Sustained loading increases the compressive strength of articular cartilage

Introduction When synovial joints are subjected to sustained or repetitive loading, fluid is driven from the articular cartilage so that it is less ab le to equalise compressive stress between opposing joint surfaces. We test the hypothesis that sustained loading reduces the compressive strength of c artilage-on-bone. Methods Forty specimens of articular cartilage-on-bone, approximately 15 mm square, were removed from the patella groove of mature bovine knees. Speci mens were loaded on a materials testing machine using a Smm-diameter plane- ended indentor. Controlled loading/unloading cycles of Is duration, and of increasing severity, were applied until failure was evident on the force-de formation graphs. Half of the specimens were 'creep loaded' for 30 min at 2 MPa before their strength was assessed. After testing, damage was investig ated using ink staining of the cartilage surface, and histology, Results Sustained loading reduced cartilage thickness by 45% and creep-load ed specimens mere 21% stronger (P = 0.01), Most specimens appeared to fail by fissuring of the cartilage surface zone. Conclusion Sustained loading strengthens cartilage by expelling water from it, reducing the tendency of the surface zone to rupture in the manner of a n over-inflated car tyre.