Changes in in vitro compressive contact stress in the rat tibiofemoral joint with varus loading

Increased compression of the tibiofemoral joint, due to increased body mass or malalignment, is a risk factor for the onset and progression of osteoarthritis. This work investigates compressive stresses and contact areas in the articular cartilage of the rat tibiofemoral joint during standing with different applied varus loads. Cadaver rat knees underwent loading of the extensors combined with varus loading (0%, 50% or 100% of bodyweight) of the tibiofemoral joint. Articular cartilage contact stress was evaluated using stereophotogrammetric measurements of biplanar radiographs, high-resolution micro-computed tomography and discrete element analysis. Random coefficients regression models were used to analyze the relationship between peak and spatially averaged contact stresses and contact areas as a function of increasing varus loadings. The contact stresses increased linearly in the medial compartment. Peak stress significantly increased 0.042 MPa (p=0.006) and spatially averaged stress significantly increased 0.029 MPa (p=0.045) for each 10% increase in varus loading. There was a trend for a small decrease in contact areas in the lateral compartment with varus loading. This is the first report of the contact stresses in a rat tibiofemoral joints under simulated weight bearing conditions. The 0.42 MPa increase in peak contact stress at the cartilage-cartilage interface of the medial compartment with 100% bodyweight varus load is similar to the reported change in peak contact stress associated with development of symptomatic knee osteoarthritis in humans. Determination of contact stresses in rat tibiofemoral joints allows comparison to contact stresses in humans with the development of osteoarthritis.