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Definition

Partial or total failure of the graft to survive and to achieve osteogenesis, osteoinduction and/or osteoconduction at the recipient site

Risk factors

 Suboptimal handling of the graft

 Donor site selection

 Use of allografts or xenogafts

 Instability at recipient site

 Morbidity at the recipient site

 Fatigue failure of implants during healing in equine long bone fracture repair

Pathogenesis

Suboptimal handling techniques of the graft during harvest and implantation (prolonged harvest‐implantation time, exposure to air, saline, and antibiotics, and/or breach of aseptic technique) will have a negative effect on graft cell viability. Selection of the bone graft harvest site is chosen based upon quantity of graft material required, intraoperative access to donor site, and desire to minimize postoperative morbidity.

Autogenous cancellous bone graft is used most commonly in the equine patient but graft rejection resulting in nonunion, fatigue fracture and implant failure has been reported [6], and rejection will be more likely with use of allo‐ or xenografts.

The slow rate of fracture healing in the adult horse contributes to poor overall survival rates for adult equine fracture patients. Adult horses often require 4 to 6 months or longer for complete fracture healing, in comparison to canine patients, which may heal in 2 to 4 months [15, 23, 24]. See Chapter 46: Complications of Orthopedic Surgery, for further details. Instability at the fracture site as well as early postoperative complications such as incisional infection, dehiscence or osteomyelitis [3, 12, 13, 18, 25] will have a negative effect on graft survival.

Utilization of bone grafts in long bone fracture repair should contribute to decrease fracture‐repair failure as a result of implant fatigue, improving prognosis for equine fracture patients. While autogenous cancellous bone grafting enhances and stimulates bone healing, fatigue failure of implants during the healing process continues to be a major postoperative complication in equine long bone fracture repair [26, 28]. The osteogenic potential of equine autogenous cancellous bone graft from various donor sites including tuber coxae, sternum, proximal tibial metaphysis, and fourth coccygeal vertebrae has been investigated [15]. During the early stages of bone healing, new bone formation at the fracture site may result from viable graft cells or cells from the environment surrounding the graft [6, 28, 29]. Therefore, transplantation of viable osteogenic cells in bone graft or donor tissue is critical to early bone healing [10, 28, 30, 31]. When the host environment is traumatized, as with most adult equine fractures, new bone formation is a product of osteogenic cells from the graft bone that remain viable following transplantation [29].

Prevention

Optimizing transplantation of tissue from a donor site to yield a greater number of viable osteogenic cells should lead to greater new bone formation [15]. Results of comparison of osteogenic potential of donor sites revealed that the tuber coxae most consistently yielded viable osteogenic cells with an acceptable percentage of osteoprogenitor cells, while the sternum and tibia were less reliable in providing osteogenic cells [15]. Two additional donor sites have been examined; the fourth coccygeal vertebra and the tibial periosteum, were tissues with good osteogenic potential, and may be considered when the tuber coxae is not accessible or does not provide an adequate amount quantity of cancellous bone.

Autografts have greater osteogenic capacity in comparison to either allograft or xenograft, and are the most commonly used type of bone graft in equine surgery [1, 32–34]. The use of allografts would eliminate the need for a second surgery to harvest the graft, thereby reducing morbidity postoperatively. However, allogeneic bone demonstrates lower osteogenic capacity and therefore slower new bone formation and may be subject to rejection by the recipient immune system. Bone allografts are subject to the same immunologic factors as other tissue grafts [6]. The rejection of bone allograft is considered to be a primarily cellular immune response, although the humoral component of the immune system may play a role as well. Host response is related to antigen concentration and total dose. Rejection of bone allograft is observed clinically and histologically as an inflammatory process with callus bridging, nonunions, and fatigue fractures [6]. The use of allogeneic bone has declined in human medicine due to concern over the possibility of viral contamination of graft material and possible transmission of disease to graft recipients [35]. Xenogenic bone is not generally considered useful as an alternative to autogenous bone, as the antigenic response elicited upon grafting results in failure of the graft in the majority of cases [32]. Partial deproteination and defatting of xenograft have been shown to decrease the antigenic response, but this process also removes the majority of osteoinductive proteins [36].

Diagnosis

Graft rejection may be recognized clinically as a non‐union fracture, slow‐healing fracture or fatigue fracture. Histologically, evidence of an inflammatory process with callus bridging may be apparent.

Monitoring

Monitoring of graft acceptance in the recipient site may be monitored indirectly with radiographic and clinical signs indicative of fracture healing. Adult horses may require 4 to 6 months for complete fracture healing.

Treatment

In cases where non‐union fracture or graft rejection result in prolonged fracture healing, further surgical intervention may be indicated, depending upon the fracture configuration and intended use of the patient.

Expected outcome

Suboptimal or failure of osteoconduction, osteoinduction, and osteogenesis processes induced by the graft will lead to instability and prolonged fracture healing. Graft rejection resulting in nonunion, fatigue fracture and implant failure has been reported [6].The consequences will depend upon the location and condition that was being treated; unstable long bone fractures will have a poor prognosis associated with increased morbidity and mortality risk, while other locations may be associated with prolonged healing and site infection and/or suboptimal cosmetic outcome but survival of the patient.