14) [36]. Radiography is the most commonly used modality (Chapter 5). Changes in bone density and architecture are monitored. It is common, especially in conservatively treated fatigue fractures, for the fracture gap to appear wider after two to three weeks due to normal osteoclastic function [37] (Figure 6.3). Soft and hard calluses can be monitored and their activity characterized over time. This allows correlation with clinical progress and can help direct rehabilitation (Chapter 15). In delayed unions, the radiographic fracture line is persistent and there is minimal callus; intramedullary opacification may also be evident [16]. Non‐unions lack osseous bridging or callus, the bone ends or margins become diffusely opaque (sclerotic) and blunt, and the fracture line persists [6].
Figure 6.3 Conservatively managed long oblique fracture of the radius (yellow arrows). (a) Presentation. (b) Five weeks post fracture demonstrating osteolysis and widening of the fracture gap and initial periosteal (white arrow heads) and endosteal (black arrow heads) callus formation. (c) Eight weeks post fracture demonstrating continued periosteal and endosteal (trabecular) callus formation resulting in medullary opacification and partial loss of demarcation of the fracture line.
Although it can present practical difficulties, nuclear scintigraphy has been advocated as the most sensitive indicator of vascular integrity at fracture sites [38]. In human medicine, nuclear scintigraphy can also be used to identify and characterize fracture‐related infection. Gallium scans, white blood cell scans and 18FDG‐PET appear to be most sensitive and specific, particularly when combined with computed tomography [36].
Volumetric imaging techniques can also be used to monitor fracture healing. In most cases, this is accomplished through computed tomography that can be used to monitor the fractured gap and the surrounding tissues. This provides more objective information than two‐dimensional radiographs and does not suffer from superimposition of normal and abnormal tissues. Implants create difficulties in interpretation, but metal reduction algorithms aid interpretation/visualization and sequences are being improved [36]. Internal sensors on implants have been developed on an experimental basis and in the future may be of clinical benefit [39].
Healing of Stress Fractures
Fatigue or stress fractures are common in young equine athletes and can occur in compacta (cortical or subchondral) or trabecular bone. Pathogenetically, repetitive stress causes microdamage accumulation in areas of rapidly remodelling/modelling bone where osteoclastic activity outpaces osteoblastic repair, leaving affected bone relatively osteoporotic and thus predisposed to further microdamage, or progression to failure [37]. Cortical stress fractures occur commonly during training in young Thoroughbred racehorses. Initially, they can result in reduced performance, but if unrecognized can progress to complete and sometimes catastrophic fracture. If discovered early, most cortical stress fractures heal completely. Even if a fracture plane cannot be imaged, this is likely to be a form of gap healing as callus production is usually seen. Some that fail to heal in a timely fashion and are in amenable locations can benefit from internal fixation. Stress fractures in subchondral compacta and trabecular bone are also common in young athletes, often leading either to articular fragmentation, articular fracture, subchondral pain or osteoarthritis [14]. Continued repetitive stress in long bone epiphyses or cuboidal bones can cause accumulation of microdamage or an attempted reparative response of bone modelling. In the absence of an outer cortex on which responsive bone can be deposited, thus increasing diameter and strength, the bone can become intensely dense (often termed sclerotic) leading to brittleness and failure/fracture or other osteoarticular damage [14]. In some situations and locations, healing can occur with rest [40].
Healing of Incomplete Fractures
When an incomplete fracture occurs, the visible gap at the site makes primary healing unlikely. If the site is relatively stable, then secondary healing can follow. Incomplete osteoarticular fractures can heal with rest, but the current consensus is that provision of interfragmentary compression, which should induce primary bone healing, produces a more predictable healing process, decreases the period of required immobilization, reduces progressive articular cartilage damage and shortens convalescence.
Healing of Complete Non‐displaced Fractures
Complete, non‐displaced fractures can also heal with conservative management. This involves secondary bone healing, but the time to pain relief, radiographic healing and return to work are often unpredictable. Complete, non‐displaced fractures are often best treated surgically to better assure quality and timing of healing. Interfragmentary compression can result in reduced pain, enhanced stability and direct bone healing, producing a more predictable outcome. Delayed surgical repair is sometimes undertaken if an initial conservative approach proves unsuccessful. In this situation, the presence of fibrous, fibrocartilaginous, cartilaginous tissue or even woven bone in and/or around the fracture gap usually makes complete compression impossible. Some form of gap healing must occur, but increased stability provided by internal fixation stimulates and enhances the process.
Healing of Displaced Fractures
Satisfactory secondary healing of displaced fractures in horses rarely occurs. The few exceptions are discussed in individual location chapters. In the appendicular skeleton, persistent pain, malunion and permanent limb dysfunction generally preclude this approach. Conservatively managed articular fractures will almost always result in osteoarthritis and chronic pain which of itself reduces healing and increases the likelihood of opposite limb overload (Chapter 14). Meticulous reduction, congruent joint surfaces and adequate mechanical support are necessary to optimize healing and prognosis.
Healing of Reduced and Repaired Fractures
The goal for reduced and repaired fractures is to create mechanical and physiological environments that promote primary bone healing. In most (equine) circumstances, repair will result in a hybrid of primary and secondary healing. As a rule of thumb, the closer to the former that the surgeon can achieve, the better the prognosis and less complicated the post‐operative care required. In primary bone healing, in which rigid internal support is provided, inflammatory and haematoma cascades still occur to some degree because there will be a delay between fracture and repair. When open approaches are used for reduction and internal fixation, fracture haematoma and any damaged bone will be removed in order to optimize reduction and architectural reconstruction. In these cases, the benefits of reduction and absolute stability outweigh potential contributions from the initial clot. It is possible that osteomac stimulation has already occurred, but surgical trauma and the inflammatory response to it should also contribute to cell signalling.
Lag screws and/or plate fixation are used to maximize friction between fracture ends or across fracture planes, thus reducing the intervening space and optimizing the environment for primary healing. With reduction and stability, the sources of healing originate from the periosteum, the endosteal tissues and the Haversian system [25]. This cascade, in which some form of inflammation
