Rotator Cuff of the Shoulder

Rotator Cuff is a group of muscles surrounding the shoulder joint, it consists of 4 muscles abbreviated in the word “SITS”:

1. Supraspinatus muscle.
2. Infraspinatus muscle.
3. Teres minor muscle.
4. Subscapularis muscle.

See Also: What is SLAP Lesion?

See Also: Hawkins Test

Rotator Cuff Muscles

	Supraspinatus Muscle
Origin	Supraspinous fossa of scapula.
Insertion	Superior facet on greater tuberosity of humerus.
Innervation	Suprascapular nerve (C4, C5 and C6) (C4,C5, C6).
Arterial Supply	Suprascapular artery.
Action	Initiates and assists deltoid in abduction of arm and acts with other rotator cuff muscles.

	Teres minor muscle
Origin	Superior part of lateral border of scapula.
Insertion	Inferior facet on greater tuberosity of humerus.
Innervation	Axillary nerve (C5 and C6) (C5, C6).
Arterial Supply	Subscapular and circumflex scapular arteries.
Action	Laterally rotate the arm; and helps to hold humeral head in glenoid cavity of scapula.

	Subscapularis muscle
Origin	Subscapular fossa of scapula.
Insertion	Lesser tuberosity of humerus.
Innervation	Upper and lower subscapular nerves (C5, C6 and C7) (C5, C6, C7).
Arterial Supply	Subscapular artery.
Action	Medially rotates the arm and adducts it; and helps to hold humeral head in glenoid cavity of scapula.

See Also: Brachial Plexus Anatomy

Rotator Cuff Muscle Function

These muscles are referred to Glenohumeral Joint as the protectors of the shoulder since, in addition to actively moving the humerus, they fine-tune the humeral head position during arm elevation.

See Also: Shoulder Range of Motion

Rotator Cuff Muscle Functions include the following:

Assist in the rotation of the shoulder and arm:

At the G-H joint, elevation through abduction of the arm requires that the greater tuberosity of the humerus pass under the coracoacromial arch. For this to occur, the humerus must externally rotate, and the acromion must elevate. ER of the humerus is produced actively by a contraction of the infraspinatus and teres minor, and by a twisting of the joint capsule.

A force couple exists in the transverse plane between the subscapularis anteriorly and the infraspinatus and teres minor posteriorly in which cocontraction of the infraspinatus, teres minor, and subscapularis muscles both depresses and compresses the humeral head during overhead movements.

In the coronal plane, there is another force couple between the deltoid and the inferior rotator cuff muscles (infraspinatus, subscapularis, and teres minor).

With the arm fully adducted, contraction of the deltoid produces a vertical force in a superior direction, resulting in an upward translation of the humeral head relative to the glenoid. Contraction of the inferior rotator cuff muscles produces both a compressive force and a downward translation of the humerus that counterbalances the force of the deltoid, thereby stabilizing the humeral head.

Reinforce the G-H capsule:

The rotator cuff muscles, together with the coracohumeral ligament, and the LHB (often referred to as the fifth rotator cuff muscle) function as contractile ligaments. For example, firing of the rotator cuff muscles increases the tension of the middle G-H ligament when the arm is abducted to 45 degrees and externally rotated.

Control much of the active arthrokinematics of the GH joint:

Contraction of the horizontally oriented supraspinatus produces a compression force directly into the glenoid fossa. This compression force holds the humeral head securely in the glenoid cavity during its superior roll, which provides stability to the joint and also maintains a mechanically efficient fulcrum for elevation of the arm.

In the shoulder midrange position, when all of the passive restraints are lax, joint stability is achieved almost entirely by the rotator cuff. In addition, as previously mentioned, without adequate supraspinatus force, the near vertical line of force of a contracting deltoid tends to jam or impinge the humeral head superiorly against the coracoacomial arch.

Compared with most joints that have a single axis on which torques are generated, the shoulder is very different, because it has no fixed axis. As a result, each muscle activation creates a unique set of rotational moments, which necessitates precise coordination in the timing and magnitude of muscle contractions.

Jenp et al. used electromyography (EMG) to detect the most specific positions of highest activation for the individual rotator cuff muscles:

• The greatest activation of the subscapularis was with the arm in the scapular plane at 90 degrees of elevation and neutral humeral rotation. The subscapularis has also been shown to be an effective humeral head depressor in ER, whereas it produces almost no A-P translation in abduction and ER.
• The infraspinatus–teres minor muscles are very effective humeral head depressors with the arm in the sagittal plane and the humerus elevated to 90 degrees in the midrange of external/IR (the socalled hornblower’s position).
• The supraspinatus could not be effectively isolated.

Rotator Cuff Footprint:

Footprint of the rotator cuff means the value of space of its insertion on the humeral head:

1. Supraspinatus: Triangular in shape, with an average maximum medial-to-lateral length of 6.9 mm and an average maximum anteroposterior width of 12.6 mm.
2. Infraspinatus: Trapezoidal in shape, with an average maximum medial-to-lateral length of 10.2 mm and an average maximum anteroposterior width of 32.7 mm.
3. Teres minor: Triangular in shape, with an average maximum medial-to-lateral length of 29 mm and an average maximum width of 21 mm.
4. Subscapularis: Comma-shaped insertion, with an average maximum medial-to-lateral length of 40 mm and an average maximum width of 20 mm.

See Also: Hornblower Sign

Acromiohumeral Distance AHD

The Acromiohumeral distance can be measured on an anteroposterior (AP) radiograph of the shoulder joint in neutral rotation, the Rockwood view, or the outlet view (v-view).

• The AHD is measured as the distance between the superior margin of the humeral head and the under surface of the acromion.
• The reference values are as follows:
  • Mean value (in healthy individual): AHD = 1.05 cm
  • Abnormal: AHD = 7 mm

If the acromiohumeral distance is at or below the cut off value of 7 mm, it should be assumed that a complete rotator cuff tear is present.

References

1. Jenp YN, Malanga GA, Growney ES, An KN. Activation of the rotator cuff in generating isometric shoulder rotation torque. Am J Sports Med. 1996 Jul-Aug;24(4):477-85. doi: 10.1177/036354659602400412. PMID: 8827307.
2. Turkel SJ, Panio MW, Marshall JL, Girgis FG. Stabilizing mechanisms preventing anterior dislocation of the glenohumeral joint. J Bone Joint Surg Am. 1981 Oct;63(8):1208-17. PMID: 7287791.
3. Culham E, Peat M: Functional anatomy of the shoulder complex. J Orthop Sports Phys Ther 18:342–350, 1993.
4. Blackburn TA, McLeod WD, White B, et al: EMG analysis of posterior rotator cuff exercises. Athl Train 25:40–45, 1990.
5. Perry J, Glousman RE: Biomechanics of throwing. In: Nicholas JA, Hershman EB, eds. The Upper Extremity in Sports Medicine. St Louis, MO: CV Mosby, 1990:727–751.
6. Sharkey NA, Marder RA: The rotator cuff opposes superior translation of the humeral head. Am J Sports Med 23:270–275, 1995.
7. Sharkey NA, Marder RA, Hanson PB: The role of the rotator cuff in elevation of the arm. Trans Orthop Res Soc 18:137, 1993.
8. Dutton’s Orthopaedic Examination, Evaluation, And Intervention 3rd Edition.
9. Millers Review of Orthopaedics -7th Edition Book.