Читать книгу Complications in Equine Surgery - Группа авторов - Страница 159

Bone grafts are most frequently used in equine surgery to facilitate healing following long bone fracture, arthrodesis, and comminuted phalangeal fractures [1]. Autogenous cancellous bone grafts are the most frequently used type of graft in the equine patient [1, 2, 3]. Bone grafts may be categorized according to their origin and location. Grafts may be harvested and applied to a different site in the same individual (autograft), to a genetically different individual of the same species (allograft), or to a member of a different species (xenograft). Grafts may be applied to an anatomically similar location (orthotopic) or different implantation site (heterotopic).

In general, bone grafts serve the function of osteoconduction or osteoinduction, depending on the type of graft [1, 4]. Osteoconduction refers to the matrix of the graft acting as a scaffold into which mesenchymal cells grow. Osteoinduction refers to a process through which signals are sent to influence new bone formation (osteogenesis) as a result of differentiation of mesenchymal cells or recruitment of viable osteoblasts and osteocytes on the surface of the bone graft. Bone morphogenetic proteins (BMPs) are molecules found within bone marrow and are responsible for signaling the differentiation of mesenchymal cells into cartilage and bone [5]. Their presence is thought to play an important role in this process.

Grafts may be composed of entirely cancellous or cortical bone, or a combination of both types. Cancellous bone grafts have been shown to have osteogenic, osteoinductive, and osteoconductive properties [1, 5, 6]. Cancellous and cortical autografts differ histologically in three respects: 1) cancellous grafts revascularize more rapidly and completely; 2) creeping substitution of cancellous bone involves an appositional bone formation phase followed by a resorptive phase; and 3) cancellous grafts repair completely with time, while cortical grafts remain admixtures of necrotic and viable bone [6]. All grafts are eventually replaced with host tissue by a process called creeping substitution, which is defined as remodeling by osteoclastic resorption and creation of new vascular channels with osteoblastic bone formation [4, 5, 7].

The successful integration of a bone graft depends on the interaction of six factors: 1) host bed; 2) viability of the bone graft; 3) volume of bone to be grafted; 4) growth factor activity of the host be; 5) metabolic activity index; and (6) homostructural function of the bone graft [1, 8, 9]. The condition of the host bed in terms of local blood flow, stability and bone marrow activity determines acceptance of the graft. Cancellous and vascularized corticocancellous bone grafts are more viable and have greater rates of acceptance in comparison to grafts with reduced vascularity. Larger volumes of bone graft take longer to be incorporated into host tissue, and therefore have a greater likelihood for development of complications such as nonunions or fatigue failure. Proliferation of perivascular connective tissue in the host bed, which facilitates osteogenesis, is induced by growth factor activity. The metabolic activity index (MAI) is correlated to the capacity of the host bed to incorporate bone grafts and repair fractures. The MAI is determined by heart rate, blood flow, metabolic rate, respiratory rate and body temperature. The MAI of horses has not been determined, but is extrapolated from other species [9]. The homostructural or support function of the bone graft influences incorporation of the bone graft. Complete incorporation of the graft into host tissue may take years [9].