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Medial subluxation of the long head of the biceps tendon is associated with injury to the subscapularis, which is innervated by the upper and lower subscapular nerves (labeled B in Figure A). Subscapularis tears were previously thought to be rare injuries of the rotator cuff; however, new evidence suggests a higher prevalence than previously thought. It usually presents as an acute avulsion in younger patients with a hyperabduction/external rotation injury or as an iatrogenic injury due to failure of repair. The subscapularis is innervated by the upper and lower subscapular nerves. The lower subscapular nerve also innervates the teres major. Adams et al. performed a study to determine a systematic approach for diagnosing subscapularis tendon tears with preoperative MRI scans. They found that 88% of patients with biceps tendon subluxation were found to have subscapularis tears. They also found that 71% of patients with a combined supraspinatus/infraspinatus tear and 68% of patients with a long head of the biceps tendon tear were found to have subscapular tears. Deutsch et al. published a study on the clinical diagnosis, magnetic resonance imaging findings, and operative treatment of traumatic tears of the subscapularis tendon. They found that pre-operative MRI detected all relevant subscapularis and bicep tendon lesions, as well as all cases of biceps subluxation or dislocation. They report that the axial image is often excluded on routine MRI examinations for suspected rotator cuff tears but is important in the evaluation of the subscspularis and biceps tendons. Figure A is a diagram of the brachial plexus with the following nerves labeled: (A) suprascapular nerve, (B) upper and lower subscapular nerves, (C) middle subscapular/thoracodorsal nerve, (D) musculocutaneous nerve, (E) axillary nerve. Illustrations A-F are axial T1-weighted MRI images demonstrating the anatomy of the shoulder. Incorrect Answers: Answer 1: The suprascapular nerve innervates the supraspinatus and infraspinatus. Answer 3: The middle subscapular nerve, also known as the thoracodorsal nerve, innervates the lattisimus dorsi. Answer 4: The musculocutaneous nerve innervates the biceps, medial brachialis, and coracobrachialis. Answer 5: The axillary nerve innervates the teres minor and deltoid.
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