A large cyst 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 age, breed, or sex predisposition. Cysts are assumed to be traumatic in origin and not part of the osteochondrosis syndrome (p 1040). 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 usually responds to intra-articular anesthesia of the distal interphalangeal joint, and may respond to palmar digital nerve anesthesia. 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 or arthroscope-assisted corticosteroid injection into accessible cyst-like lesions; extracapsular (through the hoof wall) approaches have been used in less accessible lesions. Secondary fracture of the distal phalanx (especially the extensor process) can occur due to progressive weakening of the bone. Some horses return to performance status, while others are salvaged for alternative uses such as breeding.

Last full review/revision March 2012 by Stephen B. Adams, DVM, MS, DACVS; Andrew L. Crawford, BVetMed, CertES (Orth), MRCVS; James K. Belknap, DVM, PhD, DACVS; Jane C. Boswell, MA, VetMB, CertVA, CertES (Orth), DECVS, MRCVS; Peter Clegg, MA, Vet MB, PhD, CertEO, DECVS, MRCVS; Marcus J. Head, BVetMed, MRCVS; C. Wayne McIlwraith, BVSc, PhD, DSc, FRCVS, DACVS; James Schumacher, DVM, MS, DACVS, MRCVS; John Schumacher, DVM, MS, DACVS, MRCVS; Roger K. W. Smith, MA, VetMB, PhD, DEO, DECVS, MRCVS; Chris Whitton, BVSc, FACVSc, PhD; Jean-Marie Denoix, DVM, PhD, Agregé; Joerg A. Auer, DrMedVet, Dr h c, MS, DACVS, DECVS