The impact of subchondral bone cysts on local bone stresses in the medial femoral condyle of the equine stifle joint.

Subchondral lucency (SCL), also referred to as subchondral bone cysts, can cause clinical problems in horses and humans. In humans, SCLs occur in youths and adolescents [1] due to mechanical factors (often related to athletics) and in skeletally mature individuals secondary to osteoarthritis (OA). In horses, SCL most commonly occurs in the medial femoral condyle (MFC) of growing horses (without OA), and causes lameness. The cause of equine SCL is debated, but bone trauma due to overload is the likely mechanism. Investigating the biomechanics of the healthy and cystic MFC is important to understand cyst growth and to provide a foundation for new treatment strategies. We hypothesize that SCL alters the mechanical environment of surrounding bone, which in the presence of continued loading, may lead to enlargement of the SCL. In this study, we developed and validated a finite element model of an equine stifle joint and investigated the stresses associated with varying sizes of SCL. We found substantial differences in tensile and shear stress at various stages of SCL development that suggest further bone damage leading to SCL enlargement. These data provide a first step in understanding of the altered mechanics of subchondral bone surrounding a SCL. Additional studies may provide the basis for improved treatment strategies for SCL in young horses, and may improve the understanding of SCL in humans.