An anatomic study of the effects on the suprascapular nerve due to retraction of the supraspinatus muscle after a rotator cuff tear.

The purpose of this anatomic study was to assess the risk to the suprascapular nerve by measuring the tension on the nerve and the angle between the nerve and its motor branch at the scapular notch with medial supraspinatus tendon retraction. Twelve shoulders in six cadavers were dissected to evaluate the branching point of the first motor branch of the suprascapular nerve, the change in angle between the nerve and its first motor branch at the scapular notch with retraction of the supraspinatus tendon, and the resulting tension on the nerve. The first motor branch originated at the notch in 9 cadavers, just proximal in 1, and just distal in 2. With the supraspinatus muscle in its anatomic position, the suprascapular nerve and its first motor branch angle measured 142.6 degrees at the scapular notch. After retraction of the supraspinatus, the angle markedly decreased to 98.7 degrees and 34.6 degrees with 1 cm and 5 cm of medial retraction, respectively. The motor branch was taut in all specimens at 2 to 3 cm of retraction. Medial retraction of the supraspinatus tendon drastically changes the course of the suprascapular nerve through the scapular notch, creating increased tension on the nerve. The degree of rotator cuff muscle atrophy frequently observed after a massive tear may be explained by increased tension on the nerve due to muscle retraction.