The Need for Patient-Specific “Safe Zones” in Total Hip Arthroplasty

Objectives Appropriate acetabular cup orientation is an important factor in reducing instability and maximising the performance of the bearing after Total Hip Arthroplasty (THA). Due to the large variation in patient kinematics between functional activities, current technologies lack definition of what constitutes correct target alignment. Acetabular cup orientations placed within Lewinnek’s proposed safe zone can dislocate due to functional component malalignment. Ideal acetabular cup orientation requires a more advanced analysis accounting for individual variation in functional positions. This study aims to incorporate functional imaging in the planning of acetabular cup orientation and retrospectively compare these parameters to the Lewinnek safe zone.

Methods Pre-operatively 160 consecutive total hip replacement patients underwent acetabular planning utilizing a pre-operative CT scan and functional imaging. As part of the planning, radiographic parameters including inclination, anteversion, pelvic tilt, pelvic incidence and lumbar flexion were analysed. Using these inputs, the software proposes an optimal cup orientation for each patient. Retrospectively the software proposed cup orientation was compared to the Lewinnek safe zone.

Results and Conclusion Mean age of patients was 59 years (29 to 82). Mean pelvic tilt in supine position was 4.9° (-16.8° to 28.0°). Mean pelvic tilt in standing position was 1.7° (-20.2° to 30.6°). Mean pelvic tilt in the flexed-seated position was -5.7° (-39.6° to 27.8°) Mean planned cup inclination in reference to the anterior pelvic plane (APP) was 42° (33° to 49°) and mean planned cup anteversion, referenced to the APP, was 26° (11° to 43°). Only 44 % of cups planned with functional imaging were within the Lewinnek safe zone for both inclination (40°±10°) and anteversion (15°±10°) (Fig 1).

In conclusion, our study demonstrates that historical target parameters for cup inclination and anteversion significantly differ from target values obtained with the use of functional imaging. Understanding the individual spinopelvic mobility allows for the creation of a functional and patient-specific target for acetabular component orientation.

Stichwörter Total Hip Arthroplasty, Cup orientation, Pre-Operative planning, Functional pelvic tilt, Dynamic planning

Publication History

Article published online:
15 October 2020