Background/Purpose: Incidence of knee osteoarthritis (OA) is much higher in women than in men. Previous studies have shown that bone shape is a risk factor for knee OA. However, few studies have examined whether knee bone shape differs between men and women. The purpose of the present study was to determine whether there are differences between men and women in the shape of the bones that form the knee joint.

Methods:   We used information from the NIH-funded Osteoarthritis Initiative (OAI), a cohort of persons aged 45-79 at baseline who either had symptomatic knee OA or were at high risk of it. Among participants aged between 45 and 60 years, we randomly sampled 339 knees without radiographic OA (i.e., Kellgren/Lawrence grade of 0 in central readings on baseline radiograph). We characterized distal femur and proximal tibia shape of these selected radiographs using Active Shape Modeling (ASM). ASM generates independent modes that together explain the variance in the shape of a bone across a population.  We performed linear regression analysis to examine the association between sex and proximal tibia and distal femur shape, adjusting for age, race, body mass index (BMI) and clinic site. Beta coefficients and 95% confidence intervals were estimated to represent the difference in bone shape between women and men.

Results:   The mean age was 52.7 years (±4.3 SD) for both men and women. There were 192 female and 147 male knees for the distal femur analysis. Thirteen modes were derived for femoral shape, accounting for 95.5% of the total variance.  Distal femur Mode 1 had the greatest effect size for association with sex (p<0.0001); Modes 3, 5, 6, 8 and 12 were also significantly associated.   For tibial shape, 191 female knees and 149 male knees were used for the analysis.  Ten modes explained 95.5% of shape variance. Of the significantly associated modes in the femur, Mode 2 had the greatest effect size for the association with sex (p=0.009); Modes 3 and 4 were also significantly associated.  See tables for effect sizes and descriptions of all modes significantly associated with sex.

Conclusion:  The shapes of the distal femur and proximal tibia that form the knee joint differ by sex. Additional analyses are warranted to assess whether the difference in risk of OA between the sexes arises from bone shape differences.