ARTHRITIS & RHEUMATISM Vol. 54, No. 3, March 2006, pp 802–807 DOI 10.1002/art.21659 © 2006, American College of Rheumatology The Impact of Body Mass Index on Later Total Hip Arthroplasty for Primary Osteoarthritis A Cohort Study in 1.2 Million Persons Gunnar B. Flugsrud, 1 Lars Nordsletten, 1 Birgitte Espehaug, 2 Leif I. Havelin, 2 Anders Engeland, 3 and Haakon E. Meyer 4 Objective. To investigate the effects of body mass index (BMI), height, and age on the risk of later total hip arthroplasty for primary osteoarthritis (OA). Methods. We matched screening data on body height and weight from 1,152,006 persons ages 18–67 years who attended a compulsory screening for tuber- culosis in 1963–1975 with data from the Norwegian Arthroplasty Register for the years 1987–2003. We identified 28,425 total hip replacements because of primary OA. Results. We found dose-response associations be- tween both height and BMI and later hip arthroplasty. The relative risk (RR) among men with a BMI >32 kg/m 2 versus a BMI of 20.5–21.9 kg/m 2 was 3.4 (95% confidence interval [95% CI] 2.9–4.0). The correspond- ing RR in women was 2.3 (95% CI 2.1–2.4). There was a decreasing trend in the RR with an increasing age at screening. Among men, the RR for an increase of 5 kg/m 2 in the BMI was 2.1 (95% CI 1.7–2.5) when measured at age <25 years and 1.5 (95% CI 1.3–1.7) when measured at ages 55–59 years. Among women, the corresponding RR values were 1.7 (95% CI 1.5–1.9) and 1.1 (95% CI 1.1–1.2). Conclusion. There was a strong dose-response association between BMI and later total arthroplasty for OA of the hip. Being overweight entailed the highest RR among young participants, and the participants who were overweight at a young age maintained an excess RR for arthroplasty throughout the followup period. Osteoarthritis (OA) of the hip is a disabling disease and a major cause of pain and physical impair- ment (1). In some patients, the reason for the OA is known (e.g., hip fracture, dysplasia, or rheumatoid ar- thritis). In 70% of patients, no direct cause can be discerned, and the condition is called primary OA (2). The risk of primary OA of the hip is higher in women than in men (2), and it increases with increasing age (2). We have previously shown that a high body mass index (BMI) and strenuous physical activity at work increase the risk of later total hip arthroplasty for primary OA (3). In a subsequent investigation, we could not demonstrate any positive or negative effect of weight change between the ages of 34 and 47 years on the later need for arthroplasty (4). In an investigation of female nurses, their recalled weight at 18 years of age was more predictive of later total hip arthroplasty than was their BMI measured during middle age (5). The aim of the present study was to investigate the relationship between BMI, age, and total hip arthro- plasty for primary OA. Our hypothesis was that a high BMI is more detrimental to the hip joint at a young age than later in life. SUBJECTS AND METHODS Study participants. During 1963–1975, a nationwide screening for tuberculosis was performed in Norway (6). The Supported by an OrtoMedic Charnley Fellowship to Dr. Flugsrud and by grants from the Ullevål University Hospital and the Eastern Norway Regional Health Authority. 1 Gunnar B. Flugsrud, MD, Lars Nordsletten, MD, PhD: Ullevål University Hospital, Oslo, Norway; 2 Birgitte Espehaug, MSc, PhD, Leif I. Havelin, MD, PhD: Haukeland University Hospital, Bergen, Norway; 3 Anders Engeland, MSc, PhD: Norwegian Institute of Public Health, Oslo, Norway; 4 Haakon E. Meyer, MD, PhD: Norwegian Institute of Public Health, and University of Oslo, Oslo, Norway. Address correspondence and reprint requests to Gunnar B. Flugsrud, MD, Oslo Orthopaedic Centre, Ullevål University Hospital, 0407 Oslo, Norway. E-mail: Gunnar.Flugsrud@ioks.uio.no. Submitted for publication July 10, 2005; accepted in revised form December 1, 2005. 802