JSES - 2026-07-14 - Journal Article
In Silico Biomechanical Performance of Inlay vs. Onlay Glenoid Implants in Total Shoulder Arthroplasty: A Finite Element Study.
Chapai S, Dhar U, Papp DF, Tsai CT, Miniaci A
Topics
Key Takeaway
Finite element modeling shows inlay glenoid components distribute contact force across native bone (209.4N vs. 545.8N at the implant) and generate 15-fold lower von Mises stress on glenoid bone (0.63 MPa vs. 9.39 MPa) compared to onlay designs.
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Summary
This FE study compared stress distribution and contact mechanics between inlay and onlay polyethylene glenoid components in a single healthy-shoulder CT-derived model loaded at 90° abduction in the scapular plane. Inlay components shared joint contact force with surrounding native glenoid bone (209.4N at implant vs. 545.8N for onlay) and produced substantially lower von Mises stress at the glenoid bone interface (0.63 MPa vs. 9.39 MPa). Implant undersurface stress was also lower for inlay designs (1.42 MPa vs. 2.80 MPa), suggesting a mechanistic basis for the clinically observed lower loosening rates.
Key Limitation
The model uses a single healthy glenoid CT with no bone loss, no cement mantle variation, and no cyclic loading, making it impossible to assess how inlay vs. onlay stress differences change in the pathologic glenoids (Walch B2/B3, eccentric wear) where implant choice is most consequential.
Original Abstract
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.