The subscapularis muscle is a large muscle that originates on the anterior surface of the scapula and lies in front of the shoulder. The subscapularis muscle tendon inserts into the lesser tuberosity of the humerus. The subscapularis muscle provides about 50% of the total cuff strength. The subscapularis muscle inserts into the lesser tuberosity of the humerus, while the other rotator cuff muscles have an insertion into the greater tuberosity. The long head of the biceps tendon lies in a groove anteriorly and is held in its position by the transverse humeral ligament. The action of the subscapularis muscle is adduction and internal rotation of the shoulder. The upper and lower subscapular nerves originate from the posterior cord of the brachial plexus. The suprascapular nerve is a different nerve, it innervates the supraspinatus and the infraspinatus muscles. A fall onto an outstretched arm during abduction is usually the mechanism of injury. The presentation is usually anterior shoulder pain following a forcible external rotation injury to the shoulder. Tear of the subscapularis tendon may follow anterior shoulder surgery. There may be an avulsion of the lesser tuberosity of the humerus. Subscapularis tendon tear may be isolated, or it may be associated with other rotator cuff tears. 88% of patients with biceps tendon subluxation are found to have subscapularis tendon tear. Tears can be either acute or chronic. There will be pain in front of the shoulder with weakness of internal rotation and increased passive external rotation. The diagnosis could be difficult and the condition could be missed. The transverse humeral ligament may be torn with complete rupture of the subscapularis tendon, and this may lead to medial dislocation of the biceps tendon from its groove. Lift-off test, bear-hug test, and belly press test all show weakness of internal rotation of the shoulder. When the patient is unable to lift his hand away from the lower back while the shoulder is maximally internally rotated. If you hear that there is a hyper abduction injury after an open repair of the shoulder and an inability to move the dorsum of the hand away from the back, then this is a subscapularis tendon tear. Tears of the subscapularis tendon can be diagnosed with an ultrasound or MRI. The MRI will show detachment of the subscapularis from its insertion into the lesser tuberosity of the humerus. The sagittal MRI will also show you if there is an atrophy of the muscle. If the patient has a total shoulder repair, and the patient fell down and there is an increase in the passive external rotation of the shoulder, the x-ray shows that everything is good, then you will probably need to do ultrasound evaluation of the shoulder to check the integrity of the subscapularis tendon. Arthroscopic identification of a chronic subscapularis tear can be done by the comma sign, which represents avulsion of the superior glenohumeral ligament. Chronic supraspinatus and infraspinatus tear in a young patient, and the tear cannot be repaired, then you will do latissimus dorsi transfer. Both the lift off test and the abdominal compression test needs to show that the patient has a good subscapularis muscle function before you do the latissimus dorsi transfer. Preoperative subscapularis function is necessary for good clinical outcome. In case of posterior dislocation of the shoulder in a young patient, when the humeral head defect is large but less than 50%, you may transfer the subscapularis tendon and the lesser tuberosity into the humeral head defect which is called a reverse Hill-Sachs lesion. Treatment is usually surgery. For a complete acute tear, do open or arthroscopic surgical repair. Biceps tenodesis is needed if there is subluxation of the long head of the biceps. For a chronic subscapularis tendon tear, do pectoralis major muscle transfer. When the subscapularis tear is missed and the tear is chronic, the tendon becomes retracted and atrophic and you will do subcoracoid pectoralis major tendon transfer. It may improve the function and decrease the pain. The subcoracoid position of the transfer allows redirection of the pectoralis major in a direction that is recreating the vector of the subscapularis tendon.
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