Discussion

This is a common clinical conundrum, as pathology in both these areas is a common cause of hindlimb lameness and poor performance, and the close anatomical proximity of the structures can make differentiation between pain in these sites difficult. In previous times it was usually considered that the distal tarsus was the most common site of pain, but there has been an increasing recognition of the importance of proximal suspensory desmitis, and certainly in sport horses this is more often a clinical problem. The approach to identifying pain in the areas should start by taking a thorough history and evaluating the response to any previous treatments in this area. A thorough clinical examination should then be performed but except in more acute and severe cases I think trying to use the response to flexion tests, the Churchill test or palpation over the proximal suspensory ligament to differentiate between pain in the 2 sites is difficult. I think the pattern of lameness can be quite helpful and distal tarsal joint pain can be expected to show a greater degree of lameness on a hard surface, whereas with a suspensory problem, lameness is as obvious or more obvious often on the soft. Diagnostic local analgesia is critically important. A low volume of local anaesthetic must be used in the tarsometatarsal joint to minimise the risk of blocking the suspensory ligament. A small volume of local anaesthetic should be used to block the deep branch of the lateral plantar nerve to avoid blocking the tarsometatarsal joint. Infiltration techniques are far more likely to block the distal tarsus. Anatomical imaging techniques, such as radiography and ultrasound can be somewhat frustrating in trying to differentiate between pain in both areas as there is often poor correlation between the results of imaging and degree of pain that is present. Horses with quite severe-appearing ultrasound changes may often have no lameness and in my practice the majority of horses that block well to the tarsometatarsal joint have no significant radiographic changes. Physiological imaging techniques such as gamma scintigraphy can be very helpful to determine if there is significant increased bone activity in the origin of the suspensory ligament or distal tarsal joints, but again, this can be frustrating in the more subtle lameness. I have not found thermography useful in this region. Magnetic resonance imaging is increasingly being employed, and expertise in interpretation of this region is slowly increasing.
I find comparing the response to the subtarsal and intra- articular blocks is the most accurate in differentiating pain in these areas. The normal pattern is to block the tarsometatarsal joint first and then perform a low 6-point block and then a subtarsal. If the results are still uncertain, I will then come back and block in the opposite sequence and do the subtarsal block before the tarsometatarsal joint block. If I am still uncertain about a relatively mild lameness, then assessing the response to medication of the tarsometatarsal joint can be helpful as cases with suspensory pain will often show just a transient improvement following intra- articular medication whereas true spavins show a longer duration of the horse's response to medication.