Acromioclavicular Joint

Overview

The acromioclavicular joint (AC joint) is a small yet crucial synovial joint located at the top of the shoulder where the acromion process of the scapula meets the lateral end of the clavicle. It plays a key role in connecting the upper limb to the axial skeleton, allowing the scapula to move smoothly on the thoracic wall. Despite its limited range of motion, the AC joint is essential for coordinated shoulder movement, stability, and force transmission between the clavicle and scapula. Injuries to this joint are common among athletes and individuals engaged in overhead or contact activities.

Location

The acromioclavicular joint is located at the superolateral aspect of the shoulder. It forms the articulation between the lateral (acromial) end of the clavicle and the medial margin of the acromion process of the scapula. It lies just above the shoulder joint proper (glenohumeral joint) and can be palpated easily as a small bump at the top of the shoulder. The joint marks the lateral termination of the clavicle and connects the upper limb skeleton to the trunk via the scapula and sternum.

Structure

The acromioclavicular joint is classified as a plane-type synovial joint. Its structural features include:

• Articular surfaces: The lateral end of the clavicle articulates with the medial margin of the acromion. Both surfaces are covered by fibrocartilage rather than hyaline cartilage, reflecting the joint’s load-bearing role.
• Joint capsule: A loose, fibrous capsule encloses the joint, allowing slight gliding movements. It is strengthened superiorly by the acromioclavicular ligament and inferiorly by surrounding muscle attachments.
• Articular disc: In many individuals, a wedge-shaped fibrocartilaginous disc partially or completely divides the joint cavity, cushioning and improving joint congruence. The disc may degenerate with age.
• Ligamentous support:
  • Acromioclavicular ligament: Reinforces the joint capsule superiorly, providing horizontal stability.
  • Coracoclavicular ligament: A strong extrinsic ligament composed of two parts—conoid ligament (medial) and trapezoid ligament (lateral)—that connect the clavicle to the coracoid process of the scapula. This ligament provides vertical stability and prevents the clavicle from displacing superiorly.
• Synovial membrane: Lines the inner surface of the joint capsule and secretes synovial fluid for lubrication.
• Innervation: Supplied by branches of the suprascapular nerve, lateral pectoral nerve, and axillary nerve.
• Blood supply: Receives arterial blood from the thoracoacromial artery and branches of the suprascapular artery.

Function

The acromioclavicular joint serves as a critical link between the clavicle and scapula, allowing the shoulder girdle to move as a functional unit. Its functions include:

• Facilitating scapular rotation: Enables the scapula to rotate upward and downward during arm elevation and depression.
• Permitting gliding and tilting motions: Allows small translational movements between the clavicle and scapula, accommodating changes in shoulder position.
• Force transmission: Transfers mechanical loads from the upper limb to the clavicle and through to the axial skeleton via the sternoclavicular joint.
• Stabilization: Works with the coracoclavicular ligament and surrounding muscles to maintain the alignment of the clavicle and scapula during dynamic movements.

Physiological Role(s)

• Shoulder mobility coordination: The AC joint coordinates with the sternoclavicular and glenohumeral joints to provide a full range of shoulder motion, including elevation, abduction, and rotation of the arm.
• Scapulothoracic rhythm: Plays a crucial role in the synchronized movement between the scapula and thoracic wall during arm movements, ensuring efficient biomechanics and muscle coordination.
• Shock absorption: The fibrocartilaginous articular disc helps absorb forces transmitted from the upper limb, protecting the joint surfaces and surrounding structures from stress.
• Load distribution: The joint disperses compressive and tensile forces generated during activities like pushing, pulling, or lifting, preventing localized strain on the shoulder complex.

Clinical Significance

• Acromioclavicular joint dislocation (Shoulder separation): Commonly caused by a fall onto the shoulder or direct trauma. Severity ranges from mild sprains (partial tear of the acromioclavicular ligament) to complete rupture of both the acromioclavicular and coracoclavicular ligaments, leading to elevation of the clavicle.
• Degenerative arthritis: Repetitive use or aging can lead to degeneration of the fibrocartilage and joint capsule, causing pain, stiffness, and reduced shoulder mobility. Common among weightlifters and manual laborers.
• Osteolysis of the distal clavicle: Chronic stress or microtrauma, especially from repetitive heavy lifting, may cause bone resorption at the distal clavicle, resulting in pain and weakness.
• Joint inflammation (acromioclavicular bursitis): Overuse or trauma can inflame the small bursa associated with the AC joint, producing tenderness and discomfort during shoulder elevation.
• AC joint instability: Can result from ligamentous laxity or partial tears, leading to abnormal joint motion, clicking, or pain with overhead activities.
• Radiographic evaluation: AC joint injuries are commonly assessed using anteroposterior (Zanca) radiographs or MRI for ligament and disc integrity. Treatment ranges from rest and physiotherapy to surgical reconstruction in severe cases.