English

Pronunciation

Noun

shoulders

1. Plural of shoulder

Translations

Verb

shoulders

1. third-person singular of shoulder

In human anatomy, the shoulder joint comprises the part of the body where the humerus attaches to the scapula. The shoulder refers to the group of structures in the region of the joint.

It is made up of three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone) as well as associated muscles, ligaments and tendons. The articulations between the bones of the shoulder make up the shoulder joints. The shoulder must be flexible for the wide range of motion required in the arms and hands and also strong enough to allow for actions such as lifting, pushing and pulling. The compromise between these two functions results in a large number of shoulder problems not faced by other joints such as the hip.

Joints of the shoulder

There are three joints of the shoulder: The glenohumeral, acromioclavicular, and the sternoclavicular joints.

Glenohumeral joint

The glenohumeral joint is the main joint of the shoulder and the generic term "shoulder joint" usually refers to it. It is a ball and socket joint that allows the arm to rotate in a circular fashion or to hinge out and up away from the body. It is formed by the articulation between the head of the humerus and the lateral scapula. The "ball" of the joint is the rounded, medial anterior surface of the humerus and the "socket" is formed by the glenoid fossa, the dish-shaped portion of the lateral scapula. The shallowness of the fossa and relatively loose connections between the shoulder and the rest of the body allows the arm to have tremendous mobility, at the expense of being much easier to dislocate than most other joints in the body.

The capsule is a soft tissue envelope that encircles the glenohumeral joint and attaches to the scapula, humerus, and head of the biceps. It is lined by a thin, smooth synovial membrane. This capsule is strengthened by the coracohumeral ligament which attaches the coracoid process of the scapula to the greater tubercle of the humerus. There are also three other ligaments attaching the lesser tubercle of the humerus to lateral scapula and are collectively called the glenohumeral ligaments.

There is also a ligament called semicirculare humeri which is a transversal band between the posterior sides of the tuberculum minus and majus of the humerus. This band is one of the most important strengthening ligaments of the joint capsule.

Acromioclavicular joint

The acromioclavicular (AC) joint is located between the acromion process of the scapula (part of the scapula that forms the highest point of the shoulder) and the distal end of the clavicle.

The capsule of this joint is reinforced by the coracoclavicular ligament between the scapula and clavicle at the point of articulation. The coracoclavicular ligament in further detail is created by the conoid ligament, medial from the coracoid process of the scapula and inserts on the conoid tubercle of the clavicle. Lateral to the conoid ligament is the trapezoid ligament, which runs from the coracoid process of the scapula to the trapezoid line of the clavicle. One more ligament, the coracoacromial ligament, running from the coracoid process to the acromion of the scapula contributes to the integrity of the acromioclavicular joint.

Sternoclavicular joint

The sternoclavicular occurs at the medial end of the clavicle with the manubrium or top most portion of the sternum. The clavicle is triangular and rounded and the manubrium is convex the two bones articulate. The joint consists of a tight capsule and complete intra-articular disc which ensures stability of the joint. The costoclavicular ligament is the main limitation to movemnet, therefore, the main stabiliser of the joint. A fibrocartilaginous disc present at the joint increases the range of movement. Sternoclavicular subluxation is rare, however can be caused by direct trauma.

Movements of the shoulder

The muscles and joints of the shoulder allow it to move through a remarkable range of motion, making it the most mobile joint in the human body. The shoulder can abduct, adduct (such as during the shoulder fly), rotate, be raised in front of and behind the torso and move through a full 360° in the sagittal plane. This tremendous range of motion also makes the shoulder extremely unstable, far more prone to dislocation and injury than other joints.

Major muscles

The muscles that are responsible for movement in the shoulder attach to the scapula, humerus, and clavicle. The muscles that surround the shoulder form the shoulder cap and underarm.

Rotator cuff

The rotator cuff is a structure composed of tendons that, with associated muscles (supraspinatus, infraspinatus, teres minor and subscapularis), holds the ball at the top of the humerus in the glenoid socket and provideoulder joint. The tendons of the rotator cuff muscles also connect to the capsule of the glenohumeral joint.

Two filmy sac-like structures called bursae permit smooth gliding between bone, muscle, and tendon. They cushion and protect the rotator cuff from the bony arch of the acromion.

Measurement of shoulder loads

For understanding normal and pathologic shoulder function knowledge of forces in the glenohumeral joint is essential. It forms the basis for performing fracture treatment or joint replacement surgery, for optimizing implant design and fixation and for improving and verifying analytical biomechanical models of the shoulder. With instrumented shoulder implants the joint contact forces and moments can be measured in vivo during different activities.

Additional images

Medical problems

• Shoulder problems
• Rotator cuff tear

See also

• Chip on shoulder

References

• Calais-Germain, Blandine. "Anatomy of Movement", Eastland Press, 1993. ISBN 0-939616-17-3
• Martini, Frederic; Timmons, Michael; McKinnley, Michael. "Human Anatomy", 3rd Edition, Prentice-Hall, 2000. ISBN 0-13-010011-0

The Chip-on-the-shoulder is what happens when people get in car-crashes, and their shoulders chip or flake in abnormal ways, and prohibit the normal functioning of the shoulder and its associated muscles.

External links

• Video of the shoulder carriage in motion
• NIH (article includes text from this source)
• Video of shoulder surgery, showing interior anatomy of shoulder