Acromioclavicular Articulation

Anatomy > Gray's Anatomy of the Human Body > III. Syndesmology > 6b. Acromioclavicular Articulation

The left shoulder and acromioclavicular joints, and the proper ligaments of the scapula (Gray's Anatomy, 1918).

Acromioclavicular Articulation

The acromioclavicular joint (AC joint) or articulatio acromioclavicularis is a type of synovial joint located between the acromial end of the clavicle and the medial margin of the acromion of the scapula. It is classified as an arthrodial joint (plane-type synovial joint) and plays a critical role in the movement and stability of the shoulder girdle.

Ligaments of the Acromioclavicular Joint

The stability of the AC joint is maintained by several ligaments:

• Articular capsule
• Articular disk
• Superior acromioclavicular ligament
• Inferior acromioclavicular ligament
• Coracoclavicular ligament (including the trapezoid ligament and conoid ligament)

Articular Capsule

The articular capsule (capsula articularis) surrounds the joint and is reinforced superiorly and inferiorly by the respective acromioclavicular ligaments.

Superior Acromioclavicular Ligament

This quadrilateral band strengthens the upper part of the joint. It connects the top of the lateral end of the clavicle to the upper surface of the acromion. The ligament fibers interlace with the aponeuroses of the trapezius and deltoid muscles.

Inferior Acromioclavicular Ligament

Thinner than the superior ligament, this band reinforces the lower portion of the capsule. It lies above the supraspinatus tendon and may occasionally be associated with the articular disk.

Articular Disk

The presence of an articular disk (discus articularis) in the AC joint is variable. When present, it partially or fully separates the articular cavity and contributes to joint stability.

Synovial Membrane

Typically, there is one synovial membrane, but when a complete disk divides the joint, two cavities are formed, each with its own membrane.

Coracoclavicular Ligament

Diagram of the human shoulder joint, front view

Diagram of the human shoulder joint, back view

Although not part of the AC joint proper, the coracoclavicular ligament (ligamentum coracoclaviculare) is critical for stabilizing the clavicle and suspending the shoulder girdle.

It consists of two parts:

Trapezoid Ligament

The trapezoid ligament (ligamentum trapezoideum) is flat and quadrilateral, running from the coracoid process to the trapezoid line of the clavicle.

Conoid Ligament

The conoid ligament (ligamentum conoideum) is cone-shaped, with its apex attached to the coracoid process and its base affixed to the conoid tubercle of the clavicle.

These ligaments function together to prevent excessive movement between the scapula and clavicle, particularly during upward displacement.

Movements of the Acromioclavicular Joint

Movements at the AC joint are mainly:

• Gliding motion between the acromion and clavicle
• Rotation of the scapula around a vertical axis drawn through the clavicle

The range of movement is limited by the coracoclavicular ligament:

• The trapezoid ligament restricts forward rotation.
• The conoid ligament restricts backward rotation.

These movements contribute to the elevation, depression, protraction, and retraction of the scapula and allow for greater range in shoulder movement, especially during activities like reaching overhead or throwing.

Functional Importance

The AC joint serves as a pivot allowing the scapula to rotate and adjust its position relative to the thoracic cage, enabling a wide range of shoulder movements. It ensures the scapula maintains proper contact with the ribcage during elevation or depression of the shoulder and permits smooth transmission of forces from the upper limb to the axial skeleton.

Ligaments of the Scapula

In addition to those directly associated with the AC joint, the following scapular ligaments contribute to shoulder joint stability:

Coracoacromial Ligament

This strong triangular ligament spans from the coracoid process to the acromion, forming a protective arch over the head of the humerus with these two bony projections. It is in close relation to the supraspinatus tendon and overlain by the deltoid.

Superior Transverse Scapular Ligament

Also called the suprascapular ligament, it bridges the scapular notch, converting it into a foramen for the suprascapular nerve. The corresponding artery and vein typically pass over this ligament.

Inferior Transverse Scapular Ligament

Also called the spinoglenoid ligament, it stretches across the spinoglenoid notch and transmits the suprascapular nerve and vessels to the infraspinous fossa.

Clinical Relevance

Injuries to the AC joint, including acromioclavicular joint dislocation or shoulder separation, are common in contact sports. Degenerative changes may lead to acromioclavicular joint osteoarthritis, especially in older individuals or those with a history of joint trauma.

See Also

• Clavicle
• Scapula
• Shoulder joint
• Trapezoid ligament
• Conoid ligament
• Coracoacromial ligament
• Synovial joint
• Sternoclavicular joint

External Links

• MRI images of AC joint injury
• AC Joint Osteoarthritis article
• Diagnostic ultrasound of AC joint

Gray's Anatomy

• Gray's Anatomy Contents
• Gray's Anatomy Subject Index
• About Classic Gray's Anatomy
• Glossary of anatomy terms

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Adapted from the Classic Grays Anatomy of the Human Body 1918 edition (public domain)