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 5 mm-diameter plane-ended indentor. Controlled loading/unloading cycles of 1s 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 at 2 MPa before their strength was assessed. After testing, damage was investigate using ink staining of the cartilage surface, and histology.

RESULTS

Sustained loading reduced cartilage thickness by 45% and creep-loaded specimens were 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.