Osseous cystlike lesions (OCLLs) can be identified in many bones in horses. The second most common site is the distal phalanx (also called the coffin bone or pedal bone); the most common is the medial femoral condyle. These OCLLs have been described in horses of various breeds and ages and can be identified in both forefeet and hindfeet; however, they appear to be more common in forefeet.
OCLLs in the distal phalanx are thought to be developmental when diagnosed in young horses and traumatic when diagnosed in older horses. OCLLs identified in the subchondral bone often can communicate with the distal interphalangeal joint (DIP joint), or coffin joint. OCLLs have also been identified at the insertion of the collateral ligaments of the distal phalanx or the distal impar ligament, as well as at the solar surface of the distal phalanx.
OCLLs of the distal phalanx can cause mild to severe lameness or can be clinically silent and found incidentally on radiographs. Some horses with such OCLLs may respond to hoof testers, and some horses have a positive response to distal limb flexion. Lameness can improve after regional anesthesia is administered to the palmar/plantar digital nerves or palmar/plantar nerves at the level of the proximal sesamoid bones (abaxial nerve block), or to the DIP joint (when there is joint communication with the OCLL).
Radiography can reveal OCLLs of the distal phalanx; however, additional oblique views may be necessary, depending on the location of the OCLL. Often, a rim of sclerosis can be identified around the OCLL. Advanced diagnostic imaging (CT or MRI) is sometimes needed to identify the OCLL.
Various treatments are possible for OCLLs, depending on where they are. Articular lesions can be treated conservatively (with joint medication) or by surgical debridement using arthroscopy or extra-articular approaches, depending on the OCLL location. The cysts can also be packed with bone graft, bone substitutes, or biological agents (eg, mesenchymal stem cells, platelet-rich plasma). For nonarticular OCLLs, conservative or surgical debridement methods can be performed.
The prognosis for horses with OCLLs in the distal phalanx varies. Some horses are able to return to athletic function.
See Regional Anesthesia in Horses for discussion of specific anesthetic techniques used to localize lameness.
