Surgeons’ accuracy in achieving their desired acetabular component orientation

Introduction:
Wide variability in cup orientation achieved is reported. Cup orientation dialled by the surgeon at implantation determines the resultant component orientation. The aims of this study were to determine 1. How accurate surgeons are at orientating the acetabular component and 2. Whether factors such as visual cues and side of operating table improved their accuracy.

Methods:
A pelvic model was positioned in neutral alignment on a theatre table and prepared as per a posterior approach. 21 surgeons (10 trainers and 11 trainees) were tasked with positioning an acetabular component in a series of tasks. The orientation of the component was measured using stereo-photogrammetry and the difference between the achieved and the target orientation was calculated. Tasks included: stating what surgeon's preferred orientation is and thereafter placing the cup in that orientation; reproducing visual cues (transverse acetabular ligament and alignment jig) and estimating orientation whilst on the assistant' side.

Results:
The preferred inclination/anteversion was 42/21°. On average surgeons reduced inclination by 4° and increased anteversion by 11° when tasked with replicating their desired orientation. The intra- and inter- surgeon variability (defined as 2SD) in achieving a target orientation was 10-16°. The use of visual cues, such as the transverse acetabular ligament or the alignment guide, significantly improved accuracy for both inclination and anteversion to 3°. In addition, the use of an alignment guide reduced the variability by a third. Trainees and trainers had similar accuracy and variability. There is greater variability in assessing cup inclination when standing on the assistants' compared to the surgeon's side of the table, which has implications for training.

Conclusions:
Surgeons overestimate operative inclination and under-estimate anteversion, which is of benefit as this, on average, helps achieve the desired radiographic cup orientation. Although the use of visual cues helps, conventional techniques result in a large variability in acetabular component orientation. New and better guides and methods for training surgeons need to be developed.