Abstract
Introduction
Pelvic posterior tilt change (PPTC) after THA is caused by release of joint contracture and degenerative lumbar kyphosis. PPTC increases cup anteversion and inclination and results in a risk of prosthesis impingement (PI) and edge loading (EL). There was reportedly no component orientation of fixed bearing which can avoid PI and EL against 20°PPTC. However, dual mobility bearing (DM) has been reported to have a large oscillation angle and potential to withstand EL without increasing polyethylene (PE) wear against high cup inclination such as 60∼65°.
Objective
The purpose of this study was to investigate the optimal orientation of DM-THA for avoiding PI and EL against postoperative 20°PPTC.
Methods
Our study was performed with computer tomography -based three-dimensional simulation software (ZedHip. LEXI co. Japan). The CT data of hip was derived from asian typical woman with normal hips. Used prosthesises were 50mm cup and 42mm outer head of modular dual mobility system and Accolade II 127°(stryker). Femoral coordinate system was retrocondylar plane with z-axis from trochanteric fossa to intercondylar notch. Cup orientation was described as anatomical definition. The safe zone was calculated by the required hip range of motion which was defined as 130°flexion, 40°extension, 30°external rotation, and 50°internal rotation with 90°flexion and the maximum inclination of DM cup which was 60°in consideration of withstanding EL. Cup orientations withstanding 20°PPTC were defined as the primary cup orientation which changes consistently within the safe zone with the match of 20°PPTC. And among them cup orientation with lowest inclination was defined as the optimal cup orientation.
result
The optimal orientations could be identified only within stem anteversion from 15°to 40°. The relationship between the optimal cup orientation and stem anteversion could be automatically identified. The correlation between stem anteversion and cup anteversion was linearly distributed and could be expressed as an approximated line of the formula that (stem anteversion)+(cup anteversion)=36.8. And likewise the relationship between stem anteversion and cup inclination was curved-linerly distributed and could be expressed as an approximated curved line of the formula that (cup inclination)=0.04(stem anteversion)2 2.18(stem anteversion)+74.8. Cup orientation calculated by the Widmer's combined anteversion theory is easily deviated from the safe zone by PPTC. The optimal cup orientation calculated in this study could be set more inclination and retroversion than it calculated by the Widmer's theory in contribution of large oscillation angle and admissibility of high inclination cup setting of DM. Therefore it could be possible to withstand 20°PPTC.
Conclusion
Performing THA with considering postoperative PPTC is necessary for good long term outcome without dislocation and PE wear. The solution for 20°PPTC after THA is to apply dual mobility bearing and the formula of combined orientation theory calculated in this study.
