Introduction

Bone grafts are an essential part of veterinary orthopedic surgery. They aid in the healing of acute fractures, joint arthrodeses, and revision of non‐ or delayed union fractures. Bone grafts can either be categorized depending on their composition (cancellous, cortical, or corticocancellous) or by their origin (autograft, allograft, or xenograft). Autografts are bone grafts harvested from a donor site on the patient and then transferred to another surgical site on the same patient. These are the most used forms of graft in veterinary medicine. Allografts are bone grafts harvested from another patient of the same species, and these grafts are becoming more common with the ongoing development of veterinary transplantation services. Xenografts are grafts from a different species and are the least common in veterinary medicine. Bone grafts enhance bone healing by providing a scaffold for bone regeneration and a source of bone‐forming cells, growth factors, and/or minerals that promote new bone formation. Bone regeneration is a complex process, and there are four main tenets of bone regeneration, which are osteogenesis, osteoinduction, osteoconduction, and osteopromotion. Understanding each of the tenets or grafting properties allows the clinician to understand the benefits gained from the various products, so they can choose the graft material that is most appropriate for their case.

1. Osteogenesis. Osteogenesis is the formation of new bone tissue by osteoblasts. Osteoblasts synthesize and deposit the extracellular matrix of bone, consisting mainly of collagen and other proteins. Grafts that address this tenet include fresh autogenous bone grafts and bone marrow aspirates.
   
2. Osteoinduction. Osteoinduction is the process by which undifferentiated cells are stimulated to differentiate into osteoblasts, promoting bone formation where otherwise no bone formation would occur. Certain biological substances, such as bone morphogenetic proteins (BMPs), have osteoinductive properties.
   
3. Osteoconduction. Osteoconduction involves providing a scaffold or framework that allows the ingrowth of new blood vessels, cells, and tissues to promote bone regeneration. Osteoconductive materials can be synthetic (such as porous bioceramics) or natural (such as the trabecular network of an allogenic bone graft) that serve as a scaffold for new bone formation.
   
4. Osteopromotion. Osteopromotion involves creating an optimal environment that enhances bone regeneration. It differs from osteogenesis and osteoconduction, as no cells or scaffold are added to the area to stimulate bone regeneration, and differs from osteoinduction, as placement of an osteopromotive substance will not result in bone formation when placed in a region without a healing fracture. Examples include platelet rich plasma (PRP) and biphasic calcium phosphate.^1,2

The choice of bone graft depends on the procedure being performed, the patient’s bone quality, the size and location of the bone defect, and the surgeon’s preference. This chapter discusses the benefits of bone grafts, the available types of bone grafts, and the common locations for harvesting bone grafts in canine patients.

Types of Bone Grafts

Autogenous Cancellous Bone Grafts

Autogenous cancellous bone grafts are pieces of cancellous bone harvested from a donor site on the patient with a reliably large volume of cancellous bone and then transferred to the surgery site. Cancellous autografts are preferred clinically because they have osteogenic, osteoinductive, osteoconductive, and osteopromotion qualities. Cancellous autografts are commonly used in cases when healing will be delayed (e.g., elderly patient), in patients with delayed or nonunions, and in cases where quick healing is of benefit (such as arthrodesis or in patients with expected poor postoperative compliance). In fact, cancellous autograft has been found to increase the rate of bone healing by up to four weeks when used in pancarpal arthrodesis.³ The most common locations for harvesting cancellous bone grafts in canine patients include the proximal humerus, proximal tibia, and dorsal crest of the ilial wing. The timing of graft collection should be as close to its application to the fracture site to maximize the number of viable cells present within it. The graft should be stored appropriately to prevent necrosis of the viable cells. Options for graft storage between harvest and use include within the barrel of a 5 or 12 mL syringe (Figure 54.1) or, alternatively, within a saline‐moistened sterile container (Figure 54.2). The graft should be kept moist with either saline or blood to prevent desiccation of the sample, as desiccation will lead to rapid loss of viable cells within the graft.

The clinician should be cautious in cases that are being revised for infection that gloves and instruments are changed between the primary surgery site and graft collection to prevent the seeding of infection to the graft site. Another tip is to remember to copiously lavage the recipient site prior to application of the graft, since flushing after graft placement would be detrimental. Complications following autogenous cancellous bone grafting in small animals are uncommon but have been reported to include bone fracture and early growth plate closure of the graft site.^4,5 Clinicians should be cautious when harvesting bone grafts in immature patients due to this. In addition to the potential for complications from cancellous bone graft harvest, other disadvantages include the increased surgical and anesthesia time to harvest and the low yield from small or geriatric patients.