A New Look at the Hock Joint Hilary M. Clayton, BVMS, PhD, MRCVS Mary Anne McPhail Dressage Chair in Equine Sports Medicine College of Veterinary Medicine Michigan State University East Lansing, MI 48854 Introduction The hoof ground interaction affects the transmission of weight-bearing forces throughout the horse’s limbs. One of the most frequent sites of lameness is the hock joint and various shoeing modifications are used with the objective of modifying hock motion and/or force transmission across the joint. It is not possible to evaluate the effect of these treatments in the absence of reliable data describing normal and pathological function of the joint. Since the hock is such a frequent site of lameness in performance horse, we are conducting a series of studies designed to describe the biomechanics of the normal hock, the mechanical effects of hock joint pathologies, and the effects of therapies, including farriery, on hock function. This talk will report the progress to date. Unfortunately, we have not yet reached the stage of evaluating the effect of farriery modifications on hock function, but it is hoped that the reader will benefit from a deeper understanding of hock joint mechanics. The hock is a complex joint consisting of several articulations. The majority of motion occurs at the most proximal joint, the tarsocrural joint, where the tibia rotates around the trochlea of the talus (figure 1). This joint allows a considerable amount of flexion/extension and, smaller amounts of lateral and rotational motion. Since the trochlea is oriented at an oblique angle to the long axis of the limb, the distal limb swings outward as the hock joint flexes. This outward rotation helps to avoid interference. Although the majority of tarsal joint motion occurs at the tarsocrural joint, the distal tarsal joints are thought to undergo small amounts of movement during normal locomotion and these movements may be important in the etiology of hock lameness, such as bone spavin. We are conducting a series of studies designed to investigate the movements and mechanics of the hock joint in sound horses and in horses with hock lameness. Figure 1: Bones of the right tarsus as seen from the lateral side (left) and from the front (right). The oblique axis of the trochlea relative to the axis of the limb is clearly visible in the diagram on the right. Two-Dimensional Motion and Mechanics of the Hock Joint Previous studies of the hock joint have focused on movements of the joint in two-dimensions, as seen from the side (sagittal view). This allows description of flexion and extension, but does not measure motion in other directions. It has been shown that, during the stance phase, the hock flexes by about 10o°, with the shape of the flexion curve being quite variable between individuals (Figure 2). Often the flexion cycle has two peaks and it varies between horses whether peak flexion occurs before or after midstance. The hock is maximally extended around the time of lift off; slightly before lift off in horses with straight hock conformation, slightly after lift off in horses with more angulated hocks. In the swing phase, the hock flexes through about 40°. The variability in the pattern of the angle-time diagram at the hock is unusual. Most of the horse’s joints show a similar pattern of flexion and extension, though the magnitudes of the peaks may vary between horses. Throughout the stance phase there is a torque on the caudal side of the hock due to tension in the hamstring muscles, the superficial digital flexor tendon and/or the gastrocnemius muscle. This torque controls the rate of hock flexion in early stance and causes the hock to extend in late stance. The mechanical function of the hock in early stance is to absorb concussion, especially in the dampening of impact shock. Later in the stance phase, the hock generates mechanical energy as the hock pushes off against the ground. The, hock and fetlock are all active in providing propulsion. Figure 2: Hock joint flexion/extension angles during the stance phase of the stride for four horses. Note the considerable variation in the angular patterns of the different horses. Three-Dimensional Motion of the Hock Joint As a result of the oblique orientation of the talus, movements in other directions are coupled with flexion and extension. In addition, the flattened shape of the distal bones of the hock suggests that they allow some sliding or rotational movements. Unfortunately, these bones are too small to be evaluated individually using skin markers. Instead, we have measured the three-dimensional motion of the cannon bone relative to the tibia (Lanovaz et al., 2002), which encompasses movements at all the joints of the hock. Based on computerized, anatomical measurements of the size and shape of the talus, we can separate out the motion at the tarsocrural joint to determine how much motion occurs at the distal joints. Rotations were measured in three directions: flexion-extension, abduction-adduction, internal/external rotation. In addition translation was measured in three directions: forward, sideways and vertically. Figure 3: Graph showing flexion/extension (thick gray line), abduction/adduction (thick black line) and internal/external rotation (dashed line). Extension, adduction and internal rotation are positive. The 3-D studies showed that, during the stance phase, the hock flexed through an average of 11°, which was similar to the value found in the two-dimensional studies. In addition, the cannon bone was abducted (rotated away from the midline) through 3° and internally rotated through 1.5°. At the same time the cannon bone slid forward, sideways and distally relative to the tibia. During the swing phase of the stride the hock joint underwent a considerably larger range of motion than during the stance phase. It flexed through 45° abducted through 10° and externally rotated through 5°. At the same time the cannon moved forward, sideways and distally relative to the tibia. Although most of the hock motion occurs at the tarsocrural joint, we were interested to determine how much motion occurred at the distal joints, since these are the site of bone spavin, and it seems likely that the motion patterns at these joints is related to the development of spavin. These studies have shown that, although the majority of the motion occurs at the tarsocrural joint, there is evidence that both rotational and sliding movements occur at other joints in the hock. During the stance phase, the cannon bone rotates internally at the distal joints, and in the swing phase, the cannon bone slides forward at the distal joints. A phenomenon at the hock joint that has often been reported is the “clicking” or “snapping” motion (Back et al. 1995) that occurs in early and late swing. It has been shown that the collateral ligaments of the hock play a crucial role in this action. The snapping action is described as the joint rapidly moving into maximal flexion or extension as the joint passes through a critical mid-point angle (Alexander and Trestik 1989). The snapping action is thought to come from the fact that the collateral ligaments have eccentric attachments relative to the center of rotation of the tarsocrural joint. Evidence of the snapping motion may be visible in our data. The forward sliding motion of the cannon bone becomes de-coupled with the flexion/extension motion in early swing when the joint has flexed through about 50°, and becomes coupled again later in swing as the joint passes back through the same angle. This angle corresponds almost exactly with the angle at which the snapping motion occurs. Our results indicate that movement at the distal hock joints may be affecting motion in other directions. Compensation for Lameness We studied the mechanical effects of synovitis of the distal joints of the hock, which may precede the development of bone spavin (Khumsap, 2002). Horses with synovitis showed a significant decrease in range of tarsal flexion, and in the amount of forward sliding of the cannon bone relative to the tibia during stance. The reduced sliding motion might result in repetitive loading on a focal area of articular cartilage, which may lead to the development of osteoarthritis. Horses with synovitis showed decreased weight-bearing on the lame limb, and the affected hock absorbed less concussion in early stance. There was no evidence of compensation by other joints in the lame hind limb. In addition, weight-bearing by the diagonal front limb was decreased, and there was no increase in weight-bearing by the other diagonal. The fact that the horses moved with an overall reduction in limb loading indicates that there was less vertical displacement of the horse’s body during the stride. In other words, the horse moved with a less bouncy gait. Significance of the Findings The results clearly indicate that rotation and sliding movements at the distal hock joints are a normal part of hock motion. Shoeing methods that prevent the hoof sliding forward at impact may increase the longitudinal deceleration forces. Transmission of these force through the hock joint may increase the sliding motion at the distal joints. There is some rotation at the hock during the stance phase in normal horses, and an obvious project for the future is to evaluate the mechanics of hocks that ‘wobble’ during weight-bearing and the effects of shoeing modifications in correcting the wobble without imposing further abnormal stresses on the joint. There is still a considerable amount to be learned about hock function and compensation for lameness. When these are better understood, we will be in a position to explore the effects of shoeing on hock function in normal horses and in horses with hock joint pathologies. References Alexander, R.McN. and Trestik, C.L. (1989) Bistable properties of the hock joint of horses (Equus spp.). J Zool 218, 383-391 Back, W., Schamhardt, H.C., Savelberg, H.H.C.M., Bogert, A.J. van den, Bruin, G., Hartman, W., and Barneveld, A. (1995) How the horse moves: 2. Significance of graphical representations of equine hind limb kinematics. Equine Vet J 27, 39-45 Khumsap, S. (2002) Biomechanics of the equine tarsal joint. PhD Thesis, Michigan State University, Lanovaz, J.L., Khumsap, S., Clayton, H.M., Stick, J.A. and Brown, J. (2002) Three-dimensional kinematics of the tarsal joint at the trot. Equine Vet J Supplement 34, 303-313.