Background: Metaphyseal fracture healing presents special biomechanical challenges in orthopaedic surgery. The void typically created by damage to the metaphyseal cancellous bone must usually be filled in order to recover the biomechanical integrity of the bone. While autologous bone grafting is a standard treatment for these fractures, bone graft substitutes delivered with or without pharmacologic agents may augment healing.
Hypothesis: Tricalcium phosphate (TCP) is a known osteoconductive bone filler and OP-1 an osteoinductive bone morphogenetic protein; both have been used in the past in diaphyseal fractures with success. PTH (parathyroid hormone) has been recently shown to enhance osteoblastic activity, to have a net anabolic effect on bone mass, and to enhance healing of diaphyseal fractures. Each of these agents may also enhance healing of metaphyseal fractures.
Objective: The potential of all above factors to accelerate metaphyseal fracture healing has been evaluated in a new metaphyseal fracture model developed in our laboratory in a rabbit model.
Material and Methods: A metaphyseal wedge osteotomy was created in the distal tibia of 16-week-old female New Zealand White rabbits (n=20). The osteotomy was bridged with a custom-made external fixator. The osteotomy gap was filled with TCP containing OP-1 (n=4), TCP alone with daily subcutaneous injections of 10μg/Kgr BW PTH (n=4), or TCP alone with daily subcutaneous administration of 40μg/Krg BW PTH (n=4). Two control groups, TCP alone (n=4) and normal healing (n=4), were also included. Assessment methods included biomechanical testing in both compression and torsion, radiographic examination, and QCT scans.
Results: Healing was observed in both PTH treated groups as well as in the OP-1 group at 4 weeks post-surgery. PTH appeared to have a systemic effect on bone formation, whereas the effect of OP-1 was local to the osteotomy site. In comparison, healing was delayed in the normal healing and TCP alone groups.
Conclusion: PTH and OP-1 both enhance metaphyseal fracture healing. The different systemic vs. local effects of these two agents, suggest that PTH and OP-1 may have potential synergism in accelerating healing of metaphyseal fractures.
