The shoulder girdle has only one bony attachment to the axial skeleton. The clavicle articulates with the sternum via the small sternoclavicular joint. As a result, considerable mobility is allowed in the upper extremity. Stability is provided by an intricate balance between the scapular and glenohumeral muscles and the structures of the joints in the shoulder girdle.

There are three synovial joints (glenohumeral, acromioclavicular, sternoclavicular) and two functional articulations (scapulothoracic, suprahumeral) that make up the shoulder girdle complex.

Synovial Joints

Glenohumeral Joint
The glenohumeral (GH) joint is an incongruous, ball-and-socket (spheroidal) triaxial joint with a lax joint capsule. It is supported by the tendons of the rotator cuff and the glenohumeral (superior, middle, inferior) and coracohumeral ligaments. The concave bony partner, the glenoid fossa, is located on the superior-lateral margin of the scapula. It faces anteriorly, laterally, and upward, which provides some stability to the joint. A fibrocartilagenous lip, the glenoid labrum, deepens the fossa for greater congruity and serves as the attachment site for the capsule. The convex bony partner is the head of the humerus. Only a small portion of the head comes in contact with the fossa at any one time, allowing for considerable humeral movement and potential instability.

Arthrokinematics
According to the convex-concave theory of joint motion, with motions of the humerus (physiological motions) the convex head rolls in the same direction and slides in the opposite direction in the glenoid fossa.

Focus on Evidence
Of interest and apparent contradiction of this theory, one study reported that through the mid-range of the arc of passive motion there is minimal displacement of the humeral head. However, beyond mid-range the overall displacement of the head in normal joints is anterior with shoulder flexion and posterior with shoulder extension. This cadaveric study demonstrated that the integrity of the capsular ligamentous system influenced the displacement and that both hyper- and hypomobility of the capsule changed the overall displacement of the humeral head with passive range of motion (ROM).

In another study, Howel et al., using radiographs, measured humeral head displacement in normal and unstable shoulders. These investigators reported posterior displacement of the humeral head during end-range horizontal abduction with the humerus at 90° and in full external rotation in normal subjects, yet anterior displacement in subjects with anterior instability. These studies support the importance of joint mobility testing to examine restricted accessory motions to determine if interventions with joint mobilization techniques should be used and the direction of the mobilization force, rather than just using the convex-concave rule, to determine the direction of mobilizations.

Stability
Static and dynamic restraints provide joint stability. The structural relationship of the bony anatomy, ligaments, and glenoid labrum and the adhesive and cohesive forces in the joint provide static stability. The tendons of the rotator cuff blend with the ligaments and glenoid labrum at their sites of attachment so when the muscles contract they provide dynamic stability by tightening the static restraints. The coordinated response of the muscles of the cuff and tension in the ligaments provide varying degrees of support depending on the position and motion of the humerus. In addition, the long head of the biceps and the long head of the triceps brachii reinforce the capsule with their attachments and provide superior and inferior shoulder joint support, respectively, when functioning with elbow motions. The long head of the biceps in particular stabilizes against humeral elevation and contributes to anterior stability of the glenohumeral joint by resisting torsional forces when the shoulder is abducted and externally rotated. Neuromuscular control, including movement awareness and motor response, underlies coordination of the dynamic restraints.

Acromioclavicular Joint
The acromioclavicular (AC) joint is a plane, triaxial joint that may or may not have a disk. The weak capsule is reinforced by the superior and inferior AC ligaments. The convex bony partner is a facet on the lateral end of the clavicle. The concave bony partner is a facet on the acromion of the scapula.

Arthrokinematics
With motions of the scapula, the acromial surface slides in the same direction in which the scapula moves because the surface is concave. Motions affecting this joint include upward rotation (the scapula turns so the glenoid fossa rotates upward), downward rotation, winging of the vertebral border, and tipping of the inferior angle.

Stability
The AC ligaments are supported by the strong coracoclavicular ligament. No muscles directly cross this joint for dynamic support.

Sternoclavicular Joint
The sternoclavicular (SC) joint is an incongruent, triaxial, saddle-shaped joint with a disk. The joint is supported by the anterior and posterior SC ligaments and the interclavicular and costoclavicular ligaments. The medial end of the clavicle is convex superior to inferior and concave anterior to posterior. The joint disk attaches to the upper end. The superior-lateral portion of the manubrium and first costal cartilage is concave superior to inferior and convex anterior to posterior.

Arthrokinematics
The motions of the clavicle occur as a result of the scapular motions of elevation, depression, protraction (abduction), and retraction (adduction). Rotation of the clavicle occurs as an accessory motion when the humerus is elevated above the horizontal position and the scapula upwardly rotates; it cannot occur as an isolated voluntary motion.

Stability
The ligaments crossing the joint provide static stability. There are no muscles crossing the joint for dynamic stability.

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