Instrumented Knee Implant
The instrumented knee implant measures 3 force and 3 moment components, acting on the tibial component (Heinlein et al., 2007). Its design is based on the INNEX FIXUC total knee system (Zimmer GmbH, Winterthur, Switzerland) with an ultra-congruent tibial insert and a standard femoral component. Both cruciate ligaments are sacrificed. From the load-dependent deformations of its stem, which are measured by 6 semi-conductor strain gages the 6 load components are calculated. All signals are sensed and transmitted by a custom-made, inductively powered telemetry circuit (Graichen et al., 2007).
Coordinate System
The centre of the coordinate system is fixed at the right tibial component on the extended stem axis at the height of the lowest part of the polyethylene insert (Figure). Forces and moments measured in left knees were transformed to the right side. Force components +Fx, +Fy and +Fz act in lateral, anterior and superior direction. Moments +Mx, +My, +Mz act in the sagittal, frontal and horizontal plane of the tibia and turn right around their respective axes. From the components, the resultant forces F and moments M were calculated. More detailed information on the load transformations relative to the femur can be found here.
Figure: Coordinate system of the instrumented tibia