Torques in total hip replacements

Elevated frictional torques at the articulating interface of total hip replacements (THRs), if transferred, have the potential to affect component fixation and/or induce micro motion linked to fretting and corrosion at modular junctions. Pre-clinical testing of friction behaviour in THRs has predominantly been assessed under simplified test conditions using uniaxial simulators. These methodologies and simulators are not sufficient if assessment of friction under more clinically relevant conditions, such as multi-axis motion and consideration of surgical implant positioning, are to be investigated.
To identify a suitable measuring system and develop methodologies for assessing the frictional torques in THRs, studies were conducted whereby comparisons between a newly developed electromechanical multi-axis single station hip simulator (SSHS) and an existing uniaxial hip simulator as well as a custom-built friction measuring subsystem were made. Additionally, analyses of free-body diagrams in static equilibria and functional verification testing of the SSHS was completed. The assessments conducted showed that the SSHS could detect THR frictional torques without the use of the custom-built friction subsystem; and that the sub system did not provide the additional information expected.
Methodologies for assessing THR frictional torques were developed on the SSHS and used to assess the effects of translational positioning (i.e. testing when there was offset between the head and cup centres of rotation) on THR frictional torques. Increasing the applied medial-lateral (ML) translation, as might occur in a patient when there is a lateral misalignment between the centres of the femoral head and acetabular cup, under 1kN constant load and biaxial motion increased the measured frictional torques at the bearing interface of 36mm metal-on-polymer bearings, to over 15Nm. Elevated frictional torques, if transferred to the fixation interface of the acetabular cup, may induce shear stresses that could result in cup displacement.
To determine whether the measured elevated frictional torques observed during simulator testing could cause potential displacement in an implanted press-fit acetabular cup, a load-to-failure test that applied a simultaneous 300N or 3kN femoral head load was developed. Results indicated that the elevated frictional torques measured under ML translation on the SSHS were not likely to cause cup displacement in 10 and 20pcf Sawbone blocks when either axial load was applied.
This thesis reports on developed new methodologies for assessing THR frictional torques under variations in surgical positioning and for assessing torques required to cause cup loosening under axial loading in load-to-failure tests. It was demonstrated for the first time using a multi-axis hip simulator that surgical implant malpositioning can result in elevated frictional torques at the bearing interface of THRs. However, the impact of these elevated frictional torques on cup fixation requires further work to examine surgical implant malpositioning in THRs under more representative loading and motion conditions.