A knee joint prosthesis assembly (10) according to the according to the present disclosure can include a femoral component (12), a unitary tibial component (614), a medial bearing (616), and a lateral bearing (618). The femoral component can have a medial (20) and a lateral (22) condyle portion connected by a patellar track portion (26). The femoral component can form an opening (30) between the medial and lateral condyles. The unitary tibial component can have a medial portion that includes a first posterior engagement lip (640) and a first outer securing lip (643). The tibial component can further include a lateral portion that includes a second posterior engagement lip (640) and a second outer securing lip (643). The medial bearing can have a first posterior groove (627) formed thereon that selectively engages the first posterior engagement lip and a first outer groove (629) that selectively engages the first outer securing lip.
