Cup medialisation prevents anterior overhang in total hip replacement.

Presenting Author – Sanjeev Gupta
Introduction To improve abductor efficiency after total hip replacement (THR), Charnley proposed medialisation of the acetabular cup, with respective increase in femoral offset. In contrast to cup medialisation, an alternate philosophy of reproducing the preoperative centre of rotation without medialisation, has been proposed. The aim of this study was to investigate the incidence of anterior shell overhang when the preoperative centre of rotation is reproduced, and the cup is not medialised. Methods Twenty THR patients with pre-operative CT scans were randomly selected. Acetabula with significant rim osteophyte were excluded. The preoperative femoral head centre was identified from a 3-dimensional reconstruction of the proximal femur. Cup sizing was done in 3D and confirmed by the senior author. The planned cups were virtually implanted into the acetabulum, with the shell centre coincident with the preoperative femoral head centre. Cup anteversion was aligned with the acetabular notch to mimic being parallel to the transverse acetabular ligament. Cup inclination was kept constant at 40°. With this orientation and depth, the incidence of shell overhang beyond the native anterior rim was recorded. Results Of the 20 cases, 13 had acetabular shell proud of the native anterior bony rim. This constituted 65% of patients. All cups showed posterosuperior uncoverage.
Conclusions
• Anterior shell overhang can lead to psoas irritation and groin pain in THR patients
• Failing to medialise the centre of rotation, in combination with reproducing native anteversion, can lead to anterior shell overhang in over 50% of cases.
• Some degree of cup medialisation is recommended in THR.