Abstract: Endoprosthetic knee joint devices are described in British Patent No. 1,534,263 comprising an associated pair of femoral and tibial components respectively having convex and relatively flattened articulatory bearing surfaces, and an intervening meniscal component having opposed articulatory bearing surfaces respectively substantially complementary with said femoral and tibial surfaces. It is now proposed that such devices be improved in stability by the provision of mutually slidably engageable rib and groove formations of longitudinally curved form, one each in the complementary tibial and meniscal surfaces, the groove preferably being located in the meniscal component.