The morselized and impacted bone graft. Animal experiments on proteins, impaction and load

Abstract: This thesis focuses on the healing and remodeling of the morselized and impacted bone allograft, commonly used in revision of loosened hip prostheses with osteolysis. The influence on remodeling by the impaction procedure per se was analyzed, as well as the effects of endogenous bone graft proteins, and an exogenously applied growth factor (OP-1). A bone chamber model in rats was used, with the ingrowth distance of new bone into the graft as the outcome measurement. Proteins were found to increase ingrowth, whereas impaction per se caused a decrease or delay. A rabbit knee prosthesis was developed to evaluate the influence of mechanical load on the remodeling. Morselized bone grafts were packed into the proximal tibia and either loaded or unloaded by the prosthesis. Increased remodeling was found, i.e. both graft resorption and new bone formation were increased. Four patients underwent surgery for lumbar fractures. The fractures were fixated with plates and the vertebral body was packed with morselized autografts. Biopsies were taken 1.5 years after surgery, when the plates were removed. The fractures were at that time clinically and radiographically healed, but large parts of the grafts were histologically unremodeled and not even revascularized. It was concluded that full osseous remodeling does not always occur even after a long period of time, despite the use of viable autograft instead of frozen allograft. Finally, impacted grafts were placed in the rat bone chamber model to allow fibrous ingrowth. These grafts were compared to freshly impacted grafts in a mechanical test. After fibrous ingrowth the resistance to compressive loads was doubled. In conclusion, endogenous bone graft proteins and exogenously applied OP-1 increase the new bone ingrowth distance into the graft, while impaction has a reducing effect. Load increases remodeling. In clinical cases full osseous remodeling can not always be expected, but fibrous tissue ingrowth strengthens the graft.