Shoulder Injury Guide
AC Joint Separation — Acromioclavicular Joint Injury
An AC joint separation occurs when the ligaments connecting the clavicle (collarbone) to the acromion (the bony tip of the shoulder blade) are partially or completely torn. It is one of the most common shoulder injuries in contact sports, typically caused by a direct fall onto the point of the shoulder or an outstretched hand. Severity ranges from a minor ligament sprain to a complete joint dislocation requiring surgery.
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How AC joint separations happen
The acromioclavicular joint is held together by two ligament complexes: the AC ligaments (which directly strap the clavicle to the acromion) and the coracoclavicular ligaments (which suspend the clavicle from the coracoid process of the scapula below). When a direct force drives the acromion downward relative to the clavicle — most commonly a fall onto the point of the shoulder — these ligaments are placed under sudden tensile load. Injury grade is determined by how much of this ligament complex tears.
In combat sports and grappling, the mechanism is usually a takedown landing, a throw where the top shoulder absorbs the impact, or an opponent's body weight compressing the shoulder into the mat. Defensive instinct — reaching out with an outstretched hand — transmits the force up through the arm to the AC joint and can produce the same injury pattern. Repeated lower-grade impacts without adequate recovery can also create chronic AC joint pain and early arthritic change without a single acute tear.
- Grade I: AC ligaments sprained, coracoclavicular ligaments intact — mild tenderness, no step deformity
- Grade II: AC ligaments torn, coracoclavicular ligaments sprained — mild step deformity, joint unstable under stress
- Grade III: Both ligament complexes torn — prominent step deformity, clavicle elevated, joint fully unstable
- Grades IV–VI: Severe displacement in multiple planes — rare, require surgical repair
Recognising an AC joint separation
The most reliable clinical sign is the "step deformity" — a visible and palpable bump at the top of the shoulder where the distal clavicle has risen above the acromion. This deformity is absent in grade I, subtle in grade II, and clearly visible in grade III and above. Pain is localised to the AC joint and typically worsened by cross-body movements (reaching the affected arm across the chest toward the opposite shoulder), overhead elevation past 90 degrees, and direct palpation at the joint line.
Plain X-ray confirms the diagnosis and grades severity by measuring the coracoclavicular distance. Stress views — X-rays taken with the arm hanging under a weight — can unmask grade III instability that is muscle-guarded at rest. MRI is not routinely required for diagnosis but can characterise associated labral, rotator cuff, or biceps tendon pathology in complex presentations.
Recovery and return to sport
Initial management for grades I–III follows a sling-for-comfort approach: immobilisation is not required for joint healing and prolonged sling use risks shoulder stiffness. Early range-of-motion exercises begin within days. As pain subsides, rehabilitation targets restoration of rotator cuff strength, scapular control, and closed-chain shoulder stability. The visible step deformity in grade III injuries typically persists even after full functional recovery — this cosmetic asymmetry does not indicate ongoing instability in most cases.
Return to contact sport requires not just pain-free range of motion but objective strength symmetry (>90% of the uninjured side on isokinetic testing) and demonstrated stability under sport-specific loading. For athletes in overhead or contact sports, this threshold should be met before returning to full training. Protective padding over the AC joint can reduce discomfort on return and lower re-injury risk from direct impact.
Notable cases
- Sean Strickland — UFC 328 — Shoulder separation sustained during the Strickland vs. Chimaev contest, highlighting the mechanism of AC joint loading in elite MMA exchanges.
Related guides
Frequently asked questions
Do I need surgery for an AC joint separation?
Most AC joint separations — grades I through III — are treated conservatively without surgery. Grade I and II injuries involve partial ligament damage and typically resolve with rest, physiotherapy, and a graduated return to activity over 2–8 weeks. Grade III (complete ligament disruption) is controversial: many high-level athletes return to sport without surgery, though some require surgical stabilisation if pain and instability persist beyond 3 months. Grades IV–VI involve significant displacement and almost always warrant surgical repair.
How long does an AC joint separation take to heal?
Grade I: 1–2 weeks to pain-free activity. Grade II: 4–8 weeks before return to contact sport. Grade III: 8–16 weeks depending on whether conservative or surgical management is chosen. Surgical cases require an additional 3–4 months of post-operative rehabilitation before return to full activity. Overhead strength and end-range elevation are typically the last capacities to fully return.
What does a separated AC joint feel like?
The hallmark is a prominent bump at the top of the shoulder at the point where the clavicle meets the acromion. This 'step deformity' is most visible from the front. Pain is sharp with overhead movements, cross-body actions (reaching across the chest), and direct pressure on the joint. Sleeping on the affected side is typically uncomfortable. In higher-grade separations, the entire clavicle may appear elevated relative to the acromion.
Can I still train with an AC joint separation?
Lower-body and non-contact training can typically continue within days of a grade I–II injury, provided it does not provoke shoulder pain. Overhead lifting, pressing, and contact training should be suspended until pain-free range of motion is restored and the joint is stable under load. Return to grappling or sparring depends on injury grade and stability — your physiotherapist should clear return-to-contact explicitly, as a fall or impact on an inadequately healed AC joint can worsen the separation.
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