Abstract
Introduction
Realistic knee contact forces and moments are needed for testing implant wear, fatigue and static strength, for analysing strains and remodelling at interfaces, as ‘gold standard’ for analytical musculo-skeletal models, or as input for finite element models. ISO 14243 defines the loading conditions for wear tests, but the defined loads from walking are based on very old data.
Methods
Therefore we compared the ISO loads with data obtained from instrumented tibial components with telemetric data transmission. Cruciate ligaments sacrificing total knee implants (Innex FIXUC, Zimmer) were equipped with inductively powered electronics and strain gauges to measure 6 force and moment components acting on the tibial component [1]. The coordinate system is fixed at the tibial component. -Fz acts inferiorly, +Fy anteriorly, and +Mz if the tibia is rotated outwards with the femur being fixed.
Loads were measured in 8 subjects (70 years Ø) during 10–20 repeated cycles of free walking at about 4 km/h. For each subject the load components were normalized to 75 kg body weight (BW) and averaged [2] (‘Aver75_1Subj’). These individual results were averaged again and delivered the ‘standard’ load components ‘Aver75’. From all 8 ‘Aver75_1Subj’ patterns of the resultant force Fres the highest peak value was taken and divided by the peak value of Fres in ‘Aver75’. The obtained factor was then multiplied with all ‘Aver75’ components and the BW increased to 100 kg. The final ‘standard’ components (‘High 100’) describe the highest average loads to be expected in a heavy-weight subject. This worst case but still realistic conditions should be used for wear and fatigue tests.
Results
The in vivo peak values of -Fz are up to 37% larger than in the ISO standard and the time course of the first peak is different. The first ISO-peak of Fy is totally lacking (‘?’). The second, negative peak is by 158% larger in vivo, while the third peak is by 43% smaller. The first (small) ISO-peak of Mz is nearly 3 times higher in vivo, while the second one is nearly twice as high.
Discussion
Reported are average loads which can act in heavy-weight subjects. All 3 components are much higher than in the ISO standard. Largest deviations exist for the posterior force Fy and the torque Mz in both rotational directions. Because the differences are very high, it is probable that wear in patients can be more severe than in simulator tests and may occur at different areas of the tibial plateau. During staircase walking the loads are probably even higher.
Data derived from implants cannot be transferred 1:1 to the natural knee. Fz and Fres are probably similar, but torque and transverse forces are presumably smaller in the natural knee where they are partially transferred by ligaments and soft tissues.
