In patients with rotator cuff tears lost elevation moments are compensated for by increased deltoid activation. Concomitant proximal directed destabilizing forces at the glenohumeral joint are... Show moreIn patients with rotator cuff tears lost elevation moments are compensated for by increased deltoid activation. Concomitant proximal directed destabilizing forces at the glenohumeral joint are suggested to be compensated for by 'out-of-phase' adductor activation, preserving glenohumeral stability. Aim of this study was to demonstrate causality between moment compensating deltoid activation and stability compensating 'out-of-phase' adductor muscle activation. A differential arm loading with the same magnitude of forces applied at small and large moment arms relative to the glenohumeral joint was employed to excite deltoid activation, without externally affecting the force balance. Musculoskeletal modeling was applied to analyze the protocol in terms of muscle forces and glenohumeral (in)stability. The protocol was applied experimentally using electromyography (EMG) to assess muscle activation of healthy controls and cuff tear patients. Both modeling and experiments demonstrated increased deltoid activation with increased moment loading, which was higher in patients compared to controls. Model simulation of cuff tears demonstrated glenohumeral instability and related 'out-of-phase' adductor muscle activation which was also found experimentally in patients when compared to controls. Through differential moment loading, the assumed causal relation between increased deltoid activation and compensatory adductor muscle activation in cuff tear patients could be demonstrated. 'Out-of-phase' adductor activation in patients was attributed to glenohumeral instability. The moment loading protocol discerned patients with cuff tears from controls based on muscle activation. (C) 2010 Elsevier Ltd. All rights reserved. Show less
Patients suffering tendon tears in the glenohumeral cuff muscles show activation of muscles which pull the arm downwards during arm elevation tasks. This so-called co-activation deviates from... Show morePatients suffering tendon tears in the glenohumeral cuff muscles show activation of muscles which pull the arm downwards during arm elevation tasks. This so-called co-activation deviates from healthy controls and is triggered by pain. Goal of this thesis was to demonstrate that deviating muscle activation in patients with glenohumeral cuff tears is related to shoulder instability. We hypothesized that they are compensatory for lost glenohumeral cuff functions, and restrict arm functionality. Cuff-tear simulations using a musculoskeletal model and an experimental arm loading protocol showed increased deltoid activation to compensate lost elevation moments, which jeopardizes glenohumeral stability. Activation of muscles pulling down the humeral head is required to compensate lost stability, which is counterproductive for arm elevation. There is a conflict between stability and mobility. This conflict can be solved by a tendon transfer of the teres major, which demonstrated shoulder function improvement and pain decrease. The concept of compensatory muscle activation provide insight in the underlying mechanisms of patients suffering glenohumeral cuff tears and potentially can be used, also at early symptoms like in impingement, as a diagnostic instrument or it can be applied in new treatment strategies. Show less
BACKGROUND: In massive rotator cuff tears a teres major (TMj) tendon transfer to the insertion of the supraspinatus (SSp) reverses its adduction moment arm into abduction which is supposed to be an... Show moreBACKGROUND: In massive rotator cuff tears a teres major (TMj) tendon transfer to the insertion of the supraspinatus (SSp) reverses its adduction moment arm into abduction which is supposed to be an adequate salvage procedure. Analysis of muscle function to find biomechanical ground of such success is scarce. METHODS: We compared pre- and postoperative clinical outcome of TMj transfer, i.e. Range of Motion, pain, Constant Shoulder scores and arm force. TMj activation was evaluated in 14 patients suffering massive cuff tears using activation ratios to describe the desired 'in-phase' and undesired 'out-of-phase' contribution to the external arm moment. Additionally, we analyzed activation of the latissimus dorsi (LD) and the medial part of the deltoids (DE). The activation ratios were compared to controls and TMj activation ratios were related to clinical outcome. FINDINGS: TMj tendon transfer improved arm function. Pre-operatively, we observed 'out-of-phase'abduction activation of TMj and LD. After transfer patients activated TMj according to its new anatomical position. 'Out-of-phase' LD abduction activation persisted. The clinical improvements coincided with changes in activation ratio of TMj. INTERPRETATION: 'Out-of-phase' TMj adductor activation is associated with compromised arm function in patients with irreparable cuff tears. After transfer, TMj is activated in correspondence with its new anatomical function, which was supportive for the improved arm function. Show less
Background: In massive rotator cuff tears a teres major (TMj) tendon transfer to the insertion of the supraspinatus (SSp) reverses its adduction moment arm into abduction which is supposed to be an... Show moreBackground: In massive rotator cuff tears a teres major (TMj) tendon transfer to the insertion of the supraspinatus (SSp) reverses its adduction moment arm into abduction which is supposed to be an adequate salvage procedure. Analysis of muscle function to find biomechanical ground of such success is scarce. Methods: We compared pre- and postoperative clinical outcome of TMj transfer, i.e. Range of Motion, pain, Constant Shoulder scores and arm force. TMj activation was evaluated in 14 patients suffering massive cuff tears using activation ratios to describe the desired 'in-phase' and undesired 'out-of-phase' contribution to the external arm moment. Additionally, we analyzed activation of the latissimus dorsi (LD) and the medial part of the deltoids (DE). The activation ratios were compared to controls and TMj activation ratios were related to clinical outcome. Findings: TMj tendon transfer improved arm function. Pre-operatively, we observed 'out-of-phase' abduction activation of TMj and LD. After transfer patients activated TMj according to its new anatomical position. 'Out-of-phase' LD abduction activation persisted. The clinical improvements coincided with changes in activation ratio of TMj. Interpretation: 'Out-of-phase' TMj adductor activation is associated with compromised arm function in patients with irreparable cuff tears. After transfer, TMj is activated in correspondence with its new anatomical function, which was supportive for the improved arm function. (C) 2009 Elsevier Ltd. All rights reserved. Show less
