Cartilage tolerates single impact loads of as much as half the joint fracture threshold

We hypothesized that one mechanical insult could affect cellular proliferation, matrix turnover, and the structural integrity of cartilage, and that these effects would be dose dependent and time dependent. One impact load of low impact (14.4 MPa +/- 2.1 MPa), medium impact (22.8 MPa +/- 5.8 MPa), or high impact (55.5 MPa +/- 12.6 MPa) was administered to the posterior aspect of the medial femoral condyle of New Zealand White rabbits using a previously validated pendulum device. Animals were euthanized at 2, 6, and 12 weeks after impact, and the impacted and sham (contralateral limb) cartilage were harvested. Each specimen was assessed by light microscopy and by immunohistochemical methods. Although impacted specimens had greater loss of proteoglycan staining than sham cartilage, these changes were neither dose dependent nor time dependent. No structural damage, enzymatic proteoglycan or collagen breakdown, or cellular proliferation was identified in the different impact groups. Articular cartilage is a resilient tissue, particularly in situ, and can tolerate single impact loads of as much as 45% of the joint fracture threshold without considerable disruption or degradation.