Shoulder joint structure and function


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.

Shoulder joint structure and function



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.


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.


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.


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.


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.


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




 Scapulohumeral Rhythm

Motion of the scapula, synchronous with motions of the humerus, allows for 150_ to 180_ of shoulder ROM into flexion or abduction with elevation. The ratio has considerable variation among individuals but is commonly accepted to be 2:1 (2_ of glenohumeral motion to 1_ of scapular rotation) overall motion. During the setting phase (0_ to 30_ abduction, 0_ to 60_ flexion), motion is primarily at the glenohumeral joint, whereas the scapula seeks a stable position. During the mid-range of humeral motion, the scapula has greater motion, approaching a 1:1 ratio with the humerus; later in the range, the glenohumeral joint again dominates the motion. During humeral elevation, the synchronous motion of the scapula allows the muscles moving the humerus to maintain an effective length–tension relationship throughout the activity and helps maintain congruency between the humeral head and fossa while decreasing shear forces.

Muscles causing the upward rotation of the scapula are the upper and lower trapezius and the serratus anterior. Weakness or complete paralysis of these muscles results in the scapula being rotated downward by the contracting deltoid and supraspinatus as abduction or flexion is attempted. These two muscles then reach active insufficiency, and functional elevation of the arm cannot be reached, even though there may be normal passive ROM and normal strength in the shoulder abductor and flexor muscles. During elevation of the humerus, the pectoralis minor is lengthened as the scapula upwardly rotates, retracts, and tips posteriorly. Restricted scapular movement during humeral elevation from a shortened pectoralis minor results in patterns similar to those seen in patients with impingement symptoms and could be a risk factor for development of the syndrome.


Clavicular Elevation and Rotation With Humeral Motion

It is commonly accepted that initially the first 30_ of upward rotation of the scapula occurs with elevation of the clavicle at the SC joint. Then, as the coracoclavicular ligament becomes taut, the clavicle rotates 38_ to 55_ about its longitudinal axis, which elevates its acromial end (because it is crank-shaped). This motion allows the scapula to rotate an additional 30_ at the AC joint.98 Loss of any of these functional components decreases the amount of scapular rotation and thus the ROM of the upper extremity

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