eMedicine Specialties > Sports Medicine > Shoulder

Acromioclavicular Joint Injury

L. Edward Seade, MD, Chief of Shoulder Service, Orthopaedic Specialists of Austin
Reed L Bartz, MD, Consulting Staff, Division of Sports Medicine, Nebraska Orthopaedic and Sports Medicine PC; Robert Josey, MD, Consulting Staff, Department of Orthopedic Surgery, Orthopaedic Specialists of Austin
Contributor Information and Disclosures

Updated: Oct 27, 2008

Introduction

Injuries in and around the shoulder are common in today's athletic society. Proper knowledge of the different problems and treatment options for shoulder disorders is necessary to get patients back to their preinjury state.

Background

Acromioclavicular (AC) joint injuries are common and often seen after bicycle wrecks, contact sports, and car accidents. The acromioclavicular joint is located at the top of the shoulder where the acromion process and the clavicle meet to form a joint. Several ligaments surround this joint, and depending on the severity of the injury, a person may tear one or all of the ligaments. Torn ligaments lead to acromioclavicular joint sprains and separations.¹ 

The distal clavicle and acromion process can also be fractured. Injury to the acromioclavicular joint may injure the cartilage within the joint and can later cause arthritis of the acromioclavicular joint. 

This article discusses the anatomy of the acromioclavicular joint, the diagnosis of disorders of this joint, and the different treatment options.

For excellent patient education resources, see eMedicine's Breaks, Fractures, and Dislocations Center. Also, see eMedicine's article on Shoulder Dislocation.

Related eMedicine topics:
Acromioclavicular Injury [in the Emergency Medicine section]
Acromioclavicular Joint Separations [in the Orthopedic Surgery section]
Dislocation, Shoulder [in the Emergency Medicine section]
Shoulder Dislocation [in the Orthopedic Surgery section]

Related Medscape topics:
Resource Center Exercise and Sports Medicine
Resource Center Joint Disorders
Resource Center Trauma
Specialty Site Orthopaedics
CME/CE Management of Chronic Shoulder Disorders Reviewed
Variations in Corticosteroid/Anesthetic Injections for Painful Shoulder Conditions: Comparisons Among Orthopaedic Surgeons, Rheumatologists, and Physical Medicine and Primary-Care Physicians

Frequency

United States

Injuries to the acromioclavicular joint are the most common reason that athletes seek medical attention following an acute shoulder injury. Glenohumeral dislocations (see Shoulder Dislocation) are the second most common injuries seen. Men in their second through fourth decades of life have the greatest frequency of acromioclavicular joint injuries, which are most often incomplete tears of the ligaments.¹

Functional Anatomy

The normal width of the acromioclavicula joint is 1-3 mm in younger individuals; it narrows to 0.5 mm or less in individuals older than 60 years.

The acromioclavicular joint is made up of 2 bones (the clavicle and the acromion), 4 ligaments, and a meniscus inside the joint.

• The acromioclavicular joint is surrounded by a thin joint capsule and 4 small ligaments. These ligaments mostly give joint stability to anterior and posterior translation, as well as provide horizontal stability to the joint.
• Another set of ligaments also provides vertical stability to the acromioclavicular joint. These ligaments are called the coracoclavicular ligaments, which are found medial to the acromioclavicular joint and go from the coracoid process on the scapula to the clavicle.
• Different injuries result in different tears of the 2 coracoclavicular ligaments (the conoid and the trapezoid). Torn acromioclavicular joint ligaments and/or torn coracoclavicular ligaments are seen in acromioclavicular joint sprains. The meniscus that lies in the joint may also be injured during sprains or fractures around the acromioclavicular joint.
  • The acromioclavicular capsular ligaments provide most of the joint stability in the anteroposterior (AP) direction. The conoid and trapezoid ligaments aid in providing superior-inferior stability to the joint. Compression of the joint is restrained mainly by the trapezoid ligament.

Sport-Specific Biomechanics

When a person falls onto their shoulder, the force pushes the tip of the shoulder down. The clavicle is usually kept in its anatomic position, whereas the shoulder is driven down, which injures the different ligaments or causes a fracture. When the ligaments are injured they are either sprained or, in more severe cases, torn.

Acromioclavicular joint sprains have been classified according to their severity. In a type I sprain, a mild force applied to these ligaments does not tear them. The injury simply results in a sprain, which hurts, but the shoulder does not show any gross evidence of an acromioclavicular joint dislocation. Type II sprains are seen when a heavier force is applied to the shoulder, disrupting the acromioclavicular ligaments but leaving the coracoclavicular ligaments intact. When these injuries occur, the lateral clavicle becomes a little more prominent. 

In type III sprains, the force completely disrupts the acromioclavicular and coracoclavicular ligaments. This leads to complete separation of the clavicle and obvious changes in appearance. The lateral clavicle is very prominent. A few more types of acromioclavicular joint sprains have been classified, but types I–III are the most common.

Classification of acromioclavicular joint injuries.

An acromioclavicular joint sprain is more common than a fracture after an injury. However, fractures of the distal clavicle and the acromion process may occur, so the healthcare provider must be aware of such injuries and ready to diagnose and treat them as well (see Clavicular Injuries).

Clinical

History

An acromioclavicular joint injury should be considered in any patient complaining of pain over the superior part of the shoulder. Injuries to this part of the body are painful.

• The most common mechanism for an acromioclavicular joint injury is a fall directly onto the acromion, with the arm adducted up against the body. Multiple indirect forces can result in an acromioclavicular joint injury. A fall onto an outstretched hand (FOOSH injury) and a downward force on the upper extremity have been implicated in acromioclavicular joint injuries.^{1, 2, 3}
• In the immediate setting, the patient may initially experience generalized shoulder tenderness and swelling; however, as the diffuse pain resolves, specific point tenderness over the acromioclavicular joint is usually noted. The athlete may note a significant abrasion or prominence of the distal clavicle.
• Athletes involved in weight training typically experience pain with specific exercises such as with use of the bench press and dips.
• Many individuals experience nocturnal pain and awakening when rolling onto the involved shoulder, which puts pressure on the acromioclavicular  joint.
• Rarely, the patient may report popping or catching in the region of the acromioclavicular joint.

Physical

• Patients have pain over the acromioclavicular joint. Swelling, bruising, and a prominent clavicle may be evident, depending on the type of sprain that the patient has sustained. In types I and II sprains, deformity is usually minimal. In type III, the distal clavicle is abnormally prominent. Of note, clavicle fractures, without acromioclavicular joint sprains, can also cause the clavicle to be prominent.
• The patient has poor shoulder range of motion and moderate pain when trying to raise up the arm.
• In the acute situation, the examiner may have difficulty ruling out a concomitant rotator cuff tear, as active and passive shoulder abduction maneuvers are difficult to perform in the face of an acromioclavicular joint separation.
• The most reliable physical examination test for acromioclavicular joint pathology is the cross-body adduction test. The test is performed by elevating the arm on the affected side 90º, while the examiner grasps the elbow and adducts the involved arm across the body. Although reproduction of pain with this maneuver may occur in patients with posterior capsule tightness or subacromial impingement, pain is suggestive of acromioclavicular joint pathology. Restriction of range of motion, which is rarely associated with acromioclavicular joint pathology, more likely suggests adhesive capsulitis or glenohumeral arthritis.

Causes

See History.

References

• Clavicular Injuries
• Rotator Cuff Injury
• Shoulder Dislocation
• Shoulder Impingement Syndrome
• Superior Labrum Lesions