The Importance of Off-Axis Loading and Articulation in Loosening of Cemented Glenoids

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Abstract
Background: Glenoid loosening is the primary cause of failure in anatomic Total Shoulder Arthroplasty (aTSA). Current ASTM standards evaluate loosening via standard orthogonal vectors using non-continuous monitoring, which offer limited insight into the temporal evolution of loosening along physiological, off-axis wear vectors. This study demonstrates the feasibility and utility of a novel video-based system for continuous monitoring of glenoid loosening under clinically relevant, off-axis wear vectors and subluxation translation (ST) distances. Methods: A custom electromechanical apparatus cyclically displaced a humeral head against 18 cemented 5-peg glenoids over 150,000 cycles across three orientations: superior-inferior (SI), anterior–posterior (AP), and 45-degree angle (45°), and two ST distances (75% and 105% of maximum translation). Each condition had n = 3 glenoids. A video-based motion tracking system continuously monitored micromotion and relative motion at the bone-implant interface against a 150 µm loosening threshold. Differences were analyzed using Kruskal–Wallis tests and non-parametric effect sizes (η2). Results: The system successfully captured non-linear loosening behaviors. Final relative motion at 105% ST was SI (2476.0 ± 1882.3 µm), 45° (239.6 ± 51.9 µm), and AP (265.2 ± 99.7 µm), with all groups exceeding the 150 µm threshold. At 75% ST, relative motion was SI (175.7 ± 140.7 µm), 45° (24.3 ± 69.3 µm), and AP (33.1 ± 18.4 µm). While Kruskal–Wallis tests showed non-significant differences across orientations due to small sample sizes (p = 0.11), large effect sizes (η2 = 0.393) indicated practically meaningful differences, with SI showing the greatest motion. Conclusions: This study demonstrated the feasibility of a novel method of using a video-based system for the continuous monitoring of glenoid loosening under varying mechanical conditions, provides a comprehensive platform for evaluating the biomechanics of implant loosening, and suggests that testing along intermediate axes and at higher subluxation levels may be necessary to accurately predict in vivo performance.

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