Acromioclavicular Instability
What Is the Anatomy of the AC Joint?
The acromioclavicular (AC) joint is located at the top of the shoulder, where the acromion (part of the shoulder blade) meets the clavicle (collarbone). It plays an important role in allowing coordinated movement between the arm and shoulder blade during overhead and cross-body motion. The joint is stabilised by strong ligaments — the acromioclavicular ligaments (which provide horizontal stability) and the coracoclavicular ligaments (conoid and trapezoid), which provide vertical stability. The surrounding capsule and muscles, such as the deltoid and trapezius, also contribute to overall stability.
How Does AC Joint Instability Occur?
AC joint instability most commonly occurs following trauma, such as a fall directly onto the shoulder or a collision during contact sports. The force causes separation of the clavicle from the acromion by tearing the supporting ligaments. The severity of injury is classified from a mild sprain to a complete ligament rupture with clavicle displacement.
What Is the Long-Term Consequence of Having AC Joint Instability?
If left untreated or if the injury heals poorly, AC joint instability can lead to:
Persistent pain and tenderness over the top of the shoulder.
A visible bump or deformity from clavicle displacement.
Weakness or fatigue during lifting or pushing activities.
Restricted overhead motion or difficulty with cross-body movements.
Early onset arthritis of the AC joint.
How Does AC Joint Instability Get Diagnosed?
Diagnosis is made through clinical history, examination and imaging studies:
History of shoulder trauma, pain, or deformity at the top of the shoulder.
Physical examination assessing tenderness, deformity, and stability during movement.
Comparison with the opposite shoulder for prominence or step-off at the joint.
X-rays, including special views to assess the degree of displacement.
MRI may be used to evaluate ligament integrity or associated soft tissue injury.
What Are the Indications for Treating AC Joint Instability Non-Operatively?
Non-operative management is the preferred treatment for most low-grade injuries and involves:
Minor or moderate displacement without significant functional limitation.
Low-demand or recreational athletes.
Patients preferring to avoid surgery.
Chronic instability with minimal pain or symptoms.
Treatment includes sling immobilisation for comfort (1–3 weeks), early physiotherapy to restore motion, and progressive strengthening of the deltoid and scapular stabilisers. Ice, anti-inflammatory medication, and activity modification help relieve pain during recovery.
What Are the Indications for Treating AC Joint Instability Operatively?
Surgery may be recommended in the following cases:
High-grade injuries with complete ligament rupture and significant clavicle displacement.
Persistent pain, weakness, or instability after failed non-operative treatment.
High-demand athletes or manual labourers who require shoulder strength and stability.
Cosmetic deformity causing discomfort.
Surgical techniques aim to restore normal alignment and stability of the joint by reconstructing or reinforcing the coracoclavicular ligaments, sometimes using grafts, synthetic tapes, or fixation devices. Postoperative rehabilitation focuses on restoring range of motion, strength, and function while protecting the repair during healing.
