LOCOMOTOR DISEASES Age, bodyweight, smoking habits and the risk of severe osteoarthritis in the hip and knee in men Bengt Ja¨rvholm 1 , Stefan Lewold 2 , Henrik Malchau 3 & Eva Vinga˚rd 4 1 Department of Public Health and Clinical Medicine, Umea ˚ University, Umea ˚; 2 Department of Orthopedics, Lund University, Lund ; 3 Department of Orthopedics, Sahlgrenska University Hospital, Gothenburg; 4 Section of Personal Injury Prevention, Karolinska Institute, Stockholm Accepted in revised form 22 March 2005 Abstract. Background: The objective of this study was to estimate the risk of severe osteoarthritis, with the need for arthroplasty, in the knee and/or hip according to body mass index (BMI) both within a normal range and in persons with high BMI. Fur- thermore, we wanted to study the significance of smoking. Methods: This study identifies male con- struction workers participating in a national health control program (n = 320,192). The incidence rate for joint replacement was found by matching with the Swedish hospital discharge register between 1987 and 1998. BMI and smoking habit was registered at the time of the health examination. Results: In total 1495 cases of osteoarthritis of the hip and 502 cases of osteoarthritis of the knee were identified and included in this analysis. The incidence rate was found to in- crease linearly to the BMI even within low and ,normalÕ BMI. The relative risk for osteoarthritis of the hip was more than two times higher in persons with a BMI of 20–24 than in men with a BMI 17–19. There was almost a doubling of the risk of severe knee osteoarthritis with an increase in BMI of 5 kg/m 2 . Smoker had a lower risk of osteoarthritis than non-smokers and ex-smokers. Conclusions: BMI is an important predictor of osteoarthritis even within normal BMI. A decreased risk of osteoarthritis of the hip was found in smokers, but the effect was weak compared to that of BMI or age. Contrary to studies of radiographic osteoarthritis our study indicates higher risk of hip than of knee osteoarthritis. Key words: Body mass index (BMI), Construction workers, Epidemiology, Joint replacement, Smoking Introduction Osteoarthritis of the hip and knee are common chronic diseases in the elderly leading to pain and disability. Osteoarthritis may be secondary to a trauma to the joint but, for most cases there is no trauma or other disease linked to the osteoarthritis and such cases are often called primary osteoar- thritis. The mechanisms behind primary osteoar- thritis are not fully understood. Several studies have found that enhanced age and BMI relates to an increased risk for osteoarthritis of the hip or the knee [1–6]. However, some studies indicate a less clear association between hip osteoarthritis and BMI than between osteoarthritis in the knee and BMI [5, 7, 8]. Some studies have indicated that smoking leads to lesser risk of osteoarthritis in the knee and hip but the mechanisms for this seemingly protective effect are still poorly understood [9–11]. Other risk factors for hip and knee osteoarthritis are intraarticular injury, congenital anomalies and heavy physical activity in sports or occupation [2, 6, 12]. A few studies also indicate that inherited factors are of importance [13]. The objective of this study was to estimate the risk of severe osteoarthritis in the knee and hip related to age and BMI in the same population. Furthermore, we wanted to study the claimed protective effect of smoking on development of osteoarthritis. This analysis is based on a prospectively followed cohort of Swedish male construction workers and for the diagnosis of severe osteoarthritis joint replacement was used. Subjects and methods Swedish workers in the construction industry were offered health-controls on a regular basis from late 1960th until 1993 in a national wide occupational health center (Byggha¨lsan). The results from the health controls were computerized between 1971 and 1992. The health center was organized through an agreement between the employers associations and the unions and included all workers who were employed by firms conforming to this collective agreement. This meant that almost all Swedish con- struction workers during this period were affiliated European Journal of Epidemiology (2005) 20: 537–542 Ó Springer 2005 DOI 10.1007/s10654-005-4263-x