I saw two cases today with a dropped hock. The first one was a working dog that got one of the hind limbs caught on a car door; a laceration over the tarsus was sutured by a veterinarian. The dog remained lame after several weeks and was referred for further evaluation. Watching the dog walk, it had an obvious lameness, the hock dropped closer to the ground during the weight-bearing phase of the gait cycle, and the toes were in a flexed position, kind of like a club foot. There was an obvious swelling of the common calcaneal tendon above the point of the hock, or the tuber calcis. While keeping the stifle in an extended position, the hock was flexed. The amount of flexion in the affected limb was much greater in the lame limb compared to the normal limb. Diagnosis? Rupture of the common calcaneal tendon. This was confirmed with ultrasound evaluation of the tendon. In addition, the superficial digital flexor tendon was damaged, and the two structures were "stuck together", which might further limit motion of the superficial digital flexor tendon. This was the easy case.
The second case was a former performance dog that sustained severe trauma to the tarsus, including a fracture of the calcaneus, and a luxation of the central tarsal bone. The calcaneus was repaired with a bone plate and tension band wire, and the central tarsal bone was reduced with two screws several months before referral. At the time of presentation, there was malunion of the calcaneus, but it was healed. In addition, there were changes in the lower rows of tarsal bones that resulted in a partial arthrodesis. The range of motion of the tarsus was relatively good considering the amount of trauma, but there was significant lameness while walking. During the stance phase of gait, the tarsus was dropped lower to the ground than the normal limb, and the toes were in a flexed position, almost like a club foot. Sound familiar? BUT, the common calcaneal tendon had no swelling, and was actually quite normal. Back to the drawing board. On closer palpation of the calcaneal tendons and superficial digital flexor tendons of both limbs, there was a difference between the two limbs. On the affected side, the superficial digital flexor could be followed distally, but instead of wrapping around the gastrocnemius tendon and covering the tuber calcis, it went laterally. It may be that after repair of the tarsal injuries, either the retinaculum holding the SDF in place was not sutured, or the sutures broke down, resulting in luxation of the SDF. Although the hock was not completely dropped, it is likely that the support of the SDF was missing resulting in a partial dropped hock, and caused flexion of the digits during stance.
Take home lessons from these cases? Palpate carefully, and remember your anatomy and biomechanics.