Bone Grafts in Hip Surgery

Key Points

Autologous bone graft is osteogenic, osteoinductive, and osteoconductive, with complete histocompatibility and no risk of infectious disease transmission. It is considered the gold standard for bone graft and is the most favored graft material in musculoskeletal reconstruction.
Advantages of bone allograft use include availability of materials and avoidance of donor site morbidity associated with autograft harvesting. Disadvantages of bone allograft use include lack of osteogenic cells, decreased osteoinductive factors, host immune response, and risk of infection.
Impaction bone grafting is generally excellent for treating patients with small to moderate-sized contained cavitary defects but poor with regard to implant fixation stability for patients with large uncontained segmental defects.
For such large uncontained segmental bone defects, structural bulk allograft can provide adequate support for primary implant fixation stability, with reasonable long-term outcomes despite mixed concerns over long-term resorption.
The success of graft incorporation depends on several factors—principally, graft revascularization, new bone formation (around and within the graft), and healing at the graft-host interface. These, in turn, depend on a combination of the biologic activity of the graft, the vascularity of the host bed, and the mechanical stability of the graft-host interface.

Introduction

Historical Perspective

Modern bone grafting techniques stem from an ancient era. Anthropologist A. Jagharian at Erivan Medical Centre in Armenia examined a prehistoric Khuritic skull from the ancient Centre of Ishtkun and found a piece of animal bone filling a 7-mm defect with bony regrowth around the grafted bone. Fast forward to 1668: a Dutch surgeon named Job Van Meekeren first documented a successful bone graft technique in which he inserted a fragment of a dog bone allograft into the skull of an injured soldier. By 1879, Sir William MacEwen from Scotland replaced an infected proximal two-thirds of a humerus in a 4-year-old boy with the fresh allograft tibia from another child with rickets. In 1915 F. H. Albee published his work on autologous bone graft in the United States. His research promoted the use of bone autograft as the better alternative to bone allograft transplantation. However, the difficulties in finding suitable quantities of autograft bone soon emerged. Dr. Inclan, in Cuba, began to combine autograft and allograft bone in dog and human models in 1942. In his series, he used homologous bone graft between living species of similar blood groups. By this time, immune-related problems with different tissue sources were known, and harvesting bone graft from cadavers was prohibited on religious and sentimental grounds. Modern-day bone banks emerged in the 1960s and 1970s with improvements in the processing and storage of allograft bone. Several publications also helped define the safe utility of allograft bone, which expanded the number of bone banks globally.

In recent decades, a growing interest has emerged in highly porous metal augments and bone substitutes as an alternative to bone for reconstructive hip surgery. However, autogenous and allogeneic bone remain the gold standard, especially in young patients. The most common clinical applications include restoration of bone stock in tumor surgery, difficult primary hip arthroplasty, and revision hip arthroplasty.

In this chapter, we will focus our discussion on the use of autologous and allogeneic bone grafts and will include various grafting techniques, their clinical applications and results, the scientific basis of these materials, current controversies, and future directions.

Basic Science

Biologic Properties

Bone graft around the hip may be considered, for example, to facilitate fusion of fractures or osteotomies and to replace bone defects in the acetabulum or femur secondary to trauma, tumor, pseudotumor, or wear-related osteolysis. Successful bone graft techniques rely on the three properties of bone: osteogenesis, osteoinduction, and osteoconduction.

Osteogenic graft material provides marrow-derived, preosteoblastic precursor cells and osteoblastic cells that directly produce new bone. Bone marrow aspirate and autogenous bone graft are considered osteogenic. Osteoinductive graft material contains noncollagenous bone matrix proteins, including growth factors, which induce mesenchymal stem cells to differentiate into chondroblasts and osteoblasts. Bone morphogenetic proteins (BMPs) are known for their osteoinductive properties. Osteoconductive graft materials serve as a 3-dimensionally ordered scaffold of bone mineral and collagen that facilitates structural ingrowth of capillaries, perivascular tissue, and mesenchymal stem cells. This property allows the formation of new bone in an organized fashion. Demineralized allograft bone is considered osteoconductive though is not as effective for osteoconduction as autograft bone.

Bone Grafting Options

Autograft Bone: General Biology

Autologous bone graft is bone that is taken from an individual and transplanted into another site of the same individual. Autologous bone graft has several advantages: it is osteogenic, osteoinductive, and osteoconductive; complete histocompatibility occurs without the potential for immune-related problems; and no risk exists for infectious disease transmission. As a result, autograft is considered the gold standard for bone graft and is the most favored graft material in musculoskeletal reconstruction. However, potential disadvantages to autograft use have been identified. These include limitations in amount of graft material, donor site morbidity, increased surgical time, and increased blood loss. Potentially significant donor site morbidity includes infection, pain, hemorrhage, muscle weakness, and nerve injury. Donor site morbidity is relatively common; complication rates from graft harvesting range from 10% to 25%.

Cancellous Autograft Biology

Cancellous autograft is highly osteogenic and promotes rapid revascularization and incorporation into host bone because it has a large surface area lined with osteoblasts. However, its poor initial mechanical strength often necessitates the use of early mechanical protection to allow graft-induced formation of new bone to provide early anchorage to the recipient bed.

In the primary phase of graft incorporation, immediately after bone grafting, inflammation and hematoma formation lead to coagulation. Infiltration of vascular buds into the recipient bed begins, and neovascularization promptly occurs over the next few days. In the first week, the graft is bathed in a soup of lymphocytes, plasma cells, osteoclasts, mononuclear cells, and polynuclear cells. Some early fibrous tissue forms. In the second week, fibrous granulation tissue is predominant, and osteoclastic activity increases. Macrophages arrive to remove necrotic debris within the haversian canals of the graft. Mediators released by the macrophages, combined with low oxygen tension and low pH, have a chemotactic effect on undifferentiated mesenchymal stem cells, which migrate from the host to the graft, subsequently repopulating the marrow. These primitive cells differentiate into osteogenic cells under the influence of osteoinductive agents such as cytokines, growth factors, and prostaglandins. By the fourth week, osteoblastic bone formation and osteoclastic resorption occur in a coordinated fashion.

In the secondary phase of graft incorporation, the osteoblasts cover the edges of the dead trabeculae and lay down osteoid around central cores of dead bone. This is followed by osteoclastic resorption, formation of hematopoietic marrow cells within the new marrow of the graft, and remodeling at the graft-host junction.

Radiographically, an initial increase in radiodensity is due to osteoblastic activity. This gradually decreases as necrotic bone undergoes osteoclastic resorption. Evidence of graft interdigitation can be seen radiographically by the blurring of margins between graft and host bone. This is due to bone remodeling at the graft-host junction, which may take several months to complete.

Biomechanically, cancellous autografts have increased initial mechanical strength as the result of new bone formation on a necrotic bed. Subsequently, resorption of the necrotic bed causes a decrease in strength. With remodeling of the graft-host junction, the strength of the interface between graft and host bone eventually improves.

Cortical Autograft Biology

Incorporation of nonvascularized cortical grafts involves slower revascularization and a lesser degree of osteoinduction than cancellous grafts (see Fig. 10.14 ). However, the process is similar during early stages of hematoma formation and inflammation. The cortical graft has a dense architecture that impairs revascularization and has fewer endosteal cells to promote neovascularization. Unlike cancellous autograft, incorporation is initiated by osteoclasts instead of osteoblasts.

At about 2 weeks after grafting, extensive resorption begins and increases until about 6 months before it gradually decreases to normal levels over the following 6-month period. Initial revascularization and resorption follow along peripheral haversian canals and interstitial lamellae. Resorption of inner cortical material occurs at a much slower rate. When the central osteonal canal reaches a critical size, osteoblastic new bone formation replaces osteoclastic bone resorption, which is further impaired by appositional bone growth. Eventually, creeping substitution occurs as the entire graft is resorbed and replaced by new living bone. Creeping substitution is most evident at the graft-host junction and progresses in a parallel manner along the longitudinal axis of the graft toward its middle.

Radiographically, increasing radiolucencies are seen at irregular peripheral bony margins between 6 months and 1 year. With continuing osteogenesis, a gradual increase in radiodensity occurs initially at the graft-host junction and progresses into the middle of the graft. The cortical autograft remains as a composite of necrotic and new bone for prolonged periods.

Biomechanically, the remodeling process initiated by osteoclastic activity leads to initial bone loss and a decrease in mechanical strength of up to 75%. In animal models, strength is largely restored by 6 weeks. In vascularized cortical autografts, the graft heals much more rapidly at the graft-host junction, as the remodeling process is similar to normal bone and virtually no residual weakness is noted.

Allograft Bone: General Biology

Because of limitations in the use of bone autograft, such as lack of availability and donor site morbidity, bone allograft has been used as an alternative. Bone allografting is the process by which bone is transferred from one individual to another individual within the same species. Most commonly, bone is procured from patients who underwent primary hip arthroplasty or postmortem from organ donors. It is available as fresh or preserved specimens. Preserved specimens are frozen or freeze dried. Fresh specimens elicit a higher level of immunologic response and are reserved for tumor reconstruction or joint resurfacing, when cartilage viability is important. Advantages of bone allograft include availability and avoidance of donor site morbidity associated with autograft harvesting. Disadvantages include lack of osteogenic cells, decreased osteoinductive factors, host immune response, and risk of infection. Morcellized and structural cancellous allografts are commonly used in revision hip arthroplasty to fill acetabular and femoral bone defects. Structural allografts are used primarily to support mechanical loads and resist failure at sites where structural support is desired.

Allograft Incorporation

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