Micromations at the tape junction of modular hip prostheses
S. Jauch, G. Huber, M.M. Morlock
Since 1972 modular hip prostheses were used in orthopaedic surgery. Modularity of the femoral component of total hip implants became popular because neck length and femur offset could be adjusted intraoperatively. Furthermore it is possible to combine different materials like metals and ceramics. The downside of this flexibility is an additional joining area which bears the risk of incorrect assembly as well as fretting and crevice corrosion.
So far only few studies have experimentally investigated the relative motion in the taper lock interface, which might play a role for occasionally observed prostheses failures. Micromotions in the taper lock interface can lead to a constant abrasion of the neck piece`s passivation layer resulting in fretting and fatigue fracture of the prosthesis. Contamination of the taper joining area might considerably increase the phenomenon.
In this study the fretting and crevice corrosion caused by micro motions at the mating surface is investigated. Inside into the failure mechanism and the influencing factors might help to prevent implant failures in the future.
After quantification of relative motions at the connecting element with neck pieces made of titanium and CoCrMo for different joining conditions suitable test methods for preclinical testing should be developed. A finite-element-model highlights areas with diminished contact forces which are in accordance with zones with increased probability of huge relative motions at the modular interface. Thereby it will be possible to test modular hip prostheses before clinical application in order to avoid prostheses fractures.
Presently, modular hip prostheses are embedded in methyl methacrylate according to ISO 7206-4 and loaded by a servohydraulic test machine (MTS MiniBionixII) with an axial sinusoidal load from 230N to 2300N. To determine the relative motion between the stem and the neck of the prosthesis three eddy current sensors (Micro-Epsilon) are screwed into a holder which is mounted on the stem. As a counterpart a clamp is fixed at the neck piece of the prosthesis.
This study is financially supported by Aesculap AG, Tuttlingen.