Anatomy of acromioclavicular joint

DR ZUBAIR YOUNIS
POST GRADUATE SCHOLAR

1. ANATOMY OF ACROMIOCLAVICULAR JOINT

2. Acromioclavicular joint is a plane diarthrodial joint formed by the junction of anteromedial acromion and lateral clavicle. The clavicle develops from three ossification centers , with the lateral aspect forming from a primary intramembranous ossification center beginning at 5-6 weeks of gestation although the clavicular ossification is not complete until 25 years of age. Acromion has four ossification centers, with pre-acromion, mesoacromion and meta-acromion fusing together by 18 years of age. Non fusion of these ossification centers can occur in upto 8% of indivisuals producing a condition called os acromiale , which can be clinically confused with AC joint abnormality.

3. The joint can be palpated during a shoulder examination; 2-3cm medially from the ‘tip’ of the shoulder (formed by the end of the acromion).

4. Structures of the Acromioclavicular Joint Articulating Surfaces The acromioclavicular joint consists of an articulation between the  lateral end  of the clavicle and the  acromion  of the scapula. It has two atypical features: The articular surfaces of the joint are lined with  fibrocartilage  (as opposed to hyaline cartilage). The joint cavity is partially divided by an  articular disc  – a wedge of fibrocartilage suspended from the upper part of the capsule.

5. Joint Capsule The joint capsule consists of a loose  fibrous  layer which encloses the two articular surfaces. It also gives rise to the  articular disc . The posterior aspect of the joint capsule is reinforced by fibres from the trapezius muscle. As would be expected of a synovial joint, joint capsule is lined internally by a  synovial membrane . This secretes synovial fluid into the cavity of the joint.

6. Ligaments There are three major ligaments present in the acromioclavicular joint: 1.Acromioclavicular  – runs horizontally from the acromion to the lateral clavicle. It covers the joint capsule, reinforcing its superior aspect . The AC ligaments resist 50% of anterior and 90% of posterior displacements with capsular and capsuloligamentous attachments about 2.8mm and 4.8 mm from acromial articular surface and 3.5 mm and 6.2 mm from clavicular articular surface.

7. 2. Conoid  – runs vertically from the coracoid process of the scapula to the conoid tubercle of the clavicle . It extends from posteromedial coracoid to anterior clavicle inserting about 32.1 mm medial to clavicular articular surface. 3. Trapezoid  – runs from the coracoid process of the scapula to the trapezoid line of the clavicle . It extends from anterolateral coracoid to clavicle , inserting about 14.7 mm medial to clavicular articular surface .

8. Collectively, the conoid and trapezoid ligaments are known as the  coracoclavicular ligament . It is a very strong structure, effectively suspending the weight of the upper limb from the clavicle. The CC ligaments primarily prevent superior and axial translation and secondarily resist anterior and posterior translation in the absence of acro ioclavicular ligament. The coracoacromial ligament which extends in an oblique orientation from the coracoid process to the inferior aspect of the acromion doesn’t play a significant role in AC joint stability.

9. Muscular attachments surrounding the AC joint act as dynamic stabilizers and help distribute forces imparted through the shoulder girdle, although their specific contributions remain unknown. The deltoid and trapezius muscles attach over the lateral clavicle and acromion, and their fibers over the AC joint blend with those of the superior AC ligament.

10. AP view of exposed AC joint and its supporting ligaments

11. Neurovascular Supply Vessels The arterial supply to the joint is via two vessels: Suprascapular artery  – arises from the subclavian artery at the thyrocervical trunk. Thoraco -acromial artery  – arises from the axillary artery. The veins of the joint follow the major arteries.

12. Nerves The acromioclavicular joint is innervated by articular branches of the  suprascapular  and  lateral pectoral nerves . They both arise directly from the  brachial plexus .

13. Movements The acromioclavicular joint allows a degree of  axial rotation  and  anteroposterior  movement. As no muscles act directly on the joint, all movement is passive, and is initiated by movement at other joints (such as the scapulothoracic joint ) Despite the fact that the clavicle rotates as much as 45 degrees about its axis, almost all clavicular motion takes place at the sternoclavicular articulation. Motion at the AC joint is limited to 5 to 8 degrees, predominantly because of synchronous scapuloclavicular motion, in which the clavicle and scapula move as a unit.

14. X- RAY DEMONSTRATING NORMAL ANATOMY OF AC JOINT

15. The angle of the AC joint on anteroposterior (AP) view is variable. About 49% are inclined from superolateral to inferomedial , 27% vertically oriented, 21% incongruous, and 3% laterally oriented. The joint is also inclined a few degrees from anterolateral to posterior medial on the axillary view

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