Influence of age-related bone density changes on primary stability in stemless shoulder arthroplasty: a multi-implant finite element study.

Background: Stemless implants were introduced to prevent some of the stem-related complications associated with the total shoulder arthroplasty. Although general requirements for receiving these implants include good bone quality conditions, little knowledge exists about how bone quality affects implant performance. The goal of this study was to evaluate the influence of age-induced changes in bone density, as a metric of bone quality, in the primary stability of five anatomic stemless shoulder implants using three-dimensional finite element (FE) models.

Methods: The implant designs considered were based on the Global Icon, Sidus, Simpliciti, SMR, and Inhance stemless implants. Shoulder arthroplasties were virtually simulated in Solidworks. The density distributions of 20 subjects from two age groups, 20-40 and 60-80 years old, were retrieved from medical image data and integrated into three-dimensional FE models of a single humerus geometry, developed in Abaqus, to avoid confounding factors associated with geometric characteristics. For the designs which do not have a solid collar covering the entire bone surface, ie, the Sidus, Simpliciti, SMR, and Inhance implants, contact and noncontact conditions between the humeral head component and bone were considered. Primary stability was evaluated through the assessment of micromotions at the bone-implant interface considering eight load cases related to rehabilitation activities and demanding tasks. Three research variables, considering 20 μm, 50 μm, and 150 μm as thresholds for osseointegration, were used for a statistical analysis of the results.

Results: The decreased bone density registered for the 60-80 age group led to larger micromotions at the bone-implant interface when compared to the 20-40 age group. The Global Icon-based and Inhance-based designs were the least sensitive to bone density, whereas the Sidus-based design was the most sensitive to bone density. The establishment of contact between the humeral head component and bone for the implants that do not have a solid collar led to decreased micromotions.

Discussion: Although the age-induced decline in bone density led to increased micromotions in the FE models, some stemless shoulder implants presented good overall performance regardless of the osseointegration threshold considered, suggesting that age alone may not be a contraindication to anatomic total shoulder arthroplasty. If only primary stability is considered, the results suggested superior performance for the Global Icon-based and Inhance-based designs. Moreover, the humeral head component should contact the resected bone surface when feasible. Further investigation is necessary to combine these results with the long-term performance of the implants and allow more precise recommendations.