Abstract
Introduction
The initial stress distribution in the femur after total hip arthroplasty (THA) influences the remodeling of the bone and the clinical results. We conducted thermoelastic stress analysis to evaluate the surface stress distribution in femurs after THA in vitro and elucidated the changes in the stress distribution that were due to the stem design. Using this method, we can analyze the change in the sum of the principal stresses on the basis of the changes in temperature at the bone surface and visualize the change three-dimensionally.
Objectives
We compared the thermoelastic stress analysis results with the clinical results of specially shaped stems that have two flanges, one anterior and one posterior.
Methods
For the mechanical assessment, the stems (Bicontact and Excia stems, B. Braun, Melsungen, Germany) were inserted in synthetic femurs (Pacific Research Laboratories, Vashon, WA, USA), a 0.27-Hz sinusoidal compressive load (20–1000N) was applied to the femoral head for the thermoelastic analysis, and surface stress distribution images were taken in the four planes. Furthermore, the stem positions on the computed tomography (CT) images obtained after the procedure were transferred to those obtained before it, and the contact patterns, visualized. For the clinical assessment of the Excia stem, the bone reactions around the stem were analyzed using x-ray images obtained from patients two years after THA (43 cases, 45 hips). And the contact patterns on the CT images were analyzed using a modified Gruen zone system.
Results
The mechanical assessment showed compressive stress in the medial portion and tensile stress in the lateral portion of both stems. The stress distribution of the Excia stem was observed more proximally than that of the Bicontact stem (Figure 1). This finding was similar to that obtained on the CT images (Figure 2). The clinical assessment of the Excia stem showed no stress shielding (SS) or else first- or second-degree SS in 93% of the hips and third-degree SS in 7% of them. CT imaging showed that the stem contacted the cortical bone at the front of zones 2 and 3 in 100% and 75% and at the back of zones 5, 6, and 7 in 94%, 25%, and 94% of the cases, respectively. It did not contact the cortical bone in any other zones.
Conclusions
The following can be predicted: The load is transferred to the middle and distal portions of the Bicontact stem and to the proximal portion of the Excia stem. Severe SS was not observed in the clinical results obtained for the Excia stem. The load appeared to be transmitted ideally, because the stem contacted the bone at the front of zone 2 (the anterior flange) in all cases. Thermoelastic stress analysis provided full-field visualization of the surface stress distribution in femurs after THA and the mechanical results were related to the clinical results. Therefore, thermoelastic stress analysis is an influential method that can predict the clinical results in the development of a new design stem.
