The Hock Examined the Anatomy, Conformation, and Movement of This Critical Joint

VETERINARY CONNECTION The Hock Examined The anatomy, conformation, and movement of this critical joint

BY HILARY M. CLAYTON, BVMS, PHD, MRCVS

HE HOCK JOINT IS IMPORTANT both as a key joint in deter- Tmining performance ability and tibia also as a frequent site of lameness, not tibia only in dressage horses but in horses trochlea of talus competing in all types of equestrian trochlea of talus sports. Staff and students at the central tarsal bone third tarsal bone fourth tarsal bone McPhail Equine Performance Center central tarsal bone fourth tarsal bone third tarsal bone have been actively engaged in a num- ber of studies designed to provide in- cannon bone cannon bone formation about the function and dysfunction of the hock joint. These Figure 2. The bones of the left hock joint, as seen from the lateral (left) side and from the front (right). studies have been possible thanks to a grant from the Bernice Barbour Foundation. This column will describe of the ankle and foot, which include the running) lifestyle is that the bones of the results of some of these studies and metatarsals and the phalanges, lie flat the digit have been elevated from the the implications for dressage riders and on the ground, bearing weight directly ground. The weight is borne by the trainers. from below. The underside of the foot structures on the underside of the hoof: is called the plantar side, and this type the hoof wall, which is a greatly Hock Anatomy of stance is called plantigrade stance. strengthened toe or fingernail; the sole, The hock joint is equivalent to the human One of the adaptations associated and the frog. This type of stance is ankle (Figure 1). In humans, the bones with the horse’s cursorial (adapted for called unguligrade stance, from the Latin word unguis (nail). The horse’s metatarsals and pha- Metatarsal bone langes have been raised from the ground and are relatively longer than the equivalent bones in the human. As a result, the hock joint is located mid- Phalanges way up the leg and is part of the chain of limb bones that carry the horse’s weight and provide locomotor forces. The hock is a complex joint that comprises six bones and the joints between them (Figure 2, above). These bones and joints are arranged in three main layers: 1. Upper (proximal) layer: talus and calcaneus 2.Middle layer: central tarsal bone Figure 1. Weight-bearing structures of the right foot in the horse, dog, and human. In the human, 3. Lower (distal) layer: first and sec- the ankle and the bones of the digit are flat on the ground. In the horse, weight is borne on the tip ond tarsal bones (which are usually of the toe; the metatarsal (cannon) bone and the phalanges (pastern and coffin bones) are elevated ➤ from the ground and elongated, compared with the same bones in the human. fused), and third tarsal bone. BY SUSAN HARRIS ILLUSTRATIONS

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The fourth tarsal bone spans the middle Hock Conformation One thing we’ve found is that the and distal layers at the back of the joint. In evaluating hock conformation, it’s angulation of the cannon bone is more The bones of the hock are firmly an- necessary to consider the location of informative than the angle of the tibia chored to the top of the cannon bone, the hock in relation to the entire hind in classifying the type of hock confor- the result being that the entire hock limb as well as the angulation of the mation. In horses with a small hock acts like an upward extension of the joint itself. Unless you are very expe- angle (sickle hocks), for instance, the cannon bone. The tibia (the leg bone rienced in evaluating conformation, cannon bone slopes forward when the that connects the hock to the stifle) ro- look at the horse in a standard pose: horse stands in the standard pose. This tates around the smooth, rounded from the side, with the front limbs ver- is evident in Figure 3, which shows the trochlea of the talus at the tarsocrural tical and the hind limbs positioned so angles of the limb segments in horses joint. This is the most mobile joint in that the point of the hock is beneath with large and small hock angles. We the entire hock-joint complex. Highly the point of the buttock (Figure 3, be- defined a large angle as greater than mobile joints typically have extensive low). The angle of the hock is the an- 165 degrees and a small angle as less joint capsules and copious synovial gle between the tibia and the cannon than 155 degrees. fluid, which helps to lubricate the joint. bone on the front of the joint. The lengths and angulations of the The tarsocrural joint has a voluminous In his studies of the conformation bones of the pelvis, femur, and tibia joint capsule, which sometimes be- and gaits of Swedish Warmbloods, vet- determine the location of the hock joint comes overly distended with fluid, re- erinary researcher Dr. Mikael Holm- beneath the horse’s body and its cycle sulting in a bog spavin. In this con- ström found an average hock angle of of movement as he travels. A more slop- dition, the joint capsule bulges at the 156 degrees in the general population, ing pelvis places the hip joint a little front of the joint and, to a lesser de- with elite dressage and jumping horses lower and farther forward than does gree, behind the talus. Characteris- having larger angles—around 159 de- a flatter pelvis. The femur slopes for- tically, if the swelling at the front of grees. At the McPhail Equine Perfor- ward from the hip joint: The greater the hock is squeezed, the fluid is re- mance Center, we have measured the length and the more forward the distributed and the swelling behind the standing hock angles in warmbloods, angulation of the femur, the farther joint becomes more prominent. Andalusians, Thoroughbreds, Standard- forward it places the stifle and hock The calcaneus forms the point of breds, and Arabians. We found that the beneath the body. Conversely, a femur the hock, which provides leverage to majority of horses fall within the range that is short or that has a more verti- the muscles that extend the hock joint of 155 to 165 degrees but that some cal angulation places the stifle and hock to provide propulsion during locomo- are 10 degrees higher or lower than farther back, relative to the hip joint. tion. This bone is stabilized by a liga- these values. Studies are ongoing to The tibia slopes backward from the sti- ment that runs down the back of the evaluate the effect of hock angulation fle joint: Greater length or a more hock, from calcaneus to cannon bone. on motion and performance. backward angulation places the hock The high forces exerted on the calca- farther out behind the horse. The ideal neus sometimes tear this ligament, re- sport horse has a long, forward-slop- sulting in the development of a curb, ing femur and a tibia that is neither ex- which appears as a firm swelling on cessively long nor sloping. the back of the hock. The central and third tarsal bones The Hock’s Role in Movement are shaped like broad, flat slabs; and In observing the motion of the hock their articular surfaces are flattened in joint, we evaluate both the path fol- a horizontal plane. The bones’ flattened lowed by the entire hock during the shape allows the joints between them stride and the relative motion between to rotate and slide a little bit but pre- the tibia and the cannon bone. The path cludes flexing and extending move- of the hock is assessed in terms of the Figure 3. Horses with different hock ments. “Bone spavin,” or arthritis of conformations, photographed in a standard amount of elevation (raising and low- the hock, occurs at the distal tarsal pose, with the point of the hock vertically ering), protraction (forward motion be- below the point of the buttock. At left: joints and is usually located on the in- straight hind leg. At right: sickle hock and neath the body), and retraction (pushing

ner side, at the front of the hock. forward-sloping cannon bone. out behind the body). Relative motion OF THE MCPHAIL EQUINE PERFORMANCE CENTER PHOTOGRAPHS COURTESY

18 APRIL 2003 | USDF CONNECTION VETERINARY CONNECTION between the tibia and cannon bone is indicative of movement between the in- dividual bones of the hock joint. Movement of the entire hock relative to its standing position is produced by undulations of the pelvis and by movement in the hip and stifle joints. Undulation of the pelvis is a conse- quence of flexion of the lumbosacral joint, which is located behind the sad- dle in the croup region. When this joint flexes, the pelvis rotates forward, the hindquarters tuck under, and the cycle of hind-limb motion tends to be more forward under the body mass. To vi- sualize this movement, think of a rein- ing horse performing a sliding stop with his hind limbs brought forward Figure 4. Extended trot, showing the range of motion of the hock joint. The right hind limb shows maximal extension as the horse pushes off at the end of stance. The left hind limb shows maximal under his body. flexion during the swing phase. A dressage horse’s pelvis tends to oscillate in rhythm with his limb move- gaits. What does change, however, is at contact than those with sickle hocks. ments, tucking under as his hind limbs the amount of retraction: In the highly During the early part of the stance swing forward. The forward-and-back- collected gaits, such as piaffe, the hind phase, the hock joint flexes by 12 to 15 ward swinging motion of the hind limb limbs show minimal retraction; whereas degrees. As it flexes, it absorbs con- originates in the hip joint. Suppleness in the medium and extended gaits they cussion associated with the impact of in the hip enables the hind limb to have have a wider cycle of movement that the hoof with the ground. A large a large range of motion, not only for- carries the limb farther backward. One amount of hock flexion in the stance ward and backward but also from side of the goals of dressage training is for phase gives the impression of supple to side. We like to see the hind limbs the horse to learn to provide the nec- and fluid motion and produces more work underneath the horse’s body in- essary propulsive forces without push- lowering of the haunches. In the later stead of pushing out behind because ing out too far behind his body, even part of the stance phase, the hock joint this makes them more effective in per- in the extensions. extends as it generates propulsion to forming their carrying and propulsive The movements of the hock joint it- drive the horse forward. The joint functions. Although we tend to evalu- self are primarily flexion and exten- reaches its maximal extension around ate the position and engagement of the sion around the tarsocrural joint, where the time the hoof leaves the ground hind limbs based on the positions of the tibia rotates around the trochlea (Figure 4). In some horses, peak ex- the hocks and hoofs, it is the move- of the talus. These are the movements tension occurs before lift-off; in others, ments of the hip that are the most in- that we see when we view a horse from the joint continues to extend through fluential in moving the entire hind limb the side. When the horse flexes his hind the early part of the swing phase. relative to the rest of the body. leg, the angle at the front of the hock During the swing phase, the hock The position of the hock relative to gets smaller; during extension, this an- undergoes about 20 degrees of flexion. the body changes with training as the gle increases (Figure 4, above). This raises the hoof clear of the ground horse develops the range of his gaits, During a trot stride, the hock un- as the hip flexes to protract the limb. from collection to extension. Dr. dergoes two cycles of flexion: one dur- Peak hock flexion occurs in the mid- Holmström’s research indicated that ing the stance phase and a second during dle of the swing phase, after which the there is no difference in the amount of the swing phase. When the hind hoof hock extends and lowers the hoof to protraction (forward motion) of the contacts the ground, the hock angle is the ground. We like to see a lot of hock hock—specifically, that the hock is not similar to the standing angle: Horses flexion during the swing phase, and

carried farther forward in the collected with straight hocks have larger angles the hind limb should be carried for- BY SUSAN HARRIS ILLUSTRATION

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ward with the hock in a flexed posi- find that there was no compensating tion, rather than being flexed quickly increase in weight-bearing by the other and extended prematurely. diagonal. Instead, there was an over- The front view of the talus (Figure all reduction in weight-bearing forces. 2, page 17) shows that the trochlea is The horse accomplishes this by mov- set at an angle to the long axis of the ing with a flatter trajectory, which re- limb; as a result, when the hock flexes, quires lower ground-reaction forces to the lower part of the cannon bone also lift his body into the air. This is an im- swings outward (abducts). This mech- portant finding, as it indicates that anism helps prevent the horse from in- horses with mild hock pain that are terfering, particularly during the racing not yet obviously lame may show a de- gaits when the hind hooves pass outside terioration in gait quality—a flatter, the front limbs as they swing forward. less bouncy stride. These gait changes We have also detected a small amount may be more apparent to the trainer of sliding motion between the distal than to the veterinarian, who may not tarsal bones. be familiar with the horse’s normal (sound) way of going. Under these cir- Early Signs of Lameness cumstances, it is useful to have videos Horses with hock pain modify the mo- that illustrate the subtle changes in tion and weight-bearing of their hind quality of movement. limbs. Our research indicates that horses with mild lameness that is not Hilary Clayton, BVMS, PhD, MRCVS, is a world-renowned expert on equine biomechanics consistently visible (a grade of 1 or 2 on and conditioning. Since 1997, she has held the a five-point scale) show less hock flex- Mary Anne McPhail Dressage Chair in Equine ion during the stance phase as well as Sports Medicine at Michigan State University’s less sliding of the distal tarsal joints. College of Veterinary Medicine, East Lansing. The vertical ground-reaction force, The position focuses on dressage- and sport- which measures weight-bearing, is re- horse-focused research. Dr. Clayton contri- duced, not only in the lame hind limb butes a quarterly report to USDF Connection on her team’s research efforts and findings, but also in the diagonal front limb. In which she hopes will help dressage and sport- other words, in cases of mild hock horse breeders, owners, riders, trainers, and lameness, the lame diagonal is un- caretakers to enjoy longer and more produc- loaded. We were initially surprised to tive careers with their animals.

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