The subscapularis is a large triangular muscle which fills the subscapular fossa and inserts into the lesser tubercle of the humerus and the front of the capsule of the shoulder-joint.
Structure
The subscapularis is covered by a dense fascia which attaches to the scapula at the margins of the subscapularis' attachment (origin) on the scapula.
The muscle's fibers pass laterally from its origin before coalescing into a tendon of insertion. The tendon intermingles with the glenohumeral (shoulder) joint capsule.
A bursa (which communicates with the cavity of the shoulder joint via an aperture in the joint capsule) intervenes between the tendon and a bare area at the lateral angle of the scapula/the neck of the scapula. The subscapularis (supraserratus) bursa separates the subscapularis is from the serratus anterior.
Origin
It arises from its medial two-thirds of the costal surface of the scapula, the intermuscular septa (which create ridges upon the scapula), and the lower two-thirds of the groove on the axillary border (subscapular fossa) of the scapula.
Some fibers arise from tendinous laminae, which intersect the muscle and are attached to ridges on the bone; others from an aponeurosis, which separates the muscle from the teres major and the long head of the triceps brachii.
Insertion
It inserts onto the lesser tubercle of the humerus and the anterior part of the shoulder-joint capsule. Tendinous fibers extend to the greater tubercle with insertions into the bicipital groove.
Innervation
The subscapularis is supplied by the upper and lower subscapular nerves (C5-C6), branches of the posterior cord of the brachial plexus.
Actions/movements
The subscapularis medially (internally) rotates the humerus (acting here as a prime mover) and adducts it. When the arm is raised, it draws the humerus forward and downward.
Function
The subscapularis stabilises the shoulder joint by contributing to the fixation of the proximal humerus during movements of the elbow, wrist, and hand. It is a powerful defense to the front of the shoulder-joint, preventing displacement of the head of the humerus.
Clinical significance
Examination
It is difficult to isulate the action of the subscapularis from other medial rotators of the shoulder joint; there is no satisfactory test for this muscle. The Gerber Lift-off test is the established clinical test for examination of the subscapularis. The bear hug test (internal rotation while palm is held on opposite shoulder and elbow is held in a position of maximal anterior translation) for subscapularis muscle tears has high sensitivity. Positive bear-hug and belly press tests indicate significant tearing of subscapularis.
Imaging
There is no singularly imaging device or technique for a satisfying and complete subscapularis examination, but rather the combination of the sagittal oblique MRI / short-axis US and axial MRI / long-axis US planes seems to generate useful results. Additionally, lesser tuberosity bony changes have been associated with subscapularis tendon tears. Findings with cysts seem to be more specific and combined findings with cortical irregularities more sensitive.
Another fact typically for the subscapularis muscle is the fatty infiltration of the superior portions, while sparing the inferior portions.
Since the long biceps tendon absents itself from the shoulder joint through the rotator cuff interval, it is easily possible to distinguish between the supraspinatus and the subscapularis tendon. Those two tendons build the interval sling.
Ultrasonography
Mack et al. developed an ultrasonographic procedure with which it is possible to explore almost the complete rotator cuff within six steps. It unveils clearly the whole area from the subedge of the subscapularis tendon until the intersection between the infraspinatus tendon and musculus teres minor. One of six steps does focus on the subscapularis tendon. In the first instance the examinator guides the applicator to the proximal humerus as perpendicularly as possible to the sulcus intertubercularis. Gliding now medially shows the insertion of the subscapularis tendon.