ANATOMY AND BIOMECHANICS OF THE HOCK Hilary M. Clayton Michigan State University The tarsal joint plays an important role in locomotion and is one of the most common sites of injury in horses competing in many equestrian sports. Tarsal lameness is a frequent problem in horses competing in dressage, reining, and rodeo sports. These pictures show some of the weight-bearing extremes during different sports and give an impression of the forces on the tarsal joint. This talk will review the anatomy of the tarsal joint and then describe the movements and functions of the joint based on new research information. The tarsus is a complex joint in which the bones are arranged in several rows with joints between them. . The talus and calcaneus form the proximal row. On the proximal and dorsal part of the talus is the trochlea that articulates with a corresponding groove in the tibia. Most of the motion at the hock occurs at this joint. It allows rotation in a slightly oblique plane as the joint flexes. The calcaneus lies on the plantar aspect of the joint where it provides a lever arm for the extensor muscles. As we’ll see later, the leverage afforded by the calcaneus allows the generation of large extensor torques at the tarsus. The central tarsal bone forms the intermediate row. The distal row has the fused first and second tarsal bones, the third tarsal bone and the fourth tarsal bone. The fused second and third tarsal bones are small and lie behind the much larger third tarsal bone that occupies the area between the central tarsal above and the third metatarsal below. The fourth tarsal bone bridges the intermediate and distal rows on the lateral side. Distally the third metacarpal or cannon bone transmits the weight-bearing forces, supported by the second and fourth metatarsals or splint bones. This computer model of the tarsal joint was developed by Adam Arabian, a graduate student in the McPhail Center. It is a computer solids model and can be accessed at the McPhail web site. < www.cvm.msu.edu/dressage> The morphology of the bones was based on computed tomography scans of intact and dissected tarsal joints. The joint was scanned with the ligaments intact, then the individual bones were dissected free from each other and they were rescanned. Sophisticated software was used to reconstruct the bones, paying particular attention to the articular surfaces. The outcome is a 3-dimensional model that can be viewed in its entirety and rotated to give different views of each bone and of the joint as a whole. The joint as a whole can be exploded to examine the shape and articulations of the individual tarsal bones. It has also been used to study the movements of the joints. The model has been used to predict motion at the talocrural joint based on the geometry of the articular surfaces. A further stage in the development of the model is to make a finite element model, which will be used to predict the loading based on kinematic and force data. The primary movement at the tarsal joint is flexion and extension in a slightly oblique plane. In addition there may be a small amount of sliding motion at the proximal and distal intertarsal joints and the tarsometatarsal joint. These are typically described as ‘low motion’ joints and are unlikely to contribute much to flexion and extension, though they are likely to allow a little sliding motion that may assist in dissipation of shear forces on the joint. The amount of hysteresis in the flexion-extension cycle of the tarsus suggests that there is indeed some motion at the distal joints. It is difficult to study the motions of the individual tarsal bones during locomotion. In vitro studies can be performed in which the joint is loaded from above, but this does not simulate the natural loading condition in which shear forces are also applied. This is an important consideration with regard to the development of pathologies. Studies of the overall motion of the tarsus have been performed by tracking the movements of the tibia and the metatarsus. The following description of tarsal joint motion and mechanics refers to the trot, since this is the gait used most frequently in clinical examinations. During the stance phase the tarsus flexes then extends. The joint angle is around 150° at ground contact and this angle is maintained during the impact phase when the limb as a whole is being rapidly decelerated. The joint then flexes through early stance to a minimal angle of about 135°. The joint remains flexed until midstance, then extends until the heels leave the ground at the start of breakover. Biomechanical analysis shows that during the stance phase, tarsal flexion is controlled by the extensor muscles and tendons. These structures generate a high torque on the extensor aspect as a consequence of the long lever arm of the calcaneus. Without this leverage, much higher forces would be needed in the soft tissues to avoid collapse of the tarsus under the influence of body weight. As the tarsus flexes, energy is stored elastically in the proximal part of the superficial digital flexor tendon, which is almost entirely tendinous in the hind limb. Then, as the joint extends, the stored elastic energy is released to assist in extension of the joint. Therefore the tarsus acts like an elastic spring during the stance phase. The graph shows the power profile for the tarsal joint. The negative part of the curve in early stance indicates that energy is being absorbed, by eccentric muscular action or by stretching of elastic tendons or ligaments. The positive part of the curve in late stance indicates that energy is being generated by concentric muscular contraction or by the release of stored elastic energy. The shape of this curve with a burst of energy absorption followed by a similar sized burst of energy generation is typical of elastic energy storage and release. As the limb swings forward the tarsus flexes to raise the distal limb clear of the ground. This flexion cycle peaks just after the middle of the swing phase at an angle of about 90°. In terminal swing the tarsus extends in preparation for the start of the next stance phase. In the early part of the swing phase, energy is generated on the flexor aspect of the tarsus indicating active flexion of the joint. In late swing energy is generated on the extensor aspect of the joint that is indicative of active extension of the joint. During the middle part of the swing phase the muscles are silent. When the hoof leaves the ground, the flexor muscles contract to initiate tarsal flexion, which is accompanied by abduction and Inward rotation of the metatarsus relative to the tibia. Once the distal limb has gained momentum, flexion continues through mid swing. The tarsal extensors contract during late swing to extend the joint in preparation for ground contact at the start of the next stance phase. If the stance and swing phases are compared, the tarsal joint shows a lot more flexion in the swing phase, but the torques are much larger during stance – in fact 40 times larger. Similarly, the peak power generation across the joint is about 30 times larger during the stance phase. The large torques during the stance phase emphasize the role of the stance phase in the aetiology of tarsal joint injuries and the importance of optimizing the stance phase forces. During the early part of the stance phase the hind limb acts to decelerate the forward motion of the body then in the later part of stance it acts to provide propulsion. These shear forces may be important in the aetiology of bone spavin, which will also be described. Arthrodesis of the distal joints may occur naturally or be induced surgically. It has been observed that if the most distal tarsometatarsal joint is fused, the more proximal distal intertarsal joint then develops osteoarthritis. If the distal intertarsal joint is fused, the proximal intertarsal joint develops osteoarthritis. This is an indication that the mechanical cause of the problem remains in spite of treating the painful lesion.