A knee joint prosthesis assembly according to the present disclosure can include a femoral component, a unitary tibial component, a medial bearing, and a lateral bearing. The femoral component can have a medial and a lateral condyle portion connected by a patellar track portion. The femoral component can form an opening between the medial and lateral condyles. The unitary tibial component can have a medial portion that includes a first posterior engagement lip and a first outer securing lip. The tibial component can further include a lateral portion that includes a second posterior engagement lip and a second outer securing lip. The medial bearing can have a first posterior groove formed thereon that selectively engages the first posterior engagement lip and a first outer groove that selectively engages the first outer securing lip.