The acromioclavicular joint (ACJ) is a small but important joint that joins the clavicle (collar bone) to the scapula (shoulder blade). It is visible at the lateral end of the clavicle as a small depression in slender people. It contains a fibrocartilaginous disc that slowly degenerates over time. Symptoms from the ACJ may appear early on relative to other arthritis, with patients in their 30’s and 40’s presenting due to inflammation, swelling and pain.

They usually have a typical history with pain with overhead activities that is very well localized to the joint. Palpating over the joint is often very tender. Clinicians describe the ‘drop to the floor test’, where palpating on the joint is followed by the patient dropping the shoulder down towards the floor in response.