Fractures in the Horse. Группа авторов

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Fractures in the Horse - Группа авторов


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that osteomac stimulation has already occurred, but surgical trauma and the inflammatory response to it should also contribute to cell signalling.

      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.

      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.

      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 1921) [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).

      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


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