Conformational Changes During Growth
As the limbs of the newborn foal are further developed than its skull, spine or rib cage, and nearly as long as those of the adult, there is little space for the fetus to stretch its legs during its maximal growth phase (the last 3 months of gestation). It is a minor miracle, therefore, that so few are born with congenital limb deformities. The growth rate of young foals, particularly the lighter breeds such as Thoroughbreds, is phenomenal. Daily live weight gains of 4 pounds or 2 kilograms are common in the first few weeks.
Although foals and yearlings appear to have growth spurts, one in the first 3 months of life and a second between 9 and 12 months, their increases in body weight and height plot a steady rise which gradually levels off from 15 months and which would only be stopped by severe reduction in rations or chronic illness. They can achieve over 95 percent of their adult height and 90 percent of their adult weight by 24 months of age. The age at which they level off varies considerably between individuals (Figures 9 A, B) and there is a strong hereditary link.
It is normal for the newborn foal to be knock-kneed and it is a difficult clinical judgement to decide when such a conformation is considered to be carpal valgus and abnormal (Figure 10). Some outward rotation of the forelimbs is also normal.
Carpal valgus with significant angulation is the most common congenital limb deformity but most foals correct themselves during the second month of life and require no more than limitation of exercise and remedial hoof care.
It is believed that the extra pressure of weight bearing on the outer side of the growth plate in the valgal limb stimulates growth on that side in order to correct the angulation. It is only when the deformity is too severe or the foal is over-exercised that the growth plate is damaged and the limb does not straighten spontaneously. Also, the foal’s chest broadens during the first 9 months and this naturally reduces any valgus or outward rotation of the front limbs.
FIGURE 9B. An 18-month-old Thoroughbred, which is less mature than that in 9A and as its croup is significantly higher than its wither, will grow further in height.
FIGURE 9A. An 18-month-old Thoroughbred, bred to sprint, which is quite mature for its age.
Angulation is also seen through the fetlocks and the hocks, sometimes together and in opposite directions, giving us what are known as windswept foals (Figure 11). These foals also have a great capacity for self-correction if they are allowed exercise and given appropriate foot trimming early in life.
FIGURE 10. An 8-day-old foal with a slight carpal valgus of its right forelimb, which is normal at this age.
Varying degrees of uprightness or slackness through the pasterns and fetlocks are seen in the neonate. The former may be severe enough to be classed as a congenital flexural deformity (often called incorrectly a contracture) and result in the foal being unable to stand without knuckling over. Such cases require veterinary attention and careful support. Foals that have flexural deformities of the knees can be especially difficult to manage and are likely to retain an over-at-the-knee conformation. An upright conformation is likely to persist during growth and in some foals — particularly the larger, faster growing ones — it may worsen between 3 and 5 months of age.
FIGURE 12. Physitis is apparent as prominence of the growth plates on the inside of the fetlock of a 4-month-old foal giving an hourglass appearance.
FIGURE 11. A windswept foal with valgal angulation of the left hock and varus of the right hind fetlock and hock.
This change may be associated with enlargement of the growth plates at the fetlock, giving them an hourglass shape (Figure 12). It is a response to pain in the limb and this off loading may also result in a toeing in or bandy conformation. The most common cause is physitis, which is an inflammation of the growth plate during its active ossification phase caused by excess pressure or weight bearing.
Physitis is seen at the knee between 6 and 24 months of age and — more rarely — at the hock at the same stage. Other causes of the development of an upright or knuckling conformation during growth include osteochondrosis, which arises from defective development of joints such as the fetlocks, hocks or stifles resulting in a painful arthritis. Overfeeding or overexercising can be responsible and careful management allied with regular observation is crucial for prevention and early correction. These diseases are members of a group of conditions known as developmental orthopedic disease.
Slackness or over extension of the joints of the lower limbs is due to musculo-tendinous and/or ligament weakness. Premature foals may exhibit this conformation and it gives the newborn foal an unsteady stance or gait.
It is seen more commonly in hind limbs, which usually strengthen and improve dramatically with exercise. Severely affected foals may require bandaging to support and protect their fetlock and pastern or they may need heel support with an extension if they walk on the bulbs of the heels.
Hoof And Foot Development
The shape of the hooves is apparent in the Day 65 fetus (Figure 3A) and the pedal bone within it is clear on X-ray by 80 days (Figure 4B). The hoof is unpigmented and the horn tubules grow from the coronet downward, producing leaves, which give the sole a brush-like appearance. The outer horn hardens but the leaves of soft unpigmented horn form a soft cap, which prevents the foot from damaging the uterine wall during fetal movement in later pregnancy. The leaves have a break line along which they are shed when the foal bears weight after birth (Figures 13 and 14).
FIGURE 13. The soft leaves of horn on the hoof of a newborn foal. Note the break line on the hoof wall from where they will be shed.
FIGURE 14. A sagittal section through the digit of a newborn foal. The extent of the soft leaves of horn is seen and also the open growth plates at the distal metacarpal, proximal 1st and 2nd phalanges.
The horn of the foal hoof is quite soft compared with that of the adult. Exposure to air and exercise gradually hardens it during the first 3 months of life. The conformation of the hoof soon after birth is a template of the hoof it will have as an adult. Thus the shallow heels seen at 10 days old will still be apparent 2 years later — and may be worse if the heels are allowed to collapse.
Likewise the narrow, donkey-type hoof will show very little expansion during growth however much effort is made to spread the heels and quarters. Hoof conformation is also strongly heritable.
Changes in posture will have a profound effect on the conformation of the foal’s hoof. The most common reason in lighter, faster-growing foals is the acquired flexural deformity of the distal interphalangeal or corono-pedal joint within the hoof (Figure 15). This can develop at any time within the first 6 months of life, most commonly between 1 and 4 months and only in front feet — sometimes one, sometimes both.
FIGURE 15. An acquired flexural deformity in a foal. These occur most commonly between 1 to 4 months and affect one or both front feet.
This is believed to be due to pain within the limb or foot and occurs more frequently in dry years when the paddocks will be harder. The hoof becomes more upright and its dorsal or front wall may be vertical or knuckle forward. With less weight bearing, the hoof narrows and the heels lengthen. Excess wear at the toe may lead to infection penetrating the white line. Resting these foals is important and veterinary and farriery attention are needed.
Skeletal System Adaptation By Training
Bone is a living tissue which adapts to the forces placed upon it and is replaced constantly throughout life. The lower limb of the mature animal has to withstand the stress of carrying half a ton of body weight at speed on a cannon bone and pastern bones which have a smaller cross-sectional area than the human wrist.
It has been demonstrated that the bones in the lower limb of the racehorse work very close to their breaking strain during galloping. The tube-shaped cannon bone thus develops very dense or compact bone. In direct response to the compressive and bending forces applied during training, the front part of the bone, or the shin, thickens more than its sides and back (Figures 16 A, B). This adaptation of shape and density is called remodeling. It is achieved by the work of specialized cells (osteoclasts), which reabsorb bone. New bone is then laid down in thicker and denser patterns by cells called osteoblasts. The latter cells are identical to those that modeled the bone in the embryo and developed it during growth.
FIGURE 16: (a) Cross section of the cannon bone of an untrained 2-year-old. (b) Cross section of the cannon bone of a trained 2-year-old. Note the thicker wall of the bone.
Surprisingly few exercise cycles are required for this process to be effective. Therefore the racehorse does not need or benefit from long arduous training work in order to make its bony skeleton fit. It is commonly recognized that a mismatch between bone adaptation and training results in injuries such as stress fractures and sore shins. Of course the muscular and cardio-respiratory systems may require more conditioning than is ideal for the bones and tendons within the limbs — hence the skill of training.
The joints have two main functions. The principle one is to allow movement. Almost all of the horse’s limb joints are only capable of movement in one plane, flexing and extending. The shoulder, hip and spinal joints are capable of limited rotational movement. The smooth cartilage lining, flexible joint capsule and lubrication by synovial fluid allow the surrounding muscles and tendons to implement this low friction movement while ligaments maintain the joint in a stable relationship to the rest of the limb.
Their second function is to act as shock absorbers and is equally essential to efficient motion. Concussion or pressure is absorbed by the subchondral bone plate, the bone underlying the cartilage of the articular surface. It is remodeled as described earlier and denser, more resilient bone is produced during training. The integral bones of a complicated joint like the carpus (knee) or tarsus (hock) spread the load across that joint by undergoing minimal compression and outward movement, reducing the force transmitted down the cannons to the lower limb.
Tendons are believed to lose their tensile strength and elasticity gradually with training and age. It has been shown that considerable heat is generated in the middle of the superficial flexor tendon during work and it is probable that this has a destructive effect on the mechanical properties and integrity of the tendon over time. It has to be assumed that other flexor tendons and ligaments undergo similar stress but, as they suffer strain injury less often, they must either be better adapted or, more likely, better protected by support from other structures.
In summary, the fetal limbs have two phases of rapid and critical development, the first between 30 and 50 days and the second from 250 days onward when the fetus doubles in size. This rapid growth rate continues after birth and gradually levels out between 2 and 3 years of age. The production of a sound and well-conformed athlete requires regular and careful observation, intelligent management and a lot of luck.