In order to obtain realistic loading data, a knee implant with a 9-channel telemetry transmitter was developed which enables six-component load measurements in a primary total knee replacement. Both forces in axial, medio-lateral and anterio- posterior direction and flexion-extension, varus-valgus and internal-external moments can be measured.The instrumented knee joint is a modification of the INNEXTM System, Type FIXUC (Zimmer GmbH, Winterthur, Switzerland). The standard femur component and tibial insert are used. Only the tibial component was modified to enable the integration of the electronic devices. During modification of the tibial component, the patients’ safety was deemed to be especially important.
The coordinate system of the instrumented knee implant is a
a right- handed coordinate system fixed at the right tibial implant (not at the bone!). If forces and moments are measured in a left knee, they are transformed to the right side. The coordinate system is located at the height of the lowest part of the polyethylene insert. The z-axis is aligned with the stem axis of the implant.
The force components +Fx, +Fy and +Fz act in lateral, anterior and superior direction on the tibial tray. The moment Mx acts in the sagittal plane of the tibial component and turns clockwise around the +x-axis. The moment My acts in the frontal plane and turns clockwise around the +y-axis and the moment Mz turns clockwise
around +z-axis in the transverse plane. A positive moment Mz acts if the tibial implant component (or the femur) rotates inwards and/or if the tibia bone rotates outwards. The OrthoLoad videos show the load components relative to the tibial tray. The stem axis z of the tibial implant component is rotated backwards in the sagittal plane by about 7 degree relative to the long axis of the tibia bone. This slope of the implant varies inter-individually.
Table with basic information about the knee joint patients:
|Patient||Side||Gender||Weight [kg]||Height [cm]||Age at Implantation [years]||Tibio-femoral angle||Posterior slope||Indication|
|K1L||left||m||100||177||63||3.0 – varus||5||Osteoarthritis|
|K2L||left||m||93||171||71||5.0 – varus||11||Osteoarthritis|
|K3R||right||m||95||175||70||3.5 – varus||10||Osteoarthritis|
|K4R||right||f||92||170||63||4.5 – valgus||3||Osteoarthritis|
|K5R||right||m||94||175||60||1.0 – varus||7||Osteoarthritis|
|K6L||left||f||76||174||65||4.0 – valgus||7||Osteoarthritis|
|K7L||left||f||70||166||74||6.5 – varus||7||Osteoarthritis|
|K8L||left||m||77||174||70||4.0 – varus||11||Osteoarthritis|
|K9L||left||m||100||166||75||7.0 – varus||6||Osteoarthritis|