Summary
The shoulder is highly mobile but prone to instability, often leading to dislocations. This study developed a finite element model to assess how glenoid morphology and labrum integrity affect shoulder stability. Validated against biomechanical data, the model revealed that greater joint congruence and an intact labrum increase stability, offering insights into treating recurrent shoulder instabili
Abstract
Background
The shoulder joint is the most mobile but also the most unstable joint in the human body, with a 2% dislocation rate in people aged 18-70. Stability is achieved through both passive (joint surface morphology, ligaments, and tendons) and dynamic (muscle contraction) factors. However, this balance is fragile, leading to recurrent shoulder instability in many individuals, which limits daily activities and reduces quality of life. Current knowledge about shoulder biomechanics is not as advanced as for other joints like the knee or hip, due to the shoulder's complex anatomical structure composed of four joints. While in vitro and clinical studies provide valuable insights, they are limited by the availability of patients and difficulty measuring certain parameters, making computational models an effective alternative for understanding shoulder instability.
AIMS This study aimed to develop and validate a finite element model of the shoulder joint to evaluate the impact of glenoid morphology and the labrum on shoulder stability. The goal was to understand how variations in joint geometry and the presence or absence of the labrum influence the risk of shoulder dislocation, with the ultimate aim of identifying patients at risk of recurrent instability and informing about patient-specific treatments.
Methods
A computerized 3D "ball and socket" model of the shoulder joint was developed based on anatomical data from a male population with an average age of 50. The model included the humerus and glenoid bones, glenoid labrum, and cartilage, with material properties and dimensions taken from the literature. The labrum was modeled with variable thickness around the glenoid, and the interactions between joint surfaces were considered frictionless. The model was subjected to anterior translation of the humeral head under a compressive load of 50N, simulating shoulder dislocation. The reaction forces were measured to assess joint stability under different conditions of glenoid morphology and labrum presence. The model was validated against biomechanical data from 10 cadaveric shoulders, ensuring its accuracy across a range of anatomical variability.
Results
The finite element model was validated, with results falling within one standard deviation of biomechanical data. Increased joint congruence led to higher reaction forces, indicating better stability, while decreased congruence resulted in significant reductions in reaction forces (up to 60.6%). Glenoid depth also played a role, with smaller curvature radii producing higher reaction forces. The presence of the glenoid labrum increased stability, while its absence or injury (e.g., Bankart lesion) led to reductions in reaction forces by 18.8% and 22.21%, respectively.
Discussion
This study successfully developed a geometrical finite element model of the shoulder, showing that glenoid morphology and labrum integrity significantly influence shoulder stability. Consistent with previous in vitro and finite element studies, this model provided new insights into how morphological variability in the glenoid and labrum affects joint stability. While the model's simplicity (e.g., exclusion of musculotendinous structures) may be considered a limitation, it offers a powerful tool for understanding shoulder biomechanics and can be further developed to compare surgical techniques for treating instability.