We hypothesize that variability in knee subchondral bone surface geometry will differentiate between patients at risk and those not at risk for developing osteoarthritis (OA) and suggest that statistical shape modeling (SSM) methods form the basis for developing a diagnostic tool for predicting the onset of OA. Using a subset of clinical knee MRI data from the osteoarthritis initiative (OAI), the objectives of this study were to (1) utilize SSM to compactly and efficiently describe variability in knee subchondral bone surface geometry and (2) determine the efficacy of SSM and rigid body transformations to distinguish between patients who are not expected to develop osteoarthritis (i.e. Control group) and those with clinical risk factors for OA (i.e. Incidence group). Quantitative differences in femur and tibia surface geometry were demonstrated between groups, although differences in knee joint alignment measures were not statistically significant, suggesting that variability in individual bone geometry may play a greater role in determining joint space geometry and mechanics. SSM provides a means of explicitly describing complete articular surface geometry and allows the complex spatial variation in joint surface geometry and joint congruence between healthy subjects and those with clinical risk of developing or existing signs of OA to be statistically demonstrated.
Objective
We aimed to characterize severity and occurrence of knee osteoarthritis (OA), and effects of age, sex, body weight, and reproductive status on population-level normal variation in this condition in the baboon, a natural model of human knee OA.
Methods
We visually inspected articular cartilage of distal right femora of 464 baboons (309 females, 155 males) and assigned an OA severity score (comparable to a modified Outerbridge score) from 1 = unaffected to 4 = advanced OA (eburnation). Presence/absence of osteophytes was recorded. We tested for significant effects of age, sex, weight, and, in females, reproductive status (pre-, peri-, or post-menopausal) on OA. When appropriate, analyses were repeated on an age-matched subset (153 of each sex).
Results
Knee OA was more frequent and severe in older animals (p < 0.0001), but significant age variation was apparent in each severity grade. Sex differences within the younger and older age groups suggest that males develop knee OA earlier, but females progress more quickly to advanced disease. There is a strong relationship between reproductive status and OA severity grade in females (p = 0.0005) with more severe OA in peri- and post-menopausal female baboons, as in humans.
Conclusions
Idiopathic knee OA is common in adult baboons. Occurrence and severity are influenced strongly by reproductive status in females, and by sex with regard to patterns of disease progression – providing an animal model to investigate sex-specific variation in OA susceptibility in which the environmental heterogeneity inherent in human populations is vastly reduced.
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