Increased friction coefficient and superficial zone protein expression in patients with advanced osteoarthritis

Abstract

Objective: To quantify the concentration of superficial zone protein (SZP) in the articular cartilage and synovial fluid of patients with advanced osteoarthritis (OA) and to further correlate the SZP content with the friction coefficient, OA severity, and levels of proinflammatory cytokines.

Methods: Samples of articular cartilage and synovial fluid were obtained from patients undergoing elective total knee replacement surgery. Additional normal samples were obtained from donated body program and tissue bank sources. Regional SZP expression in cartilage obtained from the femoral condyles was quantified by enzyme-linked immunosorbent assay (ELISA) and visualized by immunohistochemistry. Friction coefficient measurements of cartilage plugs slid in the boundary lubrication system were obtained. OA severity was graded using histochemical analyses. The concentrations of SZP and proinflammatory cytokines in synovial fluid were determined by ELISA.

Results: A pattern of SZP localization in knee cartilage was identified, with load-bearing regions exhibiting high SZP expression. SZP expression patterns were correlated with friction coefficient and OA severity; however, SZP expression was observed in all samples at the articular surface, regardless of OA severity. SZP expression and aspirate volume of synovial fluid were higher in OA patients than in normal controls. Expression of cytokines was elevated in the synovial fluid of some patients.

Conclusion: Our findings indicate a mechanochemical coupling in which physical forces regulate OA severity and joint lubrication. The findings of this study also suggest that SZP may be ineffective in reducing joint friction in the boundary lubrication mode at an advanced stage of OA, where other mechanisms may dominate the observed tribological behavior.

Figure 1

Immunolocalization of SZP and friction coefficient showed a dependence on femoral condyle geometry and OA severity. A, Samples were harvested from standardized anterior and posterior locations of medial (M) and lateral (L) condyles. B, The surface expression of SZP (quantified relative to bovine standards) and friction coefficient of samples from OA subjects (n = 21) varied with OA score significantly. Anterior locations MA exhibited much higher OA scores and friction coefficients compared to posterior locations MP and LP and anterior location LA [P < 0.011 ([SZP]); P < 0.015 (friction coefficient)]. C, The strong correlation of SZP concentration and friction coefficient with OA score observed with samples from OA subjects was not encountered with samples from normal subjects (n = 2).

Figure 2

SZP immunolocalization was observed in all samples at the articular surface regardless of OA severity. Histochemistry (H&E staining) revealed cell and proteoglycan patterns through the cartilage thickness that depended on OA severity. A, Samples from representative OA and normal subjects revealed the presence of SZP on exposed articular surfaces. SZP in OA samples was bound to the lamina splendens in locations LA and LP, and was observed in several cell layers into the tissue at MA and MP locations. SZP in normal samples was consistently observed in several cell layers into the tissue at MA location compared to other locations. B, From the OA study group, SZP was observed on the articular surface of samples representing the most severe OA (MA location) and minimal OA (LP location). C, In human allograft cartilage, SZP localized deeper into the tissue in the anterior location compared to the posterior location.

Figure 3

The SZP concentration and aspirate volume of synovial fluid were elevated in OA samples (n = 21). Some OA samples showed higher cytokine expressions in synovial fluid. A, The total SZP content in OA samples (quantified relative to bovine standards) was elevated compared to normal samples (see text). The concentration of SZP may be indicative of the harvest procedure and could have been underestimated in OA samples with blood (OA-b) compared to those in which blood was not detected (OA-nb). B, Western blot of synovial fluids with the monoclonal antibody S6.79 recognized the large (~345 kDa) PRG4 gene product in both OA and normal patients. The same amount of SZP was loaded in each lane (based on SZP ELISA results), with OA patients having higher SZP content (and requiring less relative loads per lane) compared to normal patients. C, Some OA samples demonstrated elevated expressions of TNF-α and IL-1β cytokines in synovial fluid.