False thoroughpin is a term used to describe a number of swellings that may develop in the distal region of the crus or tarsus and cranial to the gastrocnemius tendon. These may be seen unilaterally or bilaterally and are more common on the lateral aspect of the limb. These discrete swellings do not extend distal to the hock and cannot be balloted from lateral to medial. The swellings vary in size and may be acute or insidious in onset and may or may not be associated with lameness.
The causes of false thoroughpin vary and are not well understood. False thoroughpins are usually solitary, fluid-filled sacs; they may be multiloculated with a wall of variable thickness and may have fibrous bands traversing them. They may develop secondary to local hemorrhage or due to herniation of the tarsal sheath, gastrocnemius, or calcaneal bursae.
Ultrasonography is useful to assess the extent and nature of these swellings. Positive-contrast radiography can be used to determine whether the swellings are discrete or communicate with other structures.
In many horses, the lesions are not associated with lameness, so no treatment is required and the horses may stay in work. In some animals, low-grade lameness may occur. Intrathecal medication with corticosteroids may improve lameness in some horses, although swelling and lameness will often recur; in these cases, surgical excision may be curative.