The effect of pathological shoulder rhythm on muscle and joint forces after reverse shoulder arthroplasty, a numerical analysis.

BACKGROUND Compromised abduction ability after reverse shoulder arthroplasty is primarily linked to limited glenohumeral range of motion while scapulothoracic mobility can typically be maintained. Glenohumeral joint forces strongly depend on the resulting scapulohumeral rhythm, however, an association between the acting muscle and joint forces and the subject-specific scapulohumeral rhythm after reverse shoulder arthroplasty has not been established. METHODS Eleven reverse shoulder arthroplasty patients were divided into groups of poor and excellent abduction ability. Subject-specific models were developed and scaled for each patient using existing motion capture data in AnyBody™. Shoulder muscle and joint forces were obtained using inverse dynamics calculations during shoulder abduction to 100° in the scapula plane. The scapulohumeral rhythm, the resting abduction angle and internal body forces between the outcome groups were compared using a Mann Whitney U test. FINDINGS The mean glenohumeral and scapulothoracic contribution to overall shoulder abduction for the excellent group was on average 9.7% higher and 21.4% lower, respectively, compared to the mean of the poor group. For shoulder abduction angles between 30° and 60°, the excellent group demonstrated on average 25% higher muscle forces in the anterior deltoid which was significantly higher compared to the poor outcome patients. Scapulothoracic muscle activity did not differ significantly between the two functional groups. INTERPRETATION Accordingly, rehabilitation strategies focusing on strengthening the anterior part of the deltoid in particular may improve clinical outcomes

Effect of patient-specific scapular morphology on the glenohumeral joint force and shoulder muscle force equilibrium: a study of rotator cuff tear and osteoarthritis patients.

Introduction: Osteoarthritis (OA) and rotator cuff tear (RCT) pathologies have distinct scapular morphologies that impact disease progression. Previous studies examined the correlation between scapular morphology and glenohumeral joint biomechanics through critical shoulder angle (CSA) variations. In abduction, higher CSAs, common in RCT patients, increase vertical shear force and rotator cuff activation, while lower CSAs, common in OA patients, are associated with higher compressive force. However, the impact of the complete patient-specific scapular morphology remains unexplored due to challenges in establishing personalized models. Methods: CT data of 48 OA patients and 55 RCT patients were collected. An automated pipeline customized the AnyBody™ model with patient-specific scapular morphology and glenohumeral joint geometry. Biomechanical simulations calculated glenohumeral joint forces and instability ratios (shear-to-compressive forces). Moment arms and torques of rotator cuff and deltoid muscles were analyzed for each patient-specific geometry. Results and discussion: This study confirms the increased instability ratio on the glenohumeral joint in RCT patients during abduction (mean maximum is 32.80% higher than that in OA), while OA patients exhibit a higher vertical instability ratio in flexion (mean maximum is 24.53% higher than that in RCT) due to the increased inferior vertical shear force. This study further shows lower total joint force in OA patients than that in RCT patients (mean maximum total force for the RCT group is 11.86% greater than that for the OA group), attributed to mechanically advantageous muscle moment arms. The findings highlight the significant impact of the glenohumeral joint center positioning on muscle moment arms and the total force generated. We propose that the RCT pathomechanism is related to force magnitude, while the OA pathomechanism is associated with the shear-to-compressive loading ratio. Overall, this research contributes to the understanding of the impact of the complete 3D scapular morphology of the individual on shoulder biomechanics

The effect of pathological shoulder rhythm on muscle and joint forces after reverse shoulder arthroplasty, a numerical analysis

Background: Compromised abduction ability after reverse shoulder arthroplasty is primarily linked to limited glenohumeral range of motion while scapulothoracic mobility can typically be maintained. Glenohumeral joint forces strongly depend on the resulting scapulohumeral rhythm, however, an association between the acting muscle and joint forces and the subject-specific scapulohumeral rhythm after reverse shoulder arthroplasty has not been established. Methods: Eleven reverse shoulder arthroplasty patients were divided into groups of poor and excellent abduction ability. Subject-specific models were developed and scaled for each patient using existing motion capture data in AnyBody™. Shoulder muscle and joint forces were obtained using inverse dynamics calculations during shoulder abduction to 100° in the scapula plane. The scapulohumeral rhythm, the resting abduction angle and internal body forces between the outcome groups were compared using a Mann Whitney U test. Findings: The mean glenohumeral and scapulothoracic contribution to overall shoulder abduction for the excellent group was on average 9.7% higher and 21.4% lower, respectively, compared to the mean of the poor group. For shoulder abduction angles between 30° and 60°, the excellent group demonstrated on average 25% higher muscle forces in the anterior deltoid which was significantly higher compared to the poor outcome patients. Scapulothoracic muscle activity did not differ significantly between the two functional groups. Interpretation: Accordingly, rehabilitation strategies focusing on strengthening the anterior part of the deltoid in particular may improve clinical outcomes.ISSN:0268-0033ISSN:1879-127