Bone Grafts – Types, Mechanism, Techniques and Complications

Bone grafts are used to repair and rebuild diseased bones in your hips, knees, spine, and sometimes other bones and joints. A bone graft transplants bone tissue. Grafts can also repair bone loss caused by some types of fractures or cancers. Once your body accepts the bone graft, it provides a framework for the growth of new, living bone.

Bone has the ability to regenerate completely if provided the space into which to grow. As native bone grows, it replaces the graft material completely and results in a fully integrated region of the new bone.

Types of Bone Grafts

Depending on the source where the graft is taken from, the bone grafts can be autograft or allograft.

Autograft

Also called autologous or autogenous bone graft, this kind of graft is obtained from the same individual receiving the graft. Bone can be harvested from non-essential bones, such as the iliac crest. The iliac crest is the most common source of autografts in orthopedic surgery. Other places where graft can be taken from is fibula or metaphyses of long bones.

However, very small quantity can be taken from long ends of the bones.

Autogenous bone is the most preferred for the grafting and is deemed as the gold standard. There are no risks of graft rejection as the graft originated from the patient’s own body. It is the most efficient graft type.

However, autografting requires an additional surgical procedure. This adds to surgery stress on the patient’s body and potential for post-operative pain and complications.

Normally, a bone graft is either used in blocks or slivers after being harvested. Sometimes grafts are also harvested especially with their supplying vessel and the blood vessel is reanastomosed to the blood vessel at the recipient site.

Such a graft is called the vascularized graft.

Vascularized fibulae grafts have been used to restore skeletal integrity to long bones of limbs in which congenital bone defects exist and to replace segments of bone after trauma or malignant tumor invasion. The periosteum and nutrient artery are generally removed with the piece of bone so that the graft will remain alive and grow when transplanted into the new host site. Once the transplanted bone is secured into its new location it generally restores blood supply to the bone in which it has been attached.

Allografts

Allograft bone is harvested from an individual other than the one receiving the graft. Allograft bone is taken from cadavers that have donated their bone so that it can be used for living people who are in need of it; it is typically sourced from a bone bank.

There are three types of bone allograft available.

1. Fresh or fresh-frozen bone
2. Freeze-dried bone allograft
3. Demineralized freeze-dried bone allograft

Xenografts

Xenografts are grafts taken fro species other than humans. Till now xenografts have only historical and experimental significance.

Bone Substitutes

Bone substitutes are synthetic materials that are aimed to replace bone grafts. Partial success has been achieved with these and these are discussed separately.

A bone graft is used clinically to provide a bridge of osteogenic tissue, either in a part of the skeleton that is deficient or to establish bony fusion of the diseased joint.

Indications of Bone Grafting

In 1668 the Dutch surgeon Job Van Meekeren described first bone graft procedure. The graft was derived from a dog’s cranium and was used successfully to repair a traumatic defect in a soldier’s skull.

Duhamel is credited for the work on osteogenesis by his experiments on periosteal new bone formation in 1742.

In 1867, Ollier showed that autografts are viable and separate living bone fragments could live and grow in a suitable environment.

Bone grafts are used for various condition

• Delayed and nonunion of fractures.
• The arthrodesis of joints.
• Filling cavities or defects resulting from cysts, tumors, trauma or other pathologies.
• Fusion of growth plate cartilage.
• Bone and muscle transfer for ligamentous defects.

How Does Bone Grafts Act?

Weather transplant or implant, bone graft serves following functions:

Osteogenesis

Osteogenesis refers to new bone synthesis by graft or host-derived cells.

Osteoinduction

Bone Morphogenic Protein which is present in fresh autografts & modified allografts induce changes is host mesenchymal cells to form osteoblasts.

Osteoconduction

The graft acts as a framework or scaffold into which new capillaries, perivascular tissue, and osteoprogenitor cells grow.

Following images show the healing of lesion [aneurysmal bone cyst] in the upper tibia.

Functions of Bone Grafts

Bone grafts serve two functions, a mechanical function, and a biological function. In certain applications, one may be important than other but most of the time the two functions are intimately intertwined. The net biologic activity of a graft is the sum of its inherent biologic activity (living cells & their products) and its capacity to activate surrounding host tissues to relevant biologic activity (mediated by bioactive factors within the matrix) and its ability to support the ingrowth of host tissue.

The graft cannot exert its biological activity in isolation. It is dependent on the surrounding environment for cells to respond to its signals. The mechanical environment of the graft site is also important. Bone grafts are remodeled in response to a mechanical load to which they are subjected.

Bone grafts incorporation is a complex multifaceted process and multiple variables influence its rate, pattern, and completeness. Incorporation of a bone graft begins with homeostasis with intact bone. Unlike other tissues, bone heals by regeneration and replacement by additional bone.

How Does Bone Graft Enhance Bone Healing?

A bone graft either stimulates the recipient bone itself or causes a change in the bone to enhance bone healing. A bone graft is incorporated by following processes.

Osteogenesis

Osteogenesis is the synthesis of new bone with no indications of cellular origin i.e. either by cells of grafted bone or the host bone. Properly handled fresh cortical and cancellous autograft are able to produce new bone. Cancellous bone with its larger surface area covered with quiescent lining cells or active osteoblastic cells has a greater potential for producing new bone of graft origin than the cortical bone.

This early bone formation either by graft cells of cortical or cancellous bone is critical in callous formation during 4 to 8 weeks after surgery and accelerates the bone incorporation.

Osteoinduction

Osteoinduction is the recruitment of the mesenchymal cells of the host bed surrounding the graft to be differentiated into bone-forming cells and cartilage forming cells.

The mesenchymal cells generally arise from perivascular cells associated with proliferating blood vessels, reticular elements of bone marrow, trabecular surfaces and the cambium layer of the periosteum.

These cells then can specialize as osteoblasts, osteoclasts, chondroblasts, chondroclasts, fibroblasts and occasionally lipoblasts. The recruitment and differentiation of these cells is probably modulated by low molecular weight polypeptides such as glycoprotein bone morphogenic protein (BMP).

BMP is a hydrophobic, non-species-specific protein whose activity does not require viable graft cells and is present in fresh autografts as well as in modified allografts. The other graft-derived factors besides BMP are transforming growth factor, Insulin-like growth factor I & II, Acidic and Basic fibroblast growth factors, Platelet-derived growth factors, Interleukins, Granulocyte-macrophage colony stimulating factors etc.

The osteoinductivity of a mineralized graft is thought to be minimal except as living graft cells may be producing osteoinductive factors but the osteoinductivity of the demineralized bone matrix has been repeatedly shown.

The health of the host bed is critical in the process of osteoinduction because new osteoprogenitor cells are recruited by induction of residual mesenchymal cells in marrow reticulum, endosteum, periosteum, and connective tissue.

Osteoconduction

In addition to the above mentioned biologic functions, the graft may provide structural support until the recipient tissue can bear weight. In addition to providing mechanical supporting, they function as trellis or scaffold for the ingrowth of new host bone2.

The three-dimensional process of ingrowth of sprouting capillaries, tissue and osteoprogenitor cells from the recipient bed into the structure of the graft is termed osteoconduction. Osteoconduction may result from the active bone formation and osteoinduction e.g. in a fresh corticocancellous autograft or it may occur passively without the active participation of the graft, as is the case with most cortical allografts. The graft is resorbed gradually and is completely replaced by new host bone a process termed as creeping substitution by Phemister.

Sequence of Events in the Incorporation of Bone Grafts

1. Hematoma formation (Host-dependent)
2. Organization of hematoma by the local inflammatory response (Host-dependent)
3. Neovascularisation i.e. penetration of neocapillaries with perivascular tissues into the graft (Host-dependent)
4. Cytodifferentiation by an interaction between the host-derived cells and the grafted material (chemomodulation and environmental interplay)
5. Creeping substitution (Resorption by osteoclasts and neoosteogenesis from osteogenic cells)
6. Remodeling (continued resorption and neoosteogenesis under influence of functional stress and strain) to the trabeculated bone.

The initial event is a formation of a hematoma rich in platelet-derived growth factor, other growth factors, and cytokines. A local inflammatory response is stimulated by the implantation procedure and by the presence of the graft.

Clinically, a bone graft has been incorporated successfully when the ‘host graft-host bone’ interface unites and the affected area tolerates physiological weight bearing without fracture or pain.

Different Types of Bone Grafting

Single Onlay Cortical Bone Grafting

The term onlay bone grafting was coined by Campbell. The technique involves a bone grafting in which the transplanted tissue is laid directly onto the surface of the recipient’s bone. The technique is more of historical importance. The technique is rarely used now.

Dual Onlay Bone Grafting

This was devised by Boyd. Two cortical onlay grafts are placed opposite each other on the host bone across the nonunion and are fixed with the same set of screws; they grip the fragments like a vise. Any intervening space at the bone ends is filled with cancellous chips.

Dual grafts also have been used for other nonunions and rarely done now.

Until relatively inert metals became available, the onlay bone grafts were the simplest and most effective treatment for most ununited diaphyseal fractures.

Inlay Bone Grafting

A slot or rectangular defect is created in the cortex of the host bone and a  graft of the same size or slightly smaller is fitted into the defect. The technique is still is occasionally used in arthrodesis.

Peg Bone Grafting

The term peg is used to denote a wooden pin pushed or driven into a surface. Peg grafts are similar are used in cases of small bones like scaphoid or small fragments like lateral condyle or medial malleolus. The peg is inserted into the bone fragments after drilling a hole across the fragments.

Medullary Bone Grafting

When a bone graft is placed in the medulla of both fragments across the fracture site, it becomes a medullary graft. Medullary bone grafts have not shown good results.

Medullary grafts are now used in the metatarsals, the metacarpals, and the distal end of the radius.

Osteoperiosteal Bone Grafting

The graft tissue is removed with periosteum intact. Osteoperiosteal grafts are less osteogenic than multiple cancellous grafts and are now rarely used.

Multiple Cancellous Chip Bone Grafting

These are widely used for grafting as

• To fill cavities or defects resulting from cysts, tumors, or other causes
• To create bone blocks
• To create a wedge in osteotomies.
• To hasten the healing
• In arthrodesis of the spine

Hemicylindrical Bone Grafting

Hemicylindrical bone grafting is used when the defects in the bone are large.  A massive hemicylindrical cortical graft from the affected bone is placed across the defect and is supplemented by cancellous iliac bone.

This procedure is used in limited surgeries in case of tumor resection.

Whole Bone Transplant Bone Grafting

Whole bone transplant is used in bridging long defects in the diaphyseal portion of bones of the upper extremity unless the nonunion is near a joint. The fibula is a dispensable bone and is most commonly used for whole bone transplant.

Complications of bone grafting may occur following a procedure of bone grafting.

Bone grafting is a very common procedure in orthopedics and the iliac crest is currently the most common donor site.
Obtaining bone from the iliac crest can be associated with significant morbidity. Both the anterior and posterior portions of the iliac crest are often used for the purpose of bone grafting.

A review of the literature reveals following reported complications of bone grafting.

Complications of Bone Grafting

Arterial Injury

Superior gluteal artery injury is an infrequent but one of the severe complications of bone grafting in case of the harvest of the posterior iliac crest. The cause of injury is usually excessive muscle retraction or the placement of retractors into the sciatic notch. When occur, these arterial injuries are difficult to control, sometimes necessitating additional surgical exposure, including removal of the sciatic notch or an anterior or retroperitoneal surgical approach. The vessel may be controlled by an angiographic embolization method too.

Sometimes, injuries to the superior gluteal artery also manifest late as a gluteal artery aneurysm.

Ureteral Injury

Patients with postoperative hematuria, abdominal distention, ileus, and fever should be assessed for genitourinary tract injury. The ureter is closer to the superior gluteal artery and the sciatic notch in women, special attention be given to the protection of the sciatic notch when a graft is harvested from the posterior iliac crest in a female patient.

Hernia

Herniation of the abdominal contents through the donor site defect is a relatively common complication. Patients may present with lower abdominal pain and a sensation of fullness in the donor site.

Computed tomography usually clearly shows a hernia. Advanced age, obesity, female sex, and a graft larger than 4 cm.

Local tissue closure, Bosworth’s method of crest resection and closure, rotation of regional fascial flaps, and the use of synthetic mesh grafts have all been suggested to treat these defects.

Chronic Pain and Nerve Injuries

Chronic pain after il is one of the more common complications of bone grafting reported.

Most cases probably result from occult nerve injury or destabilization of the muscular origin at the time of harvest.

Chronic alteration in donor site sensation is one of the most commonly reported complications of bone grafting from the iliac crest.

Injury to the superior clunial nerves has been reported at the time of posterior iliac crest bone harvest. Patients with injuries to the clunial nerves have posterior pelvic pain radiating into the buttocks that is often made worse by sitting. Often these injuries can be treated with sequential nerve blocks and desensitization and occasionally necessitate exploration and neuroma resection.

Meralgia paresthetica may occur due to injury to the lateral femoral cutaneous nerve during or after harvest of the bone from the anterior iliac crest. Symptoms improve with occasional nerve blocks, analgesics, and desensitization. Some patients may require operative decompression or nerve sectioning.

Ilioinguinal nerve is vulnerable to injury due to medial retraction in this area resulting in loss of sensation to the pubic symphysis, base of the penis and proximal scrotum or labia majora, and the anteromedial aspect of the thigh.

Infection

Like any other surgical procedure, donor site wound may get infected and needs treatment to control infection.

Fractures

Fractures after iliac crest bone harvesting occur during harvesting anterior parts of the iliac crest. Most often, the fracture results in displacement of the anterior superior iliac spine. Treatment is usually nonoperative and focuses on a short period of rest followed by assisted ambulation until the fracture heals. Occasionally surgical treatment may be required.

Cosmetic defects

Removal of the superior surface of the ilium or to the development of an unsatisfactory scar. Current recommendations are to use methods that preserve superior pelvic brim like removal of a section of ilium below the crest, superior crest trapdoor and crest splitting methods.

Hematoma

Significant hematoma after iliac crest bone grafting has been reported in up to 10% of cases.

Tumor Transplantation

There are reports of musculoskeletal tumors being transplanted between two distant surgical sites.