Anatomical observations of the human acromioclavicular joint.

The condition of the acromioclavicular joint (ACJ) is considered to be one factor in the etiology of shoulder impingement syndrome, but there are few supporting morphological data. Fifty-two sides of 35 cadavers were investigated macroscopically and histologically using Safranin O, Fast green, and Weigert's iron hematoxylin staining. The ACJs were classified into three major types depending on the presence or absence of the articular disk. In type 1, the articular disk divided the articular cavity completely (n = 2; 3.8%). In type 2, the articular disk was incomplete and divided the joint cavity incompletely (n = 13; 25%). Type 2 was further divided into subtypes 2a and 2b depending on the configurations of the articular facets. In type 3, no articular disk was found in the joint cavity (n = 37; 71.2%). Type 3 was further divided into subtypes 3a, 3b, and 3c depending on the configurations of the articular surfaces. Fewer than half of the ACJ specimens (22/52 or 42.3%) demonstrated an ellipsoid character in which axial rotation was limited. Histological observation revealed that the upper part of the articular disk of the ACJ comprised fibrocartilage while the lower part comprised dense connective tissue. In cases where the ACJ appears to be an ellipsoid joint, its limited axial rotation restricts posterior tilting of the scapula during arm elevation, which could contribute to shoulder impingement syndrome.