Loose Shoulder? Understanding Acromioclavicular Joint Separation — A Guide for High-Impact Athletes and Manual Workers

Acromioclavicular (AC) joint separation (dislocation) accounts for about 10% of all other shoulder girdle injuries, and those affected are typically young athletic men aged 20 to 30. Nearly half of these shoulder injuries (40-50%) occur in contact sports, and associated shoulder injuries are found in 18% of patients.

AC joint separation refers to the separation between the clavicle and the acromion caused by injury to the acromioclavicular (AC) and coracoclavicular (CC) ligaments. Grading is based on X-ray findings and is used to assess the extent of damage to the AC joint capsule, the extrinsic AC ligaments, and the supporting structures (Type I to VI).

• Acromioclavicular ligaments: mainly provide anterior-posterior stability and fuse with the trapezius and deltoid fascia to form the AC joint capsule.
• Coracoclavicular ligaments: indirectly provide inferior stability and consist of the trapezoid and conoid ligaments.

The AC joint is a gliding joint located between the medial aspect of the acromion and the articular surface of the distal clavicle, connecting the shoulder to the axial skeleton. The AC joint coordinates three joint motions at once, including the glenohumeral joint, the scapulothoracic joint, and the sternoclavicular joint. Each joint has static and dynamic stabilizing structures.

• Static stabilizers play the most important role in AC joint stability, including the acromioclavicular ligaments and the joint capsule. The main horizontal-plane stabilizers are the superior and posterior AC ligaments, while the main vertical stabilizers of the AC joint are the conoid and trapezoid ligaments (the CC ligaments).
• Dynamic stabilizers play a secondary role in AC separation, including the anterior deltoid fibers attached to the clavicle and the trapezius attached to the acromion through fascia.

Features of Each Type

• Type I: acromioclavicular ligament sprain with partial fiber injury, but the coracoclavicular ligaments and AC capsule are intact.
• Type II: complete tearing of the acromioclavicular ligament with the coracoclavicular ligaments sprained but intact; the AC joint is widened, characterized by horizontal-axis instability, with the CC distance increased by under 25% compared with the opposite side.
• Type III: complete tearing of both the acromioclavicular and coracoclavicular ligaments with AC joint dislocation; complete disruption of the AC and CC ligaments (instability in both the horizontal and vertical axes, with the CC distance increased by 25-100% compared with the normal side), and possible injury to the trapezius and deltoid attachments.
• Type IV: complete tearing of the acromioclavicular ligament with partial or complete injury to the coracoclavicular ligaments and AC joint dislocation; the distal clavicle displaces posteriorly into the trapezius, and muscle attachments may be injured.
• Type V: complete tearing of both the acromioclavicular and coracoclavicular ligaments with AC joint dislocation and superior migration of the clavicle greater than 100%, considered a severe form of Type III.
• Type VI: rare; complete tearing of the acromioclavicular ligament with the coracoclavicular ligaments intact or injured, and inferior migration of the distal clavicle relative to the acromion or coracoid, possibly involving injury to the brachial plexus or the subclavian vessels.

Epidemiology

1. Incidence
   • AC joint separation accounts for about 12% to 16% of shoulder injuries.
   • The typical injured population is men aged 20 to 30, with men affected five times more often than women.
   • It is common in high-energy sports such as soccer, rugby, cycling crashes, wrestling, and judo.
2. Related background

• Professional or amateur athletes and manual workers are more prone to injury.
• Most cases occur with a direct impact or a fall onto the shoulder.

Pathophysiology

1. Etiology
   • Direct trauma: a direct downward impact to the acromion, causing injury to the acromioclavicular and coracoclavicular ligaments.
   • Indirect trauma: force through the hand or elbow during a fall, transmitted indirectly to the AC joint.
2. Pathological mechanism

• The severity depends on the magnitude and direction of the force and on whether muscles or ligaments provide protection.
• Early on it may involve only the acromioclavicular ligament (Type I); with greater force it also affects the coracoclavicular ligaments (Types II to VI).
• When the clavicle displaces superiorly or posteriorly, the stability between the AC joint and the coracoid is disrupted, with accompanying joint displacement or ligament tearing.

Risk Factors

1. Sport-related factors
   • Participation in high-risk sports such as soccer, rugby, cycling, skiing, or wrestling.
   • Athletes without adequate protective measures or with poor technique are more prone to injury.
2. Environmental factors
   • High-energy collisions during competition or training.
   • Wet or slippery surfaces that increase the risk of falls.
3. Individual factors
   • Young men and those with a prior history of shoulder injury are at higher risk.
   • Manual workers or those in physically demanding occupations.

History

1. Injury mechanism
   • Direct shoulder trauma, such as the shoulder striking the ground during a fall.
   • Indirect impact to the hand or elbow, with force transmitted to the shoulder.
   • Shoulder pain, swelling, and functional limitation when raising the arm or lifting the shoulder.
2. Symptom features
   • Pain located over the AC joint.
   • May be accompanied by immediate swelling, bruising, and shoulder deformity (especially in Type III and above).
3. Response after injury

• Most patients stop activity immediately and support the injured side with the arm.
• Pain intensifies in the acute phase and may be accompanied by difficulty sleeping.

Physical Examination

1. Inspection
   • Check the AC joint for swelling, bruising, or deformity.
   • A change in shoulder contour (such as superior migration of the clavicle) is especially evident in Type III and above.
2. Palpation
   • Local tenderness concentrated over the AC joint, possibly extending to the coracoid.
   • Pain is usually confirmed by a compression test that reproduces the pain to localize it.
3. Functional testing

• Cross-arm test: the patient brings the arm across to the opposite shoulder, increasing stress on the AC joint; increased pain suggests injury.
• Shoulder range of motion may be limited, but this is usually a result of pain rather than a structural restriction.

Differential Diagnosis

1. Other shoulder lesions
   • Greater tuberosity fracture of the humerus
   • Rotator cuff tear
   • Clavicle fracture
2. Other causes of shoulder pain

• Coracoid impingement syndrome
• Shoulder instability

Diagnostic Tools

1. X-ray
   • Plain shoulder anteroposterior view: assesses the AC joint space and clavicle position.
   • Stress X-ray: taken with both arms weighted, helpful for grading (Type I to VI).
2. Advanced imaging

• Ultrasound: can show the extent of ligament injury.
• CT: appropriate when a complex joint injury or fracture is suspected.
• MRI: appropriate when detailed assessment of ligament injury is needed or when an associated soft-tissue lesion is suspected.

Initial Treatment

1. Non-surgical treatment (for Type I and most Type II)
   • Ice: reduces pain and swelling.
   • Analgesics: use anti-inflammatory drugs (NSAIDs).
   • Rest and shoulder support: use a sling or shoulder brace for 2 to 3 weeks and avoid excessive shoulder activity.
   • Type I immobilization lasts 7 to 10 days; Type II immobilization may last 4 to 6 weeks.
   • Sports involving body contact and lifting may need 8 to 12 weeks after injury before returning.
2. Surgical considerations (mostly for Type IV and above)
   • Surgical repair is needed with marked clavicle displacement or severe joint dysfunction.
   • This includes ligament reconstruction and joint stabilization surgery.
3. Local injection

• In cases of chronic pain or persistent inflammatory symptoms, a local corticosteroid or proliferative injection may be considered; corticosteroids should be used with caution to avoid impairing ligament healing.

Additional Treatment

1. Physical therapy
   • Early phase: focus on controlling pain and inflammation, such as alternating cold and heat and passive motion.
   • Later phase: strengthen the shoulder muscles, especially the scapular stabilizers, to restore normal function and range of motion.
2. Assistive devices
   • Use a sling or shoulder support brace to provide joint stability and reduce range of motion to promote healing.
3. Functional training

• Gradually increase load and shoulder activity, avoiding early high-intensity exercise to prevent recurrence.

Surgery

1. Surgical indications
   • Type IV and above: marked clavicle displacement with severe impairment of shoulder function.
   • Chronic Type III: if conservative treatment for 6 to 12 weeks is ineffective, with functional limitation or significant cosmetic effect.
   • Some studies support surgery for acute Type III injuries in young patients, those with high occupational shoulder demands, or high-performance throwing athletes.
   • Type VI: emergency surgery is needed to address potential neurovascular compression.
2. Common surgical methods
   • Ligament reconstruction: uses an autograft or allograft to reconstruct the coracoclavicular ligaments and restore joint stability.
   • Fixation: uses pins, screws, or a hook plate to temporarily fix the AC joint and promote healing.
3. Postoperative rehabilitation

• Restrict shoulder activity early, then progressively restore strength and function; a full recovery takes 3 to 6 months.

Follow-up

1. Follow-up frequency
   • Type I to II: follow up every 2 to 4 weeks to assess improvement in pain and function.
   • Type III and above: close follow-up after surgery to monitor joint stability and healing progress.
2. Rehabilitation focus

• Ensure shoulder range of motion and strength recover progressively.
• Avoid high-risk sports until the shoulder is fully stable.

Prognosis

1. Type I to II
   • Conservative treatment usually works well, and most patients return to normal activity within 6 to 8 weeks.
   • Persistent pain or dysfunction is rare.
2. Type III and above
   • Non-surgical treatment: some patients may have mild dysfunction or a cosmetic concern.
   • Surgical treatment: most patients can recover near-normal function, but may need a longer rehabilitation period.
3. Long-term outcomes

• Without appropriate treatment, it may lead to arthritis, chronic pain, or dysfunction.
• Early, appropriate treatment and rehabilitation can markedly improve the prognosis.

References

• 1.4Q3 Orthop Rev (Pavia). 2024 Feb 22:16:94037.
• https://www.orthobullets.com/shoulder-and-elbow/3047/acromioclavicular-joint-injury

Further Reading

• Shoulder topic
• Sports injury overview
• PRP prolotherapy
• Rehabilitation exercise prescription
• Pain management