Abstract
Introduction
Dual-mobility (DM) liners have increased popularity due to the range of motion and stability provided by these implants. However, larger head diameters have been associated with anterior hip pain, due to surrounding soft-tissue impingement, particularly the iliopsoas. To address this, an anatomically contoured dual mobility (ACDM) liner was designed by reducing the volume of the liner below the equator (Fig1). Previous cadaver studies have shown that the ACDM significantly reduces iliopsoas tenting and trapping of the liner compared to conventional designs. We created a finite element study based on previous cadaver testing to further analyze the effectiveness of the ACDM design in reducing soft-tissue impingement, specifically the tendon-liner contact pressure and the tendon stress.
Methods
The finite element model was developed within COMSOL 4.3b. The psoas tendon was modelled as a Yeoh hyper-elastic Material, which uses 3 constants (c1-c3), density (1.73g/cm3) and a bulk modulus (26GPa)[Hirokawa,2000]. In a previous, separate study, the average stiffness of 10 psoas tendon samples (5 cadavers), were measured to be 339[N/mm] in the linear region with average width and thickness of 14mmX4mm. The 3 constants were tuned to match experimental uniaxial test data, and were 5[GPa], 0[Gpa], and 46[GPa] for c1, c2, and c3 respectively.
The implant components were rigidly modeled relative to the psoas. Cadaver specific CT models were used to create the FEA geometry. The insertion points for the Psoas were digitally determined on the proximal end of the lesser trochanter, and the psoas notch on the pelvis for hip flexion angles of −15°, 0°, 15° and 30°. These insertion points determined the length of the psoas and its relative position to the femoral head in 3D. The specific liner size and position for each cadaver was determined by implant planning with the CT models. In this abstract, we only present data for 2 specimens (left/right hips) with 44mm conventional DM, and 44mm ACDM, matching specimen anatomy. A 500N tensile load was applied to the psoas tendon proximally to simulate moderate physiological loading, the average/max stresses and contact pressures between the psoas and the two liner designs were determined.
Results
At all flexion angles from −15° to 30°, the ACDM had lower psoas-liner contact pressure and stress compared to the conventional liner. Both contact pressure and tendon stress decreased for both liners with increasing hip flexion. At −15° flexion angle, there was an average contact pressure difference of .51MPa between the conventional and ACDM designs, or 37% decrease in pressure when using the ACDM. The average difference in tendon stress was 67.9MPa, or a 59% decrease in stress when using the ACDM (fig2, fig3).
Conclusion
This study utilized cadaver specific FEA models to evaluate interaction between the iliopsoas tendon and conventional and ACDM liners. Although this abstract presented FEA models for only four hips (two specimens), the results show a notable reduction in contact pressure and tendon stress with ACDM designs. This validates findings from previous cadaver studies, suggesting that anatomically contoured designs could reduce anterior hip pain and soft tissue impingement.
