The acromioclavicular ligament is part of the acromioclavicular joint. It is divided into two parts: superior and inferior.
Superior Acromioclavicular Ligament
The superior acromioclavicular ligament is a quadrilateral band, covering the superior part of the articulation, and extending between the upper part of the lateral end of the clavicle and the adjoining part of the upper surface of the acromion. It is composed of parallel fibers, which interlace with the aponeuroses of the trapezius and deltoideus; below, it is in contact with the articular disk when this is present.
This ligament provides horizontal stability to the acromioclavicular joint
The left shoulder and acromioclavicular joints, showing the superior acromioclavicular ligament at the top, between the clavicle and the acromion.
Inferior Acromioclavicular Ligament
The inferior acromioclavicular ligament is somewhat thinner than the superior acromioclavicular ligament. It covers the under part of the articulation, and is attached to the adjoining surfaces of the two bones.
It is in relation, above, in rare cases with the articular disk; below, with the tendon of the supraspinatus.