The use of bone grafts and alloplastic materials in cranioplasty

Trephination dates from prehistoric neolithic times (10,000-7000 B.C.) and is the oldest operation known. Cranioplasty with bone allografts dates from the Stone Age Celts. Through the millennia, generations of surgeons have tried bone autografts, allografts, and rarely xenografts for cranioplasty but abandoned these in favor of alloplastic metals and plastics, most recently methylmethacrylate. Disillusionment with bone cranioplasty has followed the recurrent experience that orthotopic transplantation of bone to skull is almost invariably accompanied by a striking propensity for resorption. Resorption coupled to new bone formation is the usual process of remodeling. A unique acellular nonosteoclastic resorption, antedating invasion of the graft by osteoprogenitor cells and unrelated to the remodeling, characterizes the initial response of bone placed in a skull bed. This previously undescribed resorption in the skull likely represents passive diffusion of mineral from an altered matrix (calciolysis) and varies directly with the degree the graft is denatured by processing. There is the least amount of resorption in the fresh autograft and the most in autoclaved or chemically treated frozen or freeze-dried grafts. Remodeling of this diminished template occurs centripetally from skull defect margins through osteoconductive mechanisms only. Marrow-poor skull with thin diploe provides few osteoprogenitor cells that slowly, incompletely remodel the reduced graft over years.