Subchondral bone cysts (SBCs) are voids that can occur in the bones of young horses, especially horses intended for performance. Believed to be caused by trauma or osteochondrosis, these defects most often occur in the medial femoral condyle (MFC). Current treatments for equine SBCs have poor outcomes and have not improved over the last several decades. The gold standard for surgical treatment consists of cyst debridement and grafting. However, radiographic healing is not often reported, and when it is, only 20% of horses exhibit full radiographic healing. A novel treatment strategy has been recently introduced that places a lag screw across the SBC and has demonstrated high rates of radiographic healing. However, the mechanics of how a transcondylar lag screw could enhance SBC healing are unknown.

The goals of this study were to determine a plausible mechanism of SBC initiation and growth, as well as understand the mechanics of the transcondylar lag screw. A finite element modeling approach has been taken to examine the mechanics associated with SBCs. Using CT scans from young Thoroughbred horses, several finite element models have been developed for this study. The results of this study show that high-impact loading from gallop can cause stresses high enough to initiate bone damage in a healthy equine stifle joint. Additionally, once a small defect has manifested, stresses rise even higher and further damage is likely. Medial meniscus stress also increases with a MFC SBC, which suggests that secondary injury to the medial meniscus may be due to a disrupted load path through the MFC. Furthermore, it was determined that the transcondylar screw is able to heal SBCs by providing enough mechanical stimulus to the adjacent bone to promote bone formation. Not only is the stimulus for growth present, but the screw also aligns third principal stresses transverse to trabecular orientation across the cyst. This encourages bone to form across the void, as opposed to trabecular thickening, which results in the sclerosis typically seen in MFC SBCs. Lastly, it was determined that larger cysts respond best to the transcondylar screw. Full penetration of the screw into the cystic cavity provides the highest bone-forming stimulus, and also best aligns stresses across the void.

This work demonstrates that trauma can initiate SBCs and that the transcondylar screw provides a unique mechanism to enhance healing. Since humans are susceptible to a wide range of bone defects that exhibit similar characteristic of an equine SBC, it is believed that there is huge potential for translational applications.