The shoulder complex is composed of articulations between the humerus, glenoid, scapula, acromion, clavicle, and the surrounding soft tissue structures that connect them. The shoulder complex allows for a large degree of motion in multiple planes at the expense of stability. As a result, this compound set of articulations can present a diagnostic and treatment challenge.
The rotator interval, according to Neer,1 is the capsular tissue in the interval between the subscapularis and the supraspinatus tendons. The structures within the rotator interval contribute to the stability of the shoulder by limiting inferior translation and external rotation with the arm adducted, as well as posterior translation when the arm is forward flexed, adducted, and internally rotated.
The specific joints of the shoulder complex include the following:
- Glenohumeral (GH) joint. The most mobile joint in the body.
- Acromioclavicular (AC) joint. The ligaments providing stability to the AC joint include the superior and inferior AC ligaments, and the coracoclavicular ligaments (conoid and trapezoid). The AC ligaments are the principal restraint to anteroposterior translation, whereas the coracoclavicular ligaments restrain vertical motions. The deltoid and trapezius muscles provide dynamic stabilization to the joint.
- Sternoclavicular (SC) joint.
- Scapulothoracic articulation. A pseudojoint critical for normal biomechanics of the shoulder.
If optimal shoulder function is to occur, motion also has to be available at the cervicothoracic junction and at the connections between the first three ribs and the sternum and spine.
The shoulder complex is endowed with a unique blend of mobility and stability:
- The degree of mobility is contingent on a healthy articular surface, intact muscle–tendon units (Table 5-1), and supple capsuloligamentous restraints (Table 5-2).
- The degree of stability is dependent upon a combination of ligamentous and capsular restraints, surrounding musculature and the glenoid labrum. Static joint stability is provided by the integrity of the osseous articular structures and the capsulolabral complex, and dynamic stability by the rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) and the scapula pivoters (trapezius, serratus anterior, levator scapulae, and rhomboid major and minor).2
Table 5-1 Muscles of the Shoulder Complex According to Their Actions on the Scapula and at the Glenohumeral Joint ||Download (.pdf)
Table 5-1 Muscles of the Shoulder Complex According to Their Actions on the Scapula and at the Glenohumeral Joint
|Serratus anterior (upper fibers)|
|Scapula adductors||Levator scapulae|
|Scapula flexors||Serratus anterior (lower fibers)|
|Scapula extensors||Pectoralis minor|
|Scapula external rotators||Trapezius|
|The shoulder flexors||Coracobrachialis|
|The shoulder extensors||The triceps|
|Shoulder internal rotators||Pectoralis major ...|