There is some evidence that tibiofemoral osteoarthritis (TFJ OA) and patellofemoral osteoarthritis (PFJ OA) may have different risk factors. To investigate the possibility that these conditions are separate disease entities, we compared biomarker profiles of patients with each disease.
Serum samples were taken from 222 patients who had knee pain and X-ray signs of knee OA. Eighty-two had only medial TFJ OA and 38 only PFJ OA in one or both knees. The remaining patients had either mixed disease or equivocal radiographic evidence of OA. The following biomarkers were measured in serum samples from baseline and follow-up visits: cartilage oligomeric matrix protein (COMP), glycosaminoglycan, keratan sulphate epitope 5D4, YKL-40, osteocalcin, C-telopeptide of type I collagen, hyaluronan and C-reactive protein.
The two subsets of OA (TFJ and PFJ) had similar radiographic disease severity and there were no significant differences in the presence and patterns of pain scores (visual analogue scale and Western Ontario and McMaster Universities Osteoarthritis Index). No difference was found for the biomarkers between the two groups, with one exception. Both baseline and area under the curve per month COMP concentrations were significantly higher in the TFJ than the PFJ group (P<0.01).
The reduced serum COMP in PFJ disease compared with TFJ OA could be due to small articular cartilage volume in the latter or to a qualitative difference in cartilage metabolism.