Use of joint fluid analysis for determining cartilage damage in osteonecrosis of the knee.

Objective: To assess the value of joint fluid analysis for determining cartilage degradation and prognosis in spontaneous osteonecrosis (ON) of the knee.

Methods: Synovial fluid was obtained from 30 knees with spontaneous ON (26 medial femoral condyles, 4 medial tibial plateaus) as well as from 50 knees with medial compartmental osteoarthritis (OA) as a control. Levels of chondroitin 6-sulfate (C6S), C4S, and hyaluronic acid were measured with high-performance liquid chromatography. The lesion size, appearance of the articular cartilage, and results of magnetic resonance imaging (MRI) were compared with the results of joint fluid analysis.

Results: The mean +/- SD level of C6S was 82.2 +/- 36.6 nmoles/ml in joint fluid from ON knees, which was significantly higher than the levels in knees with grade 2 (47.2 +/- 20.0 nmoles/ml) and grade 3 (55.8 +/- 29.2 nmoles/ml) OA. The C6S:C4S ratio was highest in lesions with mild articular changes and reflected the macroscopic alteration of cartilage overlying the ON lesion. The concentration of C6S in the 9 knees with lesions that covered > or = 40% of the condyle (99.0 +/- 32.9 nmoles/ml) was higher than that in the 17 knees with lesions that covered <40% of the condyle (67.2 +/- 31.7 nmoles/ml). Knees with bone marrow edema on MRI had a higher level of C6S than did knees with a fibrous-like appearance.

Conclusion: While radiologic staging was useful for indicating the size of the ON lesion, it was less valuable for determining articular cartilage damage. Joint fluid analysis may provide more precise information about articular cartilage degradation in ON, and the findings may also be of prognostic significance.