This controlled cadaveric study used a dynamic shoulder simulator to measure how deltoid forces change across progressive rotator cuff tear patterns. It asks which tears the deltoid can compensate for and which cause abduction to collapse. Twelve shoulders were tested from intact through isolated, combined, and massive tears.
When you see a patient losing active abduction, do not assume the cuff tear is simply enlarging. This study shows the deltoid silently compensates for many tear patterns until it can no longer keep up. Deltoid fatigue or trauma can then precipitate sudden abduction loss and pseudoparalysis, regardless of tear size.
The clinical mental model: subscapularis integrity determines abduction. Anterosuperior tears involving the subscapularis collapse abduction because only the subscapularis provides anterior humeral head compression, and the deltoid cannot fully substitute. Posterosuperior tears are better tolerated because teres minor backs up posterior stability.
This reframes worsening abduction in a posterosuperior tear as a warning sign of either anterior tear extension into the subscapularis or a decompensating deltoid, not just posterior progression. It also reinforces why deltoid preservation and function matter so much in cuff-deficient shoulders and in surgical planning around the acromion.
This controlled cadaveric study used a dynamic shoulder simulator to measure how deltoid forces change across progressive rotator cuff tear patterns. It asks which tears the deltoid can compensate for and which cause abduction to collapse. Twelve shoulders were tested from intact through isolated, combined, and massive tears.
When you see a patient losing active abduction, do not assume the cuff tear is simply enlarging. This study shows the deltoid silently compensates for many tear patterns until it can no longer keep up. Deltoid fatigue or trauma can then precipitate sudden abduction loss and pseudoparalysis, regardless of tear size.
The clinical mental model: subscapularis integrity determines abduction. Anterosuperior tears involving the subscapularis collapse abduction because only the subscapularis provides anterior humeral head compression, and the deltoid cannot fully substitute. Posterosuperior tears are better tolerated because teres minor backs up posterior stability.
This reframes worsening abduction in a posterosuperior tear as a warning sign of either anterior tear extension into the subscapularis or a decompensating deltoid, not just posterior progression. It also reinforces why deltoid preservation and function matter so much in cuff-deficient shoulders and in surgical planning around the acromion.