Osteoarthritis (OA) of the hip is a disabling disease and a major cause of pain and physical impairment (1). In some patients, the reason for the OA is known (e.g., hip fracture, dysplasia, or rheumatoid arthritis). 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 arthroplasty 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.
- Top of page
- SUBJECTS AND METHODS
In a cohort of 1.2 million people who were screened during 1963–1975 at the ages of 18–67 years, a high BMI was associated with an increased risk of later total hip arthroplasty for primary OA. Being overweight when young had a higher impact than being overweight when older.
The high inclusion rate of subjects in the Norwegian Arthroplasty Register (2) as well as the very large numbers of participants and recorded arthroplasties lend strength to the findings of the present investigation. Our results are further strengthened by the measured weight and height, since self-reported data are less reliable among those with extreme values (11) as well as among the old (12).
Among the limitations of the present study, information on known risk factors for total hip arthroplasty for primary OA, such as physical activity, joint injury, and heredity, was lacking. However, in previous studies of OA of the hip (3) and knee (13), adjustment for activity or injury did not substantially alter the estimates of the effect of the BMI.
Followup in the present investigation was incomplete. It did not start at the time of screening, and it stopped while many subjects were still at risk of arthroplasty for hip OA. Participants who were ages 18–25 years at the time of screening were 46–65 years old at the end of the followup period. Since the age-specific incidence of total hip arthroplasty peaks at 70–79 years (2), we had no data on how being overweight as a young adult affects the risk of arthroplasty for OA at the age when the absolute risk for arthroplasty is highest. However, when the analysis was stratified according to the length of followup as well as when the time-dependent regression coefficients for BMI were plotted, we saw only a small diminishing of the RR through the followup period. It is therefore probable that the participants who were overweight as young adults will retain their excess risk beyond the period of our followup.
Both height (14) and BMI (6, 15, 16) are associated with age. Thus, the differences between the crude and adjusted RRs in the present study may have been affected by age as a confounder.
With regard to using total hip arthroplasty as surrogate end point for severely symptomatic OA, we need to emphasize that health care in Norway is publicly financed. Whether a person receives hip surgery is not dependent upon whether he or she has insurance, and the patient does not incur any direct costs. In a previous study (3), we discussed the annual incidence of total hip arthroplasty among Norwegians ages 35–85 years (2.2 per thousand). This value was equal to the objective need for arthroplasty as estimated in an English investigation (17), indicating that Norwegians' access to hip surgery has been reasonably adequate. However, the English investigation found that for every 100 persons cleared for surgery, there were another 17 persons with severe hip pain who were not candidates for surgery (due to medical contraindications, personal preferences, or other reasons). A Canadian investigation found underuse of hip surgery to be more prevalent among women than among men (18). In Norway, however, the rate of hip surgery is about twice as high among women as among men (2), which is consistent with the sex-specific prevalence of severe hip pain in the Canadian study (5% in women versus 2.3% in men). We conclude that although some patients with severely symptomatic OA of the hip do not undergo total hip arthroplasty, there is little reason to suspect that socioeconomic or sex differences have biased the analyses presented.
Obesity at a young age increased the RR for total hip arthroplasty more than did obesity at an older age, and obese persons maintained an increased RR throughout followup. This means that the persons who were obese when they were young carried with them an excess risk for severe OA into the age when total hip arthroplasty becomes prevalent. This is consistent with the results of the Nurses Health Study that investigated risk factors for total hip arthroplasty in 93,442 women (5). They found that recalled BMI at 18 years was a stronger risk factor than the BMI at a later age. The Johns Hopkins Precursors study found that in 1,180 men followed up for more than 30 years, BMI measured when in medical school was more strongly associated with symptomatic knee OA than was BMI reported later in life (13). The investigators could not demonstrate any association with hip OA, but with only 27 cases, the power to detect an association was limited.
Our previous finding, that the risk for arthroplasty was unaffected by weight change during the fourth and fifth decades of life (4), indicates that the effect of weight may be most prominent during the first decades of life and that the age at onset of obesity may be more important than the duration of obesity. If the relative impact of the BMI is indeed higher at a young age when controlling for all confounding factors, an explanation may be that hip joint cartilage is more vulnerable during the early stages of life.
We found a dose-response association between body height and arthroplasty, which was more pronounced among women than among men. An English investigation of persons without hip disease found that an increase in body height of 10% was accompanied by an increase in minimum hip joint space (an indicator of cartilage thickness) of only 8% among men and 7% among women (19). This suggests that taller persons may not have correspondingly thicker hip joint cartilage and may therefore be more predisposed to the development of OA.
In summary, we used objective measures of body height and weight and found a strong dose-response association between the BMI and later arthroplasty for primary OA of the hip in both women and men. The increase in RR entailed by being overweight and obese was greatest among participants who were screened at a young age. Tall persons also had an increased RR for total hip arthroplasty, but this association was unaffected by age at screening. Our findings highlight the desirability of prevention and early treatment of obesity. We have previously shown that reducing the BMI-related risk for total hip arthroplasty in a cohort of middle-aged Norwegians to that of the quartile with lowest BMI would, theoretically, reduce the need for hip surgery by 25% (95% CI 8–37%) among men and by 36% (95% CI 23–46%) among women (3). It is important to verify whether this possibly large health gain depends on intervention at a young age. The effect of weight intervention with regard to the development of hip OA remains to be shown. Experimental studies may clarify whether cartilage has a changing susceptibility to mechanical load throughout life.