Subacromial Bursa

The subacromial bursa is a large synovial fluid-filled sac located within the shoulder, between the acromion and the supraspinatus tendon. It functions as a cushion that reduces friction between the rotator cuff tendons and the overlying bony structures during shoulder movement. It is one of the most clinically significant bursae in the body, frequently involved in inflammatory shoulder conditions.

Location

The subacromial bursa is located in the lateral aspect of the shoulder, deep to the deltoid muscle and acromion, and superficial to the supraspinatus tendon and glenohumeral joint capsule. It extends laterally toward the deltoid insertion and medially beneath the coracoacromial arch.

Structure

• Flat, thin-walled synovial sac composed of an inner lining of synovial membrane and an outer fibrous layer.
• Filled with a small amount of synovial fluid under normal conditions.
• Has a variable shape and size, generally crescent-shaped or ovoid.
• May be partially or fully continuous with the subdeltoid bursa, forming a single functional unit in some individuals.

Function

• Reduces friction: Separates the supraspinatus tendon from the coracoacromial arch, allowing smooth gliding during abduction and flexion of the shoulder.
• Protects soft tissues: Acts as a cushion between tendons and overlying bone or fascia, minimizing wear and tear during motion.
• Enhances mobility: Facilitates full range of motion by reducing resistance between structural layers of the shoulder.

Physiological Role(s)

• Prevents impingement of the supraspinatus tendon during overhead movements by reducing contact stress.
• Distributes mechanical loads across the superior shoulder compartment during dynamic movement.
• Maintains synovial lubrication of interfacing structures in the subacromial space.

Relations

• Superior: Acromion, coracoacromial ligament, and deltoid muscle.
• Inferior: Supraspinatus tendon and glenohumeral joint capsule.
• Medial: Coracoid process and base of the acromion.
• Lateral: Deltoid insertion on the humerus.

Development

The subacromial bursa forms during fetal development as a cleft between tissue planes that are subject to friction. It is lined by synovial cells capable of producing lubricating fluid. In some individuals, the subacromial and subdeltoid bursae develop separately and remain distinct, while in others, they merge into a single cavity. The size and complexity of the bursa increase with postnatal shoulder use and mechanical load.

Clinical Significance

• Subacromial bursitis: Inflammation of the bursa causes shoulder pain, especially with overhead motion. Often results from overuse, trauma, or impingement syndrome.
• Rotator cuff impingement: A swollen or thickened bursa can compress the supraspinatus tendon, leading to pain, weakness, and restricted range of motion.
• Calcific bursitis: Deposition of calcium crystals in the bursa may cause acute inflammation and severe pain.
• Corticosteroid injection site: The subacromial bursa is a common target for anti-inflammatory injections in cases of chronic shoulder pain or impingement.
• Adhesive capsulitis: While primarily affecting the joint capsule, associated inflammation may involve the subacromial bursa secondarily.

Imaging

• Ultrasound: First-line imaging tool for detecting bursal thickening, fluid accumulation, or impingement. Also used to guide injections.
• MRI: Provides detailed visualization of the bursa and surrounding soft tissues, including rotator cuff pathology and subacromial impingement.
• X-ray: Cannot visualize the bursa directly, but may reveal associated findings such as acromial spurs or calcifications.

Surgical Relevance

• Bursectomy: In cases of chronic, non-responsive bursitis, surgical removal of the bursa may be performed arthroscopically.
• Subacromial decompression: Resection of acromial bone or coracoacromial ligament may be combined with bursectomy to relieve impingement.
• Rotator cuff repair: During repair procedures, the subacromial bursa is often debrided to improve visualization and reduce inflammation.

Anatomical Variations

• The size and extent of the subacromial bursa vary between individuals.
• In some people, the bursa is completely distinct from the subdeltoid bursa, while in others, it is continuous and forms a large single cavity.
• The degree of communication with adjacent bursae or joint structures may influence clinical presentation and treatment response.