by Erik Dalton Ph.D.
Acknowledging the A/C
The Acromioclavicular or A/C joint sits on the point of the shoulder lateral to the sternoclavicular and proximal to the glenohumeral. This oft-overlooked bony articulation receives little respect from most manual therapists…a regrettable omission, indeed. Both the A/C and S/C joints play vital roles in the biomechanics of throwing and other upper-limb activities. A/C joint injuries typically occur from falling directly on the point of the shoulder. In competitive sporting events a direct clash of shoulders between players often displaces or completely subluxes this joint. Some authors believe that since the A/C is such a small joint, the human shoulder (under normal circumstances) could actually function adequately without it. However, long-term A/C restrictions can have devastating affects on all upper limb functioning.
Since the acromioclavicular is a planar joint, small movements occur in all three planes. Arm elevation causes the acromion to glide superiorly on the distal clavicle. To test movement of this joint on yourself, simply glide your right index finger laterally along the superior surface of your left clavicle beginning at the sternoclavicular joint. Just before you contact the acromion you should feel a bump or a notch. Since arm elevation raises the acromion in relation to the clavicle, left arm abduction should allow your finger to palpate a “dip” as the acromion rises on the clavicle.
Approximately 20° of motion occurs at the AC joint during arm abduction with 10° occurring between 0° and 30° of arm elevation and the last 10° occurring during the last 45° of the normal range (180°) of arm elevation. At both the AC and SC joints, the clavicle posteriorly rotates approximately 45° during arm elevation. This motion is allowed by the planar configuration at the A/C joint and the fibrocartilaginous articulating disc between the sternum and proximal clavicle as discussed in the last “Sternoclavicular” newsletter.