The effects of conformity and load in total knee replacement

The effects of different conformity ratios and loads on the ultrahigh molec ular weight polyethylene stress levels acting on knee implants were examine d using a nonlinear, finite element analysis. The contact condition between a rigid cylinder with a radius of 30 mm and a polyethylene plate was model ed. Nonlinear behavior of polyethylene was assumed. The polyethylene plate was constructed with varying radii, with a minimal thickness of 6 mm and wi th a width of 40 mm. The ratio of the cylinder radius to the radius of the polyethylene plate was defined as the conformity ratio; a conformity ratio of 0 represented a flat tibial inlay, whereas the highest ratio modeled of 0.99 was nearly conforming. The conformity ratios modeled were 0, 0.2, 0.4, 0.6, 0.7, 0.8, 0.9, 0.95, and 0.99. The loads applied were 1000 N, 2000 N, 3000 N, 4000 N, 5000 N, and 6000 N. The effects of different conformity ra tios and lends on the contact area (mm(2)), the compressive surface stress (MPa), the shear stress (MPa), and the von Mises stress (MPa) were investig ated. It was found that all of these parameters were affected by changes to the conformity ratio and to a lesser extent by load changes. That is, incr easing the load from 3000 N to 6000 N resulted in a surface and shear stres s increase lower than the increase in stress caused by the small change of the conformity ratio from 0.99 to 0.95. The effect of an increasing conform ity ratio on the reduction in stress was more pronounced for conformity rat ios above 0.8. In addition, the effect of a load increase for a flat tibial inlay was two times greater than for one with near full conformity.