Occupational Strain as a Risk for Hip Osteoarthritis
A systematic review of risk assessment
; ; ; ; ; ; ;
Background: Multiple epidemiological studies have revealed an association between occupational physical strain and the risk of developing hip osteoarthritis.
Methods: To determine the association between the lifting and carrying of heavy loads or other physically demanding work and the risk of hip osteoarthritis (HOA) or total hip replacement (THR), we systematically searched the literature for primary studies on the effects of exposure to physical strain and meta-analytically reviewed the results that were amenable to comparisons across studies. We separately assessed studies that had hip pain as an endpoint.
Results: 5 cohort studies and 18 case–control studies were found suitable for inclusion. The lifting of heavy loads increases the risk of HOA or THR: exposure doubles the risk in men (relative risk [RR] 2.09, 95% confidence interval [1.4; 3.1]) and increases it by roughly 40% in women (RR 1.41 [1.0; 1.9]). Physically demanding work consisting of a combination of activities of various kinds (dealing with heavy loads, heavy manual work, or prolonged walking and standing) increases the risk by roughly 150% in men (RR 2.46 [1.3; 4.8]) and 40% in women (RR 1.38 [0.9; 2.2]). Hip pain was also reported more commonly in the exposed groups.
Conclusion: The studies are moderately to highly heterogeneous. An association exists between years of lifting heavy loads or other kinds of physical strain on the job and the risk of developing osteoarthritis of the hip. The greater the exposure, the greater the risk. The evidence base for risk assessment in women is currently inadequate.
Osteoarthritis of the hip (HOA) can cause severe pain and impairs quality of life. The burden on the healthcare system is considerable, and HOA has a negative effect on employment (1). While there are no robust data on the prevalence of HOA specific to Germany, a representative study (e1) found a lifetime prevalence of 2% for both men and women in Europe as a whole. International epidemiological studies show that the prevalence of HOA in women is 1 to 3% in the age group 45 to 75 years—depending on region and diagnostic criteria—and rises steeply thereafter. In men up to 75 years of age the reported prevalence of HOA varies between 2.5 and 8.5% (2–4). Alongside predisposing factors, the risk is increased by hip defects, age, overweight, and injury (5, 6).
The guidelines stipulate diagnosis of HOA, also known as coxarthrosis, on the basis of the triad of joint pain, impaired mobility, and radiological findings. Internationally, the most commonly used systems are the American College of Rheumatology criteria (7), the Kellgren and Lawrence radiological score (8), and function-related scoring systems such as the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) (9) and the Harris Hip Score (10). Radiological signs of arthrosis, e.g., narrowing of the joint space, do not always go hand in hand with pain and functional impairment. Only around 25 to 45% of cases of hip pain are attributable to HOA (11, 12). The overall importance of HOA will increase with the aging of the population (13).
Aim of study
The published reviews (14, 15) conclude that there is a basic link between occupations involving physically demanding work and the development of HOA. Owing to the nature of the searches and the restriction to certain types of study design, the numbers of studies included in these reviews were relatively small. Some of the searches were carried out more than 10 years ago. Meta-analyses and statements on the dose–response relationship are conspicuous by their absence. Therefore, we decided to carry out a new systematic survey of the literature with an expanded search string and including all types of studies. Study quality was assessed in uniform fashion and the data for men and women were analyzed separately. Our aim was to identify ways of preventing occupational HOA.
A systematic survey of the databases Medline, CENTRAL, Embase, and HSE-Line was carried out in March 2015. The search strategy was based on a combination of MeSH terms and text words as recommended by Mattioli in 2010 (16). We also conducted a manual search of the tables of contents of occupational medicine journals, the abstracts of international orthopedic conferences, and the references of all studies selected for inclusion. The inclusion criteria are listed in Box 1. At the beginning of the study the protocol was registered in PROSPERO (registration number CRD42015016894).
Two independent reviewers scrutinized and evaluated titles and abstracts (AB, AF, DK, MF), extracted data, and assessed the quality of the included studies (AB, DK, SU, UBA). In the case of disagreement, a third reviewer (AS) was consulted.
The quality criteria were developed separately for cohort studies and case–control studies on the basis of the Newcastle–Ottawa Quality Assessment Scale (17) and the Cochrane Handbook (18). The assessment categories are shown in Box 2. A detailed description of the quality criteria can be found in eTables 1 and 2.
To estimate the risk of HOA we compared, in each study, the highest and the second highest exposure category with the lowest exposure category. The individual studies reported odds ratios (OR), relative risks (RR), or hazard ratios (HR) with 95% confidence intervals (95% CI). Owing to the low prevalence of HOA, these estimators of effect do not differ widely from one another, so we interpreted all relative effect estimators as estimated relative risks. The effect estimators of the individual studies were merged with the random effects model in meta-analyses to produce pooled RR for the various endpoints (STATA version 14, metan procedure).
We performed separate meta-analyses for studies on the lifting/carrying of loads and studies on physically demanding work, with separate evaluation of data for men, women, and sex-undifferentiated groups. Some studies investigated several of these categories and were therefore included in various subanalyses. Four studies in which pain was the sole diagnostic criterion were not included in the meta-analyses but were evaluated separately.
The absolute risks of HOA (19) were calculated from the pooled RR and the lifetime prevalences of 1.9% for men and 2.1% for women (e1). The heterogeneity of the effect estimators was interpreted substantively and described using the I2 statistic (18).
A total of 3419 titles and abstracts were scrutinized and 247 full texts were read. After exclusion of all publications that did not fulfill our criteria, five cohort studies (eTable 3) and 18 studies with evaluation according to case–control status (eTable 4) were included (Figure).
Participants in primary studies
The 23 studies selected for inclusion were mostly carried out in Europe (n = 19), predominantly in the Scandinavian countries, the Netherlands, or the UK.
In the five cohort studies the duration of follow-up varied from 2 to 22 years. The participants were mostly of working age (20 to 70 years). Their mean age at the time of investigation was usually over 45 years. The proportions of men and women varied widely among the occupational groups. Four studies (e2–e5) included only men, and in one study (e6) all the participants were women. All of the studies included in our analysis adjusted for age and sex, and 11 of the case–control studies and four cohort studies also adjusted for body mass index or other significant confounders.
Eleven case–control studies and one cohort study investigated the exposure effect for the lifting of loads. In five of these case–control studies (e1, e7–e10) men and women were analyzed separately. Two of the case–control studies included only men (e4, e5), while one was restricted to women (e6). Five of the case–control studies evaluated men and women together (e7, e8, e11–e13). Two of the case–control studies and one cohort study diagnosed HOA on the basis of pain and were therefore analyzed separately, because pain is not a reliable diagnostic criterion (e11, e14, e15). The degree of exposure was determined by means of questionnaires or interviews: the loads ranged from 5 kg (e16) to more than 40 kg (e4). One study (e17) could not be included in the meta-analysis owing to a lack of sufficient data on categories of exposure.
Five case–control studies and three cohort studies did not describe the loads lifted or carried in concrete terms but instead defined criteria for “physically demanding work.” The definition of physically demanding work usually comprised a combination of various characteristics of activity, e.g., handling loads, heavy manual activity, and long periods of standing or walking. A sedentary occupation was generally defined as light manual labor. Some studies classified occupations as involving light or heavy physical strain. Five studies investigated the risk for men (e2, e3, e16, e18, e19), two were restricted to women (e18, e19), and one did not differentiate by sex (e20). Two cohort studies used pain as diagnostic criterion (e14, e21).
The studies included in meta-analysis employed various criteria for the diagnosis of HOA:
- Implantation of a hip joint prosthesis (total hip replacement, THR) (e4, e6–e8, e11, e13, e18, e19)
- Clinical and radiological diagnostic criteria or THR (e1, e5, e9, e15, e17)
- Exclusively clinical or radiological diagnostic criteria (e3, e10, e12, e16, e20, e22, e23).
The five cohort studies selected for inclusion scored 12 to 15 quality points on a scale of 0 to 19 (eTable 3).
Three population-based studies met all the quality criteria with regard to selection of participants. Two studies inquired about the weight of the loads lifted or carried. One frequently occurring limitation was joint analysis of men and women. The diagnosis was established by valid means in two studies, while in all other studies it rested on information provided by the probands.
The 18 included case–control studies scored 5 to 14 quality points on a scale of 0 to 15 (eTable 4). Seven studies attained at least 5 of a possible 6 points for “selection and representativeness” of the cases and control subjects. Seven studies scored maximum points for “comparability of cases and controls”. Only one study (e7) scored maximum points for determination of exposure. This study was based on assessment of exposure using a job exposure matrix (JEM). Four studies (e1, e7, e9, e11) inquired about load weight and the duration and frequency of lifting/carrying events, permitting estimation of the cumulative dose.
In men, lifting loads of the highest exposure category doubled the risk of HOA/THR (RR 2.09 [1.4; 3.1]). The second highest exposure category was associated with a 1.3-fold increase in risk (RR 1.35 [0.9; 1.9]). All studies found an increase in the risk of HOA/THR with increasing exposure. Based on a mean lifetime prevalence of HOA of 1.9% (19 per 1000), the risk in the highest exposure category rose to 40 per 1000 (95% CI: [27; 59]) (Table 1). In the second highest exposure category the risk of HOA/THR went up to 26 per 1000 [18; 36]. Therefore, a positive dose–response relationship was demonstrated both at the level of the seven studies included and at the level of the pooled estimators (Table 2).
In women, lifting loads increased the risk of HOA/THR in the two highest exposure categories by about 40% compared with the lowest exposure category (RR 1.41; [1.0; 1.9] and RR 1.40 [0.9; 2.2] respectively). The risk of developing HOA thus increased from 21 per 1000 [21; 40] to ca. 30 per 1000 [19; 46] (Table 3).
The five studies that did not evaluate men and women separately (e7, e8, e11–e13) also confirm that higher exposures are associated with an increased risk of HOA (eTable 5). The lifting and carrying of heavy loads also increases the risk of developing hip pain (RR 1.47 [1.1; 1.9]) (e11, e15). The moderate to high heterogeneity displayed by all risk estimators can be attributed to differences in study design and the variation in the weight of the loads concerned. Sensitivity analyses based on high-quality studies (scoring at least 12 of 15 points for quality) confirm the increase in the risk of developing HOA with increased exposure (e1, e4, e6–e8).
In men, physically demanding work increased the risk of HOA/THR 2.5-fold in the highest strain category (RR 2.46 [1.3; 4.8]) and twofold in the second highest strain category (RR 1.97 [1.3; 3.1]) (eTable 6). The increase in risk was somewhat lower for women: 38% in the highest (RR 1.38 [0.9; 2.2]) and 30% in the second highest strain category (RR 1.30 [0.9; 2.0]) (eTable 7). Studies that evaluated men and women together confirm these findings (eTable 7). The two cohort studies (e14, e21) that investigated hip pain found that activities involving physical strain increased the risk of pain (RR 1.77 [1.3; 2.4]).
Owing to the widely varying definitions of “physically demanding work,” the heterogeneity of the exposure effects for this form of exposure was greater than in studies of the effects of lifting loads. The study with a low-threshold definition of physically demanding work (e16) showed no increase in effects with increasing exposure, whereas very pronounced exposure effects were found in some of the studies with higher thresholds (e3). For this form of exposure too, analysis of only high-quality studies confirms that the risk increases with the level of exposure.
The studies included in our analyses show that both the lifting of heavy loads and physically demanding work in general increase the risk of HOA. This finding is in agreement with previously published reviews (14, 15, 20) reporting a moderate to high increase in the risk of the development of HOA for “heavy lifting and carrying” and “physically demanding work,” whereby these earlier publications described the exposure effects purely in narrative form. Our study involved additional meta-analytic pooling of the effects observed in comparable studies.
All analyses show that men are at greater risk of developing HOA. In all categories considered, the increase in risk is lower for women than for men. This is probably due to methodological inadequacies of the studies concerned. Most of the studies did not cover occupations where women predominate (e.g., nursing). Frequently the high exposure categories included only small numbers of women.
The frequency of lifting was usually not reported in absolute terms; rather, a minimum number of lifting events (e.g., 10 per working day) was defined. It can be assumed that men lift heavy loads more often per working day than women and thus have greater exposure. A sufficiently reliable statement on the effects for women cannot be derived from the data of the studies we analyzed. Studies with hip pain as the sole diagnostic criterion (e11, e14, e15, e21) also show an increase in risk with increasing exposure.
Examination of the effect estimators for the highest and second highest exposure categories shows that the risk of HOA increases with the weight of loads that are lifted and carried. A precise dose–response relationship could not be calculated because of the pronounced heterogeneity of data acquisition.
Four studies (e1, e7–e9) permit estimation of the minimum dose for loads. In men, the risk of developing HOA is increased after ca. 20 years’ regular lifting of loads weighing around 20 kg. For loads of 50 kg, the risk increases after only 10 years. In cumulative terms, lifting of at least 3000 to 5000 t is necessary to raise the risk of HOA to any significant extent. The weight thresholds seem be lower for women. However, the studies do not permit specification of relevant cumulative loads for women. Men in occupations involving physically demanding work have an increased risk of HOA after ca. 15 to 20 years (e2, e4).
The precision of the effect estimators is reduced by the heterogeneity of the exposure. Not all studies reported the duration of exposure. Investigations into memory distortion (recall bias) with regard to occupational exposure have shown that patients tend to overestimate their past exposure, particularly in respect of lighter loads (<5 kg), shorter work tasks (<2.5 min), and recent exposure. In the studies we analyzed, the duration of exposure was usually over 10 years and the loads mostly exceeded 20 kg, or exposure was derived indirectly from the type of occupation. In general the risk of HOA tended to be overestimated by recall bias. However, we believe that recall bias probably had only a slight impact owing to the nature of the studies included. The data on the effects in women are insufficient.
Total hip replacement is often used as a study endpoint. However, the relative frequency of this operation differs widely among countries owing to variation in healthcare structures (21). Therefore, studies of THR patients do not reflect the total incidence of HOA. Despite these limitations, the consistently comparable results of the cohort and case–control studies indicate that our findings are robust.
For men, our data demonstrate a positive association between long-term lifting and carrying of heavy loads, or physically demanding work in general, and the risk of developing osteoarthritis of the hip. For women, we take the view that more research is needed.
On the basis of our findings, recommendations for preventive measures can be proposed. Loads of 20 kg or more should not be lifted without mechanical assistance. With the goal of detecting signs of HOA as early as possible, preventive occupational medicine should include examination of the hip after no more than 15 to 20 years in a relevant occupation. Since the limited available data do not show any meaningful effect of training and exercise on the progression of HOA in the occupational context (22), any measures taken should aim at reducing the amount of strain. Workers who need to change their job should take advantage of the occupational rehabilitation programs offered by health insurance providers and pension insurance funds. Osteoarthritis of the knee is legally defined in Germany as an occupational disease (code no. 2112) with a cause–effect relationship comparable to that which is demonstrable for HOA. Thus, together with compensation in individual cases, section 3 of the Ordinance on Occupational Diseases (Berufskrankheiten-Verordnung, BKV) provides for technical aids and occupational reorientation.
This review was commissioned and financed by the Federal Institute for Occupational Safety and Health (BAuA – project F2334, www.baua.de/de/Forschung/Forschungsprojekte/f2334.html).
We thank Mirjam Fränzle and Christina Ramdohr for their assistance.
Conflict of interest statement
The authors declare that no conflict of interest exists.
Manuscript received on 29 November 2016, revised version accepted on
12 June 2017
Translated from the original German by David Roseveare
Dr. med. Annekatrin Bergmann
Medizinische Fakultät, Sektion Arbeitsmedizin
Magdeburger Str. 20, 06097 Halle, Germany
For eReferences please refer to:
Martin Luther University Halle-Wittenberg, Faculty of Medicine, Institute for Medical Epidemiology, Biometrics and Informatics: Prof. Haerting, PD Dr. Unverzagt
Regional Authority Darmstadt, Department for Occupational Safety and the Environment:
Technical University of Dresden, Faculty of Medicine, Institute and Polyclinic for Occupational and Social Medicine: Prof. Seidler, MPH; Ms Freiberg, MPH
Federal Institute for Occupational Safety and Health (BAuA), Berlin: Dr. Liebers
|1.||Günther KP, Puhl W, Brenner H, Stürmer T: Klinische Epidemiologie von Hüft- und Kniegelenkarthrosen: Eine Übersicht über Ergebnisse der „Ulmer Osteoarthrose-Studie“. Zeitschrift für Rheumatologie 2002; 61: 244–9 CrossRef|
|2.||van Saase JL, van Romunde LK, Cats A, Vandenbroucke JP, Valkenburg HA: Epidemiology of osteoarthritis: Zoetermeer survey. Comparison of radiological osteoarthritis in a Dutch population with that in 10 other populations. Ann Rheum Dis 1989; 48: 271–80 CrossRef MEDLINE PubMed Central|
|3.||Tepper S, Hochberg MC: Factors associated with hip osteoarthritis: data from the First National Health and Nutrition Examination Survey (NHANES-I). Am J Epidemiol 1993; 137: 1081–8 CrossRef|
|4.||Grubber JM, Callahan LF, Helmick CG, Zack MM, Pollard RA: Prevalence of radiographic hip and knee osteoarthritis by place of residence. J Rheumatol 1998; 25: 959–63 MEDLINE|
|5.||Jiang L, Rong J, Wang Y, et al.: The relationship between body mass index and hip osteoarthritis: a systematic review and meta-analysis. Joint Bone Spine 2011; 78: 150–5 CrossRef MEDLINE|
|6.||Pereira D, Peleteiro B, Araújo J, Branco J, Santos RA, Ramos E: The effect of osteoarthritis definition on prevalence and incidence estimates: a systematic review. Osteoarthritis Cartilage 2011; 19: 1270–85 CrossRef MEDLINE|
|7.||Altman R, Alarcón G, Appelrouth D, et al.: The American College of Rheumatology criteria for the classification and reporting of osteoarthritis of the hip. Arthritis Rheum 1991; 34: 505–14 CrossRef MEDLINE|
|8.||Kellgren JH, Lawrence JS: Radiological assessment of osteo-arthrosis. Ann Rheum Dis 1957; 16: 494–502 CrossRef|
|9.||Bellamy N, Buchanan WW, Goldsmith CH, Campbell J, Stitt LW: Validation study of WOMAC: a health status instrument for measuring clinically important patient relevant outcomes to antirheumatic drug therapy in patients with osteoarthritis of the hip or knee. J Rheumatol 1988; 15: 1833–40 MEDLINE|
|10.||Harris WH: Traumatic arthritis of the hip after dislocation and acetabular fractures: treatment by mold arthroplasty. An end-result study using a new method of result evaluation. J Bone Joint Surg Am 1969; 51: 737–55 CrossRef MEDLINE|
|11.||Birrell F, Croft P, Cooper C, Hosie G, Macfarlane G, Silman A: Health impact of pain in the hip region with and without radiographic evidence of osteoarthritis: a study of new attenders to primary care. The PCR Hip Study Group. Ann Rheum Dis 2000; 59: 857–63 CrossRef MEDLINE PubMed Central|
|12.||Morvan J, Bouttier R, Mazieres B, et al.: Relationship between hip dysplasia, pain, and osteoarthritis in a cohort of patients with hip symptoms. J Rheumatol 2013; 40: 1583–9 CrossRef MEDLINE|
|13.||Sun Y, Stürmer T, Günther KP, Brenner H: Inzidenz und Prävalenz der Cox- und Gonarthrose in der Allgemeinbevölkerung. Z Orthop 1997; 135: 184–92 CrossRef MEDLINE|
|14.||Sulsky SI, Carlton L, Bochmann F, et al.: Epidemiological evidence for work load as a risk factor for osteoarthritis of the hip: a systematic review. PloS One 2012; 7: e31521 CrossRef MEDLINE PubMed Central|
|15.||Jensen LK: Hip osteoarthritis: influence of work with heavy lifting, climbing stairs or ladders, or combining kneeling/squatting with heavy lifting. Occup Environ Med 2008; 65: 6–19 CrossRef MEDLINE|
|16.||Mattioli S, Zanardi F, Baldasseroni A, et al.: Search strings for the study of putative occupational determinants of disease. Occup Environ Med 2010; 67: 436–43 CrossRef MEDLINE PubMed Central|
|17.||Wells GA, Shea B, O‘Connell D, et al.: The Newcastle-Ottawa Scale (NOS) for assessing the quality of nonrandomised studies in meta-analyses. www.ohri.ca/programs/clinical_epidemiology/oxford.asp (last accessed on 14 May 2015).|
|18.||Higgins JPT, Green S (eds.): Cochrane handbook for systematic reviews of interventions version 5.1.0 (updated March 2011). The Cochrane Collaboration, 2011. www.cochrane-handbook.org (last accessed on 18 May 2015).|
|19.||Gyatt HG, Oxman AD, Santesso N, et al.: GRADE guidelines: 12. Preparing summary of findings tables-binary outcomes. J Clin Epidemiol 2013; 66: 158–72 CrossRef MEDLINE|
|20.||Lievense A, Bierma-Zeinstra S, Verhagen A, Verhaar J, Koes B: Influence of work on the development of osteoarthritis of the hip: a systematic review. J Rheumatol 2001; 28: 2520–8 MEDLINE|
|21.||Wengler A, Nimptsch U, Mansky T: Hip and knee replacement in Germany and the USA—analysis of individual inpatient data from German and US hospitals for the years 2005 to 2011. Dtsch Arztebl Int 2014; 111: 407–16 VOLLTEXT|
|22.||Bennell KL, Egerton T, Martin J, et al.: Effect of physical therapy on pain and function in patients with hip osteoarthritis: a randomized clinical trial. JAMA 2014; 311: 1987–97 CrossRef MEDLINE|
|e1.||Kaila-Kangas L, Arokoski J, Impivaara O, et al.: Associations of hip osteoarthritis with history of recurrent exposure to manual handling of loads over 20 kg and work participation: a population-based study of men and women. Occup Environ Med 2011; 68: 734–8 CrossRef MEDLINE|
|e2.||Roach KE, Persky V, Miles T, Budiman-Mak E: Biomechanical aspects of occupation and osteoarthritis of the hip: a case-control study. J Rheumatol 1994; 21: 2334–40 MEDLINE|
|e3.||Vingård E, Alfredsson L, Fellenius E, Hogstedt C: Disability pensions due to musculo-skeletal disorders among men in heavy occupations. A case-control study. Scand J Public Health 1992; 20: 31–6.|
|e4.||Vingård E, Hogstedt C, Alfredsson L, Fellenius E, Goldie I, Köster M: Coxarthrosis and physical work load. Scand J Work Environ Health 1991; 17: 104–9 CrossRef|
|e5.||Croft P, Cooper C, Wickham C, Coggon D: Osteoarthritis of the hip and occupational activity. Scand J Work Environ Health 1992; 18: 59–63 CrossRef|
|e6.||Vingård E, Alfredsson L, Malchau H: Osteoarthrosis of the hip in women and its relation to physical load at work and in the home. Ann Rheum Dis 1997; 56: 293–8 CrossRef|
|e7.||Rubak TS, Svendsen SW, Søballe K, Frost P: Total hip replacement due to primary osteoarthritis in relation to cumulative occupational exposures and lifestyle factors: a nationwide nested case-control study. Arthritis Care Res (Hoboken) 2014; 66: 1496–505 CrossRef MEDLINE|
|e8.||Coggon D, Kellingray S, Inskip H, Croft P, Campbell L, Cooper C: Osteoarthritis of the hip and occupational lifting. Am J Epidemiol 1998; 147: 523–8 CrossRef|
|e9.||Lau EC, Cooper C, Lam D, Chan VN, Tsang KK, Sham A: Factors associated with osteoarthritis of the hip and knee in Hong Kong Chinese: obesity, joint injury, and occupational activities. Am J Epidemiol 2000; 152: 855–62 CrossRef|
|e10.||Elsner E, Nienhaus A, Beck W: Coxarthrose und berufliche Belastungen. Z Gesundheitswiss 1995: 131–44.|
|e11.||Sobti A, Cooper C, Inskip H, Searle S, Coggon D: Occupational physical activity and long-term risk of musculoskeletal symptoms: a national survey of post office pensioners. Am J Ind Med 1997; 32: 76–83 CrossRef|
|e12.||Allen KD, Chen J, Callahan LF, et al.: Associations of occupational tasks with knee and hip osteoarthritis: the Johnston County Osteoarthritis Project. J Rheumatol 2010; 37: 842–50 CrossRef MEDLINE PubMed Central|
|e13.||Yoshimura N, Sasaki S, Iwasaki K, et al.: Occupational lifting is associated with hip osteoarthritis: a Japanese case-control study. J Rheumatol 2000; 27: 434–40.|
|e14.||Tüchsen F, Hannerz H, Burr H, Lund T, Krause N: Risk factors predicting hip pain in a 5-year prospective cohort study. Scand J Work Environ Health 2003; 29: 35–9 CrossRef|
|e15.||Pope DP, Hunt IM, Birrell FN, Silman AJ, Macfarlane GJ: Hip pain onset in relation to cumulative workplace and leisure time mechanical load: a population based case-control study. Ann Rheum Dis 2003; 62: 322–6 CrossRef PubMed Central|
|e16.||Cvijetić S, Dekanić-Ožegović D, Campbell L, Cooper C, Potocki K: Occupational physical demands and hip osteoarthritis. Arh Hig Rada Toksikol 1999; 50: 371–9 MEDLINE|
|e17.||Riyazi N, Rosendaal FR, Slagboom E, Kroon HM, Breedveld FC, Kloppenburg M: Risk factors in familial osteoarthritis: the GARP Sibling Study. Osteoarthritis Cartilage 2008; 16: 654–9 CrossRef MEDLINE|
|e18.||Rubak TS, Svendsen SW, Søballe K, Frost P: Risk and rate advancement periods of total hip replacement due to primary osteoarthritis in relation to cumulative physical workload. Scand J Work Environ Health 2013; 39: 486–94 CrossRef MEDLINE|
|e19.||Flugsrud GB, Nordsletten L, Espehaug B, Havelin LI, Meyer HE: Risk factors for total hip replacement due to primary osteoarthritis: a cohort study in 50,034 persons. Arthritis Rheum 2002; 46: 675–82 CrossRef MEDLINE|
|e20.||Juhakoski R, Heliövaara M, Impivaara O, et al.: Risk factors for the development of hip osteoarthritis: a population-based prospective study. Rheumatology (Oxford) 2009; 48: 83–7 CrossRef MEDLINE|
|e21.||Ratzlaff CR, Steininger G, Doerfling P, et al.: Influence of lifetime hip joint force on the risk of self-reported hip osteoarthritis: a community-based cohort study. Osteoarthritis Cartilage 2011; 19: 389–98 CrossRef MEDLINE|
|e22.||Heliövaara M, Mäkelä M, Impivaara O, Knekt P, Aromaa A, Sievers K: Association of overweight, trauma and workload with coxarthrosis. A health survey of 7,217 persons. Acta Orthop Scand 1993; 64: 513–8 CrossRef MEDLINE|
|e23.||Jacobsen S, Sonne-Holm S, Søballe K, Gebuhr P, Lund B: The distribution and inter-relationships of radiologic features of osteoarthrosis of the hip. A survey of 4151 subjects of the Copenhagen City Heart Study: the Osteoarthrosis Substudy. Osteoarthritis Cartilage 2004; 12: 704–10 CrossRef MEDLINE|
|e24.||Rubak TS: Cumulative physical exposure in the work environment as a risk factor for primary osteoarthritis leading to total hip replacement: exposure assessment and risk estimation. PhD thesis. Aarhus University Denmark 2010.|