Epidemiology of skeletal injuries

There are many factors that influence the risk of fractures in horses. The majority of research has focused on racehorses. In racehorses, the type of surface is influential on the type of fracture that occurs. For example, on all-weather and dirt tracks, bi-axial proximal sesamoid bone fractures are most common, whereas in horses jumping on turf tracks, lateral condylar fractures of the third metacarpal bone are most common, and in flat turf races, proximal phalanx fractures are most common.³

In the UK, the catastrophic fracture rate per 1000 starts was 0.33 in flat races, 1.4 in hurdling, and 2.3 in steeple chasing.³ Track conditions have also been shown to affect the risk of fracture with risk increasing on turf as the conditions become softer, and on dirt tracks the risk increased when the tracks became ‘muddier’.⁴ However, conversely others found that risk of fracture increased with firmer turf.⁵

In racehorses in training, the risk of non-traumatic (i.e. spontaneous) fracture in horses that are in flat race training was 1.15 per 100 horse months in training and 1.5 in national hunt (jump) training. An increased risk of fracture was found in horses that had more days in training following a 60 day non-racing period (i.e. lay-off) and the distance exercised in the two months prior to fracture were correlated with increased risk of condylar fractures. The odds of condylar fracture increased by 0.3% per day for every additional day that the horse was in training since the 60 day non-racing period.³ The risk of condylar fracture increased by 4% for each extra furlong that the horse was exercised at a ‘fast pace’. Horses that suffered fractures were those that had had been in training for longer, had higher exercise intensities and completed more races compared to controls.³

A further study showed that horses were at increased risk of fracture if they either exceeded 44 km at canter (<14 m/s) or 6 km at gallop (>14 m/sec) in a 30 day period. Increasing the distance cantered up to 50 km in a 30 day period further increased the risk of tibial and pelvic stress fractures. Distal limb fracture has been shown to be at higher risk in horses that do no fast work compared to those doing 4–10 furlongs of fast work per week. Hence, short periods of high intensity exercise may be protective against fracture by stimulating appropriate adaptation of the bone. A 30–90 day rest period has been shown to be detrimental. However, interpreting rest periods is difficult as the horse may have been rested due to an existing injury that increases the risk of future injury, or the lack of stimulation to the bone may be responsible for increasing osteoclastic activity and thus weakening the bone.³ Also the training protocol following a rest period may influence adaptation and related fracture risk.

The conformation of the individual horse has shown to influence predisposition to fractures in Thoroughbred racehorses. For example long pasterns are associated with increased risk of forelimb fracture, and carpal valgus decreases the risk of carpal fracture.⁶ In racehorses in training, gender and age were not shown to significantly affect fracture rate, whereas there was a significant difference in risk between trainers. The overall incidence of fracture for horses in National Hunt training was 1.1/100 horse months.⁷ Foals born to primiparous mares are at less risk of fracture than those of multiparous mares and the rate of fracture also decreased with increasing dam age.⁸ Thus early skeletal development can potentially affect the risk of fracture later in life.

There is less documentation of the incidence of fractures in other equestrian disciplines. In event horses, a study demonstrated that in over 60 000 cross-country starts during ‘eventing’ there were 0.12 per 1000 starts of fracture or ‘bone injury’ recorded which is dramatically lower than those described above for racing.⁹

In general, there has been a significant lag between scientific advances in the understanding of mechanisms controlling functional adaptation of bone and translation to changes in the training methods used in the equine world in general and in the horse-racing industry in particular. No doubt some of the biologic mechanisms controlling adaptation of bone to its functional environment could and should be applied to the conditioning of equine athletes. It is also probable that the empirical traditional methods of training do incorporate some scientific principles as a consequence of many years of experience and these may inform research programs to validate the methods used.

Design of the equine skeleton

The equine skeleton is a refinement of the basic mammalian pattern. The horse is an unguligrade (hoofed) animal and a member of the order Perissodactyla (odd toed), standing on the hoof of the third digit. The major muscle masses of the limbs are positioned proximally to reduce the energy required to move the limb as it swings backward and forward in cursorial locomotion. Bone mass is also minimized at the distal extremities and safety margins decrease toward the distal extremity of the limbs, which may explain why the incidence of fractures in racehorses is higher in the distal bones than those located proximally.¹⁰

Selection for high-speed gait is also reflected in the morphology of the equine skeleton. The lever arms at the elbow and hock result in small muscle contractions producing fast extensive movement of the lower limb segments. These skeletal refinements in the elite equine athlete are basically a fine tuning of the general pattern. Analysis of skeletal conformation has been used to assess the level of performance in several different types of horse. Elite show jumpers and dressage horses were found to have larger hock angles and more sloping scapulas than other horses¹¹, while another study found that in Quarter Horses the ability to generate greater torque at the hip and stifle joint was due to a greater muscle physiological cross-sectional area rather than longer moment arms.¹² However, these systems have not yet been perfected to identify potential winners at an early age. Interestingly, a recent study of the skeleton of the great racehorse Eclipse has shown that his skeleton was ‘average’ in all respects.¹³

The ability to train elite equine athletes must result from a sound understanding of many aspects of the biology of the horse, from the cell and molecular level to that of the whole animal. One of the most important systems in relation to performance is the musculoskeletal system, and bone, as the major structural support tissue, is one of the most critical aspects.

Bone matrix molecular composition

As with all connective tissue matrices, there is a high proportion of water in bone tissue in the order of 20%, depending upon the type of bone. The remaining dry weight of bone comprises approximately 65% inorganic mineral, largely hydroxyapatite, and 35% organic material. The organic component of bone is predominantly the fibrous protein collagen, imparting high tensile strength. The inorganic hydroxyapatite imparts high compressive strength.

Bone cells

Adult bone tissue comprises three major populations of cells, each with a specific functional role but with both cell-to-cell interactions and also cell-matrix interactions. It is the coordinated interaction of the activities of these cell populations that optimizes the morphology of bones in relation to changing mechanical demands. Understanding these mechanisms and applying the principles to training regimens has the potential to improve performance and minimize injury in conditioning of equine athletes.

Osteoblasts

These cells are derived from local lining cells. Osteoblast differentiation results from activation of β-catenin through Wnt signaling by the Wnt co-receptors, low-density lipoprotein receptor-related proteins 5 and 6 (LRP5, LRP6) and Frizzled.²⁷ The flattened mononuclear cells become plump when activated and synthesize bone matrix in the form of osteoid (Fig. 8.1). The osteoid then becomes mineralized over a period of weeks to form bone matrix. In rapidly forming surfaces some osteoblasts are entrapped in their own matrix and these then become osteocytes (Fig. 8.2). The osteoblasts communicate with osteoclasts and enable activation of osteoclasts to allow bone resorption. Osteoblasts also produce colony-stimulating factor that increases numbers of pre-osteoclasts from mononuclear precursors in the bone marrow and also osteoclast activation factor that activates the pre-osteoclasts and initiates resorption of bone matrix (Fig. 8.3).

Lining cells

Quiescent bone surfaces are covered by lining cells which have the capacity to respond to both mechanical and biological signals and are activated to change shape into plump, metabolically active osteoblasts. These cells are found in the osteogenic layer of the periosteum or endosteum; these membranes comprise a deep cellular layer and a more superficial fibrous layer (Fig. 8.4). Some pluripotent cells have been demonstrated in the osteogenic cellular layer and these have been shown to have the capacity to differentiate into other connective tissues such as cartilage. There is currently considerable interest in the various types of pluripotent cells in the adult as a source of ‘stem’ cells that play a role in tissue regeneration. These quiescent lining cells are activated by mechanical or hormonal stimuli to generate a bone-forming front of metabolically functional osteoblasts involved in a modeling or remodeling process to maintain or restructure the matrix. The bone lining cells are thought to have a specific role in coupling bone resorption to bone formation³⁴ and it has been suggested that this is by physically defining bone remodeling compartments.³⁵

Intercellular communication between bone cells

The activity of bone producing cells and bone resorbing cells must be balanced under normal conditions to maintain the integrity of the skeleton. The coupling of bone resorption and subsequent bone formation has been shown to involve a receptor on the osteoblast cell membrane known as receptor activator of nuclear factor κβ ligand (RANKL) which binds to RANK present on the surface of pre-osteoclasts and induces activation of the intracellular cascades to activate the osteoclasts. The RANKL can also bind to a protein called osteoprotegerin (OPG), which is stimulated by canonical Wnt signaling.³⁶ OPG prevents binding with and activation of osteoclasts. Thus a regulation of coupling of these cells and associated amounts of bone resorption and formation is effected by this system (Fig. 8.5).^{37, 38} More recent work has shown that RANKL is also expressed by mature osteocytes and this expression is necessary for normal bone structure^{39, 40} as deletion of the RANKL gene in mice results in excess bone formation. It is not clear at present how osteocytes embedded within the matrix make contact with osteoclast precursor cells.⁴¹ Factors that increase RANKL expression are important and, in less mature osteoblasts, include parathyroid hormone (PTH), parathyroid hormone-related protein (PTHrP), 1,25-dihydroxyvitamin D₃ and oncostatin M.⁴¹ In mature osteocytes, apoptosis⁴² and the protein sclerostin may stimulate RANKL production.⁴³

The osteoblasts can also secrete collagenase, an enzyme that removes the surface layer of osteoid, unmineralized bone matrix, on bone surfaces and allows osteoclasts access to the bone matrix. Hormonal influences on calcium metabolism and bone resorption act indirectly on receptors on the osteoblast which in turn regulates osteoclast recruitment and activity. Thus the osteoblast is central to the control of the bone modeling and remodeling process. Boyde et al used in vitro systems to measure bone resorption activity of osteoclasts and also to show the interactions between osteoblasts and osteoclasts.⁴⁴

The pro-inflammatory cytokine interleukin-33 (IL-33) is a newly identified factor produced by osteoblasts that inhibits osteoclast formation in vitro.45, 46 A study has demonstrated that mice deficient in the IL-33 receptor demonstrated increased osteoclast formation and low bone mass.⁴⁶

The regulation of bone resorption and bone formation also relies on coupling factors released from osteoclasts that influence the bone forming osteoblast cells. Examples are cardiotrophin-1 that stimulates bone formation⁴⁷ and the recently identified Semaphorin 4D which inhibits bone formation in female mice.^{48, 49}

Furthermore, factors released by osteocytes can influence osteoblast activity. Sclerostin, a protein released by osteocytes, is an inhibitor of bone formation and is of interest because of its potential therapeutic potential in treating osteoporosis in humans. However sclerostin mRNA and protein has been identified in other non-bone sites⁴¹ including the adult human aortic intima⁴¹ where it is suspected to inhibit aortic calcification. Sclerostin is up-regulated by calcitonin⁵⁰ and inhibited by factors that stimulate bone formation such as prostaglandin E2⁵¹, oncostatin M and cardiotrophin-1.⁵²

Bone ultra-structure

As a composite material, bone matrix is anisotropic and the architecture of the matrix in relation to the osteocytes forms the basic unit of structure, termed the lamella. The architecture of the lamellae can be observed under polarized light microscopy as the collagen is a birefringent material. The architecture of these bone lamellae in different geometrical arrangements forms the basis of the different histological types of bone. Concentric circumferential lamellae form osteons and when these are formed in the original development of the bone they are termed ‘primary osteons’. A similar architecture is also seen in the secondary osteons that are formed to repair damage such as micro-cracks or infill porosities within the cortex. Lamellae formed around vascular networks or plexi result in laminar or the less regular plexiform bone often seen in ungulates like the horse (Fig. 8.6). This type of bone allows very rapid increases in cross-sectional area with later consolidation.¹⁰ Lamellar bone can also form circumferential lamella on the entire periosteal and endosteal surfaces of individual bones (Fig. 8.7).

In embryological development and in the early stages of fracture repair a rapidly forming bone with irregular lamellae, coarse collagen fibers and large osteocyte lacunae is seen. This is called ‘woven’ bone and is rapidly remodeled to the various types of organized lamellar bone, such as primary and secondary osteonal bone, previously termed ‘Haversian’ bone (Fig. 8.8).

Bone morphology has been related to the mechanical loading requirements of different specific bones. For example, the orientation of collagen fibers in the cranial and caudal cortices of the radius reflects the mechanical requirements of this bone. Strain gauge studies have shown that this bone is loaded in both compression and bending with principal tensile strains aligned with the long axis of the bone in the cranial cortex and principal compressive strains aligned to the long axis in the caudal cortex.⁵³ The arrangement of collagen fibers in relation to this pattern of loading has been demonstrated by Riggs et al.⁵⁴ This has been supported by a comprehensive analysis of matrix morphology in the equine radius⁵⁵, in which the analysis of primary bone and bone within secondary osteones of the cranial and caudal cortices of the radius were arranged appropriately to optimize for the pattern of functional loading. This study also confirmed a predominant longitudinal arrangement of collagen fibers in the cranial cortex of this bone, to resist the functional tensile strains at this location. Any consistent changes in the magnitude and pattern of loading induce a modeling response in which the bone cell activity will modify the matrix to maintain the optimization of the overall bone architecture in relation to the new prevailing loading conditions. Matrix, and embedded osteocytes, can be removed by osteoclasts and new matrix formed by osteoblasts. This coupled cellular activity allows bone as both a material and structure to be changed in terms of mass and distribution throughout life.

Mechanical characteristics

Bone tissue as a material contributes to the mechanical characteristics of the individual skeletal elements. The architectural arrangement of the material is predominantly related to the general functional requirements of the particular skeletal elements. In the long bones the diaphysis is tubular to resist the functional loading patterns of bending and torsion. In engineering terms, these loading patterns can be sustained with maximum strength and minimal material by tubular structures with the material distributed at a distance from the neutral axis. For example, if a solid beam is subjected to bending, one surface will experience tensile stresses and the opposite surface compression stresses. At some plane between these two surfaces there will be neither tensile nor compressive stresses (the neutral axis), thus no material is required in this region. When bone tissue organizes into the components of the skeleton, its structure is based on optimization of energetic efficiency. Thus to minimize the mass of material, bone is not present in the medullary cavity (Fig. 8.9). The relative thickness of the cortices also reflects the loading patterns of a particular bone. These changes may be measured using radiographic and ultrasound transmission techniques.⁵⁸

At the extremities of the long bones and in short bones the functional loads are predominantly those of axial compression. This is reflected in a different architecture with a thin cortical shell supported by internal cancellous bone (Fig. 8.10). The plates and rods of bone are termed trabecula and formed of aligned bone lamellae (Fig. 8.11). These rods and plates are not randomly arranged but strategically placed in relation to the trajectories of principal compressive and tensile stresses. In the cancellous bone of the epiphyses, underlying the articular surfaces of the synovial joints, the trabecula are orthogonal (perpendicular) to the articular surface. This architecture of the plates and rods again allows the use of minimal material to provide maximum strength and minimize energy requirements for locomotion. In short bones that are loaded predominantly in compression, the internal structure is of strategically arranged bony trabecula. Some short bones, such as the calcaneus, are subjected to bending moments. The internal cancellous architecture reflects this with arcades of trabecular bone arranged orthogonally to resist the principal compressive and tensile stresses. Nature reflects the calculated stress-related bracing seen in some engineering structures such as the Fairbairn crane in which the loading and principal stresses resemble those of the femur (Fig. 8.12).

The increased diameter of the bones in the regions of the epiphyses and the presence of subchondral trabecular bone act both to reduce stresses on the articular cartilage and provide a compliant structure to absorb impact loads. Thus any changes in the compliance of the trabecular bone will influence the loading of the overlying cartilage and can indirectly induce changes in the articular cartilage.