Tenosynovitis of the digital flexor tendon sheath is common in all types of working horses.
The digital sheath encompasses the superficial and deep digital flexor tendons and extends from the distal one-third of the metacarpus/metatarsus distally to just proximal to the navicular bursa. Asymmetrical tendon sheath effusion is typically the first sign of a problem.
The extent of lameness due to digital sheath tenosynovitis depends on the structures involved, and it can increase with exercise. Affected horses are typically sore on firm flexion of the distal limb.
Although some cases of tenosynovitis are primary and respond to conservative therapy with or without treatment of the sheath with corticosteroids and/or hyaluronic acid, others are secondary to lesions of structures contained within the sheath.
Ultrasonographic examination of the entire length of the digital flexor tendon sheath, including the pastern region, is recommended and typically leads to a tenosynovitis diagnosis. However, marginal tears of the deep digital flexor tendon and tears of the manica flexoria (the band of tissue that connects the superficial digital flexor tendon to the deep digital flexor tendon just above the fetlock) can be difficult to diagnose via ultrasonography; they are confirmed through tenoscopic examination of the sheath.
The site of lameness should be confirmed by blocking either the tendon sheath directly or the region including the tendon sheath. The digital sheath is blocked by injecting 5–10 mL of local anesthetic under aseptic conditions after sterile preparation, using a 1- to 1½-inch, 20- to 21-gauge needle inserted perpendicular to the skin, either directly into an obvious area of distention or through the annular ligament, with the limb in a non-weight-bearing position. Research indicates that this block should be evaluated for specificity approximately 15 minutes after injection, given that with more time, structures outside the sheath might also be desensitized.
Importantly, chronic, bilaterally symmetrical digital sheath effusion (also known as “windpuffs”) can also be present in the hindlimbs, but with minimal clinical importance.
Palmar Annular Ligament Syndrome in Horses
Constriction of the digital sheath by the palmar/plantar annular ligament in horses, known as palmar annular ligament syndrome, can be primary (due to annular ligament desmitis) or secondary to long-standing tenosynovitis or enlargement of the flexor tendons contained within the fetlock canal.
Limbs affected by palmar annular ligament syndrome often show distinct tapering of the prominent digital sheath effusion at the level of the annular ligament, with effusion palpable also distal to that structure, on the palmar/plantar aspect of the pastern.
Clinical signs of palmar annular ligament syndrome are similar to those of other types of tenosynovitis in horses and include pain on palpation, swelling, and lameness, especially after forced flexion of the distal limb. Careful ultrasonographic examination is recommended to assess accompanying pathology.
Palmar annular ligament syndrome can be treated either conservatively (with steroids) or surgically (with palmar/plantar annular ligament desmotomy). Surgery is best performed tenoscopically, enabling examination of the remainder of the sheath for primary pathology and assessment of the extent of constriction.
Other common causes of tendon or ligament pathology distal to the fetlock include desmitis of the distal sesamoid ligaments (oblique and straight) or of the deep digital flexor tendon, superficial digital flexor tendon, or distal digital annular ligament. Any of these conditions can result in tenosynovitis of the digital sheath and can typically be diagnosed via ultrasonography or MRI. Importantly, chronic, bilaterally symmetrical digital sheath effusion can also be present in the hindlimbs, but with minimal clinical importance (also known as “windpuffs”).
For More Information
Jordana M, Martens A, Duchateau L, et al. Diffusion of mepivacaine to adjacent synovial structures after intrasynovial analgesia of the digital flexor tendon sheath. Equine Vet J. 2015;48(3):26-330. doi:10.1111/evj.12447
Also see pet owner content regardingjoint disorders in horses.
