Bone mechanics in transplanted osteochondral grafts

The studies in this thesis investigated the effects of cryopreservation on mechanical properties and incorporation of transplanted osteochondral allografts in ovine medial femoral condyles. In normal ovine knee joints, bone and cartilage mechanical properties varied significantly among different topographical regions. These topographical variations may be detrimental to osteochondral autograft joint repairs that take graft tissue from mismatched regions of the joint. Topographically matched allografts may be the best alternative for joint repair.

At 12 mo after transplantation of osteochondral allografts in the ovine medial femoral condyle, mechanical properties of host cancellous bone around frozen allografts was significantly stronger than the graft. In contrast, there were no differences between graft and host bone of cryopreserved allografts. Cryoprotection significantly improved bone cell recovery from frozen storage compared to bone frozen with no cryoprotection. In a second study of transplanted osteochondral allografts, unprotected freezing impaired bone remodeling in allografts for up to one month compared to fresh autografts. In contrast, cryopreserved allografts exhibited remodeling activity that was similar to fresh autografts. Despite these differences in graft remodeling, the cancellous bone architecture showed no significant differences between graft treatment groups. Rather, in all groups (including autografts), there were condyles with large graft-host differences in bone volume fraction. A finite element simulation was used to assess the effects of steep gradients in bone stiffness between graft and host regions. The graft-host disparity did not appreciably affect cartilage stress, but there were dramatic changes in the subchondral plate stress patterns. It was proposed that localized bending of the subchondral plate above the graft would result in fatigue microdamage. Subsequent reparative remodeling and changes to the zone of calcified cartilage may lead ultimately to degenerative changes in the joint.

In addition to improved chondrocyte recovery from frozen storage, cryopreservation protects bone cells from freezing damage and improves graft incorporation compared to unprotected freezing. Though techniques must still be improved, cryopreserved osteochondral allografts may ultimately be the best alternative for the surgical repair of damaged or diseased joints.