The medial part of the tarsus represents the most critical region. This is where many delicate structures like bones, joints and tendon sheaths lie directly underneath the skin, lacking any other tissues to protect them. At this location, kicking injuries may cause fractures and/or penetrating lesions into joints and tendon sheaths. Very often open fractures of the sustentaculum tali occur in conjunction with theses kind of injuries. Open fractures of the sustentaculum tali are great challenges for the veterinary surgeon, with regards to diagnosis as well as therapy.
The sustentaculum tali is part of the medial aspect of the calcaneus. Attention must be paid to the fact that it is situated directly underneath the skin and therefore is very much exposed to trauma. It is covered with a cartilaginous layer and provides a gliding surface for the deep digital flexor tendon (TBS), which is comprised of the flexor digitalis lateralis and the tibialis caudalis tendon. Both these tendons run directly over the sustentaculum tali. With the tarsus in maximal flexion these tendons are additionally stabilized in the plantar aspect of the trochlear groove of the talus.
The tendons of the flexor digitalis lateralis and the tibialis caudalis muscle are enveloped by a common tendon sheath, which starts slightly proximal to the calcaneus and reaches as far distally as the proximal third of the metatarsus. The tendon sheath and the sustentaculum tali lie immediately next to the medioplantar recess of the talocrural joint.
The most common cause of a fracture of the sustentaculum tali is a kicking injury to the medial aspect of the tarsus. This, however, does not only result in a fracture of the sustentaculum tali, but frequently opens also the tendon sheath of the TBS.
If adequate treatment is not applied immediately, purulent osteitis/osteomyelitis and tendovaginitis will develop rapidly.
The clinical signs depend on the age of the injury, the structures affected and how far the infection has progressed. Especially in cases of purulent osteomyelitis and tendovaginitis the horses are extremely lame. Apart from this, the injury on the medial aspect of the tarsus, results in a more or less diffuse swelling around the wound and an increased filling of the talocrural joint and/or the tendon-sheath can be observed.
The definitive diagnosis is made through an x-ray examination: Special projections are necessary for a better visualization of the sustentaculum tali. The best representation is obtained by the dorsomedial plantarolateral oblique (DM-PLO) and the flexed plantaroproximal/plantarodistal (PIPr-PIDi / Skyline) views. With the DM-PLO the sustentaculum tali is projected openly and this method allows a good representation and judgment of the structures involved.
To achieve the PIPr-PIDi view, the tarsus has to be flexed the same way as for a spavin test and the radiograph is taken at an almost vertical projection, centered upon the sustentaculum tali. This projection allows for precise delineation of the sustentaculum tali and especially its gliding surface.