Abstract
Shortened humeral stem implants can be advantageous as they preserve more of the patient's bone and are not limited by the canal for placement in the proximal body. However, traditional longer stems may help stabilize the implant through interaction with the dense cortical bone of the canal. We developed an FEA model to gage the contributions of design features such as stem length, coatings, and interference fit.
Models were constructed in FEMAP and solved using the NX Nastran advanced nonlinear static solver. The Turon (DJO Surgical) implant geometry was imported from a Solidworks CAD file and bone geometry was taken from a statistical shape model by Materialise representing the mean humeral geometry of 95 healthy humeri (avg age = 69.9 years). Implant and cancellous bone were considered to be linear homogeneous materials, and the cortical shell was modeled as orthotropic. Interference fits between the implant and cancellous bone surfaces were modeled using the gap feature of NX Nastran with friction coefficients corresponding to the surface finish. Loading was applied through a control node located at the center for the replacement head. Two loading conditions were analyzed, one representing torsion about the neck axis with a magnitude of 3140 Nm and one representing the peak load vector during activities of daily living.
Using resection plane nodes at the intersection of the implant and bone, the histograms of micromotion and the associated 5th, 50th, and 95th percentile values were calculated. For a traditional length stem, the dominate effect on the predicted micromotion at the resection plane was the interference fit in the coating region. The contribution of a traditional length stem to resection plane micromotion was complex and depended on the presence of the stem and the amount of interference fit in the coating region.
