Comparison of interface stresses and strains for onlay and inlay unicompartmental tibial components

There are several types of designs used for unicompartmental tibial components. The all-plastic inlay component is recessed and it preserves bone around the outer edge of the tibia. For an onlay component, the entire condyle is resected, and the plastic bearing is usually metal-backed, although all-plastic components are also available. The purpose of this study was to investigate the hypothesis that while 6-mm inlay components require less bone removal, the peak stresses and strains at the surface of the bone will be much greater when compared with 8-mm metal-backed onlay components, and that all-plastic onlays will be only a slight advantage over inlays. Tibial models were generated using computed tomography (CT) scans, while typical inlay and onlay components were modeled. Finite element analyses of bones and components were completed by assigning material properties based on the CT scans and applying loads. Results indicated that plastic inlays generated 6 times more peak stress at the tibial surface when compared with metal-backed onlays. Moreover, models using inlay components produced strain values exceeding onlay components by a factor of 13.5 due to areas of softer bone at the interface. Off-center loading toward the anterior or posterior of the components produced similar results. The stresses and strains for the 8-mm all-plastic onlay were reduced compared with the inlay but still much higher than for the metal-backed onlay. These findings indicated that metal-backed onlays may be a better option when considering load distribution on the tibial surface.