the theoretical use of stem cells to augment fracture healing, but critical experimental and clinical work around the best cell type, application method and matrix carrier is needed before recommendations can be made [71].
Growth factors have been used in human, and in some cases of equine, fracture repair. BMP has been most highly studied to date. BMP2 is known to stimulate progenitor cells to differentiate into osteoblasts, and BMP7 is known to stimulate angiogenesis. There is good clinical information in humans, although ectopic bone formation and high price are negative factors to its use [72]. In an experimental equine model, no significant increase in healing from BMP2/7 gene therapy application was seen [73]. FGF has also been shown to stimulate progenitor cell differentiation and angiogenesis. VEGF is initially released from the haematoma and promotes development of endothelial cells and vascular invasion and may be secreted by chondrocytes within callus to stimulate angiogenesis and new bone formation. Parathyroid hormone (PTH) has been shown to produce increased bone mass, bone strength and reduced bone loss, especially in cases of osteoporosis. Although PTH can stimulate fracture healing, it does not appear to act as a differentiation factor and may not be effective if the early stages of fracture healing are not optimized. Exogenous PTH has been found to be safe in horses [74]. PTH has also been implanted within a fibrin matrix into an equine subchondral bone cyst with a positive effect [75].
Overall, the use of exogenous growth factors appears logical, but a cost versus potential benefit debate is necessary before deciding on use.
Bone Grafts
Bone grafts can provide both mechanical support and enhanced osteoregeneration. Different graft types work in differing ways. The three principal properties are osteoconduction, osteoinduction and osteogenesis. Osteoconduction refers to the ability to support attachment of osteoprogenitor cells and allow migration and growth within the three‐dimensional architecture of the graft. Osteoinduction occurs when the graft itself can induce progenitor cells to develop into bone forming cells. Osteogenesis is defined as osteodifferentiation and new bone formation by donor cells derived from the host or the graft [76].
Autogenous bone grafts are most commonly used in all species. In humans, it is second only to blood transfusions as the most common tissue transfer, although there is a search for non‐autogenous bone regenerative products because of the limited supply and morbidity caused by acquisition. Horses tend to have a large supply of suitable autogenous bone, and morbidity at the retrieval site is usually minimal. Autogenous bone is the gold standard by which all other graft products are compared, and it is still the most effective for inducing bone healing. Autogenous bone grafts provide all three methods of function (osteoconduction, osteoinduction and osteogenesis). They can be either cortical, cancellous or vascularized. Autogenous cancellous bone graft is most commonly used. In horses, it is typically acquired from the tuber coxae and packed into the fracture gaps at the time of surgery. Alternative sites include the proximal tibia, humerus and sternebrae. Site is dictated by patient recumbency and surgeon preference. Autogenous cancellous bone graft contains mesenchymal stem cells, matrix proteins and a large surface area which stimulates vascularity and host integration. The graft helps form the haematoma, initial inflammation and granulation tissue bed. Neovascularization is stimulated, and osteoid forms around some of the tissue. Autogenous vascularized cortical grafts provide vascular integrity so that the grafted bone can remodel and heal. They are rarely, if ever used in horses. Autogenous cortical bone grafts are rarely used in horses and can only be incorporated through creeping substitution via osteoclastic function and long‐term osseous integration. Osteochondral grafts are commonly used in humans and occasionally in horses [77].
Allogenic grafts are acquired from a different animal of the same species with the benefit that they can be acquired, stored and used off the shelf. Allogenic bone grafts can be in the form of cancellous chips, cortical bone segments (which are used in humans to provide mechanical stability), osteochondral grafts or decalcified bone matrix. However, allogenic grafting triggers an immune response, and the consequential increased inflammatory phase may impede healing. They are rarely used in horses.
Decalcified bone matrix has been used for decades in the human field but showed no positive effect on healing in an experimental equine study [78]. Decalcified bone matrix is prepared by eliminating potential allogenic substances through a decalcification procedure that maintains the non‐calcified bone matrix including potential growth factors and allows for osteoconduction. Decalcified bone matrix is rarely used in equine fracture repair but may have a place in fractures with large residual defects [76].
Synthetic Bone Substitutes
Because of limited availability and morbidity associated with cancellous bone grafts in humans, bone substitutes are commonly used [76].
Calcium sulphate (plaster of Paris) has been used in horses principally because it can be implanted with antimicrobials. Like many bone substitutes, it is osteoconductive, biodegradable and can be combined with autogenous bone graft at a defect site. Calcium phosphate ceramic (CPC) has also been used and can be modified to a calcium hydroxyapatite implant that has some indication of stimulating bone healing. These have some potential but are rarely used in horses. Exogenous hydroxyapatite has been used in humans. It is osteoconductive and stimulates osteointegration with mechanical properties similar to cancellous bone. The implant has good porous characteristics to enhance cell migration and bone remodelling. TCP is more porous and degradable than hydroxyapatite which increase vascular invasion. There are reports of TCP use for osteochondral repair [79]. BCP is a combination of hydroxyapatite and TCP mixture, and CPC is an injectable form that has been used to enhance healing. Bioglass is a silicate‐based implant that has been shown to enhance binding to host bone. Hydroxyapatite coatings have been shown to enhance osteoprogenitor cells and protein binding. Polymethylmethacrylate (PMMA) has been used as a spacer in staged healing in humans. In horses, it is the most commonly used delivery vehicle for antimicrobials [80].
Exogenous Devices
Pulsed electromagnetic fields have been used with some success for stimulating fracture healing, especially in cases of delayed healing in people (Chapter 15). The technique has been used for over 40 years and functions by passing a current through a conductor to generate a magnetic field [81]. Although most studies have shown a positive effect used in vivo, ex vivo and clinical trials, some have shown none or negative effects. The variability may be explained by the manner of application (frequency, timing and dose), the stage of healing at which it is applied, tissue densities and application method [81]. Although there is no recent evidence to support its use in horses, some older studies reported a positive influence on bone repair. Meta‐analysis of the human literature also suggests that it can be efficacious [82].
Electric stimulation therapy has been studied for decades; although experimental use has generally shown positive effects, its efficacy in clinical trials is mixed [83]. It is theorized that bone formation is stimulated by electrical fields generated within the bone.
Low‐intensity pulsed ultrasonography (LIPUS) has also been advocated for use in fracture repair (Chapter 15). The ultrasound waves are assumed to cause material deformation of bone at the site of application and upregulate cellular and biochemical processes to stimulate bone formation [84, 85]. Meta‐analysis of clinical studies shows modest effects overall [86]. There is some evidence that LIPUS is slightly better than electrical stimulation early in fracture repair [87]. Although low‐intensity ultrasound has some evidence of efficacy in humans [88], limited use in horses has shown no positive effects [89].
There is some evidence that extracorporeal shockwave therapy (ESWT) can effectively be used to treat non‐union fractures in humans [90]. Meta‐analysis of its use in humans showed no differences in healing of acute fractures, but there was
