Assessments of different kinds of stems by experiments and FEM analysis: appropriate stress distribution on a hip prosthesis.

BACKGROUND

It is now recognized that initial stability is essential for avoidance of thigh pain in hip replacement. The initial stability corresponds to an optimal stress distribution of cementless orthopedic implants. Although the relationship between the contour and stress at the fixation site has been analyzed, guidelines on stem design have not been established.

METHODS

Finite element models of three currently-used stems were constructed for a computer simulation. Contact stress at the fixation site of a joint prosthesis was analyzed by an explicit three-dimensional finite element method. The stress immediately after applying load using a film or sensor which can measure contact stress was observed. The situation of the initial fixation about the specific part which becomes important clinically based on the results was clarified.

FINDINGS

We introduced fluctuation area as a measure to evaluate the primary fixation of femoral stems. It was found that the stress distribution on the PerFix SV stem fluctuated with a slight disturbance. On the Intra-Medullary Cruciate stem, the high stress areas were distributed on the proximal area and under the pin. The high stress area on the VerSys stem were spread on the medial side.

INTERPRETATION

This study highlights the mechanical instability of the fixation site of joint prostheses, and thus suggests that the general idea that unconditionally recommends a larger area for the fixation site of joint prostheses should be revised.