The healing of bone and cartilage

Transplantation of bone should be preceded by careful assessment of the recipient site. The function of the transplanted bone as an interposition graft, as an onlay graft, or in restoration or construction of a missing part of the skeleton must be considered. Cortical bone provides superior mechanical strength and can be incorporated with plate fixation to span interposition defects. Membranous bone used as onlay grafts for augmentation of craniofacial skeletal contour has been shown to be superior to endochondral grafts in maintaining volume. The use of rigid fixation to secure onlay grafts may eliminate the differences in resorption seen with membranous versus endochondral bone. The vascularity and quality of soft tissue at the recipient site may necessitate the use of vascularized bone or composite free tissue transfer. The calvarium is the most popular donor site for bone grafts used in craniofacial skeletal procedures. This membranous bone undergoes less resorption and revascularizes faster than endochondral bone. Cranial bone has excellent mechanical strength due to its large cortical component. The calvarial donor site causes less discomfort to the patient compared with rib or iliac crest, and the scar is well hidden. Harvesting and shaping cranial bone require special expertise, and there is potential morbidity. In cartilage transplantation, the surgeon must take into account the properties of viscoelasticity, the intrinsic balanced system of forces, and immunologic privilege. Cartilage deformed by an external force will tend to return to its original shape unless the deformation is maintained for several months. Surgical carving produces changes in the balance of intrinsic tensile and expansile forces, causing distortion in cartilage shape. Distortion can be minimized by carving in balanced cross-section. Carved cartilage grafts should be used for special indications in rhinoplasty. Autogenous cartilage is the framework of choice in ear construction. Composite grafts incorporating cartilage have been used successfully in eyelid reconstruction. Fresh autogenous cartilage is preferable to preserved allogeneic sources, as the latter undergo eventual resorption because there are no viable chondrocytes to maintain the matrix.