Mechanical Effects of Offset and Length of the Cementless Stem for Initial Fixation to the Femur.

Backgroud: Implants with different neck offsets for hip replacement surgery are now available from various implant manufacturers and have become a widely used option for achieving postoperative hip stability. This study aimed to compare the impact of neck offset on initial stem fixation and the mechanical effects of different stem lengths when using cementless stems.

Methods: We performed a finite element analysis using Mechanical Finder ver. 12.0. CAD models of the Profemur Preserve and Profemur TL cementless stems. Each stem was appropriately sized, and the von Mises stress was calculated. We defined micromotion as the relative displacement between the stem node and the surface of bone contact. The maximum micromotion values of these finite element models were compared under standing conditions.

Results: The stress per zone for both stems (Preserve and TL) was the highest in zone 5, followed by zones 3 and 4, which were almost in line with each other. The high offset (HIGH) stress was higher than the standard offset (STD) stress in each stem and zone. The micromotion of each stem was higher at each load in the following order: Preserve HIGH, Preserve STD, TL HIGH, and TL STD, with HIGH being higher than STD at each stem.

Conclusions: The choice of higher offset or shorter length stems induced higher micromotion at the interface to the bone in the early postoperative period. Therefore, surgeons should be more careful to get appropriate initial fixation using shorter stems with higher offset necks due to the relatively high incidence of loosening or fractures.