Purpose of the study: Implanting a femoral stem with a modular neck can modify the range and the position in space of hip rotation arcs. The purpose of this work was to evaluate changes in three versions of a modular neck and to define the determining criteria for the choice of the neck to implant.

Material and method: This series included 52 primary modular THA (ABGII) with ceramic bearings implanted with the HipNav 1.3 navigation system. The range of hip rotation were measured referring to the femoral saggital plane and the anterior pelvic plane. After insertion of the cup and the final ABGII stem and after choosing the length of the modular neck and the frontal inclination, the three different versions (retroversion −7, neutral 0 and anteverion +7) were tested. The range of hip rotation was measured by dynamic testing done under navigation. At the same time, the surgeon evaluated the stability and the absence of posterior impingement.

Results: In extension, mean range of rotation was 71° (102–123). It was modified by neck version. The position of the centre of rotation in relation to the reference rotation (rotation 0) depended on the version of the modular neck. The balance of the rotational arcs was better with a retroversed (−7) neck (mean centre of rotation -9) with a neutral neck (centre -13) or an ante-versed (+7) neck (centre-20). The determining factors were the version of the femoral stem and the combined (cup+stem) version. After checking the stability, the surgeon chose an anteversed neck in three cases (5.7%), a neutral neck in 25 (48%) and a retroversed neck in 24 (46.3%). The choice of the modular neck maintained the ligament balance in 71% of the hips.

Discussion: This demonstrates that the use of a prosthesis with a modular neck enables modulation of the rotational balance of the hip. This work demonstrates that work on balancing the rotational arcs of the hip in extension is a reliable operative criterion for choosing the version of the modular neck without using a navigation system.