Modular necks restore safe range of motion in total hip arthroplasty with extreme femoral version

Modular neck prostheses in total hip arthroplasty (THA) offer intraoperative flexibility for adequate femoral anteversion and offset, which is particularly beneficial in anatomically complex cases such as developmental dysplasia of the hip. However, the efficacy of current modular neck options on range of motion in addressing abnormal femoral anteversion is unclear. This study aimed to evaluate the effectiveness of current modular neck configurations with a 32-mm femoral head, in accommodating various femoral anteversion.

A prosthetic impingement analyses was conducted by collision detection using computer-aided design models of a dual modular neck femoral stem with four neck configurations: 15° anteverted, straight, 15° retroverted, and 30° retroverted. The cup safe zone, defined as the combination of cup inclination and anteversion that satisfied all the required range of motion criteria without prosthetic impingement, was assessed across stem anteversion angles (-25° to 85°).

The four modular neck configurations collectively provided continuous safe zones for stem anteversion ranging from 0° to 75°, at a clinically recommended cup inclination of 40°. In contrast, the straight neck alone provided it in limited range (10°-40°), making it inadequate for managing abnormal femoral anteversion. Notably, the constant combined anteversion concept did not hold at extreme anteversion values.

A modular neck system offering configurations from 15° anteverted to 30° retroverted, when combined with 40° cup inclination, effectively accommodates various femoral anteversion. These findings support the clinical utility of modularity in THA and underscore the importance of optimizing stem anteversion to address complex anatomical variations.