Acromion

Introduction

The acromion is a prominent bony projection of the scapula that plays a critical role in the anatomy and function of the shoulder. It serves as an attachment site for muscles and ligaments, and contributes to the protection and stability of the shoulder joint. Understanding its structure and variations is essential in both clinical and surgical settings.

Anatomy of the Acromion

Location and Orientation

The acromion is located at the lateral end of the spine of the scapula, extending anteriorly and laterally to articulate with the clavicle at the acromioclavicular joint. It forms the highest point of the shoulder and contributes to the roof of the subacromial space.

Morphology and Variations

The shape of the acromion varies among individuals and is classified into three main types:

• Flat (Type I): The undersurface of the acromion is flat, which is generally considered low risk for impingement.
• Curved (Type II): The acromion has a gentle curvature downward, which may predispose to rotator cuff compression.
• Hooked (Type III): The acromion curves sharply downward at its anterior end, associated with higher incidence of impingement and rotator cuff tears.

Surrounding Structures

The acromion is closely associated with several important musculoskeletal and soft tissue structures:

• Muscles: The deltoid muscle originates from the lateral aspect of the acromion, while the trapezius inserts along its posterior border.
• Ligaments: The coracoacromial ligament connects the acromion to the coracoid process, forming an arch over the rotator cuff tendons. The acromioclavicular ligament stabilizes the acromioclavicular joint.
• Bursae: The subacromial bursa lies beneath the acromion and reduces friction between the acromion and the underlying supraspinatus tendon.

Development and Ossification

Embryological Origin

The acromion develops as part of the scapula during fetal life. It originates from a primary cartilaginous structure that undergoes ossification in a sequential manner. This early development establishes the basic shape and orientation of the acromion in relation to the scapular spine and clavicle.

Postnatal Growth and Fusion

After birth, the acromion continues to ossify through secondary ossification centers that appear in early childhood. These centers gradually fuse with the main scapular body, usually completing fusion in late adolescence. Incomplete fusion can result in a condition known as os acromiale, which may be asymptomatic or contribute to shoulder pain and impingement.

Function of the Acromion

The acromion serves multiple functional roles in the shoulder:

• Muscle Lever Arm: It provides a lever arm for the deltoid muscle, enhancing the efficiency of shoulder abduction and flexion.
• Protection: It forms the superior aspect of the subacromial space, protecting the rotator cuff tendons and the subacromial bursa from external trauma.
• Joint Stability: By articulating with the clavicle, the acromion contributes to the stability of the acromioclavicular joint, helping maintain proper alignment of the shoulder girdle.

Clinical Significance

Acromion-Related Disorders

The acromion is involved in several clinical conditions, primarily affecting the shoulder’s function and mobility:

• Impingement Syndrome: A hooked or curved acromion can compress the rotator cuff tendons, leading to pain, inflammation, and limited range of motion.
• Rotator Cuff Injuries: Structural variations or degeneration of the acromion can contribute to partial or full-thickness rotator cuff tears.
• Fractures: Traumatic injuries can fracture the acromion, potentially compromising shoulder stability and requiring surgical intervention depending on severity.

Imaging and Diagnosis

Accurate assessment of the acromion is essential for diagnosing related disorders:

• X-Ray: Standard views, including lateral and axillary projections, help evaluate acromial morphology, joint alignment, and fractures.
• Magnetic Resonance Imaging (MRI): Provides detailed visualization of soft tissues, including rotator cuff tendons and the subacromial bursa, to identify impingement or tears.
• Computed Tomography (CT): Useful for complex fractures, os acromiale, and pre-surgical planning.

Surgical Considerations

When conservative treatment fails, surgical intervention may be necessary:

• Acromioplasty: Removal of the anterior hook of the acromion to relieve impingement and create more subacromial space.
• Fixation of Os Acromiale: Surgical stabilization of an unfused acromion to restore shoulder function.
• Arthroscopic Procedures: Minimally invasive techniques allow decompression, tendon repair, or bursa removal with reduced recovery time.

Acromion in Comparative Anatomy

The acromion is not unique to humans and can be observed in various other species, with notable differences in morphology and function:

• Primates: In many primates, the acromion is more pronounced, supporting powerful shoulder movements required for climbing and brachiation.
• Quadrupeds: In quadrupedal mammals, the acromion tends to be smaller and differently oriented, reflecting adaptations for locomotion rather than overhead arm use.
• Evolutionary Significance: The shape and orientation of the human acromion have evolved to balance stability and mobility, facilitating precise manipulation and overhead activities.

References

1. Gray H, Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Elsevier; 2020.
2. Neer CS. Anterior acromioplasty for the chronic impingement syndrome in the shoulder: a preliminary report. J Bone Joint Surg Am. 1972;54(1):41-50.
3. Bigliani LU, Morrison DS, April EW. The morphology of the acromion and its relationship to rotator cuff tears. Orthop Trans. 1986;10:228.
4. Lehtinen J, Jaroma H, Ristolainen L. Os acromiale: prevalence and clinical significance. Acta Orthop Scand. 1993;64(6):628-630.
5. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 9th ed. Philadelphia: Wolters Kluwer; 2019.
6. Flatow EL, Bigliani LU. Anatomy and biomechanics of the acromion. Clin Orthop Relat Res. 1995;(314):41-46.
7. Scheibel M, Haug L, Schroeder RJ. MR imaging of the shoulder: normal anatomy and MR appearances of common abnormalities. Eur Radiol. 2004;14(4):648-658.