Chapter 76The Tarsal Sheath
The tarsal sheath corresponds to the synovial sheath of the lateral digital flexor tendon at the level of the hock. Tenosynovitis of this sheath is a well-recognized condition1-6 and can be caused by a wide range of lesions. Nonpainful, chronic distention in the absence of obvious pathological lesions, often called idiopathic thoroughpin, is common and should be distinguished from other debilitating causes of tenosynovitis, many of which can cause persistent, severe lameness.7,8 Specific lesions within the sheath can be difficult to confirm clinically.8,9
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).
Slight to moderate distention of the tarsal sheath is often seen in young horses.8,9 It also occurs in adults, particularly in Warmbloods and Western performance horses and horses with a straight hock conformation, after extended box rest or transport. Effusion also may result from acute inflammation in nearby tissues, congestion, and edema in the distal limb (sympathetic effusion). The distention usually is not associated with signs of inflammation or lameness and tends to resolve spontaneously.8 However, the distention can become recurrent or persistent in some horses.
Tenosynovitis may be induced by trauma, leading to acute inflammation and hemorrhage in the sheath. In my experience, direct trauma to the medial plantar aspect of the sheath is common and most often results from a kick by another horse. Tenosynovitis also may result from hitting hard objects during jumping or occasionally from interference from the contralateral hind foot. These traumatic injuries may be associated with a chip fracture or fragmentation of the medial edge of the sustentaculum tali at the insertion of the plantar retinaculum.3,13 Transverse fracture of the calcaneus involving the sustentaculum tali also was described.3 In most horses no signs of direct trauma are apparent, and overstretching, or sprain, which may or may not involve the lateral digital flexor tendon, is suspected of causing acute tenosynovitis.8,9 Intrathecal hemorrhage always causes substantial inflammation and pain and can induce the formation of fibrinous adhesions. The inflammation often leads to chronic distention, with synovial thickening, fibrosis, and fibrous adhesion formation between the lateral digital flexor tendon and parietal sheath lining, which occasionally can cause persistent pain and mechanical lameness. Acute tenosynovitis without overt lameness also has been described.9
Primary Lateral Digital Flexor Tendon Injuries
Sprain injuries to the lateral digital flexor tendon do occur in the tarsal sheath region and are characterized ultrasonographically by longitudinal fraying and irregular hypoechoic lesions at the level of the sustentaculum tali. These lesions may be caused by overstretching and compression of the tendon over the bone.
Direct trauma to the medial aspect of the hock can result in breach and contamination of the tarsal sheath.3,9,13,15 The medial edge of the sustentaculum tali is the most prominent relief on the medial aspect of the tarsus, and a wound at this level often disrupts the retinaculum, thus opening the sheath.6,13 Puncture wounds are rare in my experience, but they may occur, especially in the distal caudal aspect of the crus. Iatrogenic contamination induced by intrathecal injection is also common and should be suspected in horses with worsening of the lameness and signs of inflammation after such injections.9 If untreated, suppurative tenosynovitis may lead to infectious tendonitis, destruction of the fibrocartilage, and eventually osteitis or osteomyelitis of the calcaneus.3,4,6,13 Infection also may result from extension of abscesses in adjacent tissues.