From a biomechanical perspective, the tarsus is composed of 4 joints arranged in 4 levels: 7 tarsal bones, 2 crural bones (tibia and fibula), and 4 (rarely 5) metatarsal bones below. There are multiple ligaments for structural support and mechanical function. Muscles of importance to tarsal mechanics include the following: Achilles mechanism components (gastrocnemius, superficial digital flexor, semitendinosus, gracilis, and biceps femoris), deep digital flexor, cranial tibial, peroneus longus, and long digital extensor. Injuries can occur at any of the joints (tarsocrural, talocalcaneal, talocalcaneocentral, calcaneoquartal, and tarsometatarsal); additionally, bones can be luxated or fractured. Injury to the Achilles complex and medial shearing injury causing failure of the medial collateral ligament are common soft tissue injuries.
Clinical signs of tarsal injury include lameness, swelling, and malalignment. Malalignment can include hyperextension of any of the tarsal joints, sinking or hyperflexion of the tarsocrural joint, hyperflexion of intertarsal or tarsometatarsal joints, and varus or valgus. Minor injuries will resolve with rest and a temporary splint. More severe injuries require surgery or an orthosis or both. Common surgical approaches include repair of large ligament or tendon injuries when possible or necessary, implants for fracture repair, and partial or complete tarsal arthrodesis. Orthosis options are similar to those for the carpus and include devices with and without paw segments and devices that articulate and those that do not. Although, most injuries are managed with the same 3-point corrective mechanism, orthosis design must take into account the difference in angulation of the pelvic limb compared with the thoracic limb and the subsequent mechanical implications. An orthosis is an option when surgery requires temporary support or is not appropriate, not necessary, or not possible.
Achilles mechanism injury is the second most common non- traumatic tendon injury in dogs. It results in so-called dropped hock and clawed paw (Fig 7). Traditionally, it is managed with surgery, a period of immobilization with a cast, splint, or external fixator (6-8 weeks) followed by soft padded bandage and return to function. Unfortunately, this technique does not address the clawed paw, and sometimes does not result in complete resolution of tarsal hyperflexion. In human patients with Achilles tendon injury, general standards for therapy include surgery if indicated, controlled activity, partial immobilization with hinged orthoses limiting dorsiflexion at the ankle, early weight bearing (within 2-4 weeks), and early physical therapy, which result in faster return to function and decreased disuse atrophy. McComis is credited with developing the concept of functional bracing (orthosis) as an alternative to conservative treatment for ruptured Achilles tendon in humans; bracing allows immediate weight bearing and active plantar flexion, but limits dorsiflexion at the ankle. All of this allows for healing as well as return to normal function while limiting risk of recurrence.
Similar devices are now being used in veterinary patients. An example of the device is shown in Fig 8. This orthosis protects the tendon during healing phase, allows for return of digital dorsiflexion (resolution of claw paw), serially reloads the tendon, and finally limits reinjury during rehabilitation and return to normal activity.