Shoulder Musculature and Rotator Cuff

Shoulder Musculature and Rotator Cuff

• The shoulder joint's extensive mobility is attributed to the open configuration of the glenohumeral joint, necessitating strong support for the humeral head.
• Six scapulohumeral muscles link the scapula to the humerus.
• Four scapulohumeral muscles make up the rotator cuff muscles.
• Rotator cuff muscles primarily stabilize the shoulder by supporting the humeral head within the glenoid fossa.
• Small rotator cuff muscles are prone to overuse injuries because of the substantial work they perform.

Scapulohumeral Muscles

• The deltoid muscle is one of the six scapulohumeral muscles.
• The deltoid muscle retains the humerus in the glenohumeral joint when lifting heavy objects.
• Weight is transferred from the skeleton through the trapezius, other muscles, the scapula, and clavicle, to the humerus, a process stabilized by the deltoid.
• Supraspinatus initiates upper limb abduction, followed by the deltoid and trapezius muscles.
• The deltoid can perform flexion, extension, medial rotation, and lateral rotation of the humerus at the glenohumeral joint.
• Teres major extends from the scapula to the anterior humerus, contributing to medial rotation.
• Teres major defines the posterior border of the axilla.

Rotator Cuff Muscles

• Rotator cuff muscles are situated deeper and closer to the glenohumeral joint than the deltoid and teres major.
• The components of the rotator cuff include the supraspinatus, infraspinatus, teres minor, and subscapularis.
• The supraspinatus originates on the scapula, superior to the spine, with its tendon passing under the acromion before inserting on the humerus.
• The supraspinatus initiates abduction of the humerus.
• The small space through which the supraspinatus tendon passes can lead to inflammation and tendonitis.
• The infraspinatus is inferior to the scapular spine, attaching to the humeral head and causing lateral rotation upon contraction.
• Teres minor extends from the scapula to the humerus, also causing lateral rotation, and is often hard to distinguish from the teres major during dissection.
• The suprascapular nerve innervates the supraspinatus and infraspinatus, while the axillary nerve innervates the teres minor, unlike the subscapular nerve innervation of the teres major.
• The subscapularis is situated deep to the scapula, running from its deep surface to the humerus, inserting on the lesser tubercle and medial side of the bicipital groove.
• The subscapularis causes medial rotation of the humerus.
• The tendons of the rotator cuff muscles merge and blend to support the glenohumeral joint's connective tissues and synovial capsule.
• The tone of rotator cuff muscles maintains the head of the humerus in the scapula.
• Damage to a rotator cuff muscle diminishes the joint’s strength, function, and overall effectiveness.
• Rotator cuff muscles insert high into the head of the humerus, so they do not contribute to adduction of the humerus.

Strengthening Rotator Cuff Muscles

• Elastic bands can be used to strengthen rotator cuff muscles.
• Medial rotation exercises are used to target the subscapularis.
• Lateral rotation exercises are used to target the infraspinatus and teres minor.

Innervation

• The upper and lower subscapular nerves innervate the subscapularis.
• The suprascapular nerve innervates the supraspinatus and infraspinatus.
• The axillary nerve innervates the teres minor, as well as the deltoid muscle.
• The lower subscapular nerve innervates the teres major.
• All nerves listed are branches of the brachial plexus.