The cranial bones are connected to each other by contiguous sutures, which create the rigid structure that protects the brain. The bones of the skull are constructed from two plates separated by spongiform bony tissue that is rich in blood vessels. In some areas, the plates are separated from each other without intervening bone, creating hollow spaces (sinuses).
In others, they are in direct contact and form a single unit. Skull defects can be present from birth, arise from accidents, or occur as result of the removal of benign or malignant tumors. Reconstruction of any of these osseous defects requires the complete covering by soft scalp tissue or other tissue, with the accompanying adequate blood supply. In cases where there is a defect of both scalp and cranium, it is sometimes necessary to undergo preliminary surgery to reconstruct the scalp and achieve adequate soft tissue coverage. Several months later, it is possible to undergo a second surgery to attempt the osseous reconstruction.
Reconstruction of the cranial bones can be performed by using proximal cranial bones in cases where the defect has a diameter of approximately five to eight centimeters. The most common source of cranial bone material is the parietal bone, which can be six to eight millimeters thick. This makes it possible to separate the outer bone plate from the interior plate, which remains in place. The external bone plate is shaped to fit the defect, transplanted, and affixed at its edges by plates and screws made of steel or a material that is gradually absorbed over the course of months, once a stable osseous union has been achieved. It is also possible to use a bone graft from another source, such as ribs or the pelvis, but these are not common donor sites due to the significant morbidity that results from using these grafts, and the limited ability to shape them to the defect.
Reconstruction of larger or more complex defects usually requires the use of synthetic materials. The most common solution for such defects is a delicate titanium mesh, which can be molded for an optimum fit, and affixed by appropriate screws. In more complex defects, it is possible to match a synthetic implant specially built to meet the needs of the osseous defect. These implants are made of polyethylene (Medpor), or from a methyl methacrylate polymer and calcium hydroxide (HTR). These implants are designed using CT imaging of the defect area and computerized design that is precisely matched to the defect. The unique quality of these transplants lies in the fact that they are porous and allow for the growth of connective tissue and capillary blood vessels within the implants, thereby enabling better acceptance of the implants, and relatively better long-term resistance to infection.