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 In normal foals, over 80% of the cartilage anlage has been replaced by bone at the time of birth [6].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.
Vascular Supply
Both cortical and cancellous bone are highly vascularized: it is estimated that around 10% of cardiac output is directed to bone [7]. Arterial supply is through three major sources: (i) a nutrient artery that enters the medulla through a foramen in the diaphysis, (ii) periosteal arterioles that directly penetrate the cortex throughout the diaphysis and (iii) metaphyseal arteries that typically penetrate the bone at or adjacent to the point of insertion of the joint capsule (Figure 2.3).
Dense cell populations within cortical bone require substantial blood supply to sustain high demands for oxygen and nutrients and to remove waste products associated with normal metabolism and homeostatic processes. Cortical bone is perfused by a combination of arterial blood supplied from the main nutrient artery in addition to smaller arteries in the periosteum. The nutrient artery ramifies within the medulla and anastomoses with metaphyseal vessels. Under normal conditions, the medullary circulation provides vessels that perfuse the inner 80% of the cortex. Arterioles that originate from periosteal vessels supply the outer shell of the cortex although they have the capacity to supply a much greater proportion of the bone following injury. Blood flow is predominantly centrifugal. Capillaries pass through cortical bone in Volkmann's canals, which are generally orientated perpendicular to the long axis of the bone. These branch at right angles to give rise to smaller vessels that are contained with Haversian canals that lie in the centre of osteons and are usually parallel to the long axis of the bone. Osteons, and hence vessels within them, branch regularly, thereby providing an intricate network of vessels perfusing cortical bone: osteocytes in healthy bone reside within 300 μm of a capillary. The anastomosing network between medullary and periosteal blood supplies gives cortical bone a dual blood supply. This is important following injury or surgery, when one or other of the supplies may be disrupted.
Figure 2.3 Diagrammatic illustration of blood supply to a long bone of the appendicular skeleton. Arterial supply has three sources: (i) nutrient artery, which passes through the cortex into the medulla in the mid‐diaphyseal region via a nutrient foramen, (ii) periosteal arteries, which supply the outer circumference of the cortex, and (iii) metaphyseal vessels, which supply the epiphyseal and metaphyseal regions. All three networks share anastomoses.
Disruption to blood supply and the subsequent effects on oxygen tension have a profound effect on bone cell activity. Hypoxia has been shown in vitro to increase the number, size and bone‐resorbing activity of osteoclasts and inhibit the bone‐forming activity of osteoblasts [7]. Conversely, when oxygen tension is above normal, osteoclast function is suppressed and osteoblast activity increased.
Innervation
Bone is densely innervated, although the precise nature and role of the nervous system in bone function is still being unravelled. The periosteum is richly supplied with sensory fibres and both sensory and sympathetic fibres are present on the surface of trabeculae in epiphyseal and metaphyseal bone. In cortical bone, nerve fibres are located within Haversian and Volkmann's canals. Direct contact between nerve fibres and osteocytes has been demonstrated.
The morphology and molecular phenotype of sensory neurons that innervate periosteum and the medullary cavity is consistent with a role in nociception. However, the precise mechanisms behind sensation of pain derived from bone are poorly understood [8].
There is increasing evidence that the nervous system plays a role in controlling the activity of bone cells and their homeostatic functions [9]. The mechanisms are not understood but may relate to direct effects of signalling molecules in nerve fibres through receptors expressed by bone cells, indirectly via the effects of neuromediators on bone blood flow or through regulation of cytokines expressed by cells of the immune system. There is some evidence that the nervous system may play a central role in the adaptation of bone to changes in its mechanical environment, mediated by the dense network of periosteal and endosteal nerve fibres.
Microstructure
Bone matrix is a two‐phase composite consisting of an organic component, which is synthesized and secreted by osteoblasts, and mineral. The matrix of lamellar bone makes up more than 90% of its volume, the rest being cells, cell processes and blood vessels.
The matrix undergoes mineralization as soon as it is secreted and reaches 70–80% of its final mineral density (around 65% dry weight) within approximately three weeks. Remaining water within the matrix is then progressively substituted by mineral over the ensuing months or years, resulting in the steady increase in mineral density of the bone as it ages. This is easily appreciated on microradiographs in which the relative age of different areas of the cross‐section of the bone can be determined by their radiopacity (Figure 2.4). Regulation of mineral volume fraction varies between and within bones and influences material properties such as stiffness and toughness, which have important functional consequences.
The principal structural component of the organic phase is type I collagen whose fibres are configured to form one of several different microstructures.
Woven bone describes a microstructure that is associated with relatively loosely packed, large diameter