Freezing influences the healing of rabbit medial collateral ligament autografts

Connective tissue allografts are treated commonly with deep freezing to facilitate their storage in tissue banks and to reduce their immunogenicity. The influence of freezing on the healing of these grafts is unknown. The effect of an aggressive freezing protocol on ligament graft healing in a nonimmunogenic autograft model was investigated. The femoral-medial collateral ligament-tibial complex of the rabbit knee was transplanted orthotopically into the site from which it had just been harvested. The fresh group had their grafts rinsed in normal saline, and the frozen-thawed group had their grafts repetitively frozen-thawed in liquid nitrogen immediately before reimplantation. The healing of the ligament complexes were evaluated mechanically at intervals as long as 1 year after transplantation. Repeated freezing had little effect on the initial mechanical behavior of the graft complexes but was found to be detrimental to subsequent graft incorporation. The frozen-thawed grafts were weaker than the fresh grafts overall, and this effect persisted over time. The failure load and stress of the frozen-thawed grafts reached only 87% and 75% of that of the fresh grafts after 48 weeks. Because the initial mechanical effects of freezing were minimal, subsequent effects on graft healing must be attributed to factors other than gross structural injury. Despite showing almost no signs of damage to bone-ligament-bone grafts initially, freezing procedures may have important subsequent biologic consequences that may influence the clinical success of stored connective tissue grafts used in ligament reconstruction.