The shoulder girdle is the anatomical mechanism that allows for all upper arm and shoulder movement in humans.
Muscles The shoulder girdle consists of five muscles that attach to the
clavicle and
scapula and allow for the motion of the
sternoclavicular joint (connection between sternum and clavicle) and
acromioclavicular joint (connection between clavicle and scapula). The five muscles that comprise the function of the shoulder girdle are the
trapezius muscle (upper, middle, and lower),
levator scapulae muscle,
rhomboid muscles (major and minor),
serratus anterior muscle, and
pectoralis minor muscle.
Joints The shoulder girdle is a complex of 5 joints that can be divided into two groups. 3 of these joints are true
anatomical joints, while 2 are
physiological ("false") joints. Within each group, the joints are mechanically linked so that both groups simultaneously contribute to the different movements of the shoulder to variable degrees. In the first group, the scapulohumeral or
glenohumeral joint is the anatomical joint mechanically linked to the physiological subdeltoid or
suprahumeral joint (the "second shoulder joint") so that movements in the suprahumeral joint results in movements in the
glenohumeral joint. In the second group, the scapulocostal or
scapulothoracic joint is the important physiological joint that can not function without the two anatomical joints in the group, the
acromioclavicular and
sternoclavicular joints, i.e. they join both ends of the
clavicle.
Glenohumeral joint The
glenohumeral joint is the articulation between the
head of the humerus and the
glenoid cavity of the
scapula. It is a
ball and socket type of
synovial joint with three rotatory and three translatory degrees of freedom. The glenohumeral joint allows for
adduction,
abduction,
medial and
lateral rotation,
flexion and
extension of the arm.
Acromioclavicular joint The
acromioclavicular joint is the articulation between the
acromion process of the scapula and the
lateral end of the
clavicle. It is a
plane type of synovial joint. The acromion of the scapula rotates on the acromial end of the clavicle.
Sternoclavicular joint The
sternoclavicular joint is the articulation of the
manubrium of the
sternum and the first
costal cartilage with the
medial end of the
clavicle. It is a
saddle type of synovial joint but functions as a plane joint. The sternoclavicular joint accommodates a wide range of scapula movements and can be raised to a 60° angle
Scapulocostal joint The scapulocostal joint (also known as the scapulothoracic joint) is a
physiological joint formed by an articulation of the
anterior scapula and the
posterior thoracic rib cage. It is musculotendinous in nature and is formed predominantly by the
trapezius,
rhomboids and
serratus anterior muscles. The
pectoralis minor also plays a role in its movements. The gliding movements at the scapulocostal joint are
elevation,
depression,
retraction,
protraction and superior and inferior rotation of the scapula. Disorders of the scapulocostal joint are not very common and usually restricted to
snapping scapula.
Suprahumeral joint The suprahumeral joint (also known as the subacromial joint) is a physiological joint formed by an articulation of the
coracoacromial ligament and the
head of the humerus. It is formed by the gap between the humerus and the acromion process of the scapula. This space is filled mostly by the
subacromial bursa and the
tendon of
supraspinatus. This joint plays a role during complex movements while the arm is fully flexed at the glenohumeral joint, such as changing a lightbulb, or painting a ceiling.
Movements From its neutral position, the shoulder girdle can be rotated about an imaginary vertical axis at the medial end of the clavicle (the sternoclavicular joint). Throughout this movement the scapula is rotated around the chest wall so that it moves laterally and the glenoid cavity is rotated 40–45° in the horizontal plane. When the scapula is moved medially it lies in a frontal plane with the glenoid cavity facing directly laterally. At this position, the lateral end of the clavicle is rotated posteriorly so that the angle at the acromioclavicular joint opens up slightly. When the scapula is moved laterally it lies in a sagittal plane with the glenoid cavity facing anteriorly. At this position, the lateral end of the clavicle is rotated anteriorly so that the clavicle lies in a frontal plane. While this slightly closes the angle between the clavicle and the scapula, it also widens the shoulder. The scapula can be elevated and depressed from the neutral position to a total range of ; at its most elevated position the scapula is always tilted so that the glenoid cavity is facing superiorly. During this tilting, the scapula rotates to a maximum angle of 60° about an axis passing perpendicularly through the bone slightly below the spine; this causes the inferior angle to move and the lateral angle .
Injury Shoulders are a common place for tissue injuries, especially if the person plays overhead sports such as
tennis,
volleyball,
baseball,
swimming, etc. According to Bahr's major injury related statistics, shoulder dislocations or subluxations account for 4% of injuries in adults ages 20–30 and 20% of shoulder injuries are fractures. Damage to the shoulder and adjacent features can fluctuate in severity depending on the person's age, sport, position, recurring shoulder dysfunction, and many other factors. Some other common shoulder injuries are fractures to any shoulder girdle bones i.e.
clavicle, ligamentous sprains such as
AC joint or GH ligaments,
rotator cuff injuries, different labral tears, and other acute or chronic conditions related to shoulder anatomy. Shoulder girdle pain can be
acute or
chronic and be due to a number of causes.
Inflammation or injury of associated
tendons,
bone,
muscles,
nerves,
ligaments, and
cartilage can all cause pain. Also, past injury compensation, and stress can result in complicated shoulder pain.
Disorders In humans,
winged scapula is a condition in which the shoulder blade protrudes from a person's back in an abnormal position. ==Other animals==