Relationship Between Deltoid and Rotator Cuff Muscles During Dynamic Shoulder Abduction

American Journal of Sports Medicine, May 9th, 2018

Dyrna, MD Et. Al Department of Orthopaedic Sports Medicine, Technical University Munich, Munich, Germany

Mastering the art of preserving our patient’s shoulders from rotator cuff repair by means of conservative treatment is a challenge we all face, and maybe more often than we’d like.

Dyrna Et. Al undertook an innovative, and very clinically applicable, study looking at how the deltoid compensates via increased force output in the presence of progressive rotator cuff tear size and location. They also looked at how abduction active ROM is affected, despite the deltoid doing its best to help out. The study looked at 12 cadaveric shoulders hooked up to individual shoulder simulator actuators and cameras, fluoroscopy, and 3-D motion tracking to give us some biomechanical data with regards to the rotator cuff and deltoid muscles. The study looked at 3 of the 4 rotator cuff muscles (supraspinatus, subscapularis, and infraspinatus) and categorized the complete tears into isolated, combined anterior-superior (subscapularis + supraspinatus), combined posterior-superior (infraspinatus + supraspinatus), and massive (all 3).

How the Deltoid Compensates:

During the simulations, several clinically relevant and helpful pieces of data arise when comparing a shoulder with progressive rotator cuff pathology compared to the native shoulder without any rotator cuff involvement. Overall, deltoid forces increase following any rotator cuff pathology to help the shoulder achieve active range of motion for abduction. More specifically, the study found the following:

• Anterior deltoid forces increase after anterior-superior (112% increase) and massive tears (63% increase)
• Middle deltoid forces increase after anterior-superior tears (101.3% increase)
• Posterior deltoid forces increase after anterior-superior (98.9% increase) AND posterior-superior tears (98.7% increase)

Loss of Active Glenohumeral Abduction Range of Motion with a Rotator Cuff Tear:

One particularly interesting takeaway from the study was the result of how active glenohumeral abduction was affected by different types of rotator cuff tears, even in the presence of deltoid compensation:

• Isolated subscapularis tears had little effect upon abduction active range of motion or the need for deltoid compensation.
• Although maximal abduction active range of motion was reduced, with a strong deltoid, it may be possible to attain a good amount of active abduction range of motion with an isolated supraspinatus as well as a posterior-superior rotator cuff tear.
• Despite a significant amount of increased force compensation in the deltoid, the cadaveric shoulders had a significant loss of active abduction in the presence of anterior-superior rotator cuff tears.
• Massive rotator cuff tears act similarly to anterior-superior rotator cuff tears with regards to motion loss and deltoid compensation.
• Tear of the supraspinatus may result in increased humeral head migration, which can result in the humeral head impinging upon the acromion. Basically, a hard block that the deltoid can’t overcome. Additionally, superior humeral head migration may put the deltoid at a very tough position to work.

Overall, consider that your patients with rotator cuff tears need to work on their deltoid muscle strength in order to help out the rotator cuff when it has any kind of tear. With a posterior-superior (supraspinatus and infraspinatus) tear, it may even be possible to maintain solid shoulder active abduction range of motion. Involvement of the subscapularis with the supraspinatus makes it particularly hard to achieve full active shoulder abduction even with the deltoid compensating. Remember, if you see someone with a posterior-superior cuff tear and then, boom, they all the sudden are losing active shoulder abduction, consider that they may either have deltoid weakness/fatigue or that the tear could be extending into the subscapularis.

What are you waiting for?… Hit those delts!

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