Case definition

Meaningful and unambiguous case definition for hip osteoarthritis is a challenge because it occurs in a spectrum of severity with no clear dichotomy between normality and abnormality. Moreover, the dominant symptoms of pain and stiffness correlate imperfectly with more objective radiographic evidence of the underlying pathology . As with osteoarthritis in other joints, the disorder is characterised radiologically by a reduction in joint space (reflecting a loss of the articular cartilage), osteophytosis and an increased subchondral bone density with the formation of cysts. In the most severe cases, there can also be deformity of the femoral head. Many studies have graded the severity of radiographic abnormalities according to a scheme devised by Kellgren and Lawrence, using either standardised radiographs or standardised verbal descriptors as a reference. Others have used measures of joint space, such as the minimal distance between the articular surfaces . An assessment of the validity and reliability of alternative case definitions when applied to a sample of 3585 men and women aged ≥55 years indicated a higher interrater agreement and a stronger association with symptoms for those based on minimal joint space and on comparison with standard radiographs, than for diagnosis based on verbal descriptors .

Frequency by sex, age, place and time

The prevalence and distribution of hip osteoarthritis internationally was estimated as part of the Global Burden of Disease 2010 study . In a systematic review and meta-analysis, which adjusted for differences between studies in case definition, the global age-standardised prevalence of symptomatic, radiographically confirmed (Kellgren–Lawrence grades 2–4) disease in 2010 was estimated to be 0.85% (95% uncertainty interval 0.74–1.02%), with no evidence of a change since 1990. Rates were higher in women (0.98%) than in men (0.70%), and they increased progressively with age from close to zero at 30 years to ≥10% above 80 years. They were highest in North America (1.6% in men and 2.1% in women) and lowest in East Asia (0.2% in men and 0.3% in women).

In Britain, a report published by Arthritis Research UK used data from an anonymised general practice database to estimate the prevalence of diagnosed hip osteoarthritis and pain in the hip that was not associated with a diagnosis of gout, rheumatoid arthritis or fracture, among people aged ≥45 years during 2004–2010. Rates of 4% and 7% were determined for men and women aged 45–64 years. While the diagnostic criteria may have lacked specificity (not all unexplained hip pain will have been attributable to osteoarthritis), it is unlikely that all cases in the population covered by the database will have presented to their general practitioner, and comparison of the prevalence at age ≥75 years (11% in men and 16% in women) with findings from the Global Burden of Disease study suggests that estimates were reasonably accurate.

Severe hip osteoarthritis is often treated by, and is the main indication for total hip arthroplasty. During 2012–2013, some 20,000 such procedures were performed in England over a 12-month period in patients aged 45–64 years, indicating a substantial burden in the people of working age .

Genetic predisposition

Twin and family studies point to an important role of genetic predisposition , especially in generalised osteoarthritis that affects multiple joints , and several molecular mechanisms have been identified that might be involved . Genetic factors may contribute to the variation in the prevalence of hip osteoarthritis that has been observed between different parts of the world .

Developmental abnormalities

Developmental abnormalities of the hip joint such as congenital dislocation, Perthes’ disease and slipped capital femoral epiphysis are associated with a clearly elevated risk of hip osteoarthritis . This is thought to occur because structural deformities alter the distribution of mechanical stresses in the joint. There is some evidence that more minor, sub-clinical acetabular dysplasia may also be a risk factor, although this is less certain .

Hip injury

Structural deformity may also occur as a consequence of traumatic injury to the hip, and there is some evidence that such injury increases the risk of later osteoarthritis . However, the link to injury is not as well established as for osteoarthritis of the knee.

Body mass index

Like other joints, the hip is more likely to be affected by osteoarthritis in people with a higher body mass index (BMI). However, the relationship is weaker than for knee osteoarthritis . A systematic review and meta-analysis that included 14 epidemiological studies indicated a relative risk of 1.11 (95% CI: 1.07–1.16) for an increase in BMI of 5 kg/m ² . The underlying mechanism is likely to be partly mechanical, with greater stresses on the joint in people who are heavier. However, obesity may also lead to systemic changes in metabolism that predispose to osteoarthritis.

Importance as confounders

Apart from sex and age, the most important potential confounder of associations between hip osteoarthritis and occupation is BMI. Major developmental abnormalities of the hip are relatively rare, and they account for only a small proportion of cases, while genetic predisposition would not be expected to associate strongly with occupational exposures except insofar as people may give up physically demanding work if they have developed osteoarthritis. Nevertheless, some studies of occupational risk factors have adjusted for the presence of Heberden’s nodes as a marker for systemic tendency to osteoarthritis, including from genetic predisposition.

Genetic predisposition

Twin and family studies point to an important role of genetic predisposition , especially in generalised osteoarthritis that affects multiple joints , and several molecular mechanisms have been identified that might be involved . Genetic factors may contribute to the variation in the prevalence of hip osteoarthritis that has been observed between different parts of the world .

Developmental abnormalities

Developmental abnormalities of the hip joint such as congenital dislocation, Perthes’ disease and slipped capital femoral epiphysis are associated with a clearly elevated risk of hip osteoarthritis . This is thought to occur because structural deformities alter the distribution of mechanical stresses in the joint. There is some evidence that more minor, sub-clinical acetabular dysplasia may also be a risk factor, although this is less certain .

Hip injury

Structural deformity may also occur as a consequence of traumatic injury to the hip, and there is some evidence that such injury increases the risk of later osteoarthritis . However, the link to injury is not as well established as for osteoarthritis of the knee.

Body mass index

Like other joints, the hip is more likely to be affected by osteoarthritis in people with a higher body mass index (BMI). However, the relationship is weaker than for knee osteoarthritis . A systematic review and meta-analysis that included 14 epidemiological studies indicated a relative risk of 1.11 (95% CI: 1.07–1.16) for an increase in BMI of 5 kg/m ² . The underlying mechanism is likely to be partly mechanical, with greater stresses on the joint in people who are heavier. However, obesity may also lead to systemic changes in metabolism that predispose to osteoarthritis.

Importance as confounders

Apart from sex and age, the most important potential confounder of associations between hip osteoarthritis and occupation is BMI. Major developmental abnormalities of the hip are relatively rare, and they account for only a small proportion of cases, while genetic predisposition would not be expected to associate strongly with occupational exposures except insofar as people may give up physically demanding work if they have developed osteoarthritis. Nevertheless, some studies of occupational risk factors have adjusted for the presence of Heberden’s nodes as a marker for systemic tendency to osteoarthritis, including from genetic predisposition.

Evidence for a role of occupation

Epidemiological studies have explored the association between hip osteoarthritis and various types of work and occupational activities. A systematic review published in 2012 identified 30 such investigations , 28 of which are summarised in Table 2 (the other two did not give results for osteoarthritis of the hip specifically) along with six further studies that are relevant. The total body of evidence comprises nine cohort studies, 14 case-control investigations and 11 cross-sectional surveys.

Table 2

Epidemiological studies investigating possible occupational causes of hip osteoarthritis.

Reference	Country	Study design	Study sample	Case definition and ascertainment	Exposures and methods of assessment	Confounders considered	Risk estimates (95% CIs)
Lindberg and Danielsson 1984	Sweden	Cross-sectional	332 male shipyard workers (heavy labour), 352 male white-collar workers and 438 men from general population	Previous radiograph of hip showing joint space <4 mm at <70 years, <3 mm at >70 years or ≥1 mm difference between hips	Occupational group or general population	Sex, age	No significant differences between the three groups in the prevalence of hip osteoarthritis
Typpö 1985	Finland	Case-control	Cases with hip osteoarthritis and controls without, who had attended medical or surgical out-patient clinics	Based on measurements of osteophyte size, minimal joint space and size of subchondral cysts in radiographs taken for various reasons	Classes of work and occupation determined by questionnaire. Unclear how changes of job were handled.	None	131/251 cases were heavy manual workers versus 110/254 controls 90/224 cases were farmers versus 70/255 controls 22/224 cases were construction workers versus 14/255 controls
Jacobsson et al., 1987	Sweden	Cross-sectional	85 men listed for total hip arthroplasty, 21 men with urography films showing joint space <3 mm, and 190 controls with normal joint space on urography	Group (a) Listed for hip arthroplasty; or Group (b) joint space<3 mm, identified from hospital records	Occupational history, including physical activities, by questionnaire	None	Heavy labour in 84% of Group (a) and 90% of Group (b) versus 72% of controls Farm work in 55% of Group (a) and 67% of Group (b) versus 40% of controls Heavy lifting in 85% of Group (a) and 86% of Group (b) versus 73% of controls
Thelin 1990	Sweden	Case-control	98 men aged 55–70 years with hip osteoarthritis and 201 controls from general population	Surgery for hip osteoarthritis	Years in farming from lifetime occupational history by postal questionnaire	Sex	>10 versus <1 year in farming RR: 3.2 (1.8–5.5)
Vingård, Alfredsson et al., 1991	Sweden	Cohort	207,638 men and 42,579 women from general population with same occupation in each of 1960 and 1970 censuses, followed during 1981–1983	Hospital admission for hip osteoarthritis from national discharge register	Occupation at censuses and inferred exposures to dynamic and static forces on lower extremity	Age, sex, county and degree of urbanisation	High versus low exposure to forces on lower extremity RR: 2.2 (1.6–2.8) in men born 1905–1924 RR: 2.0 (1.6–2.3) in men born 1925–1945 RR: 1.6 (0.9–3.1) in women born 1905–1924 RR: 1.1 (0.9–1.5) in women born 1925–1945 Farmers versus low exposure to forces on lower extremity RR: 3.78 (2.91–3.88) in men RR: 1.47 (0.86–2.85) in women Construction workers versus low exposure to forces on lower extremity RR: 1.66 (1.32–1.87) in men
Vingård, Hogstedt et al., 1991	Sweden	Case-control	239 men aged 50–70 years with hip osteoarthritis and 302 controls randomly selected from general population	First-time hip arthroplasty for idiopathic osteoarthritis from hospital clinics	Occupational histories to age 49 years by interview, and inferred exposures to static and dynamic forces on hip	Age, body mass index, smoking, sports activities up to age 29 years	High versus low exposure to static and dynamic forces on hip RR: 2.42 (1.45–4.04)
Vingård et al.,1992	Sweden	Case-control	140 men with hip osteoarthritis and 298 controls from general population	Receipt of disability pension with physician diagnosis of hip osteoarthritis	Occupational history by interview, and inferred exposures to physical loading	Age, sex	High versus low exposure to forces on hip RR: 12.4 (6.7–23.0) Farmers or forest workers for ≥10 years versus never in exposed occupation RR: 13.8 (4.0–48.1) Construction worker for ≥10 years versus never in exposed occupation RR: 5.3 (2.6–10.6)
Croft, Coggon et al., 1992	UK	Cross-sectional	167 men aged 60–76 years who had farmed for ≥1 year and 83 controls from mainly sedentary jobs	Joint space ≤1.5 mm or previous total hip arthroplasty	Lifetime history of work in specified jobs by questionnaire	Age, height, weight and Heberden’s nodes	1–9 years in farming OR: 4.5 (0.8–26.3) ≥10 years in farming OR: 9.3 (1.9–44.5)
Croft, Cooper et al., 1992	UK	Case-control	53 men with hip osteoarthritis on intravenous urogram and 294 controls with minimum joint space ≥3.5 mm on intravenous urogram	Previous total hip arthroplasty for osteoarthritis or minimum joint space ≤1.5 mm in at least one hip	Lifetime occupational history including detail of specified activities, by interview	Age and hospital where intravenous urogram was performed	Work for ≥10 versus <1 year in farming OR: 2.0 (0.9–4.4) Work for ≥10 versus <1 year in construction OR: 0.5 (0.1–2.3) Work involving lifting/moving weights >56 lb by hand for ≥ 20 versus <1 year OR: 2.5 (1.1–5.7)
Axmacher and Lindberg 1993	Sweden	Cross-sectional	716 farmers from across Sweden and 2500 controls from general population of Malmö, all of whom had undergone radiographic examination of bowel or urinary tract	Joint space <4 mm and/or osteosclerosis, cyst formation or other structural changes	Work in farming versus control	Age and sex	Hip osteoarthritis in 45/565 male farmers versus 10/1250 controls, and in 2/151 female farmers versus 10/1250 controls
Heliövaara et al., 1993	Finland	Cross-sectional	3322 men and 3895 women aged ≥30 years from general population	Cases defined by standardised clinical assessment in subset with questionnaire responses suggestive of musculoskeletal diseases	Exposure to each of five physically stressful activities in either current/most recent or longest duration job	Age, sex, body mass index, injury to lower limb	4–5 versus 0 physically stressful activities at work OR: 2.7 (1.7–4.4)
Roach 1994	USA	Case-control	99 men >40 years with hip osteoarthritis and 233 controls with no osteoarthritis on intra-venous pyelogram	Hip pain and Kellgren and Lawrence grade 3 or 4 from hospital records	Years in different occupational categories from postal questionnaire	Age, sex, race, previous cancer diagnosis, obesity aged 40 years, running for exercise	Heavy versus light work OR: 2.5 (1.5–5.0)
Van Dijk 1995	The Netherlands	Cross-sectional	19 female former ballet dancers aged 50–70 years and 19 pair- matched controls from surgical outpatient department with no long-term sports participation or physically demanding work	Measurement of joint space and grading to Hermodsson scale, using standardised radiographs	Work as ballet dancer versus control	Sex, age (but did not perform appropriate analysis for matched data)	No significant difference in hip osteoarthritis between dancers and controls
Vingård et al.,1997	Sweden	Case-control	230 women aged 50–70 years with hip osteoarthritis and 273 controls from general population without hip problems	Total hip arthroplasty for primary osteoarthritis ascertained from national register	Occupational history to age 50 years and associated physical activities by interview	Age, body mass index, smoking, sports activities, number of children and hormone therapy	High versus low exposure to heavy lifting RR: 1.5 (0.9–2.5)
Thelin et al.,1997	Sweden	Case-control	216 men with hip osteoarthritis who had undergone radiological examination of hip and 479 controls from general population	Joint space <3 mm	Lifetime occupational history, including specified tasks, by postal questionnaire	None	Worked on farm for >30 versus 0 years OR: 4.45 (2.90–6.83) Heavy physical work before age 16 years OR: 2.06 (1.48–2.86)
Coggon et al.,1998	UK	Case-control	210 men and 401 women aged 45–91 years with osteoarthritis and 611 controls from general population matched for sex, age and general practice	Listed for total hip arthroplasty because of primary osteoarthritis	Lifetime occupational history and associated physical activities up to 10 years before entry to study by interview	Age, sex, body mass index, history of hip injury, Heberden’s nodes	Lifting >25 kg for ≥20 versus 0 years OR: 2.3 (1.3–4.4) in men OR: 0.8 (0.4–1.5) in women Maximum lifted for ≥10 years ≥ 50 kg versus <10 kg OR: 3.2 (1.6–6.5) in men OR: 1.1 (0.5–2.5) in women
Cvijetic et al., 1999	Croatia	Cross-sectional	298 men and 292 women aged ≥45 years, randomly sampled from general population	Kellgren and Lawrence grade 2–4 on radiograph of right hip	Categories of activity in current or most recent job determined by interview	Age, sex and body mass index	Jobs with high physical strain versus mostly sedentary jobs OR: 1.15 (0.52–2.52) in men OR: 1.34 (0.52–3.04) in women
Yoshimura et al., 2000	Japan	Case-control	114 men and women aged ≥45 years with osteoarthritis, and 114 individually matched controls from general population	Listed for total hip arthroplasty for primary osteoarthritis	Lifting in main lifetime job by interview	Age, sex, district of residence, history of knee pain and age left school	Heavy lifting (weights of ≥50 kg) versus no lifting OR: 4.1 (1.1–15.2)
Chitnavis et al., 2000	UK	Cross-sectional	107 male patients with hip osteoarthritis and 52 undergoing total knee arthroplasty	Total hip arthroplasty for osteoarthritis with no identifiable cause in joint replaced	Work as a farmer by interview	Sex	16% of men with hip osteoarthritis had worked as farmers versus 7% of men with total knee arthroplasty
Lau et al., 2000	Hong Kong	Case-control	138 patients with hip osteoarthritis and 414 controls, individually matched for age and sex, from general practice clinics	Attended orthopaedic clinic with primary hip osteoarthritis of Kellgren and Lawrence grade 3 or 4	Lifting in main lifetime job by interview	Age, sex, height, weight, history of joint injury, regular sports activities	Lifting ≥10 kg > 10 times per week in main job versus no lifting ≥10 kg OR: 3.1 (0.7–14.3) in men OR: 2.4 (1.1–5.3) in women
Flugsrud et al., 2002	Norway	Cohort	50,034 participants in cardiovascular screening programme followed for 9 years	First total hip arthroplasty for primary hip osteoarthritis	Physical activity in current job at time of screening by postal questionnaire	Age at screening, sex, height, body mass index, marital status, leisure time physical activity, smoking habits	Intensive versus sedentary physical activity at work RR: 2.1 (1.5–3.0) in men RR: 2.1 (1.3–3.3) in women
Tüchsen 2003	Denmark	Cohort	All employed Danish men aged 20–59 in January 1981, 1986, 1991 and 1994 followed during 1981–1985, 1986–1990, 1991–1993 and 1994–1999, respectively	First admission to hospital for hip osteoarthritis, ascertained from national patient register	Main occupation at beginning of each follow-up period from national register	Age and sex	Self-employed farmers 1981–1985 SHR: 281 (259–304) 1986–1990 SHR: 283 (269–298) 1991–1993 SHR: 285 (268–302) 1994–1999 SHR: 286 (262–313) Self-employed in agricultural tractor pools 1981–1985 SHR: 210 (97–455) 1986–1990 SHR: 200 (122–327) 1991–1993 SHR: 192 (116–315) 1994–1999 SHR: 183 (86–387) Employees in farming and horticulture 1981–1985 SHR: 114 (89–147) 1986–1990 SHR: 138 (118–161) 1991–1993 SHR: 160 (140–183) 1994–1999 SHR: 189 (158–227)
Thelin et al., 2004	Sweden	Case-control	369 farmers with hip osteoarthritis and 369 control farmers individually matched for sex, age and residential area	Surgery for hip osteoarthritis or hip symptoms and joint space <3 mm, ascertained from medical files	Lifetime history of farming activities by interview	Handling straw, sacks, fodder and/or manure and tractor driving	Work >5 h/day in animal barns since age 30 versus no work in animal barns OR 13.3 (1.22–144.98) No associations with daily working time, working time per year or hours of tractor driving per year
Jacobsen et al., 2004	Denmark	Cross-sectional	2572 participants in Copenhagen City Heart study with no hip pain and at most only one of joint space narrowing or osteophytosis on hip radiograph	Minimal joint space determined from pelvic radiographs	Lifetime occupational history including levels of lifting, ascertained by questionnaire	None	No significant differences in mean minimal joint space by measures of occupational lifting
Thelin and Holmberg 2007	Sweden	Cohort	1220 male farmers, 1130 non-farming rural residents and 1087 urban controls matched for sex and age, followed for 14 years	First hospital care for hip osteoarthritis, ascertained from national register	Owned or rented a farm and farmed for >25 h/week	Age	Farmers versus urban controls HR: 3.0 (1.7–5.3) Non-farming rural men versus urban controls HR: 1.2 (0.8–1.6)
Riyazi et al., 2008	The Netherlands	Case-control	93 patients with familial osteoarthritis involving hip and at least one other joint site, and 345 controls from general population	Pain or stiffness in groin or hip on most days plus osteophytes or joint space narrowing on radiograph, or previous arthroplasty	Lifetime occupational activities inferred from job title ascertained by questionnaire.	Age, sex and body mass index	Ever versus never physically demanding work OR: 3.3 (1.3–8.2)
Järvholm et al.,2008	Sweden	Cohort	204,741 men aged 15–67 years employed in construction industry, followed for up to 12 years	Hospital admission for hip replacement with a diagnosis of osteoarthritis, ascertained from national register	Job title reported at entry to a health control programme	Age and body mass index	RRs >1 for 13/14 manual occupations versus white-collar workers. Highest RR 1.58 (0.93–2.68) for floor layers.
Juhakoski et al., 2009	Finland	Cohort	371 men and 469 women from general population, free from hip osteoarthritis at baseline, followed up after 22 years	Clinical diagnosis of hip osteoarthritis by physicians based on disease history, symptoms and clinical findings according to standardised criteria	Physical workload at baseline based on responses to a questionnaire	Sex, age, education, body mass index, smoking, alcohol, leisure time physical activity, injury	Heavy manual versus light sedentary work OR: 6.7 (2.3–19.5)
Allen et al.,2010	USA	Cross-sectional	2506 men and women (mean age 63.6 years) from general population	Symptomatic hip osteoarthritis defined as pain, aching or stiffness in hip on most days and Kellgren and Lawrence grade 2 + radiographic changes in same joint	Lifetime exposure to specified occupational activities elicited at interview	Age, race, gender, body mass index, prior hip injury, smoking and household tasks	Heavy work, standing versus none in longest held job OR: 1.75 (1.17–2.61) Ever versus never exposed to lifting 50 kg ≥ 10 × /week OR: 1.88 (1.20–2.92)
Franklin et al., 2010	Iceland	Case-control	1008 men and women treated for hip osteoarthritis at >60 years and 1082 controls (first degree relatives of cases of hip and knee osteoarthritis)	Total hip arthroplasty for osteoarthritis from patient records	Longest held occupation from questionnaire	Age, sex, body mass index, recreational physical activity	Farmers versus managers and professionals OR 3.6 (2.1–6.2) in men OR: 0.62 (0.36–1.0) in women
Ratzlaff et al., 2011	Canada	Cohort	2918 men and women aged 45–85 years, with no hip osteoarthritis at baseline, followed for 2 years	Self-reported diagnosis of hip osteoarthritis by a health professional	Cumulative lifetime occupational force on hip estimated from occupational history by online questionnaire	Sex, age, earlier injury, sporting activity, domestic activity	Highest versus lowest fifth of cumulative lifetime occupational force on hip HR: 1.80 (0.95–2.82)
Kaila-Kangas et al.,2011	Finland	Cross-sectional	3110 men and 3446 women from general population	Clinical diagnosis of hip osteoarthritis according to standardised criteria based on disease history, symptoms and clinical findings	Occupational activities in current and five longest held jobs by interview	Age, sex, body mass index, smoking and traumatic fractures	Manual handling of loads >20 kg for >24 versus 0 years OR: 2.3 (1.2–4.3) in men OR: 1.2 (0.7–2.1) in women
Andersen et al., 2012	Denmark	Cohort	217,055 farmers, 487,156 construction workers and 912,228 office workers, identified from a national database over 1981–2006, and followed for up to 11 years	First hip replacement surgery with a diagnosis of osteoarthritis from national patient register	Occupation from national database	Age, sex, calendar period, income and unemployment	Farmer for >10 years versus office workers HR: 3.00 (2.71–3.32) in men HR: 1.62 (1.19–2.20) in women Construction worker for >10 years versus office workers HR: 1.83 (1.68–2.00) in men HR: 1.57 (0.97–2.52) in women
Rubak et al.,2013	Denmark	Cohort	1,010,944 men and 899,549 women from general population with ≥10 years full-time employment, followed over up to 11 years	First total hip arthroplasty for primary osteoarthritis	Cumulative physical workload inferred from industries of employment in national register over up to 42 years	Age, sex, calendar year, socioeconomic status and county of residence	Highest cumulative physical workload versus never worked in industry with intermediate or high physical workload OR: 1.33 (1.17–1.53) in men OR: 1.01 (0.88–1.16) in women

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