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Workplace
Original article
Surgery for subacromial impingement syndrome in relation to intensities of occupational mechanical exposures across 10-year exposure time windows
  1. Annett Dalbøge1,2,
  2. Poul Frost1,
  3. Johan Hviid Andersen2,
  4. Susanne Wulff Svendsen2
  1. 1 Department of Occupational Medicine, Danish Ramazzini Centre, Aarhus University Hospital, Aarhus, Denmark
  2. 2 Department of Occupational Medicine, Danish Ramazzini Centre, Regional Hospital West Jutland – University Research Clinic, Herning, Denmark
  1. Correspondence to Annett Dalbøge, Department of Occupational Medicine, Danish Ramazzini Centre, Aarhus University Hospital, Aarhus, 8000, Denmark; anetaner{at}rm.dk

Abstract

Objectives We aimed to identify intensities of occupational mechanical exposures (force, arm elevation and repetition) that do not entail an increased risk of surgery for subacromial impingement syndrome (SIS) even after prolonged durations of exposure. Additionally, we wanted to evaluate if exposure to hand-arm vibration (HAV) is an independent risk factor.

Methods We used data from a register-based cohort study of the entire Danish working population (n=2 374 403). During follow-up (2003–2008), 14 118 first-time events of surgery for SIS occurred. For each person, we linked register-based occupational codes (1993–2007) to a general population job exposure matrix to obtain year-by-year exposure intensities on measurement scales for force, upper arm elevation >90° and repetition and expert rated intensities of exposure to HAV. For 10-year exposure time windows, we calculated the duration of exposure at specific intensities above minimal (low, medium and high). We used a logistic regression technique equivalent to discrete survival analysis adjusting for cumulative effects of other mechanical exposures.

Results We found indications of safe exposure intensities for repetition (median angular velocity <45°/s), while force exertion ≥10% of maximal voluntary electrical activity and upper arm elevation >90° >2 min/day implied an increased risk reaching ORs of 1.7 and 1.5 after 10 years at low intensities. No associations were found for HAV.

Conclusions We found indications of safe exposure intensities for repetition. Any intensities of force and upper arm elevation >90° above minimal implied an increased risk across 10-year exposure time windows. No independent associations were found for HAV.

  • Acromioplasty
  • Duration
  • Intensity
  • Job Exposure Matrix
  • Shoulder Disorders

https://doi.org/10.1136/oemed-2017-104511

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    What this paper adds

    • We have previously shown that the risk of surgery for subacromial impingement syndrome (SIS) is related to cumulative occupational mechanical shoulder exposures.

    • To prevent work-related SIS, it is important to identify exposure intensities that do not entail an increased risk even after prolonged durations of exposure.

    • We developed a new analytical approach, which allowed us to distil the effect of accruing one more year of a particular type (force, arm elevation, repetition and hand-arm vibration) and intensity of exposure.

    • We found indications of safe exposure intensities on measurement scales for repetition (median angular velocity <45°/s), while force exertion ≥10% of maximal voluntary electrical activity and upper arm elevation >90° >2 min/day implied an increased risk when assessed across 10-year exposure time windows.

    Introduction

    Subacromial impingement syndrome (SIS) comprises a variety of shoulder disorders.1 Occupational mechanical exposures in terms of forceful shoulder exertions, upper arm elevation >90° and repetitive shoulder movements are established risk factors for clinically diagnosed SIS.2–4 These exposures have also been identified as risk factors for surgery for SIS,5–7 which may be considered ‘the tip of the iceberg’ of shoulder symptoms and disorders. Exposure to hand-arm vibration (HAV) is less well studied for both clinically diagnosed SIS2 and surgery for SIS.6 7 In Denmark, 1–2 per 1000 working age people had first-time surgery for SIS in 2008.8

    We have previously shown that the risk of surgery for SIS was related to occupational mechanical exposures accrued over a 10-year time window.6 7 Within this time window, we found indications of cumulative effects because the risk increased gradually when we extended the number of years contributing to the cumulative exposure estimates from 2 to 10 years back in time.6 To obtain cumulative exposure measures, we used the well-known pack-year concept of smoking based on standardised measures of exposure intensity multiplied by durations of exposure (years), rather than using seniority, which does not account for exposure intensities. However, the pack-year concept ignores the fact that a given cumulative exposure may be accrued through high exposure intensities and short durations of exposure or vice versa. Since we—and others910—found indications of cumulative effects, a logical next step would be to identify exposure intensities that do not lead to an increased risk of SIS even with prolonged durations of exposure.11 We are not aware of any previous studies of SIS, where effects of exposure intensity and duration have been studied simultaneously.

    To identify safe exposure intensities, we have developed a new analytic approach. The basic idea of this approach is to let each person accumulate durations of exposure at specific exposure intensities above minimal (low, medium and high) within a fixed exposure time window (we chose 10 years) and collect this information in separate exposure duration variables. Since the 10-year exposure time windows can include a varying number of years at minimal exposure intensities, the numbers of years at intensities above minimal do not necessarily add up to 10. This means that we can include the three intensity-specific exposure duration variables in the same exposure–response analysis without oversaturating the model and thus control for durations at other exposure intensities. In this way, we can estimate the risk of surgery for SIS in relation to durations at specific exposure intensities. Additionally, we can adjust for cumulative effects of other occupational mechanical exposures, which we have not done in our previous studies.6 7

    The aim of this study was to evaluate the effect of duration of exposure (years) at specific exposure intensities to gain insight into the maximum duration of exposure at each of these intensities that does not lead to an increased risk of surgery for SIS across 10-year time windows. We hypothesised that safe exposure intensities could be identified for forceful shoulder exertions, upper arm elevation >90° and repetitive shoulder movements. Additionally, we wanted to evaluate if exposure to HAV conveys an independent risk of surgery for SIS.

    Materials and methods

    Design and population

    We used data from a nationwide cohort study, which included the entire Danish working population as described previously.6 The cohort included all persons born in Denmark (excluding Greenland) between 1 January 1933 and 31 December 1977, who were alive and lived in Denmark on 31 December 2002 according to the Civil Registration System,12 and had at least 5 years of full-time employment between 1 January 1993 and 31 December 2007 according to the Supplementary Pension Fund Register.13 Persons were excluded in case of surgery for any shoulder disorder before 31 December 2002 according to the Danish National Patient Register.12 The cohort comprised 2 374 403 persons with 14 118 first-time events of surgery for SIS during follow-up from 1 January 2003 to 31 December 2008.6 Follow-up time was calculated from 1 January 2003 or the year after they reached 5 years of full-time employment, whichever came later, until surgery for SIS or censoring due to surgery for any other shoulder disorder, death, disappearance, emigration (including change of address to Greenland) or 31 December 2008, whichever came first.6 The Danish Data Protection Agency approved the study (j. no.: 2012-41-1187). In Denmark, register studies do not need approval from the Committee System on Biomedical Research Ethics (request no. 130/2009).

    Outcome

    The outcome was first-time surgery for SIS registered with a main diagnosis in the International Classification of Diseases, 10th revision, groups M19 (other and unspecified osteoarthritis) or M75.1–M75.9 (rotator cuff syndrome, bicipital tendinitis, calcific tendinitis, impingement syndrome, bursitis and other and unspecified shoulder lesions), without a subordinate diagnosis of M75.0 (adhesive capsulitis)6 and registered with one or more Danish Nordic Medico-Statistical Committee shoulder and upper arm surgery codes in the categories KNBA (exploratory procedures), KNBE and KNBF (procedures on synovia and ligaments), KNBG, KNBH and KNBK (acromioplasty) and KNBL and KNBM (procedures on bursae and tendons).

    Exposures

    To obtain individual year-by-year information on occupational mechanical exposures, we received register information on each person’s job history (1993–2007) from the Employment Classification Module.14 The job histories were provided in terms of a Danish version of the International Classification of Occupations from 1988 (D-ISCO 88) code and number of hours worked per week for each year of employment (D-ISCO 88). To convert the D-ISCO 88 codes to quantitative exposure estimates, we used a job exposure matrix (JEM), The Shoulder JEM,5 6 15 which cross-tabulates D-ISCO 88 codes with exposure estimates for force, arm elevation >90°, repetition and HAV (see below).5 6 In case of years of employment with missing D-ISCO 88 codes, the missing exposure estimates were replaced by the mean exposure for all the person’s years with information on D-ISCO 88 codes. We adjusted the exposure estimates according to the weekly working hours using the following factors: 1 (≥37 hours/week), 0.75 (≥28 to <37 hours/week), 0.5 (≥18.5 to <28 hours/week), 0.25 (≥9 to <18.5 hours/week) and 0 (<9 hours/week).6 For years outside the labour market and years of unemployment, we used exposure estimates of zero.

    The exposure estimates in The Shoulder JEM were originally based on expert ratings.5 6 The force scores (ranging from 0 (light) to 4 (near maximal) can be tentatively converted to percentages of maximal voluntary electrical activity (%MVE) based on Moore and Garg’s rating criteria.16 A score of 0 corresponds to <10 %MVE and a score of 1.5 corresponds to approximately 20 %MVE. In an update of the The Shoulder JEM, we calibrated the expert-rated arm elevation and repetition estimates against technical measurements to obtain exposure estimates on measurement scales (‘predicted measured job exposures’) as described previously.7 15 The technical measurements were obtained using inclinometry for 36 of the 172 job groups in The Shoulder JEM and comprised 575 whole day measurements.15 We used the predicted measured job exposures in our main analyses (arm elevation >90° (min/day), repetition (median angular velocity, °/s)).

    As motivated in the introduction, we chose 10 years as the time window of relevant exposure. This means that for each year of follow-up (2003–2008), we established exposure variables for the preceding 10-year period; thus, a person could contribute to the analysis with a maximum of six 10-year exposure time windows—one for each year of follow-up. In the analyses, we compared all persons, who had surgery for SIS in a given calendar year, with all other persons, who were still at risk of surgery for SIS, with respect to their exposures within the preceding 10-year time window.

    To identify safe exposure intensities, which do not lead to an increased risk of SIS even with prolonged exposures, we created three intensity-specific exposure duration variables for each type of exposure (force, arm elevation, repetition and HAV). For each 10-year exposure time window, these exposure duration variables summed up the number of years in each of three exposure intensity categories above minimal. We used the following categories: force: >0.0–<0.5, ≥0.5–<1.5 and ≥1.5–≤3 (force scores >3 were not represented); arm elevation: >90° >2.25–<5.00, ≥5.00–<10.00 and ≥10.00–≤30.00 min/day; repetition >27–<35, ≥35–<45 and ≥45–≤70 °/s; and HAV >0–<0.5, ≥0.5–<1 and ≥1.0–≤3.0 h/day irrespective of acceleration group (accelerations >10 m/s2 were not represented). To exemplify, a person who had a force score of 0.0 for 2 years, 1.0 for 3 years and 3.0 for 5 years within a given 10-year exposure time window would accrue 0 years in the category >0.0–<0.5, 3 years in the category ≥0.5–<1.5 and 5 years in the category ≥1.5–≤3.

    Covariates

    Covariates included register-based information on age, sex, region of residence, calendar year at start of follow-up and the number of the particular follow-up year. Due to the register design, we had no information on lifestyle factors such as leisure time shoulder intensive sports, body mass index and smoking. However, socioeconomic status (SES) may be a useful surrogate for these potential confounders.17 We included SES informed by Statistics Denmark in supplementary analyses, categorised as self-employed, top managers and employees at upper level, employees at intermediate level, employees at basic level and persons outside the labour market.6 To control for cumulative effects of other occupational mechanical exposures, we calculated cumulative exposure estimates according to the pack-year concept of smoking, expressed as force-years, arm elevation-years, repetition-years and HAV-years for each 10-year time window as described previously (time-varying exposures).5 6 For instance, the results for force were controlled for arm elevation-years, repetition-years and HAV-years.

    Statistical analyses

    For each type of occupational mechanical exposure, we performed pairwise correlation analyses between the three intensity-specific exposure duration variables (e.g., three correlations between the number of years in the force intensity categories: >0.0–<0.5, ≥0.5–<1.5 and ≥1.5–≤3). We used a logistic regression technique equivalent to discrete survival analysis to calculate the OR of surgery for SIS with 95% CIs in relation to the exposure duration variables; the resulting OR can be interpreted as an HR.18 In this technique, the statistical unit is person-years. We applied models with time-varying exposures using a 1-year lag.6 For each intensity-specific exposure duration variable, the reference consisted of all person-years without exposure in the particular category of intensity across the 10-year time window. For example, the reference for the duration of a force intensity in the category >0.0–<0.5 consisted of all person-years with an exposure duration of 0 years in this force intensity category, that is, force intensities in the categories 0≥0.5–<1.5 and ≥1.5–≤3. In this way, all person-years were included in all analyses. Partially adjusted models included the following covariates: the two other exposure duration variables for the specific type of exposure, age (five categories) as a time-varying variable, sex, region of residence (five categories), calendar year at start of follow-up (continuous) and the number of the particular follow-up year. Fully adjusted models further included cumulative exposures to other occupational mechanical exposures as time-varying variables.6 In supplementary analyses, we additionally controlled for SES. Since this might imply overadjustment, we also performed alternative analyses, where we restricted the population to employees at intermediate level. We chose the intermediate level because this is a large group (57% of the men and 67% of the women in the cohort),6 which represents all types of exposure and all exposure intensities. Ten years at a specific exposure intensity imply that exposure at this intensity was also accrued in the two most recent years of the time window while, for example, 8 years at a specific exposure intensity might have been accrued earlier on. To control for the potential effect of recent exposure, we created three variables for each type of exposure, which counted the number of years at low, medium and high exposure intensities within the two most recent years in each 10-year time window. Finally, we repeated the main analyses replacing the predicted measured job exposures with measured job exposures when available, and further restricting the study population to persons with a least 5, 7 and 10 years with measured job exposures in the period 1993–2007 as a way of evaluating the validity of our main analyses. We also repeated the analyses for men and women separately except for HAV due to uncertain estimates for women. The analyses were performed on Statistics Denmark’s research platform using STATA V.14.

    Results

    Table 1 shows characteristics of the total number of person-years of follow-up according to the four categories of force intensity. Men were increasingly over-represented in the two highest categories of force intensity, and the oldest age group was over-represented in the lowest category. As expected, top managers and employees at upper level were also over-represented in the lowest category, while employees at basic level were over-represented in the two highest categories. The percentages with high intensities of arm elevation and repetition increased with increasing force intensities and high intensities of exposure to HAV were increasingly over-represented in the two highest categories of force intensity. In total, 22.3% of the person-years were exposed to HAV but for short durations per day. Only 5% of the person-years were exposed to HAV at accelerations ≥3 m/s2 (result not shown). Pairwise correlations between the three exposure duration variables for force ranged from −0.09 to −0.26. Corresponding correlations for arm elevation, repetition and HAV were −0.04 to −0.13,–0.04 to −0.14 and −0.08 to 0.06. Thus, the exposure duration variables hardly correlated, which allowed us to include them together in the analyses. We had measured job exposures for 41% of the observation years (48% for men and 33% for women).

    View this table:
    Table 1

    Characteristics of 13 332 922 person-years (PY) of follow-up (Denmark 2003–2008) according to categories of force intensity (the mean of five experts’ ratings ranging from 0 (light) to 4 (near maximal)). Occupational mechanical exposures were estimated using The Shoulder JEM.5 6 15 Numbers in cells are percentages.

    Figure 1 shows ORs of surgery for SIS in relation to duration of exposure for the three categories of force intensity above minimal (represented by green, yellow and red curves) for 10-year exposure time windows. For each mechanical exposure, graphs to the left show partly adjusted ORs, while graphs to the right show fully adjusted ORs.

    Figure 1
    Figure 1

    ORs with 95% CIs of surgery for subacromial impingement syndrome (Denmark 2003–2008) in relation to duration of exposure (years) at different exposure intensities for force, arm elevation, repetition and hand-arm vibration (HAV) across 10-year exposure time windows. For each type of exposure, graphs to the left are partly adjusted ORs*, while graphs to the right are fully adjusted ORs†. *Each curve is adjusted for durations of exposure in the two other intensity categories above minimal, age, sex, region of residence, calendar year at start of follow-up and the number of the particular follow-up year. †Additionally adjusted for cumulative effects of other occupational mechanical exposures.

    Force

    We found exposure–response relationships for durations of force at all exposure intensities, reaching a maximum fully adjusted OR of 2.5 (95% CI 2.1 to 2.9). The figure can be used to calculate ORs for persons with different 10-year exposure profiles. For example, you can calculate the OR for persons with a 10-year time window, which includes 1 year as minimally exposed, 2 years with a force intensity >0.0–<0.5 (green colour curve) and 7 years with an intensity >1.5–≤3 (red colour curve) as the product of the ORs for the exposed years, that is, 1.2 (for 2 years with a green force intensity) × 1.7 (for 5 years with a red force intensity)=2.04. In supplementary analyses, we controlled for SES, which reduced the maximum fully adjusted OR to 2.3 (95% CI 1.9 to 2.7). In the analysis restricted to employees with an SES at intermediate level, a maximum fully adjusted OR of 2.5 (95% CI 2.1 to 3.0) was found. For the three variables capturing recent exposure at low, medium and high exposure intensities, we found fully adjusted ORs of 1.0 (95% CI 0.9 to 1.1), 1.1 (95% CI 1.0 to 1.2) and 1.2 (95% CI 1.1 to 1.3), which suggests that recent exposures did not inflate the maximum ORs for long exposure durations.

    Arm elevation

    Exposure–response relationships were found for durations of arm elevation at all exposure intensities above minimal, reaching a maximum fully adjusted OR of 1.7 (95% CI 1.5 to 2.0). Controlling for SES and restricting the analysis to employees with a SES at intermediate level both reduced the maximum fully adjusted OR to 1.4 (95% CI 1.2 to 1.6). For the three recent exposure variables, fully adjusted ORs of 0.9 (95% CI 0.9 to 1.0), 1.0 (95% CI 1.0 to 1.1) and 1.1 (95% CI 1.0 to 1.2) were found. After replacing the predicted measured job exposures with measured job exposures when available, we found a maximum fully adjusted OR of 2.0 (95% CI 1.7 to 2.3). Restricting the study population to persons with a least 5, 7 and 10 years of measured job exposures yielded maximum fully adjusted ORs of 1.9 (95% CI 1.6 to 2.2), 1.9 (95% CI 1.7 to 2.1) and 2.1 (95% CI 1.8 to 2.4).

    Repetition

    The green and yellow curves representing median angular velocities of >27–<35 and ≥35–<45°/s did not show an increasing trend, while the red curve (>45°/s) did so. For exposure intensities ≥45°/s, the fully adjusted OR reached 1.5 (95% CI 1.3 to 1.8) after 1–2 years with no subsequent increase. Controlling for SES reduced the maximum fully adjusted OR to 1.4 (95% CI 1.2 to 1.6), and in analyses restricted to employees with an SES at intermediate level, the maximum fully adjusted OR was 1.3 (95% CI 1.1 to 1.5). For the three recent exposure variables, we found similar results as for force and arm elevation. Using measured job exposures when available, we found a maximum fully adjusted OR of 1.5 (95% CI 1.4 to 1.8). When restricting the study population to persons with at least 5, 7 and 10 years of measured job exposures, we found maximum fully adjusted ORs of 1.5 (95% CI 1.3 to 1.7), 1.5 (95% CI 1.2 to 1.7) and 1.4 (95% CI 1.2 to 1.7).

    HAV

    For HAV, we found no exposure–response relationships after controlling for cumulative effects of other occupational mechanical exposures.

    Sex-specific results

    In sex-specific analyses, the curves were closely identical for men and women (results not shown). We found maximum fully adjusted ORs of 2.6 for both sexes for force (95% CI 2.1 to 3.1 for men and 1.9–3.8 for women), 2.1 for both sexes for arm elevation (95% CI 1.8 to 2.5 for men and 1.5–3.0 for women) and 1.6 for both sexes for repetition (95% CI 1.4 to 1.9 and 1.3–2.0, respectively).

    Discussion

    We found indications of safe exposure intensities for repetition in terms of median angular velocities <45°/s, while any intensity of force and arm elevation >90° above minimal implied an increased risk across 10-year exposure time windows. These results applied to both men and women. No associations were found for HAV when adjusted for cumulative effects of other occupational mechanical exposures.

    Our new analytical approach enabled us to gain insight into the maximum duration of exposure at specific exposure intensities that did not lead to an increased risk of surgery for SIS across 10-year time windows. This new approach also allowed us to adjust for cumulative effects of other occupational mechanical exposures. The study benefited from the fact that we were able to estimate both exposure intensity and exposure duration without recall-bias by linking registered D-ISCO 88 codes with The Shoulder JEM, which has shown good predictive validity in four studies.5–7 19 The exposure intensities of arm elevation and repetition were calibrated into measurement scales,15 and force intensities could be tentatively transformed into %MVE.16 The study also benefited from the fact that information on D-ISCO 88 codes, surgery for SIS and covariates was gathered through high-quality national longitudinal registers. A limitation of the study was that the available register information did not allow us to adjust for body mass index and smoking, for which we have previously found associations with surgery for SIS in a case–control study nested within the cohort.7 Of importance, these factors did not confound the main results7 and in the present study, adjusting for SES and restricting the cohort to people with a SES at intermediate level only changed the results marginally.

    Effects of recent exposures may have inflated the maximum ORs for long exposure durations. To the extent that the risk of surgery for SIS diminished with time since exposure, we may also have underestimated effects of recent short periods with high exposure intensities (and overestimated effects of remote short periods with high exposure intensities) because the analyses ignored the temporal distribution of years at different exposure intensities within each 10-year time window (eg, 2 years at a high force intensity might be accrued at the beginning, middle or end of the time window). However, the ORs were close to 1 for the variables that captured recent exposure, which suggests that 1 year at a high exposure intensity counts equally across 10-year time windows. However, a healthy worker effect may have attenuated the risk associated with 10 years at a high exposure intensity. The sensitivity analyses, where we restricted the study population to years with 5, 7 and 10 years with measured job exposures, tended to show slightly larger maximum adjusted ORs, but agreed well with our main results, which is reassuring with respect to the use of predicted measured job exposures from The Shoulder JEM.

    Effects of the rate of exposure delivery behind a given cumulative exposure have been compared in epidemiological studies of cancer risk.11 20 For example, it has been reported that for intensities of smoking <20 cigarettes per day, the cancer risk per pack-year increased with intensity, whereas for higher intensities, the risk decreased with intensity; in any case, the risk increased as a function of cumulative exposures (pack-years).11 20 A limitation of this approach is that exposure intensity is calculated as lifetime mean exposure, which does not account for year-by-year variation in exposure intensity. In our study, the three curves for each type of exposure in figure 1 represent different cumulative exposures, which means that they do not show how the rate of exposure delivery behind a given cumulative exposure influences the risk of surgery for SIS. We developed our new approach because we aimed to identify exposure intensities, which could be considered safe across 10-year time windows. Our focus was to distil the effect of accruing one more year of a particular type and intensity of exposure, which is also why we chose not to examine interactions between exposure intensity and duration as has been done previously.21

    For force and arm elevation >90°, our results suggested that any duration of exposure >0 years with an intensity above minimal implied an increased risk. To identify safe exposure limits for force, a viable way to proceed might be to subdivide the lowest force intensity into 2–3 exposure categories. Regarding arm elevation, we intend to conduct further analyses of the present data set in order to reveal which durations that can be considered safe for the categories >30°−60° and >60°−90°. For repetition, median angular velocities <45°/s seemed safe according to the present study. Examples of job groups that exceed 45o/s are postal workers (58o/s) and kitchen assistants (59o/s).15 In our case–control study, where we used cumulative exposure measures,7 a mean intensity of approximately 40o/s across a 10-year time window yielded a significantly elevated ORadj of around 1.5, but the risk might have been overestimated as we were unable to control for cumulative effects of other occupational mechanical exposures. A recent cross-sectional study pointed to a suitable exposure limit of 50o/s,22 but this was the exposure intensity that was associated with a doubling of the prevalence, which may not be a sufficient requirement for preventive purposes, and the study did not consider cumulative effects. Moreover, the outcome was not SIS, but infraspinatus tendinitis.22 Thus, it seems reasonable to point to 45o/s as a threshold limit value. We are currently evaluating cumulative effects of combinations of two or more occupational mechanical exposures on the risk of surgery for SIS using the present data set.

    While mechanical exposures cannot accumulate in the body, their effects on specific tissues can. Work-related pathomechanisms of SIS are believed to involve microtrauma of subacromial structures. Short-term exposures at high intensities and long-term exposures at intensities even within the tendons’ immediate physiological limits are thought to induce repeated microdamage, which eventually exceed the capacity for tendon repair, and at some point, the injury becomes clinically apparent.23 In our study, the observed increase in risk with increasing intensity and duration of force and arm elevation supported both parts of this hypothesis: 1 year of exposure at high intensities was sufficient to induce a modest increase in risk, while a substantial increase in risk was seen after 8–9 years of exposure. For repetition, short-term exposure (1–2 years) at a high intensity seemed sufficient, and we observed no further increase in risk with prolonged duration.

    In aetiological research, studies employing complex case definitions (eg, based on physical signs) have typically yielded similar results to studies that have employed simple symptom-based case definitions.24 This suggests that our results with respect to surgery for SIS (‘the tip of the iceberg’) should be generalisable to clinically diagnosed SIS and probably even self-reported anterolateral shoulder pain. In Denmark, we have a public and tax-paid healthcare system, which minimises socioeconomic differences in access to surgery, and the results should also be generalisable to other countries with relatively equal access to surgery for SIS. Importantly, the results only apply to the shoulder, and further studies are needed to identify safe limits with respect to other upper extremity disorders and in relation to informing policy and future legislation. Our analytical approach can be used for that purpose.

    In conclusion, we found indications of a safe exposure intensity for repetitive shoulder movements, while forceful shoulder exertions and upper arm elevation >90° above minimal implied an increased risk of surgery for SIS across 10-year exposure time windows. No associations were found for HAV when adjusted for cumulative effects of other occupational mechanical exposures.

    Acknowledgments

    We would like to thank Professor Morten Frydenberg, Department of Public Health – Section for Biostatistics, Aarhus University, for statistical assistance.

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    View Abstract

    Footnotes

    • Contributors Conceived and designed the study: AD, PF and SWS. Analysed data: AD in close collaboration with PF and SWS. Interpreted data: AD, JHA, PF and SWS. Drafted the paper: AD in close collaboration with PF and SWS. All authors have reviewed the paper for important intellectual content, approved the final version of the manuscript and take responsibility for the integrity of the work as a whole.

    • Funding This study was a part of the DOC*X study, which was supported by the Danish Working Environment Research Fund (project no. 43B2014B03).

    • Competing interests None declared.

    • Provenance and peer review Not commissioned; externally peer reviewed.

    • Data sharing statement No additional unpublished data from the study is available.