Subchondral Cystic Lesions in Horses

Subchondral cystic lesions (also called osseous cystlike lesions) in horses most commonly occur in the stifle in the medial femoral condyle. Osseous cystlike lesions also occur, rarely, in the proximal tibia. The pathogenesis of these cysts is poorly understood; however, they might develop after trauma to the articular surface or as a result of osteochondrosis.

Subchondral cystic lesions often present in young horses but can occur at any age. The severity of lameness varies from mild to severe and can be acute in onset. Lameness can be intermittent, particularly in older horses.

Some horses with subchondral cystic lesions also have mild effusion of the medial femorotibial joint; in many cases, however, localizing signs are absent. Intra-articular anesthesia of the femorotibial joints can produce partial to substantial improvement.

Diagnosis is usually confirmed by radiographic evaluation. Medial femoral subchondral cystic lesions are most evident on caudocranial projections and can appear as variably sized round or oval radiolucent defects in the subchondral bone (see bone cyst image) or extending further proximally into the condyle. Some lesions are surrounded by an obvious sclerotic rim.

Subchondral bone cyst, stifle, radiograph, horse

Image

Courtesy of Dr. Sushmitha Durgam.

A very small defect in the subchondral bone of the medial femoral condyle that is radiographically evident might be a precursor to subchondral cystic lesions. Defects in the surface of the medial femoral condyle are evident ultrasonographically in some horses. Subchondral cystic lesions in the proximal tibia are usually smaller and most evident on lateral, lateral oblique, or caudocranial radiographic projections.

Nonarticular osseous cystlike lesions or small lesions might respond to conservative treatment, including rest, systemic NSAIDs, and intra-articular or intra-lesional corticosteroids. Arthroscopic debridement of the lesion along with (or without) administration of biologics such as platelet-rich plasma (PRP) or mesenchymal stem cells is a common treatment. Transcondylar screw treatment (4.5-mm cortical screw placed in lag fashion across the cystic lesion) has been successful for treating nonarticular osseous cystlike lesions in some horses (see lag screw placement image). Intralesional corticosteroid injection without debridement, performed under standing sedation and with ultrasonographic guidance, is often an effective treatment.

Transcondylar lag screw placement, radiograph, horse

Image

Courtesy of Dr. Sushmitha Durgam.

Regardless of treatment method, complete radiographic resolution of subchondral cystic lesions might not be attained and might not be necessary for soundness.

The prognosis for return to athletic function in horses with subchondral cystlike lesions of the medial femoral condyle depends on the age of the horse, articular cartilage involvement, and cyst size. Horses with smaller cysts or with cysts that have a narrower subchondral plate defect generally have a better prognosis.

One study reported the prognosis for racing as approximately 70% after treatment, regardless of which of three treatment methods (debridement alone, intralesional corticosteroid injection, or intralesional mesenchymal stem cell injections) was used (1). Another study documented an appreciably worse prognosis for horses > 3 years old at the time of arthroscopic debridement (2).