•The average dimension of Acromioclavicular joint is 9mm superoinferiorly and 19mm anteroposteriorly. Joint is straight and vertical in 30% of population and in 70% the joint is oblique downwards and medially.
•Articular cartilage of clavicle end becomes fibrocartilaginous after 17 years of age and the acromial end becomes fibrocartilaginous at 23 years.
•A fibrocartilaginous disk of variable size and shape attached to the superior capsule is present between the articular surfaces. The fibrocartilaginous disk is partial and meniscus like in 50%, remnant like in 30%, absent in 20% and complete in less than 2%.
•Anteroposterior stability of ACJ is provided by the ACJ capsule and ligaments. The superior and anterior capsule and ligaments are the strongest.
•Superoinferior stability is provided by the coracoclavicular ligaments. Coracoclavicular ligaments have trapezoid and conoid parts. They are confluent at the coracoid and separate at the clavicle side. They form the primary suspensory complex of the upper limb.
•Conoid ligament is conical and is attached to the posterior-medial aspect of coracoid process. Trapezoid ligament is quadrangular and attaches to coracoid shaft. Trapezoid ligament is attached 2.5cm from the lateral end of clavicle to the trapezoid ridge and the conoid portion is attached 4.5 cm from the lateral end of clavicle to the conoid tubercle. This is important for anatomical reconstruction of coracoclavicular ligaments.
•Conoid footprint is posterior and measures 25-30mm. Conoid portion provides 60% of the superoinferior stability.
•Deltoid inserts to the superior capsule and anterior surface of lateral 1/3rd of clavicle. Trapezius inserts to the superior capsule and dorsal surface of lateral end of clavicle. Their attachments are also detached in higher grades of ACJ injury.
•Joint allows axial rotation of clavicle and anteroposterior and superoinferior sliding of acromion.
Mechanism of injury
•Acromioclavicular joint injuries occur due to direct trauma over the acromion with the arm in the adducted position.
•Once the ligaments are ruptured, the acromion is pulled down by the weight of the upper limb and the lateral end of clavicle is pulled up by the sternocleidomastoid and the trapezius.
•Zanca view showing both Acromioclavicular joints is the best view for diagnosis. Measure and compare the coracoclavicular distance on either side.
•Zanca view taken in the standing position. Should show both ACJ. It is a true AP view with 10-150 cephalic tilt.
•Axillary view is needed for diagnosis of posterior displacement in type IV injury. Axillary view taken in 70-900 abduction with the beam directed cranially.
•Normal distance from superior border of coracoid to inferior border of clavicle ranges 1.1cm to 1.3 cm. Side to side difference of > 25% diagnostic of injury.
•Stress x-rays taken with 10-15 pounds hanging from the forearm with the shoulder muscles relaxed. But it is impractical in acute cases due to pain.
•Associated injuries to the glenohumeral joint especially SLAP lesions are common. Hence MRI scans may be necessary in higher grades of injury.
Rockwood & Young classification
2.Acromioclavicular ligaments ruptured and coracoclavicular ligaments intact. Joint displaced to a third of the width of clavicle.
3.Acromioclavicular and coracoclavicular ligaments ruptured with 25-100% displacement.
4.Acromion displaced posteriorly into the trapezius.
6.Acromion displaced below the coracoid.
To continue click the following link