Pathomechanics of glenohumeral capsule and scapula in idiopathic frozen shoulder: a study using a three-dimensional finite element model.

Background: The pathomechanics of primary frozen shoulders are not yet fully understood. There is ongoing uncertainty regarding the optimal extent of surgical release, with concerns about both over- and under-release of the joint capsule. In the frozen shoulder, different ranges of motion of the shoulder joint experience varying areas of stress, contributing to stiffness and limited movement. The ultimate goal of treatment is to restore the full range of motion while carefully addressing the specific areas of pathology. The purpose of this study was to analyze the kinematics of the glenohumeral joint capsule in a frozen shoulder and to investigate scapulothoracic motion patterns. Methods: Ten patients with unilateral idiopathic frozen shoulders confirmed by a computed tomography (CT) arthrogram were enrolled in this study. All patients were scanned with additional high-resolution CT at maximum humeral abduction position. The modelling programs were used to simulate glenohumeral and scapulothoracic motion based on reconstructed CT images. The finite element models of the glenohumeral capsule were also constructed based on the CT arthrogram at 0° abduction. We evaluated the changes in scapular position between 0 to maximal humeral abduction angles and measured the degree of scapular abduction, external rotation, and posterior tilt. The tension changes and stress patterns of the capsule during various shoulder motions were also assessed. Results: In maximal humeral abduction, abutment of the humeral head against the lateral acromion was found in all frozen shoulder models. The scapula showed an increment in scapular upward rotation and a decrement in posterior tilt but inconsistency in internal/external rotation in the humeral abduction position. The abduction and internal rotation behind the back position of the humerus caused an increase in the stress at the anteroinferior capsule, compared to other portions of the capsule. Meanwhile, the anterosuperior capsule showed a prominent stress in external rotation at 0° humeral abduction. Conclusion: Pathomechanics of the frozen shoulder characterised by glenohumeral motion limitations should be considered complicated, as confirmed by high tension in the anteroinferior glenohumeral capsule and altered scapular motion. Selective surgical release of the anteroinferior part of the glenohumeral capsule with scapular mobilization can regain glenohumeral motion in the idiopathic frozen shoulder.