Extensor Tendons

Transection of an extensor tendon below the carpus produces a reduced ability to extend the digit, which is detected as an exaggerated, rapid (uncontrolled) dorsal flip of the hoof at the walk. This subtle change is easiest to detect if the lateral and common (or long) digital extensor tendons are transected completely. Intermittently the horse knuckles at the fetlock joint and places the digit on the dorsal surface of the pastern and fetlock joint. The gait abnormality is more obvious in a hindlimb and with lacerations in close proximity to the fetlock. Remaining peritendonous fascial attachments provide some support in horses with more proximal injuries. Horses with extensor tendon lacerations bear weight fully in a normal posture, unless other aspects of the wound create lameness and pain.

Transection of extensor tendons proximal to the carpus and at, or just proximal to, the tarsus also commonly occurs. Proximal to the carpus, transection of the extensor carpi radialis and common digital extensor tendons is most frequent. Flexion of the carpus may cause pain. The tendon sheath is often involved.

Proximal or dorsal to the tarsus, transection of the long digital extensor, cranialis tibialis, and fibularis (peroneus) tertius tendons is most frequent. If the fibularis tertius is disrupted, the hock can be extended while the stifle is flexed, indicating loss of the reciprocal apparatus. The gastrocnemius tendon develops a characteristic wrinkle in this extended position (see Figure 80-2 and page 802). The degree of gait abnormality may be mild. Transection of all the extensor tendons over the tarsus still allows full weight bearing with the foot flat on the ground. A greater tarsal extension during the swing phase of the stride and intermittent knuckling of the digit can be detected.

Digital Flexor Tendons

Transection of the digital flexor tendons below the carpus or tarsus produces pain on weight bearing and therefore lameness and gait abnormality. Transection of the SDFT is most common because it has the most superficial position of the two digital flexor tendons. The suspensory ligament (SL) is deep to the deep digital flexor tendon (DDFT) and is therefore least commonly injured with lacerations. A horse with complete transection of the SDFT may stand normally, or it may bear weight on the toe of the hoof to minimize movement of the tendon ends with fetlock joint extension. Complete transection of one branch of the SDFT in the pastern may not alter the stance during full load bearing. Administration of phenylbutazone for pain may eliminate lameness, and a gait abnormality may become hard to detect. The DDFT and SL support the fetlock joint together with the SDFT. Therefore slight hyperextension of the fetlock because of disruption of the SDFT may be difficult to detect unless the contralateral limb is picked up. The greater the number of structures transected, the less support to the fetlock and other distal joints and the greater the likelihood of vessel and nerve transection. Elevation of the toe is pathognomonic for transection of the DDFT.