Mean age at start of follow-up was 55 years. During follow up, 799 participants received their first total hip replacement. The indication for surgery was primary osteoarthritis for 572 (235 men and 337 women). The 227 participants who had a total hip replacement for other indications were censored. The most common reasons other than primary osteoarthritis for total hip replacement were sequelae to a hip fracture (n = 95) and rheumatoid arthritis (n = 26). Participants were also censored during follow up as a result of emigration (n = 32) or death (n = 3107). The majority of participants were censored at the end of follow up (n = 34,930).
The rate of weight change ranged from −4.4 kg/year to 8.2 kg/year. The 25th, 50th, and 75th percentiles were −0.07 kg/year, 0.18 kg/year, and 0.45 kg/year, respectively, for men, and −0.11 kg/year, 0.14 kg/year, and 0.41 kg/year, respectively, for women. In both men and women, those with the highest weight gain had a higher mean BMI and mean body weight, and showed somewhat lower levels of physical activity (Tables 4 and 5). The proportion of exsmokers was highest among participants who gained most in weight. The incidence of total hip replacement showed no association with weight slope (Table 6). The age-adjusted and multivariate-adjusted relative risks for total hip replacement were also unaffected by weight slope.
We divided the cohort into quartiles according to BMI as recorded at the first screening, and performed the Cox analysis for each subgroup separately. There was no association between weight change and risk for later total hip replacement in any of the BMI categories. Including information on cardiovascular disease, diabetes mellitus, sick leave, or sick leave pension in the regression model did not alter the results (data not shown).
We tested for effect modification of weight change by each of the other variables used in the model. Only for weight change and age at screening in women did the interaction term reach statistical significance (P = 0.025). Stratifying the analysis on both sex and age at the second cardiovascular screening, none of the subgroups showed any association between high weight gain and risk for later total hip replacement. On the contrary, among women who were 50 years old or more at the second cardiovascular screening, those who were in the third and fourth quartiles of weight change showed a somewhat reduced risk for later total hip replacement compared with the lowest weight change quartile (relative risk [RR] = 0.60, 95% confidence interval (CI) = 0.38-0.95 for the third quartile, and RR = 0.51, CI = 0.32-0.82 for the fourth quartile).
In another analysis, we used weight change as a continuous variable in the multivariate analysis, and found a relative risk for men with a weight increase of 10 kg per 10 years of 0.93 (CI 0.69-1.25) compared with a man with no weight change. For women the corresponding relative risk was 0.85 per kg/year (CI 0.70-1.04).
In additional analyses, we compared the 5% of participants with the highest rate of weight gain (male >0.96 kg/year, female >0.94 kg/year) with the rest of the cohort and then the 5% of participants with highest rate of weight loss (male <-0.45 kg/year, female <-0.59 kg/year) with the other 95%. We found no differences in the risk for later total hip replacement (data not shown).
In a further analysis, we used the continuous variable absolute weight change rather than weight slope as an indicator of weight change. A man with 10-kg weight reduction from the first to the last screening had a relative risk for total hip replacement of 1.11 (CI 0.91-1.52) compared with a man with no weight change. For women the corresponding relative risk was 1.11 (CI 1.00-1.35). Using relative weight change as the indicator of weight change, there was no effect on the risk for total hip replacement (data not shown).
To detect any change, with increasing age, on the association between BMI and risk for later total hip replacement, we first found the relative risk for total hip replacement related to BMI at the tuberculosis screening (2.84 per 10 kg/m2 CI 1.79-4.41 for men, and 2.37 per 10 kg/m2 CI 1.97-3.11 for women). We then computed the relative risk for total hip replacement related to BMI as recorded at the next 2 screenings. There were no differences among these relative risks (Table 7).
Adult weight change between the ages of 34 and 47 years did not affect subsequent risk for total hip replacement. This was true irrespective of sex, initial BMI, and the magnitude of weight change. The investigation was strengthened by the population-based design and high response rates, and by the fact that we had screening information on most known confounders. However, information on hormone replacement therapy, previous hip injury, and bone mineral density was not available.
Comparisons between the National Arthroplasty Register and other official Norwegian registries indicate a high level of reporting. After excluding 1 hospital with poor reporting (not located within the counties of the present investigation), the completeness nationwide has been reported as 97%.22 During 1991-1995, 69% of the total hip replacements recorded in the Norwegian Arthroplasty Register were the result of primary osteoarthritis.6 This is in good agreement with the Swedish National Hip Arthroplasty Register, which during 1992-1996 reported that 69% of 48,007 total hip replacements were done for this indication.26 We have previously compared the Norwegian incidence of total hip replacement with an estimate of the actual demand for the same surgery in England and found similar values.7
Participants were 46-67 years old at the start of follow up and 56-77 at the end. Of all total hip replacements resulting from primary osteoarthritis recorded in the Norwegian Arthroplasty Register from the 3 counties, 3.8% were performed on patients younger than 55 years and 40.6% on patients aged 55-70 years (B. Espehaug, written personal communication, 1999). From 1991-1995, 13% of total hip replacements in the Norwegian Arthroplasty Register were the result of sequelae after fracture of the femoral neck, 8% the result of sequelae of hip dysplasia, and 4% the result of rheumatoid arthritis. This is very similar to the frequencies of indications recorded in the present investigation, suggesting that the total hip replacements that occurred during our follow up are representative of all total hip replacements that our study population would sustain during their lifetime.
Body weight and BMI are risk factors for osteoarthritis in the knee and hip. The association is considered to be stronger for knee osteoarthritis.27,28 However, the association we previously reported between BMI and hip osteoarthritis21 is very similar to that described between BMI and knee osteoarthritis. In the Framingham osteoarthritis study weight loss over the previous 10 years was associated with reduced risk for symptomatic knee osteoarthritis in women approximately 60 years of age with baseline BMI 25 kg/m2 or more, but weight gain did not increase the risk.20 This effect might have been the result of reduced joint pathology, but might also be explained by less weight provocation of already pathologic joints. In another Framingham study (64% women), weight gain was associated with increased risk for radiographic knee osteoarthritis.21 In that report the screening period covered 4 years, ending 4 years before index radiographs taken at a mean age of 71 years.
At the last screening 95% of the participants were aged 54 years or less, which is younger than the usual age of onset for primary osteoarthritis of the hip.28 There was no difference between operated and nonoperated participants with regard to change in physical activity from the first to the second cardiovascular screening. This supports the assumption that symptoms of osteoarthritis were negligible at all 3 screenings and that hip pain did not influence weight change from the first to the last screening.
For the majority of the study group body weight and BMI remained fairly constant throughout the screening period (and possibly even up to the age of hip replacement). However, even in the subgroups with extreme weight gain or weight loss we could not demonstrate any change in risk for later total hip replacement resulting from osteoarthritis. In the study population 3524 participants (with 51 total hip replacements) had a decrease in BMI from first to third screening of at least 2 kg/m2, and 1126 participants (with 24 total hip replacements) had a decrease in BMI of at least 3 kg/m2. There were 10,107 participants (145 total hip replacements) with a BMI increase of at least 2 kg/m2, and 5244 participants (78 total hip replacements) with a BMI increase of at least 3 kg/m2. The power to detect a 67% risk reduction (or a 3-fold increase) among the most extreme of these groups was greater than 95%.
If one could determine at what age a high BMI is most detrimental to the hip joint, this information would help to focus a preventive strategy. Case-control studies of male and female recipients of total hip replacements indicated that recalled BMI at 40 years was more predictive of later hip osteoarthritis than recalled BMI at 20, 30, and 50 years of age.29,30 These reports indicated that BMI impact peaked at age 40 years, but the differences were very small. We found that the association between BMI and later total hip replacement resulting from primary osteoarthritis remained unchanged during the fourth and fifth decades of life.
In this study we found no association between adult weight change and the risk for later total hip replacement resulting from primary osteoarthritis. Future research should focus on weight reduction during the first 3 decades of life or after the age of 55 years to define a preventive strategy against disabling osteoarthritis of the hip.
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Keywords:© 2003 Lippincott Williams & Wilkins, Inc.
osteoarthritis; hip; body mass index; weight gain; weight loss; obesity; Norway