U01.11.002 Rotator cuff muscles

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Learning Objectives

  • Identify the four SItS muscles that stabilize the glenohumeral joint.
  • Correlate each rotator cuff muscle with its specific innervation (C5-C6) and primary action.
  • Recognize the clinical presentation of a Supraspinatus tear and its diagnostic physical exam.

1. The SITS Muscles: Anatomy and Function

The rotator cuff is a group of four muscles that provide structural stability to the shoulder. They are primarily innervated by the C5-C6 nerve roots.

  • Supraspinatus: Abducts the arm initially (the first 15°) before the deltoid takes over.
    • Nerve: Suprascapular nerve.
    • Clinical: Most common rotator cuff injury. Often caused by impingement between the humerus and the acromion.
  • Infraspinatus: Externally rotates the arm.
    • Nerve: Suprascapular nerve.
    • Clinical: Frequently injured in athletes (e.g., pitching injuries).
  • Teres minor: Adducts and externally rotates the arm.
    • Nerve: Axillary nerve.
  • Subscapularis: Internally rotates and adducts the arm.
    • Nerve: Upper and lower subscapular nerves.

2. Clinical Assessment: The “Empty Can” Test

Because the supraspinatus is the most commonly torn muscle, specific physical exam maneuvers are high-yield for the USMLE.

  • The Maneuver: The patient’s arm is abducted to 90° and angled forward, with the thumb pointing down (as if emptying a soda can).
  • Positive Result: Weakness or pain during this movement suggests supraspinatus tendinopathy or tear.

Activity

3. Comparative Anatomy Table

Muscle Primary Action Innervation Attachment Site
Supraspinatus Initial Abduction Suprascapular Greater tubercle
Infraspinatus External Rotation Suprascapular Greater tubercle
Teres minor Ext. Rotation / Adduction Axillary Greater tubercle
Subscapularis Internal Rotation / Adduction Subscapular (Upper/Lower) Lesser tubercle
Clinical Notes & Corrections:

  • Impingement Syndrome: The supraspinatus tendon passes through the narrow subacromial space. Chronic overhead movement can lead to subacromial bursitis or tendon fraying.
  • Lesser vs. Greater Tubercle: Note that only the Subscapularis inserts on the lesser tubercle. The other three “SIt” muscles insert on the greater tubercle.

Activity: Rotator Cuff Muscle Challenge

Memory Hooks:

SItS: Small ‘t’ is for teres minor (not Teres major).

Supraspinatus: Superior position = Abduction.

Subscapularis: Sub = Under = Internal rotation.

Activity: