Читать книгу Fractures in the Horse - Группа авторов - Страница 23

The skeleton is comprised of a set of bones that together form the axial skeleton, including the skull, ossicles, hyoid, vertebrae, ribs and sacrum and the appendicular skeleton, which includes the limb bones.

The cells of individual bones express a genetic blueprint that governs their overall shape at an early stage of embryogenesis. For instance, the developing femur of an embryonic mouse transplanted in utero to the spleen still goes on to form a bone that with minimal mechanical environment is still recognizable as a basic femur (Figure 2.1) (John Chalmers, personal communication). The macroscopic architecture of each bone has evolved to meet functional demands that vary between different species and different anatomical locations in the same species. However, long bones, which make up the majority of the appendicular skeleton, share a fundamentally similar blueprint. The majority of bones comprise a tubular shaft (the diaphysis), optimized to use minimal mass for the greatest strength in resisting bending and twisting. The diaphysis flares at each end, the metaphyses, to form a more bulbous terminus, the epiphysis, with broad, sculptured end surfaces that articulate with adjacent bones. The epiphyses are optimized, to resist compressive loading and reducing pressure and impact loading on articular surfaces. The cortex of the diaphysis and outer shell of the epiphysis is formed of cortical bone that appears solid and has an apparent density (volume fraction [V _f] = volume of bone matrix per unit volume of tissue) of approximately 90%. The medulla of the diaphysis is filled with marrow that is comprised predominantly of adipocytes. The cortex steadily thins as it flares towards the epiphysis while the medulla becomes filled with cancellous (trabecular) bone, which becomes progressively more dense (V _f) towards the articular surface. The cortical shell may be less than a millimetre thick below articular cartilage and is directly supported by the underlying cancellous bone across the entire joint surface. The trabeculae within the epiphysis are generally arranged in arrays that transmit load from the joint surface to the cortex as it thickens towards the diaphysis. Spaces between the trabeculae are filled with blood and lymphatic vessels, nerve fibres, adipocytes and haematopoietic tissue.

Figure 2.1 Normal femur of an embryonic mouse (a) and one that was transplanted in utero to the spleen (b).

Cuboidal bones of the carpus and tarsus, the distal phalanx, navicular bone, proximal sesamoid bones and patella, share a different structural template, which consists of a thin cortical shell that encloses a network of cancellous bone throughout the medulla. The relative density of cancellous bone varies between different regions of individual bones depending on their loading history [1–3].

A soft tissue layer, the periosteum, covers the majority of the outer surface of most bones. Periosteum is absent where articular cartilage and ligamentous insertions are present. The periosteum is comprised of two layers: an outer fibrous sheath and an inner, cellular sheet frequently referred to as the cambium layer that is highly vascularized. The cambium layer is abundant in osteoprogenitor cells, which, combined with its rich blood supply, make it important in fracture healing. The inner (medullary or endosteal) surface of a bone is lined with endosteum, which is comprised of a thin membrane, only 10–40 μm thick, consisting of connective tissue and a few layers of cells. The endosteum also contains osteoprogenitor cells and has an important function in fracture healing.

The medulla of long bones is filled with haematopoietic tissue and fat. The proportion occupied by either tissue shifts towards fat in older animals. It contains osteogenic stem cells, and the fat may play an important role in bone biomechanics and absorption of impact loads [4].