Osseous cyst-like lesions in the distal phalanx can result in a lameness that varies from mild to severe and may be unresponsive to anti-inflammatory medication. There is no apparent breed or sex predisposition. The cysts are commonly first diagnosed in young horses (1–3 yr old) but may be diagnosed in older horses. It is unknown whether the cysts have a developmental or traumatic origin. They are most commonly located in the subchondral bone either in the extensor process or along the joint surface close to the midline; the cysts may communicate with the distal interphalangeal joint. The lameness may be exacerbated with distal limb flexion, and it usually responds to intra-articular anesthesia of the distal interphalangeal joint if the cysts communicate with the joint. The lameness may respond to palmar digital nerve anesthesia but more commonly requires a more proximal nerve block (eg, abaxial sesamoid) for resolution of the lameness. Diagnosis is confirmed by radiography and/or CT. Differential diagnoses include keratoma, navicular disease, and primary degenerative joint disease of the distal interphalangeal joint. Surgical treatment includes arthroscopic debridement; extracapsular (through the hoof wall) approaches to the cysts have been used in less accessible lesions. Secondary fracture of the extensor process has been reported to occur due to a cyst in that region. Some horses return to performance status, whereas others are used for alternative purposes such as breeding.
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