ANATOMY AND BIOMECHANICS OF THE COXOFEMORAL JOINT AND STIFLE JOINTS Hilary M. Clayton Michigan State University The coxofemoral or hip joint is the proximal articulation of the hind limb with the pelvis. The head of the femur forms a ball and socket joint with the deep ilial acetabulum bounded by a rim of fibrocartilage. The ligament of the femoral head anchors the head of the femur within the acetabulum and is assisted by an accessory ligament present only in horses and donkeys among the domestic species. The accessory ligament detaches from the prepubic tendon, runs across the ventral side of the pubis in a shallow groove, passes through the acetabular notch and inserts on the head of the femur. The femoral and accessory ligaments restrict rotation and abduction of the hip joint. The ball and socket construction suggests that movement will occur in any direction, but the restrictive nature of the ligaments restricts motion to primarily flexion and extension. The caudal part of the greater trochanter of the femur increases the lever arm of the middle gluteal muscle, which is a powerful extensor of the hip joint. The following description of the motions and mechanics of the hip joint are for the trot. During the stance phase of the stride, the hip joint extends gradually from about 70° at ground contact to about 100° at lift off. As the hoof leaves the ground the hip joint starts to flex, reaching peak flexion of about 50° just after the middle of the swing phase, then extending in preparation for ground contact at the start of the next stance phase. Mechanical analysis has shown that there is a large torque on the extensor side of the hip joint through most of the stance phase. There is a burst of energy generation by the hip extensor muscles between 20 to 90% of the stance phase, with two distinct peaks in power production. These findings indicate that the hip extensor muscles are actively extending the hip joint during the stance phase which has the effect of retracting the hind limb and pulling the trunk forward over the limb. In the swing phase there is a flexor torque throughout the period of flexion and an extensor torque during extension. This means that the flexor muscles are generating energy during the period of flexion and the extensor muscles are generating energy during extension. Throughout the stride, the power profile at the hip joint is positive. This indicates that the hip joint plays an important role in driving the motions of the entire hind limb during trotting. The stifle joint is composed of the femorotibial and femoropatellar joints. The femorotibial joint, between the condyles of the femur and the tibial plateau allows flexion and extension together with some translation in which the tibia slides in a rostrocaudal direction. The menisci increase the congruence of the articular surfaces, while the cruciate ligaments restrict translation. The patella glides up and down the trochlea as the femorotibial joint extends and flexes. The stifle joint flexes by about 10° during the early part of the stance phase at the trot, then maintains this flexion through the middle part of stance before extending prior to the start of breakover. In the swing phase, there is a single flexion cycle that shows peak flexion of about 90° just after the middle of the swing phase. Biomechanical analysis shows that there is a torque on the flexor aspect throughout the stance phase. This is contrary to the generally accepted opinion that the stifle is prevented from collapsing under the influence of gravity by the quadriceps muscle. In fact, the net torque is provided by the flexor muscles. A burst of energy generation in early stance coincides with a burst of energy absorption on the extensor aspect of the tarsus and is most likely due to the action of the superficial digital flexor tendon. During the swing phase, the extensor muscles control the rate of flexion of the stifle joint in early swing and the flexor muscles control the rate of extension in late swing. During most of the stride, mechanical energy is being absorbed across the stifle joint. The main bursts of energy absorption at the stifle coincide with bursts of energy generation at the hip and tarsal joints. This is indicative of energy transfers across the joints, which contribute to locomotor efficiency.