Rotational stability based on displacements obtained by simulation and measurement when torsion loading is applied to...

The rotational stability of cementless hip prostheses corresponds to the design of the fixation parts. The appropriate design of a femoral stem is important for secure primary fixation. The relative displacement of the bone and stems in the rotational direction should be used to evaluate the initial fixability and stability of stems. This article addresses the issue of the fixation method of hip stems and their rotational stability.

Specimens comprised four kinds of hip prosthesis. Rotational displacement that reproduced a torsion moment was analyzed and measured. Various finite element models (FEMs) of the four femoral stems were constructed for computer simulation. The measurement models consisted of an artificial femur and real stems used clinically. Common conditions of analyses and measurements were as follows: (1) a torsion load of 18.9 Nm was applied to the proximal femur as the intrarotation; (2) a stepping load of 1800 N was applied to the proximal tip of the stem; and (3) rigid contact existed between the distal end of the model femur and the rigid base.

It was found that the relative rotational displacement obtained by FEMs was 0.21 mm for the Intra-Medullary Cruciate stem, 0.10 mm for the VerSys stem, 0.67 mm for the PerFix SV stem, and 0.03 mm for the Duetto SI stem. The relative rotational displacement obtained by the laser sensor was 0.37 mm for the Intra-Medullary Cruciate stem, 0.25 mm for the VerSys stem, 1.87 mm for the PerFix SV stem, and 0.17 mm in the Duetto SI stem.

Judging from rotational displacement values obtained by the two approaches, three types of stem (Intra-Medullary Cruciate stem, VerSys stem, Duetto SI stem) provided rotational stability.