The bone was one of the first tissue transplants performed and remains one of, if not the most, abundant tissues transplanted. Originally, autograft was used to unite fractures and fuse joints . Allograft use increased in prevalence in orthopaedic oncology as limb-salvage techniques improved. In revision hip and knee surgery, allograft bone is used when conventional methods of reconstruction are inadequate and because autograft is in short supply. As a tissue, bone has unique properties that are critical for success. Osteogenesi s is the ability to produce a new bone and is accomplished by osteoblasts. Bone proteins, such as bone morphogenic protein that are a part of the bone matrix, stimulate new bone formation through the recruitment and differentiation of pluripotent mesenchymal stem cells into osteoblasts. This characteristic is known as osteoinduction . Lastly, osteoconduction is the graft’s ability to act as scaffolding for the ingrowth of blood vessels and cells from the host bed. The process known as creeping substitution is the gradual resorption and replacement of this scaffolding with the host bone. Autograft and fresh allograft possess all of these properties. Fresh allografts, however, are rejected by the host immune system, resulting in complete graft resorption or marked delay in incorporation. Therefore, allograft used in revision joint surgery is processed and possesses the property of osteoconduction only. The success of allograft depends largely on its ability to heal to and incorporate with the host bone. Histologically, these events are similar to fracture healing. Inflammation predominates early on. Unlike autograft, in which surviving surface osteoblasts contribute bone, allograft incorporation depends on osteoblasts differentiated from pluripotent cells brought in by vasculature from the host bed. Thus, the process is similar to autograft incorporation but slower. This early phase is similar for the cancellous as well as cortical bone. Creeping substitution characterizes the incorporation of cancellous bone. That is, bone formation and resorption occur concomitantly. Eventually the entire graft may be replaced by the host bone. In the cortical bone, formation only occurs after resorption. Consequently, the graft is weaker than the normal bone for a long period of time and must be protected from excessive loading. In theory, this remodeling process eventually involves the entire structural graft. In reality, these grafts have little biologic activity outside of the graft-host junction [12].

Although animal studies have supplied most of our knowledge of the basic science of allograft, human retrievals have given the most insight into the biologic behavior of processed allografts in humans. Enneking et al. studied 16 retrieved massive human allografts that had been in situ for 4–65 months [13]. They demonstrated that union between allograft and host took place slowly at cortical-cortical junctions and more rapidly at cancellous-cancellous junctions. Internal repair was confined to the superficial surfaces and ends of the grafts and had involved only 20% of the graft by 5 years. The deep portions of the graft retained their architecture. Parks and Engh’s study of allografts in revision knee arthroplasty retrievals had similar findings with no evidence of revascularization, resorption, or remodeling beyond the graft-host union [14] (Figs. 8.3 and 8.4).

Ultimately, the biology depends greatly on the clinical situation and the type of graft used. As discussed, bone loss in revision surgery can be cavitary or segmental . A cavitary lesion with a well-vascularized bed is ideal for the cancellous bone, and complete incorporation is to be expected. With increasing bone loss and decreasing vascularity, a more inconsistent incorporation is to be expected. In contrast, segmental loss requiring large structural allograft relies on cortical-to-cortical contact between host and graft. The majority of the graft is surrounded by soft tissue that is usually avascular scar. Here the allograft can unite to the host bone, but there will be little if any internal remodeling of the graft [13].

In reviewing the biology of allografts, we see that union with the host bone is the first step toward success. Unfortunately, failure can and does still follow all too often. Fracture of structural allograft is reported to be as high as 16.5% [15]. It goes without saying that the biomechanical behavior of the graft is of critical importance in determining success or failure. The individual factors that influence the physical properties of transplantable bone are analyzed in this section.

The ability of a graft to withstand loads is largely determined by the original properties of the bone at the time of donation. Although supply often limits surgeons’ options when choosing donor material, the factors that influence these properties should be known. For instance, bone tissue is strongest in the 20- to 39-year-old age groups and typically weakens thereafter. However, even in the 70- to 79-year-old age group, 70% to 85% of the maximum strength is maintained [16]. The surgeon can more closely control other factors, such as the method of preservation and sterilization.

The more common methods of preserving and storing specimens until they are required for implantation are freezing and freeze-drying . Both alter the immunogenicity of the graft, but freeze-drying has a more substantial effect on the physical properties [16, 17]. Freeze-drying causes little change or a slight increase in compressive strength but lowers the bending and torsional strength substantially [15, 18, 19]. Cracks have been observed in rehydrated freeze-dried specimens, which might explain the observed reduction in strength [18]. Freezing alone has little if any effect on the physical properties of the bone [18, 20]. These observations suggest that fresh frozen bone would be best when large torsional and bending loads can be expected. Clinically, this would be seen at the host-graft junction when a whole distal femur was used. Conversely, in a situation in which the graft will see primarily compressive loads, freeze-dried graft should be biomechanically sound. Most cavitary or isolated metaphyseal lesions fall into this category.

Sterilization of a graft prior to implantation can be done either of two ways. The grafts can be sterilely harvested and stored, or nonsterile grafts can be secondarily sterilized with high-dose radiation. Radiation below 3 megarads appears to cause little change in bone strength; however, above this level, significant alterations in the physical properties occur, resulting in a decrease in the compressive, bending, and torsional strength of the graft. These effects are magnified when combined with freeze-drying [19, 20].

Once retrieved, preserved, stored, sterilized, and implanted, a bone graft is subjected to load. The bone can fail under the single application of a large load if a fall or some other trauma ensues. However, fatigue failure secondary to repetitive smaller loads is more common with large allografts. Live bone is capable of remodeling when subjected to these loads. Until transplanted bone becomes vascularized, it does not have this capability. Because retrieval studies have shown that outside of the host-graft junction little remodeling occurs, it is imperative that large allografts be protected with adequate internal fixation to prevent fatigue failure. Intramedullary fixation with stemmed components is preferred over plates and screws because the stress risers made by screw holes weaken the graft, thus increasing the fracture risk.

Although extremely rare, transmission of an infectious agent through allograft bone transplantation remains a relevant concern. Most of this has centered on transmission of human immunodeficiency virus (HIV). The risk estimate for HIV transmission in 1990 was 1 in 1.6 million [21, 22]. With improved screening tools and sterilization methods and stricter donor criteria, this risk may be even less today. In the early 2000s, disease transmission from allograft bone received significant attention after a Minnesota man died from a Clostridium infection 4 days after an osteochondral transplant, and the Centers for Disease Control (CDC) uncovered 26 cases of infection from orthopaedic allograft transplants related to poor techniques in procurement, processing, and storage. Utilizing a tissue bank that is a member of the American Association of Tissue Banks reduces this risk of bacterial contamination , but these risks should be considered when counseling patients on surgical options.

Bone deficiency in many revisions is minor and contained. After component removal, bone loss is limited to punctate cancellous defects . Minor defects have been defined differently in terms of size. In general, it is assumed that cancellous metaphyseal bone is in sufficient supply and quality to support primary implants. In these cases, defects can be filled with cement, particulate autograft from bone cuts, or particulate allograft if autograft supply is insufficient. Outcome will be similar regardless of management.

For smaller defects under 5 mm in depth that remain contained, the use of cement can be utilized to successfully fill the void [23]. This has the advantage of being readily available, given the fact that majority of revision implants are cemented. Additionally, the use of cement to fill bony contained defects can be supplemented by the use of cortical screws to enhance the strength of the construct. However, in a young patient, filling the bony defect with morselized allograft can restore bone stock facilitating any future revision that might take place.