ACJ Joint

AC Joint
Anatomy

variability in joint inclination, from 0 to 50o when visualized from the front
incongruency may exist in a significant number
clavicle may lie superior to the acromion
intra-articular fibrocartilaginous disk projects into the joint from the superior part of the articular capsule, and may be complete or meniscoid
joint surface covered by fibrous articular cartilage
joint is surrounded by a thin capsule
stabilized by capsular (acromioclavicular) and extracapsular (coracoclavicular) ligaments; also supported by the deltoid and trapezius muscles
Capsular ligaments – anteriorly, posterior, inferior, superior acromioclavicular ligaments, the strongest of which is the superior

Coracoclavicular ligament – runs from the outer, inferior surface of the clavicle to the base of the coracoid process of the scapula. Has two components – the conoid and trapezoid ligaments (conoid is more medial). They help to prevent the acromion from being driven downward and medially.

The clavicle rotates 40-50o around its longitudinal axis during full abduction – the connection to the scapula via the coracoclavicular ligament mediates scapulothoracic rotation

Stability

Fukuda et al. – load displacement tests with sequential ligament sectioning
acromioclavicular ligaments are most important for horizontal stability
coracoclavicular ligaments are most important for vertical stability

Motion

Although the clavicle rotates 40-50o during full elevation, motion through the AC joint is probably only 5-10o. Most of clavicular rotation is coupled to scapular rotation, rather than being pure AC joint motion.

Mechanism of injury

direct force on the acromion, forcing it down and medial
the shoulder is down

Classification
AC Dislocations

(originally Tossy and Allman-3 types)
Type I-AC ligament sprain
Type II-AC ligs torn,C/T intact
Type III-AC,C/T all torn
Type IV-AC,C/T all torn and distal clavicle buttonholes postero-superior into trapezius-marked/fixed dislocation
Type V-IV + deltoid and trapezius tear thus v.v.displaced
Type VI-AC,C/T all torn and distal clavicle goes inferior and trapped under coracoid process
Type VII-Total clavicular dislocation (AC and SC dislocation)