The biology of bone grafts

The clinical outcome of bone grafting procedures depends on many factors, including type and fixation of the bone graft as well as the site and status of the host bed. Bone grafts serve one or both of two main functions, as a source of osteogenesis and as a mechanical support. Autografts, both cancellous and cortical, are implanted fresh, provide a source of osteoprogenitor cells, and are osteoinductive. The latter is a process whereby the transplanted tissue induces mesenchymal cells of the recipient to differentiate into osteoblastic cells. Cortical grafts, whether autogeneic or allogeneic, at least initially act as weight-bearing space fillers. All bone grafts are initially resorbed; cancellous grafts are completely replaced in time by creeping substitution, whereas cortical grafts remain an admixture of necrotic and viable bone for a prolonged period of time. The three-dimensional framework that supports invasion of the bone grafts by capillaries and osteoprogenitor cells, termed "osteoconduction", is another important function of both autografts and allografts. Because fresh allographs evoke both local and systemic immune responses that diminish or destroy the osteoinductive and conductive processes, freezing or freeze-drying of allografts is used clinically to improve incorporation. Graft incorporation is also influenced by the vascularity and composition of the host bed. Thus, the interaction of the host and the bone graft determines the success of these procedures, which ultimately is to provide a mechanically efficient support structure.     

Knochen-Tissue-Engineering in der klinischen Anwendung

Treatment of severe bone defects remains a challenge in orthopaedic surgery and traumatology. Surgical techniques should provide primary stability to reach osseous integration and secondary remodeling of bone grafts and substitute materials. None of the currently available substitute materials provides osteoconduction and osteogenesis comparable to those of human allografts and autografts. To enhance osteoinductive and osteogenetic properties of these implants mesenchymal stem cells are used successfully in bone tissue engineering approaches. The aim of this report is to summarize the currently available data on bone tissue engineering and preliminary experience with a tissue engineered graft in acetabular revision surgery after loosening of a hip replacement.     

Natural history of autografts and allografts

The clinical outcome of bone grafting procedures depends on many factors, including type and fixation of the bone graft as well as the site and status of the host bed. Bone grafts serve one or both of two main functions, as a source of osteogenetic cells and as a mechanical support. Autografts, both cancellous and cortical, are usually implanted fresh and are often osteogenetic, whether by providing a source of osteoprogenitor cells or by being osteoinductive. The latter is a process whereby the transplanted tissue induces mesenchymal cells of the recipient to differentiate into osteoblastic cells. Cortical grafts, whether autogeneic or allogeneic, at least initially act as weight-bearing space fillers or struts. All bone grafts are initially resorbed, but cancellous grafts are completely replaced in time by creeping substitution, while cortical grafts remain an admixture of necrotic and viable bone for a prolonged period of time. The three-dimensional framework, which supports invasion of the bone grafts by capillaries and osteoprogenitor cells, termed "osteoconduction", is another important function of both autografts and allografts. Fresh allografts are more slowly and less completely replaced by host bones because they invoke both local and systemic immune responses that diminish or destroy the osteoinductive and conductive processes. Although freezing or freeze-drying of allografts improves acceptance, their failure rate is still too high. These processes are also influenced by the vascularity and composition of the host bed. Thus, the interaction of the host and the bone graft determines the success of these procedures, which ultimately is to provide a mechanically efficient support structure.     

Incorporation of impacted morselized bone allografts in rabbits

Morselized bone allografts have been used for the treatment of bone stock loss in orthopedic revision surgery with encouraging results. However, several parameters can influence the graft incorporation including the processing treatments. This experimental work used a cavitary bone defect in 90 rabbits to evaluate the sequence of incorporation of three different kinds of morselized bone allografts: uncryopreserved cancellous bone, freeze-dried cancellous bone, and totally demineralized cortical bone each of which were prepared in accordance with our rigid protocol. Revascularization and remodeling of the transplanted bone grafts were evident upon histological evaluation. Bone apposition and bone resorption resulted in a mixture of graft and new bone. Mineralized cancellous grafts showed great osteoconductive capacity, whereas demineralized cortical grafts showed an intense osteoinductive capacity and a weak osteoconductive capacity. In a general evaluation, cryopreserved cancellous bone grafts showed superior biological efficacy for reconstruction of experimental bone defects, closely followed by freeze-dried cancellous bone grafts, and, finally, by demineralized cortical bone grafts. These results validate our protocol for the processing and preservation of these three kinds of bone grafts.     

Quantitative comparisons of healing in cranial fresh autografts, frozen autografts and processed autografts, and allografts in canine skull defects

Adult dog skull defects larger than 17 mm do not spontaneously heal. A quest for a potentially viable, cosmetically, mechanically, and technically acceptable template for human cranial reconstruction prompted a comparison of processed autogeneic and allogeneic bone implants with a fresh autograft control in the dog. Quantitative reproducible observations demonstrated that fresh calvarial bone autografts were superior to the nonviable implants in volume percent defect filled, mm2 new cortical bone, mm2 new and old cortical bone, and cortical bone porosity. Frozen autografts achieved 75%, antigen-extracted, autolyzed, partially demineralized auto- and allografts, 50% of the overall efficiency of fresh autografts. Fresh cancellous bone added to allografts did not improve long-term repair. Remodeling of all grafts appeared consistent with osteoconductive invasion by peripheral host endosteal and diploic elements; host external periosteum and dura contributed less. Central osteoinductive recruitment of mesenchymal cells from muscle or dura seemed not to occur in the adult dog. Partially demineralized dog calvarial grafts were resorbed without acting as a template for new bone formation. Surface demineralization, antigen extraction, and autolytic digestion of autografts and allografts, with or without fresh iliac bone, did not improve calvarial bone regeneration in adult dogs.     

Bone graft materials. An overview of the basic science

Autograft, allograft, and synthetic bone graft substitute materials play an important role in reconstructive orthopaedic surgery, and understanding the biologic effects of these materials is necessary for optimum use. Although vascularized and cancellous autograft show optimum skeletal incorporation, host morbidity limits autograft availability. Experimental studies have confirmed an immune response to allograft bone, but the clinical significance of this response in humans still is unclear. Small amounts of cancellous allograft in humans usually are remodeled completely; large allografts become incorporated by limited, surface intramembranous bone formation suggesting that these graft are primarily osteoconductive. Several synthetic skeletal substitute materials also are osteoconductive, and may show remodeling characteristics similar to allograft. Demineralized bone matrix and some isolated or synthetic proteins can induce endochondral bone formation, and therefore are osteoinductive. The extent and distribution of remodeling of bone graft materials are influenced by many factors, including the quality of the host site and the local mechanical environment (strain). Graft materials are likely to become more specialized for use in specific clinical applications, and composite preparations may soon provide bone graft materials with efficacy that equals or exceeds that of autogenous grafts.     

The biology of bone graft repair

Cancellous and cortical autografts histologically have three differences: (1) cancellous grafts are revascularized more rapidly and completely than cortical grafts; (2) creeping substitution of cancellous bone initially involves an appositional bone formation phase, followed by a resorptive phase, whereas cortical grafts undergo a reverse creeping substitution process; (3) cancellous grafts tend to repair completely with time, whereas cortical grafts remain as admixtures of necrotic and viable bone. Physiologic skeletal metabolic factors influence the rate, amount, and completeness of bone repair and graft incorporation. The mechanical strengths of cancellous and cortical grafts are correlated with their respective repair processes: cancellous grafts tend to be strengthened first, whereas cortical grafts are weakened. Bone allografts are influenced by the same immunologic factors as other tissue grafts. Fresh bone allografts may be rejected by the host's immune system. The histoincompatibility antigens of bone allografts are presumably the proteins or glycoproteins on cell surfaces. The matrix proteins may or may not elicit graft rejection. The rejection of a bone allograft is considered to be a cellular rather than a humoral response, although the humoral component may play a part. The degree of the host response to an allograft may be related to the antigen concentration and total dose. The rejection of a bone allograft is histologically expressed by the disruption of vessels, an inflammatory process including lymphocytes, fibrous encapsulation, peripheral graft resorption, callus bridging, nonunions, and fatigue fractures.     

Bone graft for revision hip arthroplasty: biology and future applications

Revision total hip arthroplasty often presents surgeons with difficult bone loss problems. The selection of an appropriate bone graft is influenced by the size of the bone defect, the location, the biology of the bone graft site, and whether the graft is required for structural support. Autogenous bone graft remains the gold standard bone graft material but there only is a limited amount available and there is morbidity associated with the harvesting of these grafts. The most frequently used bone graft materials include autogenous iliac crest bone graft, cancellous allograft chips, demineralized bone matrix, and bulk structural allografts (femoral head, distal or proximal femoral allograft, whole acetabuli, and femoral strut grafts). It often is difficult to determine on plain radiographs whether nonstructural grafts actually incorporate into the host bone. Recently, attention has focused on the use of new materials for bone grafting including: new demineralized bone matrices, ceramics, autologous platelet concentrates, recombinant proteins, and stem cells. The purpose of this review was to assess the biologic potential of these nonstructural grafts in revision hip arthroplasty and their limitations. Are these agents actually incorporating into host bone? Future bone graft options, including tissue engineering and gene therapy will be discussed briefly.     

The use of bone graft substitutes in large cancellous voids: any specific needs?

Bone graft is the second most common transplantation tissue, with blood being by far the commonest. Autograft is considered ideal for grafting procedures, providing osteoinductive growth factors, osteogenic cells and an osteoconductive scaffold. Limitations, however, exist regarding donor site morbidity and graft availability. Allograft on the other hand poses the risk of disease transmission. Synthetic graft substitutes lack osteoinductive or osteogenic properties. Composite grafts combine scaffolding properties with biological elements to stimulate cell proliferation and differentiation and eventually osteogenesis. We present here an overview of bone graft substitutes available for clinical application in large cancellous voids.     

Cavitary acetabular defects treated with morselized cancellous bone graft and cementless cups

The use of impacted morselized cancellous bone grafts in conjunction with cementless hemispherical acetabular cups for treatment of AAOS type II acetabular cavitary deficiencies was evaluated in a retrospective study of 23 primary and 24 revision total hip arthroplasties, at a mean follow-up of 7.9 and 8.1 years, respectively. All primary hips received autografts, while all revision hips received allografts. Modified Harris Hip Scores for primary and revision hip replacements increased from a pre-operative mean of 37 and 47 to a postoperative mean of 90 and 86, respectively. All 23 autografts and 23 out of 24 cancellous allografts were radiographically incorporated without evidence of resorption. There were no instances of infection, component migration, or cases requiring subsequent acetabular revision. We conclude that impacted morselized cancellous bone-graft augmentation of cementless cups is a viable surgical option for AAOS type II cavitary acetabular defects.     

Microsurgical free bone transfer in the dog

Bone grafts have been used extensively for the reconstruction of skeletal injury in dogs. Cancellous and corticocancellous chip autografts are used for their osteoinductive and osteoconductive properties. Fresh, fresh-frozen, freeze-dried, and ethylene oxide-sterilized cortical allografts have been recommended for the reconstruction of segmental cortical defects. Donor sites for microsurgical bone transfer have been identified experimentally in the dog, but these remain to be used clinically. This paper reviews the anatomy and structural characteristics of donor sites for microsurgical bone transfer. Potential clinical indications for microsurgical bone transfer in the dog are described.     

Autogenous bone grafts: nonvascular and vascular.

Nonvascularized autogenous bone grafts have been used for at least 150 years. Vascularized bone grafts were first done only 17 years ago. We now know that bone grafts act through osteoconductive and osteoinductive mechanisms to produce new bone. Autogenous bone grafts remain the gold standard against which allografts and artificial graft materials should be measured.     

Bone substitutes: an update

Autograft is considered ideal for grafting procedures, providing osteoinductive growth factors, osteogenic cells, and an osteoconductive scaffold. Limitations, however, exist regarding donor site morbidity and graft availability. Allograft on the other hand, posses the risk of disease transmission. Synthetic graft substitutes lack osteoinductive or osteogenic properties. Composite grafts combine scaffolding properties with biological elements to stimulate cell proliferation and differentiation and eventually osteogenesis. We present here an overview of bone grafts and graft substitutes available for clinical applications.     

Viability of impacted bone allografts under metal mesh at the calcar in revision surgery of the hip

Metal meshes are used in revision surgery of the hip to contain impacted bone grafts in cases with cortical or calcar defects in order to provide rotational stability to the stem. However, the viability of bone allografts under these metal meshes has been uncertain. We describe the histological appearances of biopsies obtained from impacted bone allografts to the calcar contained by a metal mesh in two femoral reconstructions which needed further surgery at 24 and 33 months after the revision procedure. A line of osteoid and viable new bone was observed on the surface of necrotic trabeculae. Active bone marrow between these trabeculae showed necrotic areas in some medullary spaces with reparative fibrous tissue and isolated reactive lymphocytes. This is interpreted as reparative changes after revascularisation of the cancellous allografts. These pathological findings are similar to those reported in allografts contained by cortical host bone and support the hypothesis that incorporation of morcellised bone under metal meshes is not affected by these devices.     

Bone grafting and substitutes in spine surgery

The increase of spinal procedures over the last decades has made the long-term problems, such as pseudarthrosis, apparent. This demands optimized strategies, techniques and technologies. Modern fixation systems have been developed as an adjunct to spinal fusion, and several generations of different synthetic cages have proved to be reasonable alternatives to autologous bone or allografts. The development of recombinant bone morphogenetic proteins (BMPs) is of promise, because of their great osteoinductive capabilities. While spine surgeons are familiar with autologous and synthetic grafts as well as allografts, these comparably new evolving growth-factor-based technologies are of high interest. This was a selective literature review. Alternatives to autologous grafts include allograft bone, synthetic cages and growth-factor based bone substitutes, BMPs being the most-studied among them. Autologous iliac crest alone provides all of the required capabilities of an ideal bone graft, i.e. osteoconduction, osteoinduction, osteogenesis, but each of the alternatives can produce excellent results in a number of indications. If combined, these alternatives can cumulatively provide all required graft capabilities. Nevertheless, all of the available grafts have specific characteristics and can feature certain complications. Alternatives to autologous grafts circumvent donor-site morbidity and are available in a larger amount than autologous bone for extensive surgery. New technologies offer excellent possibilities of new bone formation, but there are also severe risks and high costs to be considered. The indication for bone grafting must be clearly defined, the graft selection should be individually adapted, and the risks, efforts and costs of the selected fusion procedure should be carefully considered.     

Morphometric and physical investigations of segmental cortical bone autografts and allografts in canine ulnar defects.

Cortical bone grafts were implanted for six months in mature dogs using an osteoperiosteal 3-cm defect in the ulna to evaluate their respective morphometric and physical values compared with autografts. The bone-grafting material included fresh auto- and allografts, frozen and thimerosal preserved allografts, and partially demineralized bone allografts. The grafts were evaluated by roentgenograms, microradiograms, photon absorptiometry, porosity, fluorescence labeling measurements, and torsional loading at failure. Autografts achieved a better union score than the allografts, but intracortical bone porosity, percentage of cumulative new bone, and mineral apposition rate were not variables with statistical significance. Lamellar bone was found earlier and in greater quantity in autografts. Within the graft, new bone was deposited at a slower rate than in the recipient bone. Autografts showed less peripheral resorption and a greater torsional resistance than allografts. Photon absorptiometry demonstrated that nondemineralized allografts underwent a substantial loss of peripheral bone. This marked reduction in the outer diameter of the graft had more influence on torsional resistance than did the intracortical porosity of the graft. Demineralized allografts were osteoinductive in only 28% of the cases and appeared to respond in an all-or-nothing pattern. Frozen and thimerosal preserved allografts were the most acceptable substitutes to autografts.     

Rinsing morselized allografts improves bone and tissue ingrowth

Bone defects in revision hip surgery can be reconstructed with impacted morselized bone grafts. Rinsing these trabecular allografts may enhance graft incorporation by washing out immunogenic factors present in blood, marrow, and fat. However, it has been proposed that impaction of the graft releases biologically active factors, which can provide sufficient activity to stimulate new bone formation. Rinsing before impaction could enhance bone allograft incorporation, but rinsing after impaction could diminish the incorporation process of impacted bone graft. To study the effect of rinsing and impaction of morselized bone grafts on bone ingrowth, a bone chamber study was done in goats. Autografts and allografts were divided into three treatment groups: (A) impacted; (B) rinsed and impacted; and (C) rinsed, impacted, rinsed, and impacted again. Ten goats received three bone chambers in each proximal tibia. The chambers were filled with either allograft or autograft, yielding six different implants per goat. After 6 weeks, histologic analyses were done and bone and tissue ingrowth were measured. New bone and total tissue ingrowth were higher in autografts than in allografts, especially in the nonrinsed group. With rinsing, total tissue ingrowth increased in the allograft group to approach that of autografts. Rinsing after impaction did not additionally alter bone ingrowth. The current findings show that incorporation of allografts can be improved by rinsing the grafts before impaction.     

Acetabular bone stock reconstruction with frozen femoral head allografts and the use of a cementless screw cup in total hip revision surgery. A 5 years' clinical, radiologic and scintigraphic follow-up

This prospective study reviews the behaviour of a cementless conical screw cup in total hip revision surgery with or without the use of bone grafts to reconstruct acetabular bone stock. Clinical and radiologic results are given. For proper radiographic interpretation a new classification system is suggested, based on histologic features, the principles of which are explained. The results of auto- or allografts for the treatment of the same pathological condition are compared. The difference between both grafts successively in bone incorporation, biochemical behaviour, morbidity and mortality rate is discussed. The difference between a cancellous and a cortical bone transplant in the mechanism of repair is discussed. This study also analyzes acetabular bone graft activity using serial technetium-99m bone scans. Some guidelines for rehabilitation programs can be deduced from these observations. Finally, a correlation between graft incorporation and the acetabular deficiency to be reconstructed can be postulated.     

Different healing rates of bone autografts, syngeneic grafts, and allografts in an experimental rat model

Matching of donors and recipients for tissue antigens is vitally important for successful transplantation of essentially all organs and tissues, the major exception being bone. The importance of tissue-typing for the healing of bone allografts remains, however, a controversial issue as development of both humoral and cell-mediated immunity against the grafted bone has been observed in some experimental systems. In the present study, we compared the healing patterns of frozen antigen-mismatched allografts, frozen antigen-matched allografts (syngeneic grafts), and fresh cortical bone autografts in an experimental rat model. Histomorphometry of the graft-host interface revealed that new bone formation started significantly earlier in autografts than in allografts or syngeneic grafts. By 2 weeks, the level of new bone formation in the syngeneic grafts had reached that in autografts. Antigen-mismatched allografts, however, continued to exhibit a retarded formation of new bone throughout the union process. These histomorphometric observations were confirmed by molecular biologic analyses for the mRNA levles of type I collagen, which increased earlier and reached a higher level in autografts than in allografts. Use of syngeneic grafts resulted in a longer persistence of type I collagen mRNA expression in the healing tissue than in antigen-mismatched allografts. No apparent differences were seen between allografts and autografts in the expression of type III collagen. No cartilage-specific type JI collagen mRNA was observed, indicating that antigen-mismatching or preservation by freezing did not alter the basic mechanism of the interface healing process, although it did slow down the beginning of the process. The experiments suggest that a major antigen mismatch between donor and recipient affects the temporal gene expression of extracellular bone matrix and delays new bone formation at the graft-host interface of cortical bone allografts.     

Bone grafting: role of histocompatibility in transplantation

The role of histocompatibility matching in bone allografting was studied in two canine bone graft models. In a cancellous ulnar segmental replacement model, frozen bone allografts exchanged between closely matched dogs were significantly better incorporated by radiographic and histologic criteria than were strongly incompatible grafts. Frozen allografts from disparate donors in recipients receiving immunosuppression appeared indistinguishable 6 months later from those in the untreated closely matched groups and from fresh autografts. Fresh vascularized orthotopically placed fibular bone grafts were evaluated by quantitative blood flow assessment, microangiography, and fluorochrome histomorphometry. Revascularized grafts exchanged between untreated closely matched dogs demonstrated preservation of blood flow and a pattern of repair that was delayed but not otherwise different than vascularized autografts. These results suggest that fresh vascularized grafts in the judiciously matched or immunosuppressed recipient offer attractive clinical possibilities.