Abstract
Introduction
All current methods of cup placement use anterior pelvic plane (APP) as the reference. However, the majority of studies investigating the measurement of anteversion (AV) and abduction angles (AA) are inaccurate since the effect of pelvic tilt and obliquity are not considered. The aim of this study was to describe a reproducible, novel technique for functional cup positioning using internal and external bony landmarks and the transverse acetabular ligament (TAL).
Methods
The pelvic obliquity and tilt are measured on the pre-operative weight bearing AP and lateral pelvic radiographs. Intra-operatively, the highest point of the iliac crest is identified and a line is drawn to the middle of the greater trochanter with knee flexed to 90 degrees and leg thigh horizontal to the floor, parallel to the APP. The cup is placed parallel to the TAL and inside the anterior acetabular wall notch, and then is adjusted for the femoral anteversion, pelvic tilt and obliquity. The angle between the drawn line and the cup handle is the operative anteversion. 78 consecutive total hip replacements (76 patients) were performed using this technique. The functional cup orientation was measured on post-operative weight bearing pelvic radiographs using EBRA software.
Results
The mean follow-up was 1.2 ± 0.3 years. There were no fracture, dislocation or infection. The mean functional AV and AA were 17.9° ± 4.7° (7.8–28.7) and 41.7° ± 3.8° (33.4–50), respectively. The mean pelvic tilt and obliquity were −3.1° ± 9.7° (−25–9) and −1.5° ± 3.2° (−9.9–7.4), respectively. 96% of functional AV and 100% of functional AA measurements were within the safe zone.
Discussion and Conclusion
This is an easy, accurate, and reproducible technique, which uses bony landmarks and TAL, adjusted for femoral anteversion and pelvis tilt and obliquity. Weight-bearing radiographs should be used to standardise the measurements with the goal to reproduce the functional cup orientation within the safe zone.
