Dr Peter Gillespie. BVSc MACVS.
Fifty five million years of evolution has seen the horse develop from a small, dog sized creature with four toes on each leg into the tall, elegant, fleet-footed animal we know today. The requirement for speed as a means of eluding predators has lead to unique skeletal adaptations. Compared to most grazing animals a horse’s legs are disproportionately long and light. By keeping the heavy muscle mass at the base or pivot of the limb, the leg is able to swing like a pendulum. The net result is a relatively small amount of muscular effort at the shoulder and hip results in a large range of motion at the foot. This energy efficient apparatus allows the horse to maintain high speeds over longer distances compared to other mammals.
The necessity for the bones to be light enough to be transported efficiently without wasting energy has to be offset against the need for strength and rigidity – after all, structural support is the most important role of the skeleton. The long bones of the distal limbs have unique design features that allows them to effectively resist the upwards forces of impact and loading and the opposing downward forces of the horse’s own bodyweight. The hoof mechanism, tendons, muscles and ligaments help in this role.
To understand what happens to the forces of impact, it is helpful to study the stride during motion. Each stride can be divided into 2 parts; the contact phase when the foot is on the ground and the swing phase when it is off.
The contact phase can be further subdivided into (fig 1);
The impact phase occurs during the first few milliseconds after the hoof contacts the ground. During this phase the limb undergoes rapid deceleration. This sends a force as a series of waves through the limb which are initially absorbed by the hoof mechanism, followed by the bones and joints. Not surprisingly it is during this phase that most bone and joint injuries occur.
The loading phase follows the impact phase. As the horse’s weight passes over the stationary hoof, the tendons and ligaments undergo maximum loading. Their inherent elasticity enables them to absorb the loading forces. Most tendon and ligament injuries occur during the loading phase.
The break-over phase begins when the heel of the foot leaves the ground and begins to rotate around the toe which is still in contact. Break over at the centre of the toe begins the swing phase of the stride. The forces on the limb during the swing phase are minimal.
Speed, conformation, hoof balance and track surface all have an effect on the magnitude and distribution of the impact and loading forces.
Speed has the greatest effect; remember the equation force = mass x acceleration? Obviously mass or weight remains constant during exercise but the faster a horse accelerates, the faster the limb decelerates at impact. Consequently the forces absorbed by the bones and joints increase. A horse travelling at race speeds is subjected to forces equivalent to three times its bodyweight.
Conformation and hoof balance play an important part as to how the forces are absorbed. Correct alignment of the bones and joints in relation to the foot and to each other are necessary to avoid uneven distribution. The foot has a ‘centre of gravity’ as does the limb as a whole. Ideally both should be directly in line with each other and joined by a straight column of bones.
A good track surface should have a natural ‘springiness’ that matches the stride. During the impact phase the surface should give slightly (a galloping horse should leave a surface impression approximately 50mm deep). It should start to rebound half way through the loading phase and reach full rebound at the start of the break-over phase. In this way the track works in unison with the stride. A surface that is too hard rebounds too quickly during the loading phase which adds to the forces on the limb. If the surface is too soft it will rebound too late to be of any benefit in reducing impact.
Serious bone injuries are generally attributed to normal bone reacting to abnormal circumstances, the so called ‘bad step’ on the track. Using the latest bone scanning technology, equine researchers have been able to show that this is often not the case. Most serious bone injuries are in fact caused through abnormal bone reacting to normal impact and loading. Many apparently sound horses are in fact unsound – they have areas of weakened bone along with small stress fractures that predispose to more serious injuries.
Bone is a dynamic tissue, constantly remodeling itself in response to the forces of impact and loading. Bones are made up of two types of bone tissue (fig 2). Cortical bone is the dense bone that gives bones their shape and strength. It makes up 80% of the adult skeleton. Trabecular bone is the ‘mesh like’ or honeycomb bone that forms in the ends of the long bones surrounding the bone marrow.
Both types of bone are made up of a collagen matrix along with osteoblasts (bone producing cells) and osteoclasts (bone resorbing cells).
Collagen accounts for approximately one quarter of the body’s protein. It comprises 30% of mature bone as well as being a major component of connective tissue and cartilage.
Osteoblasts are bone forming cells that are interspersed throughout the collagen matrix. They secrete a ‘glue like’ mineralized substance known as osteoid which contains calcium and phosphorus.
Osteoclasts are bone resorbing cells. They work in unison with osteoblasts to remodel cortical bone. These cells are activated when cortical bone tissue is subjected to impact and loading forces. Remodeling increases bone density by removing existing bone and adding new bone to areas where the forces are greatest. Remodeled bone is stronger than modeled bone; i.e. the bone that is laid down during normal growth periods.
Although attention to conformation, foot balance and track design all play an important part in preventing bone injuries, increasing cortical bone density – in other words, building stronger bone is vital. This can be achieved through a combination of training and nutrition.
Several studies have shown the effect training has on increasing bone density in young horses. One study compared weanlings that had been boxed with those that had been paddock raised and exercised daily. It was found that the paddock raised horses had 33% higher bone densities.
Scanning technology has shown that horses with a greater cross-sectional area of cannon bone combined with more bone in the dorsal cortex are able to better withstand the forces of impact and loading.
Research in thoroughbreds has shown that bone density increases more with training over shorter distances (400metres) at speeds around 13.5secs /furlong. It was found that in most cases (with all other factors being normal) the cannon bone has a fatigue life of around 50,000 strides, the equivalent of 5 months training. Once this point is reached, the incidence of shin soreness was found to increase dramatically.
Bone scans have shown that the shape and composition of bone is also affected by lack of exercise. Horses returning from the spelling paddock were found to have lower bone densities than when they were turned out. It was found that during the first 60 days of training, bone density decreased even further.
Nutrition is important in maximizing bone density. Skeletal growth is rapid during the first 12 months. A study of growth rates in young thoroughbreds showed that at 6 months of age a horse can reach 84% of its mature height but only 46% of its mature weight. At 12 months it can attain 94% of its height and 65% of its weight. By 22 months it has virtually stopped growing in height (97%) and has usually reached 90% of its mature weight. Skeletal growth therefore occurs faster than weight gain. Other studies have followed total bone mineral content (BMC) during the same growth period. At 6 months of age bone mineralisation is 68% complete, by 12 months it has increased to 76%. Maximum BMC is not reached until a horse is 6 years old.
Bone mineralisation begins during the last three months of pregnancy. At this point the foetal bones are simply cartilage models of the adult bones. The skeleton of the newborn foal contains only 17% of the adult bone mineral content.
The mineral content of milk is highest during the first week of lactation and then declines steadily. If the mare’s calcium intake is not adequate during gestation and lactation, it will result in skeletal mineral loss in the foal. Foals born of calcium deficient mares have smaller cannon bone diameters at birth.
Calcium and phosphorus are the main minerals found in bone. Daily requirements are dependent on growth rate. A six month old weanling gaining 0.65kg/day requires approximately 38g calcium and 25g phosphorus/day. A 12 month old yearling gaining 0.5kg/day requires around 45g calcium and 30g phosphorus. Lucerne hay is the best feed source of calcium containing around 12g/kg while grains are the best source of phosphorus (3.0g/kg). Bran contains high levels of phosphorus but only 20% is available for absorption.
The ratio of calcium to phosphorous is just as important. Diets should contain ratios of between 3:1 and 1:1 calcium:phosphorous. Studies have shown that high ratios of around 6:1 fed over a long period resulted in reduced bone density. Inverse ratios can also have a detrimental effect. These can occur on high grain/low lucerne diets. Studies have shown that feeding extra calcium has no effect on bone density.
Magnesium is essential for the formation of the collagen matrix as well as bone mineralization. Around 60% of the magnesium in the body is found in bone tissue. Human studies have shown a positive correlation between bone mineral density and dietary magnesium. Low magnesium is associated with reduced activity by osteoblasts and osteoclasts, the cells involved in bone remodeling. Good feed sources are lucerne (3.0g/kg) and soyabean-meal (2.7g/kg).
Copper, zinc and boron are trace minerals that play an important part in bone development. Studies have shown that copper supplementation of mares and foals can play an important part in skeletal development. Zinc is essential for bone and cartilage formation. It can interfere with the uptake of copper consequently high dietary levels are detrimental to skeletal development. Boron is a trace element that up until recently has received little attention. Human research indicates that boron aids the uptake of calcium and magnesium into bone as well as increasing vitamin D3 levels, important for bone mineral metabolism.
Silicon is one of the most common elements on earth and is essential for normal body function. Most of the silicon found in nature is in a form of sand and is unable to be absorbed by the body. Plants can take silicon up from the soil consequently the forage and grains that horses consume contain small amounts.
Silicon is involved in the formation of the collagen matrix as well as bone mineralisation. Experiments on chickens fed silicon deficient diets resulted in lower bone collagen levels, resulting in abnormal bone growth. The same series of experiments also showed that silicon supplementation increased the rate of bone mineralization as well as increasing the glycosaminoglycan levels in cartilage, important in the prevention and treatment of degenerative joint disease.
Studies carried out at the University of Texas demonstrated the importance of silicon in maximizing bone density in young racehorses. In these studies horses on silicon supplemented diets were able to train and race for longer and sustained less bone injuries than non supplemented horses. This was attributed to an overall improvement in bone density. It was noted that silicon supplementation had no effect on growth rate therefore was not connected with any risk of growth related diseases like OCD.
The studies also showed that if an absorbable form of silicon is given to lactating broodmares the levels in the milk increase which results in more silicon available to the foal. Interestingly human studies on milk mineral composition have shown that zinc, copper, iron and silicon are the trace elements found in the greatest concentrations during early lactation. As lactation advances, zinc, copper and iron levels decrease while silicon stays the same.
Dietary mineral supplementation along with careful design of training programmes will result in improved bone density and in turn, improved skeletal durability. Young horses will able to stay in training for longer without the interruptions that bone injuries can cause.
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