Mechanical disruption of human patellar cartilage by repetitive loading in vitro

Plugs of cartilage and subchondral bone from patellae were subjected to cyclic compression consisting of rapid ramp loading for 0.3 seconds followed by a 2.7 second pause. At 1000 psi cyclic load, surface abrasion of the cartilage was noted at 250 cycles of compression. Primary fissures, which penetrated to the calcified cartilage, developed at 500 cycles. Secondary fissures, emanating from the primary fissures at 30°-90° angles, and penetrating to varying depths, were observed at 1000 cycles. Coalescence of fissures and undermining of cartilage fragments were noted at 8000 cycles. With greater loads, the same sequence of events occurred with fewer cycles except that primary fissures appeared before the surface was abraided. Fissure formation did not occur in specimens subjected to loads of 250 psi to 500 psi even if the superficial 100 μm of cartilage was removed and specimens were loaded for 120,860 cycles. The deeper layers of cartilage appear to be of prime importance in resisting fissure formation.