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 able to equalise compressive stress between opposing joint surfaces. We test the hypothesis that sustained loading reduces the compressive strength of cartilage-on-bone. Methods Forty specimens of articular cartilage-on-bone, approximately 15 mm square, were removed from the patella groove of mature bovine knees. Specimens 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-deformation graphs. Half of the specimens were 'creep loaded' for 30 min at 2 MPa before their strength was assessed. After testing, damage was investigated using ink staining of the cartilage surface, and histology, Results Sustained loading reduced cartilage thickness by 45% and creep-loaded 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 an over-inflated car tyre.