A unicompartmental knee prosthesis for implantation into a knee joint includes a body having a substantially elliptical shape in plan and a pair of contoured opposed faces including a first face and a second face, the second face including a concave surface. The prosthesis is free of fixation to a permanent location within the knee joint compartment. A peripheral edge extends between the faces and has a first side, a second side opposite the first side, and a first end and a second end opposite the first end, the body including a cusp comprising a reverse curve downward from to the first face at one of the first and second ends. A first dimension D is defined by the first end and the second end, and a second dimension F is defined by the first side and the second side. A coordinate system origin (CSO) is at an intersection of D and F, the body including a material thickness at the CSO, where the dimension F is from about 0.25 to about 1.5 of the dimension D.