The European Society for Biomechanics (ESB) awarded our publication about the “Standardized tibio-femoral implant loas and kinematic” Dreyer M. and Trepczynski A., Nasab S.H.H., Kutzner I., Schütz P., Weisse B., Dymke J., Postolka B., Moewis P., Bergmann G., Duda G.N., Taylor W.R., Damm P. and Smith C.R. with the M.S. Perren Award 2022.
Severe compression fractures of a vertebral body or a tumour in the region of the spine sometimes require the replacement of a vertebral body by an implant. The loads on such an implant are not well known. In order to measure these loads, the commercially available vertebral body replacement ‘SYNEX’ was modified. It allows the in vivo measurement of three force components and three moments acting on the implant. The 9-channel telemetry transmitter developed in our biomechanics laboratory was placed into the cylinder of the implant together with 6 load sensors and a coil for the inductive power supply. Usually, the spine is in addition stabilized dorsally by an internal spinal fixation device implanted from the back side.
The bone-based coordinate system was chosen according to ISO 2631. The x- axis in the median plane points anteriorly, the y-axis in the frontal plane to the left side, and the z-axis cranially.
The forces and moments are presented in the measuring units N and Nm.
Table with basic information about those patients who had vertebral body replacements:
Little was known about the loads acting on internal spinal fixators. In order to measure the loads, a commercially available implant was modified. A measuring cartridge was integrated into the longitudinal rod containing six load sensors, an 8-channel telemetry transmitter, and the secondary coil for the inductive power supply.
Both telemeterized fixators transmit their load values as a radio frequency pulse train outside the body. For the measurements, a flat power coil, fixed to the patient’s back, supplies the energy needed by both fixators. The power coil has an integrated antenna which delivers the signals to the external components of the telemetry system.
The internal fixators were implanted pairwise. All reported data came from the left implant and are reported in a right-handed coordinate system.
The measured load components act at the centre of the cylindrical part of the implant. The z-axis is
the long axis of the fixator and points upwards. The y-axis is parallel to the axis of the Schanz screw and points ventrally. The x-axis is perpendicular to both others and is directed to the right side. All force components Fx, Fy, Fz act in axis directions while the moment components Mx, My, Mz turn clockwise around the axes.
Due to the anatomical conditions at the implantation site this coordinate system does not coincide exactly with the sagittal and frontal plane of the upper body. The forces and moments are presented in the measuring units N and Nm.
Table with basic information about those patients who had instrumented spinal fixators:
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 componentsrelative 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: