Читать книгу Fractures in the Horse - Группа авторов - Страница 25

Long bones of the appendicular skeleton form in the embryo as cartilage rudiments that are invaded by blood vessels and bone cells. Centres of ossification form within the anlage and progressively replace the cartilage model. Ossification usually begins at foci in the mid‐diaphysis and then the epiphyses. As a rigid tissue, bone can only grow or change shape through appositional growth, involving the addition or resorption of tissue at existing surfaces. The presence of articular cartilage at the ends of long bones prevents longitudinal growth, as new bone cannot be deposited at these surfaces. Conversely, cartilage expands by interstitial growth. Retention of a transverse section of growth cartilage, the physis, at a point where the fronts of diaphyseal and epiphyseal ossification centres meet permits the continued growth of the bone along its long axis. In addition, a layer of growth cartilage is retained between the epiphyseal centre of ossification and overlying articular cartilage to facilitate radial expansion of the epiphysis during growth. By the time of birth, functional loading necessitates that the proportion of cartilage remaining in the weight‐bearing locations of the skeleton is relatively low. In precocial animals that undergo locomotion immediately after birth, such as the horse, bones of the distal limb, e.g. the third metacarpal bone, have effectively reached their adult length by the time of parturition and retain little growth cartilage in weight‐bearing locations (Figure 2.2). Growth cartilage at the physis and around the epiphysis is eventually replaced by bone, at which stage the skeleton is considered to be mature. In the horse, this occurs relatively early in bones of the distal limb (e.g. 6 months in the third metacarpal bone) and considerably later in bones of the proximal limb (e.g. 24–36 months in the humerus). The previous location of the physis remains visible grossly and radiologically for many years as a roughening on the periosteal surface of the bone and as a transverse linear radiopacity termed the ‘physeal scar’.

Cuboidal bones of the carpus and tarsus ossify in the last two months of gestation. In normal foals, over 80% of the cartilage anlage has been replaced by bone at the time of birth [6]. The extent of ossification may be significantly less in foals born prematurely or those that are dysmature or suffering hypothyroidism. The majority of cuboidal bones ossify from a single centre and grow centrifugally. However, the third tarsal bone has two centres located in the body of the bone and dorsally. The point where the two ossifying fronts meet represents a line of potential weakness in foals in which the ossification process is retarded at birth.

Figure 2.2 Third metatarsal bones from a neonatal Thoroughbred foal (left) and that of adult Thoroughbred. Note the similar length of the two bones.