Effects of Internal Fluid Pressure on Stresses in Subchondral Bone Cysts of the Medial Femoral Condyle.

The etiology of subchondral bone cysts (SBCs) is not fully understood. Mechanical trauma and fluid pressure are two mechanisms believed to cause their formation and growth. The equine stifle joint provides a natural animal model for studying SBCs. Computed tomography images of an extended yearling cadaveric stifle joint were segmented using ScanIP to isolate bones and relevant soft tissues. Three model geometries were created to simulate cyst sizes of approximately 0.03 cm (C1), 0.5 cm (C2), and 1 cm (C3). A uniform pressure resulting in 3000 N force was applied at the proximal end of the femur. Two types of simulations, filled-cyst and empty-cysts with uniform pressure loads, were used to simulate fluid pressurization. Our models suggest that shear stresses are likely the cause of failure for the subchondral bone and not pressurized fluid from the joint. Bone stresses did not begin to increase until cyst pressures were greater than 3 MPa. For all cyst sizes, fluid pressure must rise above what is likely to occur in vivo in order to increase bone shear stress, shown to be most critical. Synovial fluid pressure acts upon a porous trabecular bone network, soft tissue, and marrow, so the continuum nature of our model likely overestimates the predicted effects of fluid pressures.