mass index, smoking, or other health-related lifestyle factors.
We found no scientifically admissible study or studies that examined the effect of genetic factors on the prognosis of neck pain in workers.
We found no scientifically admissible study or studies that explored the prognostic role of cultural factors among workers with neck pain.
Two factors—prior treatment for 2 disc levels, and requiring additional surgery—were both predictive of prognosis in a group of military service personnel who had undergone cervical disc herniation surgery. Those with a history of prior treatment and who needed more surgery were more likely to be assessed as medically unfit for duty later on. 
Like neck pain in workers and neck pain in WAD, neck pain in the general population is frequently persistent and/or recurrent. [9, 10] Studies suggest that between 50% and 85% of people in the general population (or in primary care setting) who experience neck pain at some initial point will report neck pain 1 to 5 years later. Estimates varied by populations studied and across case definitions for neck pain. Moreover, in all but 1 study (Côté et al),  it was impossible to determine what proportion of participants experienced persistent versus recurrent neck pain. This particular study assessed 6-month period prevalence of neck pain at 6-month intervals in a general population, and thus provides more detailed information on the course of neck pain.  In this sample, approximately 10% of subjects with initially mild or intense but non-disabling neck pain reported neck pain that became disabling over the follow-up period; whereas one-fifth experienced recovery followed by worsening; and almost 40% experienced persistent levels of neck pain. However, even in this study, it is unclear what proportion of individuals experienced continuous neck pain. Even so, the evidence is clear that most people with neck pain do not experience a complete resolution of this problem.
Like the studies examining gender as a risk factor for new onset of neck pain in these populations,  the evidence on gender as a predictor of recovery from neck pain is equivocal—approximately half the studies examining this issue found that women had poorer outcomes compared to men, whereas the remaining studies noted no effect of gender. Interestingly, although Bot et al reported a higher incidence of neck pain among women found no gender differences in prognosis. [19, 29] No study found that men with neck pain had a poorer prognosis. However, even in the studies reporting poorer prognosis for women with neck pain, the impact was generally modest. We conclude, therefore, that gender is, at best, a weak predictor of recovery for neck pain in this population.
The evidence regarding age as a prognostic factor in neck pain was consistent: young age was associated with better prognosis in all studies examining this issue. However, again, there was not a large impact of age, suggesting that although older age played a consistent role in predicting poorer prognosis for neck pain, it was a weak predictor of recovery. Although most studies of age dichotomized younger versus older or examined age as a continuous variable, 1 study which trichotomized age (divided people into 3 age groups) reported the largest impact on recovery in the middle group (those aged 45–59 years).  This group was almost 4 times more likely to experience chronic, recurrent, or continuous neck pain compared with the older or younger groups. This is congruent with the literature on risk,  and provides preliminary evidence suggesting that the highest risk and poorest prognosis for neck pain is during the middle aged years. This should be confirmed in further studies.
There was a wide variety of indexes of health and health-related factors in the reviewed literature. In general, prior pain and/or injuries and poor health predicted greater presence and/or greater intensity of neck pain at follow-up, although the associations were modest. Despite preliminary findings that regular physical activity protects against onset of neck pain in the general population,  and that exercise is an important component of treatment for neck pain,  the prognostic studies evaluating this factor found that initial levels of exercise were not associated with persistence or with recurrence of neck pain at follow-up. In fact, regular bicycling was associated with poorer prognosis in 1 study.  However, physical fitness and exercise levels are difficult to measure in self-report questionnaires; and, moreover, increases in exercise levels over the follow-up period have not been evaluated. Prognosis may also depend on whether or not the exercises themselves were designed to impact the neck and shoulder areas.
Better psychologic health (measured in a variety of ways) and greater social support predicted a better outcome in primary care and general population samples with initial neck pain, whereas passive coping predicted a worse outcome. The associations between these psychologic factors and continued pain at follow-up were generally stronger than the associations seen for other types of prognostic factors. These findings are in keeping with the best evidence on risk for new episodes of neck pain, where poor psychologic health was associated with neck pain and was also a risk factor for new episodes. Although psychologic functioning is a potentially modifiable prognostic factor (and therefore a potential intervention target for trials), few trials have explored the impact of psychologic interventions alone, outside the context of multimodal treatment approaches to neck pain.
In summary, to the extent that common factors have been studied, the evidence suggests that most factors which predict poor outcome in persons with neck pain are consistent with those factors that increase the risk for new neck pain (or neck pain episodes).
State of the Literature and Study Limitations
The scientifically admissible literature on prognostic factors within the general population and for persons in primary health care settings is not extensive, although the studies included in the best evidence synthesis were well designed. It is likely that the 2 populations considered here—the general population and patients seeking primary care—are quite similar with respect to prognostic factors for neck pain outcome. Most studies used multivariable analyses to identify the presence and strength of prognostic factors. However, our best evidence synthesis of prognosis in these settings is based on only 6 studies. A seventh study used a distinct sample of persons with longstanding, functionally-limiting pain which had led them to be referred for inpatient rehabilitation. One might expect prognostic factors in this particular population to be different from those in general populations and in primary health care settings.
Identification of prognostic factors can guide expectations for recovery and, where these prognostic factors are modifiable, can guide considerations of what intervention targets will be the most productive. This is especially true when the effect of that factor on neck pain outcome is large. Most of the prognostic factors identified in this literature had only a modest association with outcome of neck pain. In addition, we should point out the wide variety of case definitions of neck pain and prognostic factors that we encountered in the literature. This suggests we should use caution in drawing firm conclusions at this time.
One promising exception involves psychologic factors, although few studies looked at the same psychologic constructs. The importance of this group of prognostic factors needs to be confirmed in additional phase III studies; in addition, if these are to be studied in the context of intervention trials, we need confirmation that they are indeed modifiable. Some such evidence exists for example coping strategies in chronic pain samples have been reported to be amenable to change via cognitive-behavioral or multimodal interventions. [30–32]
Limitations in the literature we reviewed are outlined above. However, the methodology used in the synthesis of the best evidence also has some limitations (outlined in more detail elsewhere). 
In particular, although there is a large overlap among journals indexed in Medline and in other electronic health databases, it is possible that using only Medline resulted in missing studies that may have informed this best evidence synthesis. There is also controversy about whether systematic literature reviews should report findings from all relevant studies or use a best evidence synthesis, as we did. We believe that using a best evidence synthesis approach, that is, reporting evidence only from those studies we judged to have adequate validity, increases the validity of the conclusions.
There are also some limitations specific to our systematic review of prognosis of neck pain in the general population. First, although all articles used in our analysis were judged to be scientifically admissible, the quality and methodology of the studies still varied considerably. In particular, the adequacy of control of confounders varied widely among studies. We attempted to address this potential source of bias by classifying studies into phase I, II, and III and by giving greater scientific weight to studies that explicitly controlled for confounders (i.e., phase III studies).
Second, we made no conclusions about some prognostic factors, indicating that the evidence varied too much among studies to reach firm conclusions. In some cases, this variability may have been due to an attempt to combine studies which diverged because of population-specific effects (that is, the strength and direction of the association varies in the populations in question), and that there was no genuine contradiction between studies.
We propose the following high priorities for future research in this area.
Research priorities and recommendations to improve the quality of prognostic studies are outlined in more detail in Carroll et al. 
There should be closer tracking of the usual course of neck pain. Although continuous neck pain and recurrent neck pain both reflect failure to recover, there may be different consequences and prognostic factors for these states.
Studies are needed to confirm or refute the suggestion that middle-aged persons are at the greatest risk of persistent neck pain.
Studies to assess the role of degenerative disc changes in recovery from neck pain are required.
Studies examining the role of compensation systems, policies, and legal factors in prognosis for recovery are also required.
Studies examining the effect of genetic factors on neck pain prognosis would be useful.
Further studies are needed to examine the role of exercise and fitness levels on the outcome of neck pain episodes. Given uncertainty about self-reports of fitness or exercise levels, these factors should preferably be assessed using objective criteria rather than self- report. It might also be important to determine the type of exercise as part of this research.
Most people (50%– 85%) in the general population with neck pain do not experience a complete resolution of this problem.
Younger people have a better prognosis, and 1 study suggests that those in middle age have the poorest prognosis.
Poor health and prior pain episodes are associated with a poorer prognosis; however, the effect of these factors was modest. Psychologic factors are important in prognosis for neck pain in the general population. Poor psychologic health, and worrying, becoming angry, or becoming frustrated in response to neck pain, were associated with poorer prognosis. Greater optimism, coping that involves self-assurance, and having less need to socialize, were all associated with better prognosis. The impact of psychologic factors was of at least moderate strength (i.e., most ORs in these studies were between 2 and 6).
There is preliminary evidence from 2 studies that general exercise at baseline is not associated with prognosis; however, 1 study found those who engage in regular bicycling have a poorer prognosis.
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