Last updated: 11/24/2009

Bone tissue is a matrix-like structure primarily composed of a protein called collagen. It is strengthened by hydroxyapatite, deposits of calcium and phosphate salts. Four types of bone cells are located within and around this matrix. Together, these...

four types of cells are responsible for building the bone matrix, maintaining it, and remodeling the bone as needed. The four types of bone cells are:

• Osteoblasts, which produce the bone matrix.
• Osteocytes, mature osteoblasts that maintain the bone.
• Osteoclasts, which break down and remove bone tissue.
• Bone lining cells, which cover bone surfaces.

There are three ways that a bone graft can help repair a defect.

• Osteogenesis, the formation of new bone by the cells contained within the graft.
• Osteoinduction, a chemical process in which molecules contained within the graft (bone morphogenetic proteins, abbreviated as BMP) convert the patient's cells into cells capable of forming bone.
• Osteoconduction, a physical effect whereby the graft matrix configures a scaffold on which cells in the recipient form new bone.

The term "graft" commonly refers to an autograft or allograft. A graft made of bone from the patient's own body (e.g., hip bones or ribs) is an autograft. To obtain a piece of bone for an autograft, the patient undergoes surgery under general anesthesia. An incision is made over the crest of the hip bone, a piece of bone is removed, and the incision is stitched closed.

An allograft uses bone from a cadaver, which has been frozen and stored in a tissue bank. Allografts are used because of the inadequate amount of available autograft material, and the limited size and shape of a person's own bone. Bones for allografts are usually available from organ and tissues donated by healthy people who die unexpectedly. Occasionally, allograft bone may be provided by a living donor. Allograft bone is commonly used in reconstructive surgery of the hip, knee, and long bones, as well as cases of bone loss due to trauma or tumors. Using allograft tissue from another person eliminates the need for a second operation to remove autograft bone or tendon. It also reduces the risk of infection, and safeguards against temporary pain and loss of function at or near the secondary site.

To place an autograft or allograft, the surgeon makes an incision in the skin over the bone defect, and shapes the bone graft or replacement material to fit into it. After the graft is placed into the defect, it is held in place with pins, plates, or screws. The incision is stitched closed, and a splint or cast is often used to prevent movement of the bones while healing.

After the bone graft has been accepted by the body, the transplanted bone is slowly converted into new living bone or soft tissue, and incorporated into the body as a functional unit.

Bone grafts for spinal fusion

In surgery of the spine, especially spinal fusion, (also called arthrodesis), surgeons may decide to use bone grafts to assist in the healing and remodeling of the spine after surgery. Normally, small pieces of bone are placed into the space between the vertebrae to be fused, and sometimes larger solid pieces of bone provide immediate structural support. Spinal fusion involves the surgical treatment of abnormalities in the vertebrae, such as curvatures, scoliosis or kyphosis, or injuries (fractures). Bone grafts may be used in spinal fusion surgery involving the lower (lumbar) or upper (cervical) spine. Cervical spinal fusion joins selected bones in the neck. This surgery may also be performed by other means, such as metal rods, which would not require bone grafts.