Acromioclavicular Joint
Clinical Overview
The AC joint is very prone to injury. Subluxation is more common than fracture. The degree of subluxation depends on the ligaments involved. Low grades of subluxation are common and do not necessarily require treatment.
Ligaments around the joint are the superior and inferior acromioclavicular ligaments. The lateral clavicle is also constrained by the conoid and trapezoid ligaments.
- Degeneration is common especially in athletic individuals
- Erosions may occur due to chronic trauma, especially in the rugby scrum
- The joint is frequently involved in RA and crystal arthropathy
X-Ray
Most cases can be diagnosed with radiographs. More than 2cm coracoclavicular distance indicates ligament tears.
Classification of ACJ Injury
| Grade | Description |
|---|---|
| I | No separation |
| II | Separation less than 100% of the opposing surfaces |
| III | Separation more than 100% of the opposing surfaces |
| IV | Posterior displacement of the clavicle |
| V | Clavicle dislocated under coracoid |
MRI
Best for detecting tears of the coracoclavicular ligaments. Use sagittal images and look for the conoid and trapezoid ligaments. STIR shows oedema. T1 T2 show ligament disruption a little better.
Ultrasound - Dynamic Assessment
Minimal subluxation can be detected by dynamic ultrasound as the patient moves their hand to and fro from the ipsilateral knee to the contralateral shoulder. Normally some approximation of the acromion to the lateral end of the clavicle is present.
- Higher grades indicate ligament laxity
- Significant inferior–superior movement is not detected unless there are ligament tears
- As the joint moves, synovial tissue and fluid may be extruded
- Fluid in the subacromial subdeltoid bursa may communicate and result in a large synovial cyst
- This is referred to as the "geyser phenomenon"