Читать книгу Fractures in the Horse - Группа авторов - Страница 144

Unlike other tissues, the mechanical environment of bone has a significant influence on the healing environment. Roux initially coined the term ‘developmental mechanics’ in which he hypothesized that the cell type involved in healing is based on the mechanical load [27]. Wolff described skeletal tissue as organized to optimize strength in response to loading [28]. Pauwels took this further to show that progenitor cells differentiate in response to the nature of mechanical load [29]. This classic work explains the influence of the mechanical environment on bone adaptation and demonstrates that the stability of a fracture will dictate the type of cells and tissue matrix that will occupy the site and thus determine the quality of healing.

The mechanical strain that occurs at the time of fracture can lead to significant vascular changes. Increased strain results in continued vascular damage, decreased oxygen tension and consequent stimulation of chondrocyte formation [18, 30, 31]. Local strains of less than 5% lead to intramembranous ossification, strains of less than 15% lead to endochondral ossification and strains greater than 15% result in formation of fibrous tissue and hence lead to non‐union. In contrast, excessive reduction in the local strain environment due to over stabilization by a repair (which has never been documented in horses) can lead to reduced bone healing. Loss of low‐level strain can lead to reduced external callus formation, fracture end osteolysis and adverse remodelling [32]. The vascular response to fracture, since it is dependent on local strain, will cause differences in healing type. With physiologically sound rigid stability and compression of fracture ends, local vascularity is enhanced and bone formation can occur [18, 30, 31]. However, in secondary bone healing, increased strain and a void between the fracture ends will lead to a relatively hypoxic area in which only chondrocytes can thrive. Progenitor cells differentiate into chondrocytes, and as these fill the fracture gap (the soft callus phase of healing), strain decreases due to a relative increase in stability and the environment becomes conducive to the formation of hard callus.

As stability is a major factor in influencing healing, meticulous attention must be paid to adequate reduction, debridement, and application of fixation principles for optimal stabilization. With the advent of minimally invasive procedures, debridement through open reduction is often not necessary. In minimally displaced fractures, the requirements for meticulous reduction to produce stability is overcome by enhanced rigidity of the locking plate system. As experience with minimally invasive techniques increases, the limits of reduction will be tested and further guidelines will evolve.

Discussion of the mechanical environment raises the question of appropriate time for implant removal. At the later stages of bone healing, it is possible that implants can shield stresses that may be necessary to complete healing and restore full bone strength. This continues to be debated in human and veterinary medicine as clinicians constantly question when the mechanical strength of the bone (without the implants) is optimal for removal without reinjuring the fracture site [33]. This is difficult to determine objectively. In most equine repairs, either a staged removal occurs for example if two plates are used or the animal may be exercised with the implants in place in order to apply some stress to the bone before removal. The form of exercise can vary according to individual circumstances. A progressive transition in mechanical environment can follow implant removal to apply gradually increasing loads. Care must be taken from the clinical perspective to be assured that the healing bone is not overloaded. However, at this time there are no objective means of determining bone strength or resilience, and judgement must be made on the basis of clinical signs and results of imaging.