Treatments and Prognosis for Subchondral Cystic Lesions in the Distal Extremities in Thoroughbred Prospect Racehorses

SIMPLE SUMMARY: Subchondral cystic lesions (SCLs) are spheric/oval-shaped demineralized cavities that develop in equine bones at a young age (3–18 months of age). SCLs are a form of developmental bone disease, and, while their etiology is incompletely understood, they may be a manifestation of osteochondrosis (OC). This subchondral bone absence in SCLs impacts the distribution of weight-bearing loads, which can result in lameness and poor performance in racehorses. Also, the communication between SCLs and the affected joint can create a step defect in the cartilage, which may trigger arthritis. Several surgical and medical treatments have been developed to improve the athletic prognosis of horses with SCLs. We compared four treatments for SCLs in the metacarpal, metatarsal, and phalangeal bones of young prospect racehorses and found that all treatments have similar prognoses. We also found that horses treated for SCLs have similar racing careers compared to those of their siblings, despite having a difference in purchase price as yearlings. Horses with higher and wider SCLs were found to have fewer wins and places during their racing careers, indicating that SCLs negatively affect the quality of racing. ABSTRACT: Subchondral cystic lesions (SCLs) in equines and their treatments have been mainly studied in the medial femoral condyle of the femur. SCLs in the distal extremities affecting the fetlock or interphalangeal joints are frequent, but treatment or prognosis studies in horses are currently sparse. Our objective was to compare four treatments for SCLs in the distal extremities (intralesional injection of corticosteroids, transcortical drilling, cortical screw placement, and absorbable hydroxyapatite implant placement) and report the racing prognoses for affected thoroughbred yearlings. Data from 113 thoroughbred yearlings treated for SCLs in the distal extremities were collected from 2014 to 2020. Age at surgery, sex, bone affected, radiographic SCL measurements, SCL shape, and type of treatment were recorded. Sale data and racing performance were collected for the operated horses and for 109 maternal siblings that were free of SCLs. An analysis was conducted to assess if SCL size affected racing prognosis and to detect differences in sale value and selected racing parameters between the cases and controls. The outcomes for the different treatments, the different bones affected, and the SCL shape type were also analyzed. There was no difference in the ability to start in a race between the cases and controls (60.2% vs. 69.7%, respectively). The auction value of the treated horses was significantly lower than that of their siblings. The bone affected did not impact any of the racing variables studied, whereas the height of the SCLs negatively affected the number of wins and placed races. The type of treatment for the horses affected by SCLs did not have an impact on sale prices, ability to start a race, race starts, wins, and places, or age at the time of the first start. In conclusion, yearlings diagnosed with an SCL in the distal extremities had lower auction prices and decreased racing performances, with lower numbers of wins correlated with larger SCL heights compared to the siblings. Similar racing performance was found regardless of the treatment received.