Instrumented joint prostheses offer the possibility of measuring in vivo loads during activities of daily living. To analyze the complex kinetic situation in the knee joint, a six degree-of-freedom measurement is essential. A tray-in-a-tray tibial baseplate design was developed to measure the resultant forces and moments. The strain distribution within the double wall central stem of the baseplate is measured by means of strain gages. In combination with a pre-operative calibration procedure the forces and moments in the knee joint are subsequently calculated. Unfortunately, the same resultant force may deform the baseplate and subsequently the hollow stem differently, depending on the medial/lateralload distribution and the corresponding lever arms. Thus, the resulting measuring error depending on different implant geometries should be analyzed by means of a finite-element-analysis (FEA).

Different baseplates were designed using a 3D CAD-software (Unigraphics V18, EDS). These models were imported into the finite-element package (Patran 2001r3, MSC; Abaqus, HKS). The tibial baseplate was meshed automatically using tetraeder elements. The refinement of the mesh was controlled by means of mesh seeds for the central hollow stem. A 2 mm thick ring of bone, simulating the cortical shell, supported the tibial base-plate. No trabecular support was modeled to create a worst-case scenario for the implant. Tibiofemoral forces were applied in 3 directions on two contact areas, representing the femoral condyles. In the transversal plane the location of these contact areas was varied, simulating ML-movement and axial rotation. The resultant forces and moments were kept constant.

The proposed design shows an influence of the load transfer mode on the strain distribution in the stem, which is below 2%.

The accuracy of the proposed design is further encouraging the development of an instrumented knee prosthesis.