Stimulation of subchondral bone cyst healing by placement of a transcondylar screw in the equine medial femoral condyle

OBJECTIVE: To predict the bone formation stimulus of a transcondylar screw across an equine subchondral bone cyst (SBC) in an equine medial femoral condyle (MFC). STUDY DESIGN: Finite element modeling (FEM) of an equine MFC with a 2 cm3 SBC under several transcondylar screw conditions. SAMPLE POPULATION: The right stifle of a yearling thoroughbred without stifle disease that had been euthanized for reasons unrelated to this study and donated to the University. METHODS: The FEM was derived from computed tomography of a yearling thoroughbred and analyzed in ABAQUS v6.14. The transcondylar screw was modeled as a 4.5-mm stainless steel cylinder. The region of interest was the centrodistal MFC, and bone stimulus was calculated. The stimulus threshold for bone formation (BFT) was >60 MPa and is presented as the percentage of total bone surface area (BFA) and frontal plane maps. Principal compressive stress vectors were also determined. Tested variables were daily cycles, load, and screw compression and position. RESULTS: At 750 cycles and 900-N load,  BFT occurred proportionally with load, screw compression, and daily cycles (steps). Compressive stress was oriented vertically on the SBC surface without a screw but aligned with the long axis of well-placed lag screws. Screw placement through the void also increased the number and magnitude of compressive vectors. CONCLUSION: This model predicted that a transcondylar lag screw across an MFC SBC increased surface BFA stimulation and reoriented the compression vector. Increasing screw compression, load, and steps per day increased the bone formation stimulus. CLINICAL SIGNIFICANCE: This study provides evidence that supports the use of a lag screw thorough an MFC SBC to promote bone formation.