Finite Element Analysis and Torsional Stability Testing of Reference (Metaphyseal and Diaphyseal Fixation) and Experimental (Metaphyseal Fixation Only) Cementless Femoral Stems

Kim, Young Hoo

Abstract

PURPOSE
To report the details of the findings of finite element analysis and torsional stability testing of a reference stem (fixation in both metaphysis and diaphysis) and an experimental stem (fixation in the metaphysis only). MATERIALS AND METHODS: Finite element ABAQUS software (version 5.5) was used for all analyses. A Newton Raphson iterative solution scheme was used to calculate the nodal displacements and to solve the contact equations. Finite element models of the reconstructed proximal femur were developed. A model of bone geometry was developed from the digitized sections of a right adult human femoral specimen. Two femoral stems (reference and experimental stems) were designed in accordance with the bone geometry. Rotational micromotion of the implant relative to the proximal femoral cortical surface was measured using a single, linearly variable differential transducer (LVDT). Three piezoelectric transducers were used to detect the displacement of the stem in the femur. RESULTS: Under all interface conditions, the compressive stresses of the coating surface were below 1.5 MPa. Shear stresses in the two friction models were below 0.5 MPa, and below 1 MPa in the bonded model. The stress exerted over the cortex in the experimental model was 50% of that in the reference model. The relative displacement of the stem in the coated region was less than 0.05 mm, but it increased distally in a linear fashion and was 0.45 mm at the stem tip. A stress concentration in the proximal femoral cancellous bone was noticeably higher in the experimental model than in the reference model. However, the overall characteristics of stress transfer were not changed by stem shape modification. Experimental stem was found to have significantly less rotational micromotion and total permanent rotational displacement values (between 10 and 29 N.m) than the reference stem. CONCLUSION: The torsional stability of the experimental stem is enhanced by: increasing stem thickness in the anteroposterior plane; adding a lateral flare to the stem; and a congruent fit between the proximal medial portion of the stem and calcar femorale. A role of short and tapered distal stem of the experimental stem was negligible in providing with the stem stability and, therefore, the femoral distal stem can be removed. Distal stem removal can minimize stress shielding related bone resorption and can avoid thigh pain.