Abstract
Background
Clinical studies purport an increase in internal rotation strength after transfer in subscapularis deficient patients. The present study aimed to establish the internal rotation biomechanics of a pectoralis major muscle transfer for the treatment of subscapularis muscle insufficiency.
Methods
A computational study was performed utilizing a three-dimensional, validated, computational model of the upper extremity. The pectoralis major was modeled as three distinct components. Moments were calculated from combined force (active and passive) as well as muscle moment arms for each of the internal rotators throughout a physiologic range of motion. The results were compared to both normal and subscapularis deficient simulations.
Results
A deficient subscapularis decreases the internal rotation moment by a maximum of 55% (16.9Nm). Transfer of the clavicular component in isolation and transfer of both the clavicular and sternal components decrease the sum internal rotation moment about the shoulder in a subscapularis deficient patient by a maximum of 5.3% (1.6Nm) and 10.6% (3.2Nm) respectively.
Conclusion
The clinical significance of this study is that the strength improvement demonstrated in clinical studies is likely a dynamic stabilizing effect of the transfer. This affirms the concept that the glenohumeral joint must have stability before the surrounding musculature can act effectively.
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