Tenosynovitis of the digital flexor tendon sheath is common in all types of working horses. Chronic digital sheath tenosynovitis may be bilaterally symmetric in the hindlimbs in horses with minimal clinical significance (“windpuffs”). 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. Asymmetric tendon sheath effusion typically indicates a problem. Lameness degree is variable, depending on the structure(s) involved, and may increase with exercise. 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 digital flexor tendon sheath, including the intersesamoidean ligament and distal sesamoidean ligaments, is recommended and typically leads to a diagnosis. However, marginal tears of the deep digital flexor tendon (typically dorsolateral in the pastern) and tears of the manica flexoria can be difficult to diagnose via ultrasound but are confirmed through tenoscopic examination of the sheath. Verification of site of lameness should be confirmed via intrathecal injection of analgesia.
Palmar/plantar annular ligament constriction can be primary due to desmitis of the ligament or secondary to longstanding tenosynovitis or enlargement of the flexor tendons contained within the fetlock canal. Clinical signs are similar to those of other causes of tenosynovitis 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. Treatment can be either conservative (ie, steroids) or surgical (palmar/plantar annular ligament desmotomy). Surgery is best performed tenoscopically, which allows visualization of the remainder of the sheath for primary pathology and assessment of the degree of constriction.
Other common causes of tendon or ligament pathology distal to the fetlock include desmitis of the distal sesamoidean ligaments (oblique and straight), deep digital flexor tendon, superficial digital flexor tendon, and the distal digital annular ligament. Any of these conditions can result in tenosynovitis of the digital sheath and can typically be diagnosed using ultrasonography or MRI.