Elsevier

The Journal of Hand Surgery

Volume 39, Issue 2, February 2014, Pages 303-311
The Journal of Hand Surgery

Scientific article
Computational Sensitivity Analysis to Identify Muscles That Can Mechanically Contribute to Shoulder Deformity Following Brachial Plexus Birth Palsy

https://doi.org/10.1016/j.jhsa.2013.10.027Get rights and content

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.

Section snippets

Musculoskeletal model

We used a 3-dimensional computer model of the upper limb musculoskeletal system20 to perform a sensitivity analysis to assess the impact of 2 potential deformity mechanisms, strength imbalance and impaired growth, on shoulder biomechanics. The model, implemented for dynamic musculoskeletal simulation in the OpenSim 3.0 software platform (Stanford University, Stanford, CA),21 has been widely used to evaluate both healthy and pathological upper limb function.17, 18, 22, 23, 24, 25, 26

Potential contributors to osseous deformity: strength imbalance mechanism

Through the simulated strength imbalance mechanism, several muscles increased the glenohumeral joint force in the axial plane and were therefore mechanically capable of contributing to osseous deformity (Fig. 2). Muscles that increased the posteriorly directed, compressive glenohumeral joint force included the infraspinatus, subscapularis, long head of biceps, latissimus dorsi, teres major, teres minor, and posterior deltoid. The infraspinatus produced the highest glenohumeral joint force

Discussion

Muscles that can alter shoulder forces may provide a mechanical stimulus for the development of shoulder deformity. In our computational simulations, the posteriorly directed, compressive glenohumeral joint force was increased the most by the infraspinatus, subscapularis, latissimus dorsi, and long head of biceps, which are therefore mechanically capable of contributing to osseous deformity. Likewise, because they reduced shoulder ROM the most, the subscapularis, anterior deltoid, pectoralis

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    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.

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