Prognostic factors in lateral epicondylitis: a randomized trial with one-year follow-up in 266 new cases treated with minimal occupational intervention or the usual approach in general practice

J. P. Haahr and J. H. Andersen

Department of Occupational Medicine, Herning Hospital, DK-7400 Herning, Denmark.

Correspondence to: J. P. Haahr. E-mail: hecjph{at}ringamt.dk


    Abstract
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Objectives. To determine whether minimal intervention by occupational specialists involving information about the disorder, encouragement to stay active and instruction in graded self-performed exercises could enhance the prognosis of lateral epicondylitis compared with the treatment usually given in general practice, to quantify workplace factors associated with the prognosis, and to consider treatments given in general practice.

Methods. A randomized controlled trial was performed in a cohort of 266 consecutive new cases of lateral epicondylitis diagnosed in general practice. Workplace factors were assessed with questionnaires at the time of inclusion, and patients completed follow-ups at 3, 6 and 12 months. Status at 1 yr was assessed as overall improvement and pain reduction compared with the time of diagnosis. General practitioners (GPs) registered the treatments given for both cases and controls during follow-up. Numbers of contacts with GPs and physiotherapists were obtained from the National Health Insurance registry. Prognostic factors were analysed by multiple logistic regression analysis.

Results. After 1 yr, 83% of cases showed improvement in the condition, but the intervention was found to have had no advantage. Poor overall improvement was associated with employment in manual jobs [odds ratio (OR) 3.0, 95% confidence interval (CI) 1.0–8.7], a high level of physical strain at work (OR 8.5, CI 1.0–74.7) and a high level of pain at baseline (OR 2.3, CI 1.0–5.3). Pain reduction less than 50% was associated with manual jobs (OR 2.3, CI 1.1–5.1), high physical strain at work (OR 3.6, CI 1.0–12.9), high baseline distress (OR 1.9, CI 1.0–4.0) and tennis elbow on the dominant side (OR 3.1, CI 1.4–6.8). The intervention group received less treatment and fewer treatment modalities, but the intervention was not followed by a reduction in the number of visits to GPs and physiotherapist clinics during 12 months of follow-up.

Conclusions. Poor prognosis at 1 yr of follow-up for lateral epicondylitis was related to manual work and high baseline pain, whilst no relation was found between the type of medical treatment given/chosen and prognosis. This may have implications for the future management of lateral epicondylitis in terms of a greater focus on interaction with the workplace regarding job modification to reduce physical demands during recovery.

KEY WORDS: Tennis elbow, Workplace factors, Health-care utilization, Primary care, Prevention.


    Introduction
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Lateral epicondylitis (tennis elbow) is a well-known diagnosis in general practice [1] and one of the most frequently diagnosed upper-extremity musculoskeletal disorders. For example, during the 1990s in the USA, whilst Washington State saw a reduction in the overall incidence rate for claims for non-traumatic soft tissue disorders, the rate increased for epicondylitis [2]. Tennis elbow is characterized by pain on the lateral aspect of the elbow, which is increased by strenuous use of the hand and forearm. Clinical examination reveals lateral epicondylar pain and tenderness, and pain on resisted extension of the wrist [3]. The pathogenesis and pathoanatomy remain uncertain, although there are indications that degeneration is involved, at least in more longstanding cases [4]. There is evidence that a combination of physical strain and perhaps psychosocial factors is associated with an increased risk of tennis elbow or forearm pain [2, 59]. The clinical disorder has been described thoroughly, but information on its natural course has been scarce [10, 11]. Labelle et al. [12] in 1992 found that the scientific evidence in favour of any specific treatment was poor, and according to The Cochrane Library (2002, Issue 4) this has not changed, and patients with lateral epicondylitis are offered a number of different treatments [13]. The treatments offered ranged from `wait and see', information and general advice, through medication consisting mainly of non-steroidal anti-inflammatory drugs, to a variety of physical treatments, local corticosteroid injection, exercise or the use of different appliances or bandages. Furthermore, a number of surgical procedures are offered. Methodologically rigorous research on prognosis has been recommended in an earlier review, where only four studies were found to provide moderately good evidence for prognosis that was given [11, 14]. The lack of evidence of any long-term beneficial effects in terms of improvement following specific treatment regimens raises the question of whether other, stronger prognostic factors are at work; for instance, the same factors that may be considered risk factors in the development of lateral epicondylitis [15, 16].

The primary aims of this study were to examine the effect of a minimalist intervention in lateral epicondylitis by an occupational physician, involving information about the disorder, instruction in graded self-performed exercises, and encouragement to stay active, and whether this could improve the prognosis compared with treatment in general practice. A secondary aim was to describe the prognosis at 1 yr of follow- up, and to quantify workplace factors associated with the prognosis at 1 yr. Finally, we wanted to consider treatments given in primary care.


    Materials and methods
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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Participants, recruitment and participant flow
From May 1998, a randomized clinical trial with 1 yr of follow-up was performed in Ringkjoebing County after a population-based case-referent study of risk factors for the development of tennis elbow [9]. Consecutively diagnosed patients with tennis elbow, aged 18–66 yr, who consulted their general practitioner (GP) with a new episode of lateral epicondylitis were recruited from 104 participating GPs. The recruitment and flow of patients are shown in Fig. 1. Eligibility criteria for participation was fulfilment of the diagnostic criteria for lateral epicondylitis, i.e. the presence of pain in the elbow region and direct and indirect tenderness at or within 2 cm of the lateral humeral epicondyle on resisted extension of the wrist and/or the third finger. Cases with either a history of elbow pain for more than 1 yr, a previous elbow operation or a known inflammatory rheumatic disorder were excluded. Three hundred and sixty eligible cases were invited to participate. Informed and written consent was obtained from 289 cases, which were then randomized by the general practices to either an intervention or a control group. Twenty-three of the randomized cases did not return the baseline questionnaire, regretted participation or did not receive the intervention. A further 10 cases were lost during follow-up.



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FIG. 1. Number of participants and dropouts submitting questionnaires (figures in boxes) at baseline and 3, 6 and 12 months of follow-up, by intervention group, among 289 new cases of tennis elbow randomized in general practices.

 
The appropriate ethics committees approved the study protocol.

Intervention
Participants in the control group received treatment as preferred and agreed upon by the patient and the patient’s GP. Cases allocated to the intervention group were seen at the local department of occupational medicine within 1 week of inclusion. A brief history was taken, physical examination was performed and information was given, the main message being that lateral epicondylitis, according to our best knowledge, is mainly a self-limiting condition with a favourable prognosis, though the pain often varies in intensity. Patients were informed that, to our present knowledge, no specific treatment seems to improve the overall long-term prognosis. Advice was given against complete rest and the patients were encouraged to stay active, but with advice to avoid activities found to exaggerate the pain. In cases with a history of strenuous job tasks, the patient was encouraged to adjust work conditions if possible. The patients were then seen by an ergonomist, who gave instructions in performing a graded exercise programme, which was to be used as long as symptoms persisted [17]. Patients were allowed to use analgesics available over the counter without prescription and elbow bandages at their own discretion. In cases with continuation or worsening of symptoms, the patients were allowed to consult their GP again, and no restrictions were imposed on subsequently selected treatments.

Outcomes
The primary outcome measures were self-reported overall development of the condition reported in follow-up questionnaires at 3, 6 and 12 months after inclusion in the trial (‘How do you evaluate the overall development of your lateral elbow pain since the first contact with your doctor 3 months/6 months/1 yr ago?’, with response categories of ‘much better’, ‘better’, ‘neither better nor worse’, ‘worse’ and ‘much worse’). As an alternative measure of outcome, we used a reduction of at least 50% in a combined pain and function score on the affected side (for details see below).

As secondary outcomes, GPs registered all treatments given within the 12-month follow-up period for both cases and controls and reported the results at the end of the follow-up period. GPs also reported on sickness absence, whether the patients had ceased working or changed job, started some kind of education or other rehabilitation activity, and whether a labour compensation claim had been submitted. For all cases, the number of visits to GPs and physiotherapists was recorded from the National Health Insurance registry for two periods of time: 52 weeks before diagnosis and inclusion in the study, and 52 weeks after inclusion.

Randomization
Randomization to intervention or control was performed in advance by the project coordinator in blocks of four according to a predetermined random sequence of numbers, and was supplied to each general practice in sealed envelopes, which were opened by the GP after eligible cases had signed an informed consent form. Whenever the practice had included more than two cases, it was supplied with supplementary envelopes. In the envelopes were also included a complete and prenumerated set of the necessary forms and a baseline questionnaire for the patient to complete. On the envelopes were printed instructions to the GP, including eligibility criteria.

Blinding
Due to the design, no blinding to treatment could be done. The primary outcome assessment was done independently, based on patient self-reports in follow-up questionnaires.

Prognostic factors
At baseline, cases filled in a questionnaire on demographic factors (gender, age, body weight, height, educational background, dominant hand, physical activity), workplace factors (present job title, physical and psychosocial factors) and clinical factors. Regarding workplace factors, the authors classified the present job by title as being potentially strenuous or not for the upper extremities. Baseline physical strain to the upper extremities was calculated as an index (no strain, low, medium or high strain) from questions on posture of the arms and hands (two items), repetitive movements of the arms and hands (two items), and force calculated from five questions on the use of tools (heavy tools weighing more than 1 kg). Regarding psychosocial workplace factors, social support at work was assessed by four questions from the Karasek and Theorell Job Content Questionnaire (help from colleagues, colleagues listening, help from closest superior, listening by superior). Questions on job demands (three items), job control (14 items) and job satisfaction (eight items) were also included [18].

Clinical factors included the presence of pain in the shoulders, forearms or hands within the past 3 months, high baseline distress (distress in the past 4 weeks, measured by the Setterlind two-item symptom scale, e.g. the presence of a fast heartbeat, breathlessness, dizziness, feeling nervous, inability to relax, sleeping problems, etc.) [19], and pain score [(pain in the elbow region in the past 3 months assessed with a numerical box complaint scale ranging from 0 (no complaints at all) to 9 (pain as bad as could be)], indicating (i) the severity of pain at worst, (ii) the average level of pain within the last 3 months, (iii) the severity of impairment in daily activities within the last 3 months due to elbow pain, and (iv) the average level of pain within the last 7 days. These four scores were summed to give a range of 0–36 [20]. Further clinical factors were tennis elbow located in the dominant arm and the treatment group. The construction of the factors or variables has been described in detail elsewhere [9]. Whenever scales were dichotomized, the upper quartile was used as the cut-off point.

Statistical methods and analysis
Cases with bilateral tennis elbow (n = 16) were assigned the pain score from the dominant arm. The difference in pain score between the two groups was tested by analysis of variance (ANOVA), and the difference in overall development was tested by the {chi}2 test. Crude and adjusted odds ratios (ORs) for successful outcome at 1 yr were calculated by logistic regression analysis. A baseline pain score of 27 or higher was defined as high (among the 25% with the highest scores). Variables found to be significantly associated with positive case status (risk factors) in the case–referent study were also entered as prognostic factors [9]. The difference in the number of treatment modalities registered by the GPs was tested with the {chi}2 test. The difference in the number of visits to primary care within the treatment groups was evaluated with the paired-samples t-test, taking into account the individual baseline intensity of visits expressed as the number of visits in the year before inclusion. Based on prior power calculations, we intended to include 400 new cases in expectation of some dropouts (the probability of detecting a clinical difference in the two groups of 15% being 80% with 250 participants). The recruitment period was estimated to be 1.5 yr. However, recruitment was stopped, after 2 yr, at 289 cases, due to a substantially decreasing inclusion rate. In order to increase power, the scores of overall improvement for 12 cases registered at 6 months but missing at 1 yr were included in the analysis. The same was done for 10 cases with regard to a 50% reduction in pain score.

Pain and impaired function associated with lateral epicondylitis may fluctuate over time. In analyses, baseline pain and impaired function on average during the past 3 months had similar distributions of score values. Pain at worst during the past 3 months and average pain during the past 7 days were skewed to the right. During follow-up, mean score values decreased and the skewing of the distribution was to the left, though it was less for average pain during the past 7 days. Furthermore, the language/wording of the question concerning impaired function includes the concept of pain. Therefore, the combined score was maintained as an outcome measure. The inter-individual variation in scoring pain and function may be high compared with the intra-individual variation on repeating the score. Therefore, rather than using the total reduction in score value as the outcome, we used the proportional reduction in the individual score.


    Results
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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Baseline data
Baseline characteristics of the 266 cases are shown in Table 1 according to treatment group. For most factors, no major differences were found between the intervention and control groups, although in the intervention group only 16 (11%) had less than 11 yr of education whilst in the control group 28 (22%) had less than 11 yr of education (P = 0.01). Fifty-six per cent were employed in potentially strainful work according to their job title, whereas 29% had high physical strain at work measured as the work strain index.


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TABLE 1. Distribution of baseline characteristics among 266 participants according to treatment group

 
Ninety per cent of the cases indicated that within the past 3 months they also had pain in the shoulder, forearm or hand. In 70% the dominant elbow was affected.

Pain score and global improvement at 1 yr of follow-up
Pain within the preceding 3 months (as indicated at 12 months of follow-up) showed a mean score of 1.8 (S.D. 1.9), and the severity of impairment in daily activities within the last 3 months due to elbow pain showed a mean score of 1.7 (S.D. 2.2). Pain on average within the last week scored 1.7 (S.D. 2.3), and pain at worst within the past 3 months scored 2.5 (S.D. 2.4). The overall combined pain score (sum-score of the four subscales) at 12 months was reduced from 22.1 on inclusion (S.D. 7.6) to 7.8 (S.D. 8.3) after 1 yr.

Information and instruction in self-performed exercises given to the intervention group was not followed by any difference in pain score or perceived overall development compared with the control group. Mean scores on the scales of pain/function at each follow-up assessment decreased rapidly in the beginning and were approximately halved at 6 months. The values are shown in Fig. 2. The proportion of cases who indicated overall improvement followed the same pattern and increased rapidly in the beginning, from 69% at 3 months to 73% at 6 months and 83% at 12 months of follow-up (Fig. 3). The Pearson correlation coefficient between the overall pain/function score and global improvement at 12 months was 0.67 and significant at the 0.01 level.



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FIG. 2. Mean pain/function score (minimum 0, maximum 36) with 95% confidence intervals in a cohort of 266 new cases of tennis elbow from general practices according to treatment group at baseline and 3, 6 and 12 months of follow-up.

 


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FIG. 3. Proportion with perceived improvement compared with baseline state among 266 new cases of tennis elbow at 3, 6 and 12 months of follow-up according to treatment group.

 
Prognostic factors
Absence of perceived global improvement at 12 months was significantly associated with high physical strain at work [OR 8.5; 95% confidence interval (CI) 1.0–74.7] and high baseline pain score (OR 2.3, CI 1.0–5.3) (Table 2). Lack of overall improvement at 1 yr was also associated with high baseline level of distress, age older than 40 yr and pain in the shoulder, forearm, wrist or hand within the past 3 months. The significant associations disappeared, however, on mutual adjustment. Gender, short education, poor baseline social support at work, tennis elbow in the dominant elbow, high body mass index and physical inactivity were not related to perceived overall improvement.


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TABLE 2. OR of perceived unchanged or worse overall development in tennis elbow state at 12 months of follow-up in 266 new cases of tennis elbow according to baseline characteristics

 
Continued high pain score (and low function) with reduction at 1 yr of less than 50% was significantly associated with high physical strain at work (OR 3.6, CI 1.0–12.9), tennis elbow located in the dominant elbow (OR 3.1, CI 1.4–6.8) and high baseline distress (OR 1.9, CI 1.0–4.0) (Table 3). Continued high pain score was also associated with female gender, age older than 40 yr, short education and physical inactivity, but the significant associations disappeared on mutual adjustment. Baseline poor social support at work, presence of pain in the shoulder, lower forearm, wrist or hand during the past 3 months, treatment group and body mass index were not associated with this outcome.


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TABLE 3. Odds ratio (OR) of high pain score (less than 50% reduction) at 1 yr of follow-up in 266 new cases of tennis elbow by baseline characteristics

 
Working in a job classified as being potentially strenuous (manual worker), when adjusted for all other factors except physical strain, was associated with absence of general improvement (OR 3.0, CI 1.0–8.7) and less than 50% reduction in pain score (OR 2.3, CI 1.1–5.1).

Filing a labour compensation claim was not associated with any of the two primary outcomes.

Primary health-care utilization
The intervention group received the same treatment modalities as the control group, but the GPs registered fewer treatments and fewer treatment modalities per case in the intervention group (Table 4). In the intervention group, one-third received one or several treatments classified as passive (ultrasound, laser, acupuncture, glucocorticoid injections, supporting bandages or other passive treatments) as opposed to three-quarters in the control group. The control group also received more physiotherapy. However, 44% of the control group also received advice or instruction about exercises during the follow-up period, which indicates that this approach has been widely adopted by GPs. No differences were found between the two groups regarding sickness absence, job changing or starting education.


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TABLE 4. Treatment modalities, other actions prescribed and registered by GPs, and number of visits to the GP among 266 cases of lateral epicondylitis during 1 yr of follow-up according to treatment group: number (%)a

 
The number of visits to a GP, as recorded by the National Health Insurance registry within the same period, did not differ significantly between the two groups during follow-up. However, both groups had significantly more visits during the first 12 months of follow-up compared with the past 12 months. The intervention group made 2.6 visits more and the control group 3.1 more visits per case during the first year after recruitment, but the difference was not significant. The same pattern was found for the number of visits to physiotherapists.


    Discussion
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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Main findings
Providing standardized information and encouragement to stay active and perform exercises in this cohort of patients with newly developed and diagnosed epicondylitis did not seem to have any lasting advantageous effect on global improvement or pain improvement. Study participants with high physical strain at work, who were employed in manual jobs, reported high baseline distress and a high pain score, and in whom tennis elbow was located in the dominant arm had the poorest prognosis after 1 yr. Age above 40 yr was also consistently associated with poor outcome. The intervention group received less passive treatment, and cases in the intervention group received fewer treatment modalities. However, the intervention was not followed by fewer visits to a GP than in the control group, and the two groups made the same number of visits to physiotherapists.

Validity
The study should ideally have included a no-treatment group, but under these conditions it may be hard to recruit participants, and rather than imposing restrictions on treatments during follow-up we relied on registration of treatments carried out. The power of the study was reduced by exercise being encouraged by GPs in the control group also (44%), and by low compliance in the intervention group.

The assessment of the outcome is dependent on the reliability of the outcome measure. In using a combined score covering a period of 3 months rather than 1 week, we compensated for fluctuations in pain and function. In the Danish PRIM study (Projects on Research and Intervention in Monotonous work), weekly pain reports for 12 weeks among employees at a postal centre showed very good agreement with retrospective measures covering 3 months. For the elbow region, the {kappa} value was 0.79 among all subjects and 0.66 among subjects with complaints [21]. Cole et al. [16], in their study of musculoskeletal pain and interference with activity among newspaper workers, found that pain increased during follow-up, whereas work interference was unchanged. The questions used were almost identical to ours, but the period of time covered was restricted to the past week for both pain and function. In our study the combined outcome score was not related to clinical examination findings, which are often considered a gold standard, but the score related well to perceived overall improvement level at follow-up (r = 0.67).

Exercises involving a contract–relax–stretch model have been used with apparent success among cases of lateral epicondylitis referred to an orthopaedic clinic [17]. In our study, cases in the intervention group were instructed during a single session to perform exercises within limits determined by pain level, and the instruction was not repeated systematically on later occasions. In a recent study, comparison of corticosteroid injections, naproxen and placebo showed a good initial effect of injections on pain. However, no difference was found after 1 yr [22]. Also, at 1 yr no significant difference was found between an initial ‘wait-and-see’ policy and physiotherapy [23]. Regarding the overall development of lateral epicondylitis, it seems to be a consistent finding that improvement is greatest at the beginning of follow-up (3–6 months). Like Hay et al. [22] and Smidt et al. [23] in their studies of treatment with corticosteroid injections, physiotherapy and placebo, we found that improvement occurred in just over 80% of cases after 1 yr [22, 23]. Lewis et al. [15] measured pain and function during the preceding 24 h of follow-ups, using a score from 0 to 9. A pain score of 1.5 was seen at 12 months, while our study found a score of 1.7. One reason for the similarity in findings here after 1 yr may be that the disease takes its course almost regardless of treatment. However, the perceived intensity of symptoms and dysfunction are probably related to daily activities and the work situation.

In this study, the strongest negative prognostic factors were physical strain at work, affection of the dominant elbow, and high baseline distress and pain. Lewis et al. [15] found that clinical improvement was significantly greater among non-manual workers at 4 weeks and 6 months of follow-up [15]. Smidt et al. [23] found that concomitant neck pain, severity of pain on the day of examination, and social class were associated with outcome at 1 yr follow-up. Both Smidt et al. [23] and Sölverborn [17] found that previous duration of symptoms was related to outcome, but this finding was not replicated in the present study, possibly because only new cases with symptoms with duration of less than 1 yr were included. Also, we found age above 40 yr to be positively but, after adjustment, non-significantly associated with an unfavourable outcome.

It is indeed thought-provoking that medical passive treatment, including physiotherapy, which was found to account for the majority of health care costs for work-related enthesopathies of the upper extremity in an American setting, hardly has any impact on this study's main negative prognostic factors gender, dominant arm, age, social class and profession [24]. Also, it is noteworthy that, among these prognostic indicators, high physical strain is now accepted as an important risk factor for the development of lateral epicondylitis [5, 7], which is supported by recent studies [6, 9]. From this perspective, we find it interesting that cases in the intervention group obtained equally good results at 1 yr of follow-up, even though they received less passive treatment than controls. The findings support (i) the adoption of a wait-and-see policy, and (ii) the encouragement of patients to stay as active as possible, as recommended for instance in the Netherlands [25]. One may think that the treatment given in the doctor’s and physiotherapist’s offices is less important than what happens outside these offices afterwards, i.e. by how information concerning the disease, treatments, and prognosis, as well as advice on possible actions, is understood and applied by the patient. A Canadian study of health-care utilization among newspaper workers with neck and upper limb problems showed great variation in the interventions used and the professionals consulted [26]. This indicates that persons experiencing upper extremity problems shop around in seeking the solutions that they find best in the given situation. In this respect, the services provided by the medical profession may be seen as only one option among several.

Strengths and weaknesses
The fluctuating intensity of symptoms and the multiplicity of advice sought by patients during the course of the disease make it difficult to conduct methodologically rigorous studies. As mentioned above, we discarded the possibility of conducting a randomized, controlled trial with a no-treatment control group. Furthermore, we allowed the GPs to use whatever treatment they thought wise, because we could not expect GPs and patients to stick to any protocol for more than a short time. Like Hay et al. [22], we had hoped that GPs would capture all eligible cases, but found that 20% of the GPs had recruited 60% of the cases. The main objective of testing the efficiency of the intervention in terms of long-term global improvement and pain reduction could still be achieved, as could the identification of other prognostic factors and primary health-care utilization. The strengths of the approach chosen were the capture of cases as close as possible to debut, diagnosis by doctors using uniform criteria, and the classification of cases by severity at baseline. Furthermore, follow-up time and response rates were satisfactory according to the recommendations of Cole and Hudak [14]. A further strength is the use of information from the National Health Insurance registry to assess the number of visits to GPs and physiotherapists. All cases in the study population were registered with only one general practice and the National Health Insurance registry records all visits in order to refund doctors’ fees. The information on visits is therefore of high reliability. Furthermore, this approach made it possible to study three important aspects simultaneously within the same framework: intervention effects, other prognostic factors, and primary health-care utilization.

Conclusions
High physical strain at work or being employed in jobs with manual tasks, high baseline perceived distress and a high level of pain and dysfunction seem to predict an unfavourable outcome after 1 yr. In this cohort of participants with newly diagnosed lateral epicondylitis, standardized information and encouragement to stay active and perform exercises, given during a single visit to the local department of occupational medicine, gave no advantage in global improvement or pain. However, the intervention group received less treatment during follow-up, especially less passive treatment, and fewer treatment modalities at the general practice, although the number of visits was unchanged.

We conclude that high physical strain, established as a risk factor for developing lateral epicondylitis, also seems to act as a negative prognostic factor. This may have important implications for future treatment strategies. Patients with elbow pain may benefit from shifting the focus more towards adjustment at the workplace and staying active rather than receiving passive medical treatment.


    Acknowledgments
 
We thank the patients and GPs for participating in the study. Funding was from The Research Council of the National Working Environment Authority, The Danish Insurance Association and The Medical Research Unit of Ringkjoebing County.

Conflict of interest

The authors have declared no conflicts of interest.


    References
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 

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Submitted 19 September 2002; Accepted 4 March 2003