The association of proximal femoral shape and incident radiographic hip OA in elderly women

Abstract

Purpose: Variations in femoral head shape are reported to predict incident hip osteoarthritis (OA). This study evaluated if proximal femur shape at baseline was a risk factor for incident radiographic hip OA (RHOA) after 8.3 years of follow-up in a cohort of elderly Caucasian women.

Methods: Supine pelvic radiographs were obtained as part of the Study of Osteoporotic Fractures (SOF) at baseline and Visit 5 (8.3 years later), and were scored for RHOA. A nested case-control study was performed: hips were eligible for inclusion if they had no prevalent RHOA in either hip at baseline. Cases of incident RHOA were defined as no RHOA at baseline and RHOA in their right hip present at Visit 5 [or right total hip replacement (THR) for OA between baseline for follow-up] and a random selection of one half of all incident RHOA cases plus right THR cases (n=102) were chosen. A random selection (n=249) of control subjects who had no RHOA in their right hip at both baseline and follow-up visit were included for comparison. The shape of the right proximal femur was outlined on a digitized baseline radiograph and a statistical image analysis technique, Active Shape Modeling (ASM), was used to generate 10 unique and independent "modes" or variations in shape, which explained 95% of the variance in the shape of the proximal femurs studied. Any hip shape was therefore described as the average shape plus a linear combination of these 10 independent modes of variation. The values for each of these 10 modes for each hip analyzed were entered into a logistic regression model as independent predictors of incident RHOA adjusting for covariates.

Results: The incident RHOA cases were slightly taller, heavier and had higher total hip bone mineral density (BMD) than control subjects (P<0.05), but were otherwise similar demographically. Results of ASM showed that Modes 1, 2 and 3 together explained 81% of the variance in proximal femur shape among all subjects analyzed. Modes 3, 5, 9 which accounted for 8.9%, 3.3% and 0.8% of the variance respectively, were significant predictors of incident RHOA with adjusted odds-ratios ranging from 1.61 to 1.99 (P<0.001) for every 1 standard deviation (SD) increase in the mode score.

Conclusion: These results suggest that variations in the relative sizes of the femoral head and neck at baseline are modest determinants of incident RHOA in elderly Caucasian women.

Fig. 1

Outline of proximal femur. 60-point outline of proximal femur from the lesser trochanter to the opposite shaft. These point coordinates were inputted into the ASM program to generate the 10 modes of shape variation.

Fig. 2

Figures showing the main effects of proximal femur shape variation in the Modes 3, 5 and 9. Each figure shows the average (—), +2 SD (- - - -) and −2 SD (..) shapes in each of these modes. In each mode the +2 SD outline is associated with a higher risk of incident RHOA. (A) Mode 3: +2 SD shape associated with variations in the relative size of femoral head and neck length. (B) Mode 5: +2 SD shape associated with variations in relative size and width of femoral neck and greater trochanter. (C) +2 SD shape associated with variations in relative size of both femoral head and greater trochanter (see text for details).