An implantable knee prosthesis for unicompartmental implantation into a knee joint includes a body having a substantially elliptical shape in plan and a pair of opposed faces. A peripheral edge extends between the faces and has first and second opposed sides and first and second opposed ends. At least one keel member extends from one of the faces. A first dimension D is defined by the first and second ends. A second dimension F is defined by the first and second sides. The dimension F is from about 0.25 to about 1.5 of the dimension D.