Bone grafting is a surgical procedure by which new bone or a replacement material is placed into spaces between or around broken bone (fractures) or holesin bone. Bone grafting is used to repair bone fractures that are extremely complex, pose significant risk to the patient, or fail to heal properly. Bonegrafts are also used to help fusion between vertebrae, correct deformities, or provide structural support for fractures of the spine. In addition to fracture repair, bone grafts are used to repair defects in bone caused by birth defects, traumatic injury, or surgery for bone cancer.
Bone is composed of a matrix, mainly made up of a protein called collagen. Itis strengthened by deposits of calcium and phosphate salts, called hydroxyapatite. Within and around this matrix are four types of bone cells. Osteoblasts produce the bone matrix. Osteocytes are mature osteoblasts and maintain thebone. Osteoclasts break down and remove bone tissue. Bone lining cells coverbone surfaces. Together, these four types of cells are responsible for building the bone matrix, maintaining it, and remodeling the bone as needed.
There are three ways in which a bone graft can help repair a defect. The first is called osteogenesis, the formation of new bone by the cells contained within the graft. The second is osteoinduction, a chemical process in which molecules contained within the graft convert the patient's cells into cells thatare capable of forming bone. The third is osteoconduction, a physical effectby which the matrix of the graft forms a scaffold on which cells in the recipient are able to form new bone.
New bone for grafting can be obtained from other bones in the patient's own body (an autograft) or from bone taken from other people that is frozen and stored in tissue banks (an allograft). Natural and synthetic replacement materials are also used instead of bone, including natural collagen protein, polymers such as silicone, some acrylics, hydroxyapatite, calcium sulfate, and ceramics. A new material, called resorbable polymeric grafts is also being studied. These resorbable grafts provide a structure for new bone to grow on, thenslowly dissolve, leaving only the new bone behind.
To place the graft, the surgeon makes an incision in the skin over the bone defect and shapes the bone graft or replacement material to fit into the defect. After the graft is in place, it is held in place with pins, plates, or screws. The incision is closed with stitches and a splint or cast is used to prevent movement of the bones while healing. The time required for convalescencefor fractures or spinal fusion may vary from 1-10 days, and vigorous exercise may be limited for up to three months.
Most bone grafts are successful in helping the bone defect to heal. The extent of recovery depends on the size of the defect and the condition of the bonesurrounding the graft at the time of surgery. Severe defects take longer toheal and may require additional attention after the initial graft. Less severe bone defects usually heal completely without serious complications.
The drawbacks of autografts include the additional surgical and anesthesia time to obtain the bone for grafting, the added costs of the additional surgery, pain and infection that might occur at the site from which the graft is taken, and the relatively small amount of bone that is available for grafting.
The drawbacks of allografts include variability between lots, since the boneis harvested from a variety of donors. The bone may take longer to incorporate with the host bone than an autograft would, and the graft may be less effective than an autograft. There is the possibility of transferring diseases from the donor to the patient. Other complications may result from the immune response mounted by the patient's immune system against the grafted bone tissue. With the use anti-rejection agents (drugs to combat rejection of grafted bone tissue) immune rejection is less of a problem.