The subscapularis muscle is a large muscle that originates
on the anterior surface of the scapula and lies in front of the shoulder. The
muscle passes to its insertion into the humeral head underneath the arch formed
by the coracoid process and the combined origins of the coracobrachialis muscle
and short head of the biceps. The subscapularis muscle is the largest of the
four rotator cuff muscles, and it 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 subscapularis muscle acts as a dynamic stabilizer of the
humeral head and aids in lifting across the chest. The function of the
subscapularis muscle is to adduct and rotate the arm medially. At the insertion
of the subscapularis tendon into the humerus lies the transverse humeral
ligament. The long head of the biceps tendon lies within the bicipital groove
and is held in place by the transverse humeral ligament. When a complete
rupture of the subscapularis tendon occurs, the transverse humeral ligament may
also become torn causing medial dislocation of the biceps tendon from the
bicipital groove. Tears of the subscapularis tendon may be diagnosed by using
MRI or ultrasound. With ultrasound imaging, the probe is placed transversely
over the bicipital groove to identify the groove and biceps tendon while the
arm is in a neutral position. The arm is then externally rotated to view the
subscapularis tendon. Tears are not uncommon and can be missed. Subscapularis
rupture can be either acute or chronic and can also be partial or complete. The
patient will have pain, anterior shoulder swelling, decreased range of motion,
weakness of internal rotation, increased external rotation of the shoulder
compared to the other side. For the lift off test, the patient places the hand
behind their back at the lumbar level and lifts the hand away from the back
when the patient has an intact subscapularis tendon. If the patient is unable
to lift the hand off of the lower back, then a tear of the subscapularis tendon
is suspected. For the lift off lag test, the examiner will hold the patient’s
hand away from the back at the lumbar region and let go. Patient will be unable
to keep the hand away from the back if the tendon is torn. For the belly-press
test, the patient presses the palm of the hand against the abdomen with the
wrist in a neutral position. This is an example of an intact subscapularis
tendon. A positive sign for the belly-press test occurs if the patient is
unable to press his belly without wrist volar flexion or the elbow falling
posteriorly. Treatment for a complete tear of the subscapularis tendon is
surgical repair; repair may be either open or arthroscopic. Biceps tenodesis
during repair is associated with improved outcomes. Biceps tenodesis is usually
done if the biceps is involved in the process, otherwise subluxation of the
biceps will stress and fail the repair. Pectoral major muscle transfer is the
procedure of choice for chronic muscle tear. Posterior dislocation of the
humeral head with a “reverse Hill-Sachs lesion” is a rare condition. The
condition can be repaired after reduction of the dislocation with the
Mclaughlin procedure utilizing the subscapularis tendon if the lesion is
between 20-40% of the humeral head. The subscapularis tendon is used to fill
the reverse Hill-Sachs lesion using suture anchors or screws inserted in the
humeral head defect. The screws are inserted into a portion of the lesser
tuberosity that is attached to the subscapularis. The subscapularis muscle is
supplied by the upper and lower subscapular nerves. The upper and lower
subscapular nerves originate from the posterior cord of the brachial plexus.
The subscapular artery, which is the largest branch of the axillary artery,
supplies the subscapularis muscle.
