The Scapula and Acromioclavicular Joint Separation and Arthritis

Abstract

The acromioclavicular (AC) joint serves many key roles in shoulder function. The AC joint is an important component of the screw axis mechanism that imparts normal shoulder motion [1, 2]. The AC joint stabilizes the clavicular strut and permits anterior and posterior translation, superior and inferior translation, and rotation. The AC ligaments control most of the anterior and posterior translation of the AC joint with some contributions by the coracoclavicular ligaments. The AC ligaments, especially posterior and superior AC ligaments, confer horizontal stability but also play a role in vertical and rota-tional stability [3]. The superior AC ligament (56%) and posterior AC ligament (25%) contribute to resistance of posterior translation [4], and the anatomical position of the superior AC ligament also suggests a role as a tension band helping control lateral tilt of the acromion. The conoid with its more medial and posterior insertion on the clavicle contributes to 60% of vertical stability [4], while the trapezoid with its more lateral and anterior insertion on the clavicle contributes to vertical and rotational stability at the AC joint.