formation occurs on a microscale, theoretically bypasses the instability and hypoxia that leads to soft and subsequently hard callus formation. It is likely that at a histologic level these stages also occur as some local degree of hypoxia is likely to occur until vascularity is restored. However, this is quite rapid as the surrounding soft tissues, periosteum, endosteal tissues and Haversian system are sources of neovascularity. Acute restoration and maintenance of stability by fracture repair improves the speed of neovascularization from these sites [18] and, as long stability is maintained, direct bone remodelling across the fracture will lead to a relatively rapid healing process.
Gap healing is a form of primary healing that occurs when fractures are not in perfectly uniform alignment and/or contact. This is likely to occur in many complete equine fractures, regardless of how stable they are, because perfect architectural reconstruction is often not possible. It is inevitable in complex and comminuted fractures and is probably also the situation in all complete fractures at a histologic level. In this case, the gap may heal through more of a secondary healing process even though the entire bone is stabilized. Alternatively, there may be areas of healing in a single fracture which are closer to primary repair and others in which secondary healing predominates. A (unheard of in equine patients but is recognized in small animals) theoretical concern with gap healing is that some fractures can be over‐stabilized, and consequently non‐union or atrophic union results. Mechanical loading and a degree of micromotion are necessary stimuli for secondary bone healing. Deprivation of these in the presence of a fracture gap prevents both primary union and the cascade of secondary healing.
Secondary bone healing follows use of external fixators or internal fixation in which the architecture of the original bone is not perfectly realigned and stabilized. In secondary healing, the inflammatory and haematoma phase is more prolonged than in primary repairs due to relative instability and soft tissue trauma. Locking plates, especially if placed in a minimally invasive fashion, allow for stabilization with gap healing while maintaining the clot and the inflammatory mediators for an optimized local environment. Stability is the most important influence on the effectiveness and timing of the stages in secondary bone healing, although other factors such as contamination and/or infection can impact negatively on the process.
Healing of Repaired But Non‐reduced and/or Unstable Fractures
The size, physiology and behavioural characteristics of horses are such that long bone fractures commonly fall into this category. Despite improvements in implant design and surgical techniques, many repairs are to some degree incompletely reduced (often due to missing or avascular fragments) and/or slightly unstable. In these scenarios, secondary healing, or at best some gap healing, is likely. It is then a race between the stable fixation (the combination of reduced/stable areas and the implants engaging them) providing enough mechanical stability to brace the unstable areas until they are supported by secondary healing. The balance between these two processes dictates outcome. If there is evidence of gross instability, revisionary surgery or external support should be considered to restore some stability to the microenvironment of the fracture.
Effects of Internal Fixation on Bone Healing
In the horse, internal fixation, whether through open reduction or minimally invasive techniques, is the most commonly used method for repairing fractures. In simple, usually articular fractures, one or more lag screws can be used to re‐appose the joint surface and provide compression to promote primary fracture healing. In these cases, the severity of articular deficits and/or articular cartilage damage usually dictates prognosis as the fractures are usually stable. In equine athletes, this commonly occurs through pathologic bone as seen in the carpus (Chapter 24) or in the metacarpo/metatarsophalangeal joints (Chapters 19–21) [14]. In these locations fractures usually heal, but the pathologic bone commonly influences the articular surface and consequently reduces the prognosis for an athlete.
The stability and process of fracture healing following plate fixation, whether by open reduction or minimally invasive approaches, is highly dependent upon anatomic location and the quality of reduction and stabilization at the site. Even with meticulous reconstruction of a long bone fracture, perfect anatomic reduction usually does not occur, and some areas undergo gap healing. It is generally accepted that the proportion of load that can be borne by bone has a direct bearing on outcome. The role of gap healing on cyclic fatigue of implants is unknown but is a potential factor in determining the risk of repair failure.
Intra‐osseous nails are used in anatomically appropriate situations to convert highly unstable fractures to ones with sufficient stability to permit secondary bone healing. Strict anatomic reduction does not occur. They maintain bone length, i.e. prevent diaphyseal overriding and reduce bending and torsional forces. Rush pins have similar goals, but in horses rarely are able to be of benefit.
In limb fractures, wires are sometimes used to help maintain reduction, especially in long oblique fracture repairs. However, small screws and/or countersunk lag screws are often most appropriate. Wire can also be used to create a tension band, usually as a supplement to other fixation techniques in order to optimize the biomechanics of repairs. In fractures of the mandible and maxilla, wires are used to close fracture gaps and increase stability in order to improve the environment for secondary bone healing (Chapter 36).
Effects of External Fixation on Bone Healing
In the horse, in which bed rest cannot be enforced and because of their large size, external fixation (Chapter 13) is only used when no internal fixation techniques are viable, such as with highly comminuted fractures or occasionally with an open fracture. The typical ring external fixator applied in other species is not commonly used in horses, and instead either a pin cast or customized external fixator is employed [41]. The technique is dependent on secondary bone healing, and providing sufficient stability for this process is critical to success. Pin breakage or bone failure in the region of the pins are not uncommon sequelae and can lead to failure [42]. Pin loosening can also result in reduced stability at the fracture site, which impedes healing, and increases pain. External fixation is inevitably a race between fracture healing (decreasing pain) and pin and/or bone failure.
Intrinsic Factors That Affect Healing
The most common, and likely the greatest, source of negative effects that inflammation can have in equine fracture healing is infection. Infection can result from contamination at the time of fracture or at the time of repair. The presence of foreign material (i.e. implants) commonly makes resolution impossible until these are removed. A number of procedures exist to prevent and combat infection (Chapters 9, 11, and 14). At a tissue level, the development of chronic inflammation can lead to compromised vascularity, impaired cell signalling, instability and persistent pain [26].
In other species, host factors have been correlated with the quality of fracture healing. In people, age, immune status, metabolic status and social behaviours can all have negative impacts [10]. Although there has been no correlation between quality of fracture healing and systemic metabolic conditions in the horse, it appears plausible and logical. In humans and other species, ageing has a negative effect [43]. This may be mediated through the immune system and/or negative influences on angiogenesis, mesenchymal stem cell activity and reduced progenitor cell numbers and activity [43]. Clinical experience suggests that age also appears to be correlated with fracture healing in horses. It
