Читать книгу Fractures in the Horse - Группа авторов - Страница 150

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 and haematoma 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.