Bone grafting is used to repair bone fractures that are extremely complex, pose a significant risk to the patient, or fail to heal properly. Bone grafting is also used to help fusion between vertebrae, correct deformities, or provide structural support for fractures of the spine. In addition to fracture repair, bone grafting is used to repair defects in bone caused by congenital disorders, traumatic injury, or surgery for bone cancer. Bone grafts are also used for facial or cranial reconstruction.
Degenerative diseases of the spine increase with age. People over age 50 are more likely to need a bone graft if their condition requires surgery. Traumatic injuries occur most often in people 18–44 years.
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.
The surgeon does a clinical examination, and conducts tests to determine the necessity of a bone graft. Diagnostic tests determine the precise location of damage. These tests include x rays, magnetic resonance imaging (MRI), and computed tomography (CT) scan. They provide an image of the affected area, and indicate the exact amount of damage that has occurred due to the fracture or defect.
Orthopedic surgeries pose varying degrees of difficulty. The patient is instructed on what will take place during the procedure, as well as risks involved. A consent form is obtained before surgery.
The following activities will help the patient prepare for surgery.
- thorough physician consult before surgery
- banking some of his or her own blood in case a transfusion is needed
- eating well to achieve good nutritional status before and after surgery
- following a recommended exercise program before and after surgery.
- maintaining a positive attitude
- smoking cessation
Pain is normal for a few days following surgery, and medication is given regularly to alleviate this problem. The patient will likely have a urinary catheter.
The time required for convalescence after bone grafts due to fractures or spinal fusion varies from one to 10 days. Vigorous exercise may be limited for up to three months. Children heal faster than adults.
If a spinal fusion was performed, the patient may be discharged from the hospital with a back brace or cast. The family will be taught how to provide home care for the patient. A splint or cast prevents injury or movement while healing.
There is little risk of graft rejection for autografts, but there are drawbacks:
- additional surgical and anesthesia time (typically 30 minutes per procedure) to obtain or harvest the bone for grafting
- added costs for the additional surgery
- pain and infection at the site from which the graft is taken
- the relatively small amount of bone available for grafting
- surgical complications, such as infection and pain that sometimes last a longer period of time than the primary surgery (up to two years)
Allografts also have drawbacks:
- Bone variability because it is harvested from a variety of donors.
- Grafted bone may take longer to incorporate with the host bone (than in an autograft).
- Graft may be less effective than an autograft.
- Possibility of transferring diseases to the patient.
- Potential immune response complications (patient's immune system fighting against the grafted bone tissue). This problem is lessened through the use of anti-rejection drugs.
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 bone surrounding the graft at the time of surgery. Severe defects take some time to heal, and may require further attention after the initial graft. Less severe bone defects should heal completely without serious complications. Repeat surgery is sometimes required if the condition recurs or complications develop.
If the bone graft is done on the face or head, the surgeries usually result in a more normal appearance.
Morbidity and mortality rates
Although bone harvested from the patient is ideal, postoperative morbidity is sometimes associated with hip bone or fibula (part of the knee) autografts. Morbidity of allografts is usually related to the graft incorporating more slowly, and less completely, into the body.
In one study of over 1,000 patients who received very large allografts after bone cancer surgery, researchers found that approximately 85% were able to return to work or normal physical activities without crutches. However, approximately 25% required a second operation because the first graft did not heal properly.
Infections associated with bacterial contamination of allografts are rare. However, they can result in serious illness and death.
Despite the increase in the number of procedures requiring bone grafts, there is no ideal bone graft substitute. However, there are a variety of natural and synthetic replacement materials used instead of bone, including collagen (the protein substance of the white fibers of the skin, bone, and connective tissue); polymers, such as silicone and some acrylics; hydroxyapatite; calcium sulfate; and ceramics. Several new products are available or in development. They function as bone graft substitutes or extenders. Demineralized bone matrix (bone that has had its calcium removed) possesses some of the properties that the body uses to induce bone formation. Calcium hydroxyappetite products or coral have structures similar to bone, and act as scaffolding for new bone.
New BMP products are expected to be strong inducers of bone growth (osteoinductive). These new products will be relatively expensive, but will grow bone better than the patient's own bone, eliminating the need for bone graft harvesting. Bone morphogenetic proteins have been extracted from natural tissues and produced in the laboratory to stimulate bone production in animals and humans. Because they do not have the same drawbacks as grafts, surgeons are hopeful that they will soon be able to use BMP and laboratory produced BMP to aid in the generation and repair of bone.
The INFUSE Bone Graft (rhBMP-2) has received Food and Drug Administration approval, and has demonstrated better patient outcomes than hip autografts with regard to length of surgery, blood loss, hospital stay, reoperation
Advances in tissue engineering have provided polymer based graft substitutes with degradable, porous, three-dimensional structure. New bone may be grown on these products; the grafts then slowly dissolve, leaving only the new bone behind.
See also Disk removal.
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American Association of Tissue Banks. 1350 Beverly Road, Suite 220-A, McLean, VA. 22101. (703) 827-9582, fax: (703) 356-2198. E-mail: email@example.com. <http://www.aatb.org/menu.htm>.
National Institutes of Health. 9000 Rockville Pike, Bethesda, MD. 20892. (301) 496-4000. Email: NIHInfo@OD. NIH.GOV. <http://www.nih.gov/>.
North American Spine Society. 22 Calendar Court, 2nd Floor, LaGrange, IL 60525. (877) Spine-Dr. E-mail: info@ spine.org. <http://www.spine.org>.
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Lisa Christenson, Ph.D. Crystal H. Kaczkowski, M.Sc.
WHO PERFORMS THE PROCEDURE AND WHERE IS IT PERFORMED?
Bone grafts are performed by orthopedic surgeons, neurosurgeons, craniofacial surgeons, and periodontists. These physicians have specialized training in their field of expertise. They have completed additional education beyond a general medical residency. A physician becomes board certified after completing training in a specialty area, passing examinations, and meeting certification requirements. To become certified, the physician must:
- Complete the education required for a medical doctor or doctor of neurosurgery.
- Complete three to seven years of training in a residency program in the specialty field.
- Pass a written, and sometimes an oral, test given by the specialty board.
- Prepare for periodic recertification (required in most fields).
Specialty boards certify that physicians have met certain standards. Not all specialists are certified; certification is voluntary.
The surgeon and his or her surgical team will perform the surgery in a hospital on an inpatient basis.