The equine tarsal joint (or hock) is composed of 4 joints: the tibiotarsal, proximal intertarsal, distal intertarsal and tarsometatarsal joints. The tibiotarsal joint performs 98% of the movement of the hock during movement and stance. The proximal intertarsal joint performs 2% of the movement and the distal intertarsal and tarsometatarsal joint are relatively immobile. The movement of the hock is necessary for shock absorption during gallop and to provide propulsion during a trot.
The primary cause of hock lameness in horses is distal tarsitis, sometimes also referred to as bone spavin. This condition is defined as the presence of significant swelling and articular damage to the distal tarsal articulation. In addition to the articular damage, there is often significant shear and rotational stress over these joints during contraction of the tibialis cranialis muscle, especially at the time of heel-off. In many cases, a firm enlargement of the hock is visible on a weight-bearing lateromedial and a flexed dorsoplantar radiograph of the talus. This enlargement is referred to as the Churchill’s Hock Test and is a diagnostic tool that is used to determine the degree of hock pain.
Like the shoulder joint, the tarsal joint is a synovial joint divided by menisci into lateral and medial compartments. The menisci are fibrocartilaginous structures that act as a shock absorber, reducing concussion and incongruity of the articular surfaces. In contrast to the shoulder joint, which has a sheath around the bicipital tendon to protect it against friction within the joint cavity, the tarsal joints do not have a sheath surrounding the tendon. Instead, the tendons are protected by an intertubercular bursa that fills the space between the humeral tubercles and cushions the tendons.
In normal tarsal movement, the tibia is guided along a circular path by a series of tarsal ridges. There is a slight outward rotation of the third metatarsal bone during gait and a small inward rotation at the point of landing. Radiopharmaceutical uptake is greatest dorsally and laterally in normal horses, suggesting greater adaptive bone modeling in response to loading in these areas.
To diagnose tarsal pain, regional anesthesia can be used to identify the source of the pain using a diagnostic block in conjunction with desensitization. A tibial nerve block followed by a peroneal nerve block will localize the tarsal articulation and allow the horse to be placed on a weight-bearing radiograph of the talus, which will be easily visualized for signs of synovial effusion. If no effusion is seen, further diagnostic blocks can be performed on the flexed dorsoplantar or lateromedial view of the talus. Typically, a firm enlargement will be visualized over the distal tarsal articulation and this will be identified as the source of pain on the flexed dorsoplantar and lateromedial views of the talus. Similarly, a tarsal luxation will be easily identified on the flexed dorsoplantar view as it is often noted by a “snap-joint” appearance of the distal tarsal articulation. X-rays and ultrasound are often used to assess the articulation for damage to the articular cartilage or bone and to assess the degree of shear and rotational stress over the distal tarsal arthritic joint.