A knee joint prosthesis assembly comprising: a femoral component having a medial and a lateral condyle portion (20,22) connected by a patellar track portion, the femoral component forming an opening between the medial and lateral condyles; a unitary tibial component having a first outer securing lip located on a medial portion and a second outer securing lip located on a lateral portion; a medial bearing having a first posterior groove formed thereon; and a lateral bearing separately formed and independent from the medial bearing and having a second posterior groove formed thereon; characterised in that the medial portion of the unitary tibial component includes a first posterior engagement lip and the lateral portion of the unitary tibial component includes a second posterior engagement lip, wherein the medial bearing is configured to be slid into engagement with the medial portion of the unitary tibial component such that the first posterior groove formed on the medial bearing engages the first posterior engagement lip and a first outer groove formed on the medial bearing engages the first outer securing lip; and the lateral bearing is configured to be slid into engagement with the lateral portion of the unitary tibial component such that the second posterior groove formed on the lateral bearing engages the second posterior engagement lip and a second outer groove formed on the lateral bearing engages the second outer securing lip; wherein the knee joint prosthesis assembly further includes a locking bar configured to be selectively located between the tibial component and the medial and lateral bearings, the locking bar being adapted to secure the medial and lateral bearings to the tibial component. a femoral component having a medial and a lateral condyle portion (20,22) connected by a patellar track portion, the femoral component forming an opening between the medial and lateral condyles; a unitary tibial component having a first outer securing lip located on a medial portion and a second outer securing lip located on a lateral portion; a medial bearing having a first posterior groove formed thereon; and a lateral bearing separately formed and independent from the medial bearing and having a second posterior groove formed thereon; characterised in that the medial portion of the unitary tibial component includes a first posterior engagement lip and the lateral portion of the unitary tibial component includes a second posterior engagement lip, wherein the medial bearing is configured to be slid into engagement with the medial portion of the unitary tibial component such that the first posterior groove formed on the medial bearing engages the first posterior engagement lip and a first outer groove formed on the medial bearing engages the first outer securing lip; and the lateral bearing is configured to be slid into engagement with the lateral portion of the unitary tibial component such that the second posterior groove formed on the lateral bearing engages the second posterior engagement lip and a second outer groove formed on the lateral bearing engages the second outer securing lip; wherein the knee joint prosthesis assembly further includes a locking bar configured to be selectively located between the tibial component and the medial and lateral bearings, the locking bar being adapted to secure the medial and lateral bearings to the tibial component.