Socioeconomic status, occupation, and risk of hospitalisation due to coxarthrosis in Denmark 1981-99.(Extended Report)

Objectives: To predict the relative risk and time trend in hospitalisation due to coxarthrosis (CA) among groups of different socioeconomic status and occupations in order to test existing aetiological hypotheses.

Methods: Four consecutive cohorts of all gainfully employed Danish men were followed up for CA. Standardised hospitalisation ratios (SHR) and time trends were calculated.

Results: Self employed men had an SHR due to CA of 140 (95% confidence interval (CI) 130 to 151) in 1994-99. It changed--1.2% (95% CI -2 to -0.4) annually from 1981 to 1999. Self employed farmers had an SHR of 286 (95% CI 262 to 313) during 1994-99, increasing 0.14% a year (95% CI -0.9 to 1.1) from 1981 to 1999. Unskilled men had an SHR of 121 (95% CI 113 to 130) in 1994-99. The SHR increased 1.6% annually (95% CI 0.7 to 2.4) from 1981 to 1999. Employed agricultural workers had an SHR of 189 (95% CI 158 to 227) from 1994 to 1999. The SHR increased 3.7% annually (95% CI 1.2 to 6.2) from 1981 to 1999.

Conclusions: The relative risk of hospitalisation due to CA was consistently high among farmers in four successive analyses spanning an 18 year period. The relative risks remained stable over time despite the fact that the number of farmers decreased. The risk increased among employed workers in agriculture and horticulture over that period, and an increased risk was also found among tractor drivers and a few other occupations.

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According to a Danish-Swedish study 40% of ideopathic cases of coxarthrosis (CA) can be attributed to physical workload, 55% to sports, and 15% to being overweight. (1) Altogether the three factors account for 80% of CA. The calculations are based on a case-control study of 239 male recipients of a hip prosthesis and 302 men randomly selected from the general population. March and Bachmeier estimated the costs connected with musculoskeletal disorders, (2) of which CA is the most common, to be 1-2.5% of the gross national product in Western industrialised countries. Clearly, identification of risk factors, monitoring of high risk groups, and prevention are very important public health issues. (3)

Two reviews from 1997 conclude that there is an association between farming and CA. (3 4) Lievense el al systematically reviewed two retrospective cohort and 14 case-control studies published between 1993 and April 2000 relating CA to the workload. (5) Cases of CA were defined as patients with radiological and/or clinical CA, or a (total) hip replacement, or who were on the waiting list for one. The authors evaluated a possible publication bias by means of a funnel diagram of the odds ratios (OR) by sample size but found none. The studies included found ORs between 1.1 and 13.8. The OR varied between 2 and 14 in those studies in which the physical workload was farming. The six studies on heavy lifting reported ORs between 1.5 and 3.5 compared with no or low exposure to lifting heavy weights. The authors found moderate evidence for a threefold risk of heavy workload in patients with CA. They also found moderate evidence for a positive relationship between a history of farming and the occurrence of CA. Further evidence to support the part that jobs and occupational physical activities play in the occurrence of CA was recently reported by Schouten et al. (6) Lau et al found increasing OR with body height, and a high OR in men with a history of joint injury (OR = 25; 95% confidence interval (CI) 3.5 to 181), and occupational exposure to walking, kneeling, climbing stairs frequently (OR = 12.5; 95% CI 1.5 to 104), and lifting [greater than or equal to] 110 kg more than 10 times a week (OR = 5.3; 95% CI 1.8 to 15.8). (7)

A relatively old case-control study published in Swedish and not included in the reviews should be mentioned. (8) Cases were 98 people with total hip replacement due to (CA). Ninety eight hospital controls with inguinal hernia and 202 population controls were included. Compared with the hospital controls the risk ratio in farmers was found to be 4.9 (95% CI 3.0 to 8.1). The author suggested that CA in farmers may be due to tractor driving in a twisted position. We will look at self employed in tractor pools to test that hypothesis. Also cow milking was associated with an increased risk (risk ratio = 1.8; 95% CI 1.1 to 3.4).

Interesting new hypotheses about specific work tasks of farmers are tested in a case-control study by Thelin et al, based on a cohort of farmers affiliated with the Swedish Farmers' Safety and Preventive Health Association. (9) Despite a limited exposure contrast because all the controls were farmers too, they found strong and significant evidence for an association between CA and several occupational burdens. Farmers with larger dairy and swine confinement operations had an increased risk of acquiring OA of the hip. In a multivariate logistic regression model those who milked more than 40 cows daily had an increased risk (OR = 4.5; 95% CI 1.9 to 11.0) compared with those who did not work in dairy production. Those who had worked more than five hours daily in animal barns over a long period of time also had an increased risk (OR = 13.3; 95% CI 1.2 to 145.0) compared with those who did not work with animals. In another model those who had large farm areas (> 100 ha) had a significantly decreased risk (OR = 0.14; 95% CI 0.05 to 0.43) compared with those who had smaller farm areas.

Further evidence for the role of CA was found in a recent study of hip pain. (10) Hip pain is both an antecedent and a symptom of CA, and the study showed that hip pain was predicted by obesity, physically demanding work, and self reported whole body vibrations five years earlier.

Although farming had already been reported to be associated with CA in 1966, (11) we found no studies of time trends for different socioeconomic groups or specific occupational groups such as farmers. A Swedish study suggests that the prevalence of CA as seen on radiographs in subjects who have had a normal …