Background: The primary fixation of cementless hip prostheses is related to the shape of the stem. When there is a complication of loading in the rotational direction, the mechanical fixation of a hip stem is considered to provide good primary fixation. The purpose of this study was to evaluate whether the Intra-Medullary Cruciate stem with a characteristic fixation method, which was developed by a group at Kitasato University, contributes to primary fixation by finite element analysis.
Methods: Analysis was performed at a friction coefficient of 0.1 with automatic contact, under the restriction of the distal femoral end. The following three loading conditions were applied: (1) step loading of the joint resultant force in the region around the hip stem; (2) loading in the rotational direction, simulating torsion; and (3) loading of the femoral head equivalent to that during walking. Displacement of an Intra-Medullary Cruciate stem and a reference stem along the x-, y-, and z-axes and rotational direction was calculated by simulation, and the stress distributed on the stem and femur was determined.
Results: Relative displacement along the z-axis of distal parts, which is a clinical problem with hip prosthesis stems, was lower for the Intra-Medullary Cruciate stem than for the reference stem. Displacement of the stem along the z-axis direction was low, indicating a low risk of sinking. The interlocking mechanism, which is a characteristic of the Intra-Medullary Cruciate stem, functioned to suppress its displacement, indicating that the locking method of this stem contributed to its stability. Because no stress concentration was detected in certain regions, it was thought that there are no risks of breakage of the Intra-Medullary Cruciate stem and femur.
Conclusions: It was suggested that effective fixation of the Intra-Medullary Cruciate stem can be achieved because its displacement is lower than that of the reference stem and displacement of the stress level is appropriate for primary fixation.