The determinants of change in patella cartilage volume in osteoarthritic knees.

OBJECTIVE: The rate of change in patella articular cartilage and factors influencing it, in subjects with osteoarthritis (OA), is unknown. We performed a cohort study to determine this. METHODS: One hundred ten subjects with OA had baseline skyline and lateral radiographs and magnetic resonance imaging (MRI) on their knee. They were followed 2 years later with a repeat MRI of the same knee. Patella and tibial cartilage volume was measured at baseline and followup. Risk factors assessed at baseline were tested for their association with change in patella cartilage volume over time. RESULTS: The annual percentage loss of patella cartilage was 4.5 +/- 4.3%. Sex, body mass index (BMI), and pain score at baseline were associated with an increase in cartilage loss. The rate of patella cartilage loss was greater in women than men, 5.3% versus 3.5% (p < 0.03), independent of age, BMI, and pain score. No association was seen between change in patellar cartilage volume and change in either medial or lateral tibial cartilage volume (r = 0.02, p = 0.86 and r = 0.08, p = 0.43, respectively). CONCLUSION: In OA, patella cartilage volume is lost at 4.5 +/- 4.3% per year. The main factors affecting this are sex, BMI, and baseline pain score. The poor correlation between patella cartilage loss and cartilage loss in the tibial compartment suggests that the pathogenetic mechanisms for OA in the patellofemoral and tibiofemoral joint may differ. Further work will be required to determine whether the rate of patella cartilage loss in OA is steady or phasic, and to determine which factors can be modified to reduce cartilage loss.