Purpose
Two mechanisms, strength imbalance or impaired longitudinal muscle growth, potentially
cause osseous and postural shoulder deformity in children with brachial plexus birth
palsy. Our objective was to determine which muscles, via either deformity mechanism,
were mechanically capable of producing forces that could promote shoulder deformity.
Methods
In an upper limb computational musculoskeletal model, we simulated strength imbalance
by allowing each muscle crossing the shoulder to produce 30% of its maximum force.
To simulate impaired longitudinal muscle growth, the functional length of each muscle
crossing the shoulder was reduced by 30%. We performed a sensitivity analysis to identify
muscles that, through either simulated deformity mechanism, increased the posteriorly
directed, compressive glenohumeral joint force consistent with osseous deformity or
reduced the shoulder external rotation or abduction range of motion consistent with
postural deformity.
Results
Most of the increase in the posterior glenohumeral joint force by the strength imbalance
mechanism was caused by the subscapularis, latissimus dorsi, and infraspinatus. Posterior
glenohumeral joint force increased the most owing to impaired growth of the infraspinatus,
subscapularis, and long head of biceps. Through the strength imbalance mechanism,
the subscapularis, anterior deltoid, and pectoralis major muscles reduced external
shoulder rotation by 28°, 17°, and 10°, respectively. Shoulder motion was reduced
by 40° to 56° owing to impaired growth of the anterior deltoid, subscapularis, and
long head of triceps.
Conclusions
The infraspinatus, subscapularis, latissimus dorsi, long head of biceps, anterior
deltoid, pectoralis major, and long head of triceps were identified in this computational
study as being the most capable of producing shoulder forces that may contribute to
shoulder deformity following brachial plexus birth palsy.
Clinical relevance
The muscles mechanically capable of producing deforming shoulder forces should be
the focus of experimental studies investigating the musculoskeletal consequences of
brachial plexus birth palsy and are potentially critical targets for treating shoulder
deformity.
Key words
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Article info
Publication history
Published online: December 16, 2013
Accepted:
October 30,
2013
Received:
June 27,
2013
Footnotes
This work was supported by a research grant from the Pediatric Orthopaedic Society of North America and the Orthopaedic Research and Education Foundation.
No benefits in any form have been received or will be received related directly or indirectly to the subject of this article.
Identification
Copyright
© 2014 American Society for Surgery of the Hand. Published by Elsevier Inc. All rights reserved.
