Rotator Cuff

The rotator cuff is a group of four muscles and their tendons that stabilize the human shoulder and enable its extensive range of motion. These muscles—supraspinatus, infraspinatus, teres minor, and subscapularis—form a functional unit collectively known as the SITS muscles. They arise from the scapula and insert into the humerus, creating a musculotendinous cuff that surrounds the glenohumeral joint.

Anatomy

The rotator cuff comprises four distinct muscles, each originating from the scapula and inserting onto the proximal humerus. The glenohumeral joint is a ball-and-socket joint with a large spherical humeral head and a small glenoid cavity. This anatomy makes the joint highly mobile but inherently unstable. The rotator cuff provides dynamic stability by compressing the humeral head against the glenoid².

Supraspinatus

The supraspinatus originates from the supraspinous fossa of the scapula, above the spine. It passes above the glenohumeral joint and inserts on the superior facet of the greater tubercle of the humerus. The supraspinatus is the only rotator cuff muscle that does not rotate the humerus—it initiates abduction of the arm for the first 0 to 15 degrees. Beyond 15 degrees, the deltoid takes over abduction¹².

The supraspinatus is the most commonly injured rotator cuff tendon⁷.

Infraspinatus

The infraspinatus originates from the infraspinous fossa below the spine of the scapula and inserts on the middle facet of the greater tubercle. It is a powerful external rotator of the humerus and works with the teres minor to externally rotate the arm. The infraspinatus is innervated by the suprascapular nerve (C5-C6)¹⁴.

Teres Minor

The teres minor originates from the lateral border of the scapula (below the infraglenoid tubercle) and inserts on the inferior facet of the greater tubercle. It is the smallest of the four rotator cuff muscles and is innervated by the axillary nerve (C5). The teres minor assists with external rotation and adduction of the arm¹⁵.

Subscapularis

The subscapularis is the largest rotator cuff muscle. It originates from the subscapular fossa of the scapula and inserts on the lesser tubercle of the humerus and the anterior capsule of the shoulder joint. It is the primary internal (medial) rotator of the humerus and is innervated by the upper and lower subscapular nerves (C5-C6)²⁵.

Tendon Insertions

The four tendons insert over the greater tuberosity with a mean anteroposterior length of 37.8 mm and mean mediolateral distance of 14.7 mm. The supraspinatus inserts on the superior facet, infraspinatus on the middle facet, and teres minor on the inferior facet. The subscapularis inserts on the lesser tubercle. These tendons converge to form the rotator cuff tendon, which blends with the joint capsule and surrounds the glenohumeral joint¹¹.

Functions

The primary biomechanical role of the rotator cuff is to stabilize the glenohumeral joint by compressing the humeral head against the glenoid. The four muscles work together to perform the following functions³:

• Supraspinatus: Initiates arm abduction (first 15 degrees), depresses the humeral head
• Infraspinatus: External rotation of the shoulder
• Teres Minor: External rotation and adduction of the arm
• Subscapularis: Internal rotation of the shoulder, depresses humeral head

The rotator cuff muscles also provide fine motor control, allowing different portions to activate independently during complex movements. When the deltoid abducts the arm past 15 degrees, the rotator cuff centering forces are essential for smooth humeral head motion²⁶.

Stability and Shoulder Mechanics

Together with the deltoid, the four rotator cuff muscles form a force couple. The inferior rotator cuff (infraspinatus, teres minor, subscapularis) balances the superior moment created by the deltoid during abduction. This dynamic stabilization allows for the shoulder's extensive range of motion while preventing subluxation⁸.

Clinical Significance

Rotator Cuff Tears

Rotator cuff tears are the most common upper extremity condition seen by primary care and orthopedic surgeons. Nearly 2 million people in the United States visit their doctors each year because of rotator cuff tears⁹.

The supraspinatus tendon is the most commonly torn, followed by infraspinatus, subscapularis, and teres minor. Teres minor tears are the least common and most often occur in conjunction with other tendon tears¹⁰.

Epidemiology

Rotator cuff tears increase with age. Full-thickness tears are present in approximately 28% of people over 60 and 65% of those over 70. The dominant arm is more commonly affected⁸¹².

Traumatic tears occur in younger patients following falls or shoulder dislocations, while degenerative tears are more common in older patients and result from wear over time⁸.

Tear Classification

Rotator cuff tears are classified as partial-thickness or full-thickness. Partial tears do not completely detach the tendon. Full-thickness tears can be incomplete (partially detached) or complete (a hole through the tendon)⁹.

Tear size is classified as: small (0-1 cm), medium (1-3 cm), large (3-5 cm), or massive (greater than 5 cm involving 2 or more tendons)⁸.

Clinical Examination

Each rotator cuff muscle can be evaluated independently through specific physical examination tests²:

• Supraspinatus: Jobe test (empty can test)—patient raises arm to 90 degrees abduction and 30 degrees forward flexion with thumb pointing down; examiner pushes down
• Infraspinatus: External rotation against resistance with elbow at side; drop arm test
• Teres Minor: Hornblower's test—arm at 90 degrees abduction with external rotation against resistance
• Subscapularis: Lift-off test and bear hug test—patient attempts to lift hand off lower back against resistance

Treatment

Treatment depends on tear severity. Partial tears and small full-thickness tears are often treated conservatively with physical therapy, anti-inflammatory medications, and rest. Larger tears may require arthroscopic or open surgical repair. Massive irreparable tears may require reverse shoulder arthroplasty or tendon transfer procedures¹².