Acromioclavicular (AC) Joint: Normal Distance, Measurement, and Clinical Significance

What is the normal distance between the clavicle and the acromion?

The normal distance between the clavicle and the acromion, specifically referring to the acromioclavicular (AC) joint space as seen on radiographs, typically ranges from 1 to 6 millimeters (mm), with an average of approximately 3 mm. This measurement can vary based on individual anatomy, age, and the specific imaging technique used.

Anatomical Context: The Acromioclavicular (AC) Joint

To understand the "normal" distance, it's crucial to first understand the anatomy involved. The clavicle (collarbone) and the acromion (the bony process forming the highest point of the shoulder blade, or scapula) articulate to form the acromioclavicular (AC) joint. This is a diarthrodial (synovial) joint, meaning it's highly mobile and designed for movement, albeit with limited range compared to the glenohumeral joint.

While the joint itself is relatively small, its stability is critical for overall shoulder function and the smooth transfer of forces from the arm to the axial skeleton. Stability is primarily provided by:

• The AC joint capsule: Surrounds the joint.
• Acromioclavicular ligaments: Reinforce the capsule, providing horizontal stability.
• Coracoclavicular ligaments: Comprising the conoid and trapezoid ligaments, these are the primary stabilizers, preventing superior displacement of the clavicle relative to the acromion and providing vertical stability.

The "distance" in question refers to the space between the articulating surfaces of the clavicle and the acromion, which is occupied by articular cartilage and a small amount of synovial fluid.

Defining "Normal" Distance

The "normal" distance is generally assessed via medical imaging, most commonly plain film radiographs (X-rays). It's important to recognize that this is not a fixed number but a range, influenced by several factors.

On a standard anteroposterior (AP) shoulder X-ray, the AC joint space is typically measured as the clear distance between the inferior cortex of the distal clavicle and the superior cortex of the acromion. As noted in the summary, studies indicate a normal range of 1 to 6 mm, with many individuals falling around 3 mm.

It's crucial to distinguish this measurement from the coracoclavicular distance, which is the vertical distance between the superior aspect of the coracoid process and the inferior aspect of the clavicle. While related to AC joint stability, it's a different measurement, typically ranging from 11-13 mm, and often increases significantly in more severe AC joint separations.

Measurement Techniques

Accurate assessment of the AC joint space relies on precise imaging and measurement techniques:

• Plain Radiography (X-rays):
  • Anteroposterior (AP) View: The most common view, though often limited by superimposition.
  • Zanca View: A specialized AP view with a 10-15 degree cephalic tilt, designed to project the AC joint free of overlying structures like the acromial base, offering a clearer view.
  • Stress Views: Involve the patient holding weights in each hand (typically 5-10 lbs) to apply inferior traction, which can reveal subtle AC joint instability or separation that might not be apparent on routine views. Measurements are then compared to the contralateral (opposite) side.
• Magnetic Resonance Imaging (MRI): While not typically used for initial AC joint space measurement, MRI provides excellent soft tissue contrast, allowing for detailed visualization of the articular cartilage, joint capsule, and surrounding ligaments, which is crucial for assessing internal derangement or subtle injuries not visible on X-ray.
• Computed Tomography (CT) Scan: Offers superior bony detail and can be useful for evaluating complex fractures involving the distal clavicle or acromion, or for surgical planning.

Standardization of patient positioning and radiographic technique is paramount to ensure consistent and comparable measurements.

Factors Influencing AC Joint Space

Several factors can influence the measured AC joint space:

• Age: The AC joint space may slightly decrease with age due to degenerative changes, cartilage thinning, and the formation of osteophytes (bone spurs).
• Sex: Minor differences may exist between sexes, though often not clinically significant in the normal range.
• Dominant vs. Non-dominant Arm: Some individuals may exhibit slight asymmetry between their dominant and non-dominant shoulders, though significant differences warrant further investigation.
• Pathological Conditions:
  • AC Joint Separation/Sprain: Trauma to the shoulder can disrupt the AC and/or coracoclavicular ligaments, leading to widening of the AC joint space and superior displacement of the clavicle. This is graded based on the severity of ligamentous injury and displacement.
  • Osteoarthritis (Degenerative Joint Disease): Chronic wear and tear can lead to narrowing of the joint space due to cartilage loss, along with subchondral sclerosis and osteophyte formation.
  • Distal Clavicle Osteolysis: A condition, often seen in weightlifters or those performing repetitive overhead activities, where the end of the clavicle erodes, leading to a widened joint space.

Clinical Significance of AC Joint Space

Understanding the normal AC joint space is vital for clinicians in diagnosing and managing various shoulder conditions:

• Diagnosis of AC Joint Injuries: A widened AC joint space, especially when compared to the contralateral side or on stress views, is a hallmark sign of an AC joint separation. The degree of widening helps classify the injury severity (e.g., Rockwood classification grades).
• Assessment of Degenerative Changes: Narrowing of the joint space, combined with other radiographic findings, aids in the diagnosis of AC joint osteoarthritis.
• Guiding Treatment Decisions: The extent of AC joint widening and instability plays a significant role in determining whether conservative management (e.g., rest, ice, physical therapy) or surgical intervention is warranted.
• Correlation with Pain and Dysfunction: Abnormal AC joint spacing, whether too wide or too narrow, can correlate with pain, clicking, grinding sensations, and limitations in shoulder range of motion and strength.

When to Seek Professional Advice

While normal variations exist, any persistent shoulder pain, swelling, deformity, or loss of function following an injury or developing over time warrants professional medical evaluation. A healthcare provider, such as a physician, physical therapist, or sports medicine specialist, can accurately assess your condition, order appropriate imaging, and interpret the findings in the context of your symptoms and medical history.

Conclusion

The normal distance between the clavicle and the acromion, specifically the AC joint space, is a critical anatomical measurement, typically ranging from 1 to 6 mm on radiographs. This seemingly small space plays a crucial role in shoulder biomechanics. While variations are normal, significant deviations can indicate injury or degenerative disease. For accurate diagnosis and appropriate management of any shoulder concerns, consulting with a qualified healthcare professional is always recommended.