Infraspinatus Advancement via Remplissage Improves Dynamic Anterior Stability but Reduces External Rotation Moment Arm: A Computational Study.

Abstract

The addition of remplissage to arthroscopic Bankart repair has demonstrated meaningful decreases in recurrent instability. Mechanisms for its effectiveness have been proposed but not validated. Similarly, hypothetical concerns surrounding overconstraint of the shoulder with limitations of external rotation have not been demonstrated on a large scale. This computational study sought to quantify the change in force vector of the infraspinatus if incorporated into remplissage fixation, and the subsequent effect of that on anterior stability and external rotation. Sixteen shoulders (13 males, 3 females; 9 right, 7 left; age 25.8 ± 7.0 years) with Hill-Sachs lesions were computationally modeled using ArtiSynth software. To capture the infraspinatus muscle-tendon's line of action, four groups of muscle fibers were modelled with identical origin points at the scapula but varying insertion sites on the humeral head for remplissage fixation: the native insertion, the lateral border, the center, and the medial border of the Hill-Sachs lesion-representing progressive medial advancement. Fiber wrapping around the humeral head and scapula was simulated with the shoulder in 90/90 abduction and external rotation (ABER). From these simulations, the effective force vector exerted on the humeral head was extracted. Posterior force component, abduction and external rotation moment arms were calculated and compared across the four groups using one-way repeated measures ANOVA. Average medialized distances were 11.3 ± 3.8 mm, 18.0 ± 3.8 mm and 23.8 ± 3.9 for the lateral, middle, and medial groups, respectively. There was significant effect of insertion site location on all three variables of interest (all F (3,45)>71.1, p<.001). Linear regression showed that each millimeter of medialization was associated with a 2.3% increase in infraspinatus posterior force vector (R²=0.849, p<.001), with reductions in external rotation and abduction moment arms by 6.9% (R²=0.839, p<.001) and 6.1% (R²=0.914, p<.001), respectively. Incorporation of the infraspinatus tendon within remplissage fixation was shown to significantly increase the posterior force vector of the infraspinatus in the ABER position. Additionally, this advancement resulted in a reduced moment arm for external rotation and abduction. Surgeons should be aware of the biomechanical advantages and drawbacks of infraspinatus incorporation and medialization, either purposeful or inadvertent, when performing remplissage. Basic Science Study, Computer Modeling.