In Silico Biomechanical Performance of Inlay vs. Onlay Glenoid Implants in Total Shoulder Arthroplasty: A Finite Element Study.

BACKGROUND: While total shoulder arthroplasty (TSA) provides long-lasting relief for many patients, glenoid loosening remains one of the more common reasons for TSA failure and can lead to revision surgery in the mid- to long-term. Some studies show better biomechanical performance of inlay glenoid components over onlay glenoid components, indicating that onlay glenoid components have a higher chance of loosening. Clinical studies suggest better longevity of inlay implants versus onlay glenoid designs when considering both concentric and eccentric glenoid bone loss situations. Given these clinical and biomechanical findings, we created 3D finite element (FE) models to elucidate biomechanical loads and better understand the differences between the two designs.

METHODS: We created 3D finite element (FE) total shoulder arthroplasty models with both inlay and onlay glenoid components in combination with a traditional spherical humeral head design using a healthy shoulder CT as a baseline. The model aligned the humerus on the scapular plane with 90° abduction. Preloaded spring components acted as muscle elements, providing the compressive load at the joint.

RESULTS: 3D finite element modeling demonstrated that inlay glenoids share contact loads (contact force and pressure) at the interface between the glenoid component and the surrounding glenoid bone, while onlay glenoids take the contact loads on the glenoid component solely. Results show the total contact force at the joint as 539.1N in the inlay and 545.8N in the onlay configuration. Sharing the load with surrounding native glenoid bone, inlay glenoid experienced 209.4N in contact force, while onlay glenoid received the whole joint contact force. The model predicted average von Mises stresses of 0.6332MPa and 9.386MPa on the glenoid bone for the inlay and the onlay configurations, respectively. It predicted stresses of 1.416MPa and 2.801MPa (inlay vs. onlay) on the undersurface of the glenoid components (implant bone interface with implant fixed to the glenoid bone), and 1.059MPa and 1.921MPa (inlay vs. onlay) on the glenoid components' contact surface.

CONCLUSIONS: Using a 3D FE model, inlay glenoid components faced lower loads and stresses when compared with onlay glenoids. This may explain why inlay components demonstrate less glenoid loosening when compared with onlay glenoid components.

LEVEL OF EVIDENCE: Basic Science Study, Computer Modeling.