Functional Anatomy

The deep digital flexor tendon (DDFT) in horses is formed by the fusion in the proximal metatarsal region of the thin medial digital flexor tendon and the larger lateral digital flexor tendon.^10-13 The two tendons pass within separate sheaths. The lateral digital flexor muscle covers the caudal aspect of the tibia and is joined by the tibialis caudalis muscle in the distal aspect of the crus. The tendon starts 2 to 4 cm proximal to the tarsocrural joint and passes medial to the tuber calcanei over a fibrocartilage-covered groove on the plantar aspect of the sustentaculum tali of the calcaneus. The lateral digital flexor tendon passes over the thick plantar ligament on the distal, medial aspect of the tarsus, medial to the superficial digital flexor tendon (SDFT), before being joined by the medial digital flexor tendon 1 to 3 cm distal to the tarsometatarsal joint.

The tarsal sheath is 16 to 20 cm long and starts near the musculotendonous junction of the lateral digital flexor tendon in the distal caudal aspect of the crus. At this level the tarsal sheath forms a large pouch between the lateral digital flexor muscle and the common calcanean tendon. The distended pouch is largest over the lateral aspect of the crus. At the level of the tarsocrural joint the sheath extends laterally to surround the lateral digital flexor tendon. Cranially a rigid groove is formed by fibrocartilaginous thickening of the tarsocrural joint capsule. The sheath terminates as a recess dorsomedial to the DDFT in the proximal third of the metatarsal region.

The tarsal sheath is enclosed at the level of the sustentaculum tali by a thick, transversely oriented ligament, the plantar retinaculum (Figure 76-1), and in the distal tarsus by a superficial fascia. The plantar nerves and vessels run within the retinaculum, in the plantar two thirds of its width, that is, plantar and plantaromedial to the lateral digital flexor tendon. The sheath is lined by a parietal synovial membrane with few villi, except distally. This membrane reflects plantarly to wrap around the tendon, leaving a thin but continuous membrane, or mesotendon, along the plantaromedial aspect of the lateral digital flexor tendon. This membrane carries vessels to the tendon and therefore should be preserved during surgery.

Fig. 76-1 Transverse section of the proximal tarsus, showing the lateral digital flexor tendon (LDFT) in the fibrocartilaginous groove (F) on the plantar aspect of the sustentaculum tali (Sust). Lateral is to the left. Vessels and nerves course within the retinaculum (2), lateral to the attachment of the mesotendon (1). MDFT, Medial digital flexor tendon; SDFT, superficial digital flexor tendon.

Causes of Tarsal Tenosynovitis

Distention of the tarsal sheath (Figure 76-2) is commonly termed thoroughpin, or true thoroughpin,¹⁴ but the condition may have distinct causes (see Figure 6-30).

Fig. 76-2 Distention of the tarsal sheath, giving the typical thoroughpin swelling, medially in the distal caudal aspect of the crus and proximal metatarsal region (white arrows) and laterally, caudal to the tibia (black arrow).