Supraspinatus

The supraspinatus is one of the four muscles of the rotator cuff and plays a crucial role in shoulder stability and movement. It is primarily responsible for initiating arm abduction and contributes significantly to maintaining the integrity of the glenohumeral joint. Understanding its anatomy and function is essential in diagnosing and managing shoulder injuries.

Anatomy

Location

The supraspinatus muscle is located in the posterior aspect of the shoulder, occupying the supraspinous fossa of the scapula. It lies above the spine of the scapula and is covered by the trapezius muscle. Anteriorly, it is separated from the subscapularis by the scapular spine and acromion, and it runs laterally towards the humerus to join the rotator cuff tendons.

Origin

The muscle originates from the medial two-thirds of the supraspinous fossa of the scapula. The fibers arise from the bony surface and its surrounding fascia, providing a strong base for contraction and force transmission to the humerus.

Insertion

The supraspinatus tendon inserts onto the superior facet of the greater tubercle of the humerus. This insertion allows the muscle to exert force for arm abduction and contributes to the dynamic stability of the shoulder joint.

Structure

• The muscle fibers are predominantly parallel and converge into a strong tendon anteriorly.
• The tendon passes beneath the acromion, forming a critical part of the subacromial space.
• It is closely associated with the joint capsule and the other rotator cuff muscles, ensuring coordinated shoulder movement.

Innervation and Blood Supply

Innervation

The supraspinatus muscle is innervated by the suprascapular nerve, which arises from the upper trunk of the brachial plexus, specifically from the C5 and C6 nerve roots. The nerve passes through the suprascapular notch beneath the superior transverse scapular ligament to reach the muscle. Motor fibers stimulate muscle contraction, while sensory fibers provide proprioceptive feedback from the shoulder joint.

Blood Supply

The primary arterial supply to the supraspinatus muscle comes from the suprascapular artery, a branch of the thyrocervical trunk. Venous drainage occurs through the accompanying suprascapular vein. The region also benefits from collateral circulation from the dorsal scapular and circumflex scapular arteries, which helps maintain muscle perfusion even if one vessel is compromised.

Function

• Abduction of the arm: The supraspinatus initiates the first 15 degrees of arm abduction before the deltoid muscle becomes the primary mover.
• Stabilization of the glenohumeral joint: It holds the head of the humerus firmly within the shallow glenoid cavity during arm movements.
• Contribution to rotator cuff mechanics: The supraspinatus works in synergy with the other rotator cuff muscles to maintain smooth and coordinated shoulder motion and prevent superior displacement of the humeral head.

Clinical Significance

Common Injuries

• Supraspinatus tendon tears: Partial or complete tears often result from acute trauma or chronic degeneration, leading to pain and weakness in arm abduction.
• Tendinopathy and overuse injuries: Repetitive overhead activities can cause inflammation and microtears in the tendon, commonly seen in athletes and manual laborers.
• Impingement syndrome: Compression of the supraspinatus tendon under the acromion can result in pain, restricted movement, and inflammation.

Diagnostic Evaluation

• Physical examination tests: The Empty Can test, Drop Arm test, and Neer impingement test are commonly used to assess supraspinatus function and detect tears or impingement.
• Imaging studies: Magnetic Resonance Imaging (MRI) provides detailed visualization of tendon integrity, while ultrasound can evaluate dynamic movement and tendon pathology.

Treatment and Management

• Conservative management: Includes rest, physical therapy, nonsteroidal anti-inflammatory drugs, and activity modification to reduce symptoms and promote healing.
• Surgical repair: Indicated for severe tears or failed conservative treatment. Options include arthroscopic or open tendon repair.
• Postoperative rehabilitation: Gradual strengthening and range-of-motion exercises are essential to restore function and prevent recurrence.

Variations and Anatomical Considerations

• Accessory muscles or tendon variations may alter the presentation of injuries or surgical approaches.
• Knowledge of anatomical variations is important for planning surgical interventions and avoiding iatrogenic nerve or vascular injury.

References

1. Gray H. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Elsevier; 2020.
2. Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Churchill Livingstone; 2020.
3. Neer CS 2nd. Anterior acromioplasty for the chronic impingement syndrome in the shoulder. J Bone Joint Surg Am. 1972;54(1):41-50.
4. Warner JJ, Beim GM, Higgins L. Diagnosis and management of rotator cuff tears. J Am Acad Orthop Surg. 2000;8(6):357-367.
5. DePalma AF, Callery G. Surgery of the Shoulder. 4th ed. Philadelphia: Saunders; 2011.
6. Mohtadi NGH, Hollinshead RM. Clinical Anatomy of the Shoulder. New York: McGraw-Hill; 2015.
7. Yamamoto A, Itoi E, Minagawa H, et al. Tears of the supraspinatus tendon: evaluation with ultrasound and MRI. Radiology. 2002;225(3):721-726.