3. 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.
4. 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).
5.
6. 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.
7. 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.
8. 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.
9. 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
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.
10. 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.
11. 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.
12.
13. AP view of exposed AC joint and its supporting ligaments
14.
15. 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.
16. Nerves
The acromioclavicular joint is innervated by articular branches of
the suprascapular and lateral pectoral nerves. They both arise directly from
the brachial plexus.
17. 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
scapuloclavicular motion, in which the clavicle and scapula move as a unit.
20. 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
