Bone grafting is a surgical procedure in which an orthopaedic surgeon transplants bone tissue. Bone grafts are used to repair fractures that are complex or have failed to heal, to replace missing bone following trauma or tumor removal, and to correct deformities. They are also used in spinal surgery to help fuse vertebrae. Bone grafts work because, given sufficient space and proper scaffolding, bone tissue has a remarkable ability to regenerate.
Osseous, or bone, tissue consists of protein fibers called collagen embedded in a matrix of intercellular liquid hardened by deposits of calcium and phosphate salts. Within and around the matrix, 3 types of bone cells build, maintain, and remodel bone. These include osteoblasts, or immature bone cells, which produce the bone matrix; osteocytes, or mature bone cells, which serve to maintain the matrix; and osteoclasts which break down and remove bone tissue (Fig. 1).
A bone graft grows and repairs a defect according to 3 different processes. The first is osteogenesis, or the formation of new bone by living cells within the graft such as osteoblasts. The second, osteoinduction, is a chemical process in which protein molecules within the graft recruit and stimulate the patient’s undifferentiated cells to become osteoblasts. Lastly, osteoconduction is the process by which the matrix of the graft serves as a scaffold to maintain space so the recipient’s cells can generate new bone tissue. Bone grafts thus provide a structure for bone to grow, but then slowly dissolve, leaving behind only the new bone.
An autograft, or bone obtained from a patient’s own body, is considered the gold standard in bone grafting procedures largely because native bone is osteogenic as well as osteoinductive and osteoconductive. It is also non-immunogenic, which means it is compatible with the patient’s own tissues and the body will not attack or reject it. As all bone requires an adequate blood supply, depending on the graft size and transplant site, a section of the periosteum (the thin layer of connective tissue that covers the bone) and its accompanying blood vessels may be included with the autograft and reattached at the site to ensure its blood supply.
There are additionally 2 types of bone that can be used for an autograft: cancellous or soft bone and cortical or hard bone. Compared to cortical bone, cancellous bone has greater surface area and is much more porous, allowing cells to infiltrate and blood vessels to form. It thus has more bone-forming potential than cortical bone, but cortical bone, being harder, can provide immediate structural support for new bone. Common sources of cancellous autograft bone are the iliac crest (upper portion of each side of the pelvis), upper tibia (shinbone), and the radius (wrist). Cortical autografts can be harvested from the diaphysis, or shaft, of the fibula (outer bone of the lower leg) and used to reinforce reconstructions, such as for spinal injuries or to replace segments where bone has been lost due to a trauma or tumor removal.
While autografts are the preferred material, harvesting bone from the patient’s own body necessitates additional surgical time and blood loss, and the amount of bone that can be harvested is always limited. The chief drawbacks to an autograft, however, are potential complications at the harvest site, such as infection and possible nerve injury.
Allografts and synthetic materials
A graft obtained from donor (cadaveric) bone is called an allograft. After being harvested, the bone tissue is tested for disease, cleansed, frozen, and stored in tissue banks. Allografts are not osteogenic, but they are able to stimulate cells to become osteoblasts and to provide a scaffold for bone growth. Unlike autografts, they do not require the patient to undergo an additional surgery; this reduces the risk of infection and precludes pain and loss of function at a second surgical incision site.
A variety of natural and synthetic replacement materials can also serve as substitutes for natural bone or can be mixed with either allograft or autograft tissue as bone extenders. These include ceramics like calcium phosphates, bioglass, and calcium sulphate. All of these materials are biologically active to some degree. For example, allograft bone that has been treated with a strong acid to remove the inorganic mineral deposits, known as demineralized bone matrix, possesses some osteoinductive properties. Additionally, coral, which has a biochemical and physical structure similar to bone, can act as a scaffold for new bone. Natural substances, such as bone morphogenetic proteins (BMPs), which contain growth factors, can be added to these synthetic materials to enhance their biological activity.
Allografts, either alone or mixed with extenders or enhancers, are often used in pelvic, knee, and femur (thighbone) reconstructions, but, ultimately, the type of bone graft used depends on the site and the exact nature of the injury being repaired. Since allograft tissue is not the patient’s own, the bone quality may vary. Moreover, the graft may take longer to incorporate with the patient’s native bone than an autograft. There is also a greater risk of reabsorption of the graft as well as the possibility of immune response complications, though taking anti-rejection drugs helps diminish this. Likewise, advanced testing and cleansing methods have greatly reduced the risk of transferring a disease along with an allograft.
Bone grafting procedures
During a bone grafting procedure, the orthopaedic surgeon will either place the graft material directly into the bone defect or lay it across to bridge the area to be fused. In some instances, the bone graft is held in place with pins, plates, or screws. When additional stability or protection is needed during the healing process, a splint, cast, or brace can be applied.
Recovery from a bone grafting procedure can take from 2 weeks to more than a year, depending on the size of the defect and the condition of the surrounding bone at the time of the surgery. More severe cases take longer and can require follow-up surgery.
Certain behaviors and conditions can affect the outcome of a graft. For instance, smoking can diminish outcomes because the carbon monoxide in cigarettes reduces local blood flow, decreasing osteoblast formation and bone metabolism at the graft site. Diabetes mellitus, which can cause peripheral nerve and vascular problems, may negatively impact fracture healing and bone grafting. Deficiencies in dietary calcium and vitamin D impair bone metabolism while metabolic conditions, such as thyroid problems and low growth hormone levels, have been associated with high rates of nonunions (fractures that fail to heal). Additionally, some drugs, such as nonsteroidal anti-inflammatory medications and corticosteroids, can interfere with bone healing. By inhibiting osteoclasts, bisphosphonates used to treat osteoporosis (low bone density) can decrease the rate of bone remodeling. Overall, bone grafting is highly successful in patients who do not smoke and follow their surgeon’s instructions when it comes to medications and activity modification.
Author: Thomas N. Bernard, Jr., MD | Columbus, GA
Reprinted with permission from the Hughston Health Alert, Volume 29,Number 1, Winter 2017.