This study evaluated the repair mechanism of massive human allografts. Seventy-three specimens were examined between 2 and 156 months post implantation. Initial healing over the first 12 months occurred by host-derived external callus, which resulted in a 40% surface coating by one year and 80% by two years. Primary internal cortical to cortical bone healing occurred more slowly and incompletely. Internal repair penetrated the allograft cortex up to only 20% by five years. Allograft fractures, although always through necrotic bone, showed a strong propensity for internal healing. Autogenous bone graft in contact with allografts resulted in intense external and internal bone formation. Bone cement was very effective for fixation of implants with no evidence of osteolysis or implant loosening. Osteoarticular allografts showed no chondrocyte survival, but well preserved articular height up until 3 years post implantation. With time, a fibrovascular pannus progressively covered the chondral surface and ultimately resorbed the articular surface. These changes progressed with time but were less advanced in the upper extremity. Degenerative changes in articular cartilage were more advanced in unstable joints.
Massive allografts used for tumor and traumatic reconstruction incorporate in a slow and incomplete fashion. Little or no repair tissue penetrated deep into the haversian canals of the allograft even after 5 years and the majority of the allografts remained indolent. This essentially protects the cemented implants from loosening or failing. Allograft fractures, however, occur in areas of stress concentration, such as the ends of fixation devices or at the edge of an area of accelerated resorption commonly seen in the second year. Specimens in situ for greater than 3 years exhibited greater variation in the extent of cortical repair indicating a strong immunological component to the extent of resorption and subsequent repair. Frozen osteoarticular allografts initially are well preserved despite a lack of chondrocyte survival but will ultimately degenerate as fibrovascular pannus covers the articular surface and resorbes the articular cartilage. This degenerative process is accelerated in unstable joints. Therefore, all attempts should be made to implant an osteoarticualr allograft in maximum stability when required for reconstruction in the upper extremity.