Shoulder 29. December. 2012 Thursday



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Dr. Kaan Yücel http://yeditepeanatomy1.wordpress.com Yeditepe Anatomy


Shoulder

29. December.2012 Thursday

The shoulder is the region of upper limb attachment to the trunk.

Shoulder is the proximal segment of the limb that overlaps parts of the trunk (thorax and back) and lower lateral neck. It includes the pectoral, scapular, and deltoid regions of the upper limb, and the lateral part (greater supraclavicular fossa) of the lateral cervical region. It overlies half of the pectoral girdle. The pectoral (shoulder) girdle is a bony ring, incomplete posteriorly, formed by the scapulae and clavicles and completed anteriorly by the manubrium of the sternum (part of the axial skeleton).



The bone framework of the shoulder consists of:

  • the clavicle and scapula, which form the pectoral girdle (shoulder girdle); and

  • the proximal end of the humerus.

The superficial muscles of the shoulder consist of the trapezius and deltoid muscles, which together form the smooth muscular contour over the lateral part of the shoulder. These muscles connect the scapula and clavicle to the trunk and to the arm, respectively.

Joints

The three joints in the shoulder complex are the sternoclavicular, acromioclavicular, and glenohumeral joints.

The sternoclavicular joint and the acromioclavicular joint link the two bones of the pectoral girdle to each other and to the trunk. The combined movements at these two joints enable the scapula to be positioned over a wide range on the thoracic wall, substantially increasing "reach" by the upper limb.

The glenohumeral joint (shoulder joint) is the articulation between the humerus of the arm and the scapula.

Muscles

The two most superficial muscles of the shoulder are the trapezius and deltoid muscles. Together, they provide the characteristic contour of the shoulder:



  • trapezius attaches the scapula and clavicle to the trunk;

  • deltoid attaches the scapula and clavicle to the humerus.

Both the trapezius and deltoid are attached to opposing surfaces and margins of the spine of the scapula, acromion, and clavicle. The scapula, acromion, and clavicle can be palpated between the attachments of trapezius and deltoid.

SUPERFICIAL POSTERIOR AXIOAPPENDICULAR (EXTRINSIC SHOULDER) MUSCLES

The superficial axioappendicular muscles are the trapezius and latissimus dorsi.



Trapezius

The superior fibers of trapezius, from the skull and upper portion of the vertebral column, descend to attach to the lateral third of the clavicle and to the acromion of the scapula.

The superior and inferior fibers work together to rotate the lateral aspect of the scapula upward, which needs to occur when raising the upper limb above the head. Movement to 180 degrees (elevation) is brought about by rotation of the scapula upwards by the trapezius and serratus anterior (see “Movements of the shoulder girdle” on page 7). The trapezius also braces the shoulders by pulling the scapulae posteriorly and superiorly, fixing them in position on the thoracic wall with tonic contraction; consequently, weakness of this muscle causes drooping of the shoulders.

Latissimus dorsi

This large, fan-shaped muscle passes from the trunk to the humerus and acts directly on the glenohumeral joint and indirectly on the pectoral girdle (scapulothoracic joint). The latissimus dorsi extends, retracts, and rotates the humerus medially (e.g., when folding the arms behind the back or scratching the skin over the opposite scapula).

In combination with the pectoralis major, the latissimus dorsi is a powerful adductor of the humerus and plays a major role in downward rotation of the scapula in association with this movement. It is also useful in restoring the upper limb from abduction superior to the shoulder; hence the latissimus dorsi is important in climbing. In conjunction with the pectoralis major, the latissimus dorsi raises the trunk to the arm, which occurs when performing chin-ups (hoisting oneself so the chin touches an overhead bar) or climbing a tree, for example. These movements are also used when chopping wood, paddling a canoe, and swimming (particularly during the crawl stroke).

DEEP POSTERIOR AXIOAPPENDICULAR (EXTRINSIC SHOULDER) MUSCLES

The deep posterior thoracoappendicular muscles are the levator scapulae and rhomboids. These muscles provide direct attachment of the appendicular skeleton to the axial skeleton.



Levator scapulae

True to its name, the levator scapulae acts with the descending part of the trapezius to elevate the scapula, or fix it (resists forces that would depress it, as when carrying a load. With the rhomboids and pectoralis minor, the levator scapulae rotates the scapula, depressing the glenoid cavity (rotating the lateral aspect of scapula inferiorly). Acting bilaterally (also with the trapezius), the levators extend the neck; acting unilaterally, the muscle may contribute to lateral flexion of the neck (toward the side of the active muscle).

Rhomboid minor and major

The rhomboids retract and rotate the scapula, depressing its glenoid cavity. They also assist the serratus anterior in holding the scapula against the thoracic wall and fixing the scapula during movements of the upper limb. The rhomboids are used when forcibly lowering the raised upper limbs (e.g., when driving a stake with a sledge hammer).



SCAPULOHUMERAL (INSTRINSIC SHOULDER) MUSCLES

The six scapulohumeral muscles (deltoid, teres major, supraspinatus, infraspinatus, subscapularis, and teres minor) are relatively short muscles that pass from the scapula to the humerus and act on the glenohumeral joint.All the intrinsic muscles but the deltoid and the subscapularis are muscles of the posterior scapular region.



Deltoid




The deltoid muscle is large and triangular in shape, with its base attached to the scapula and clavicle and its apex attached to the humerus. It originates along a continuous U-shaped line of attachment to the clavicle and the scapula, mirroring the adjacent insertion sites of the trapezius muscle. It inserts into the deltoid tuberosity on the lateral surface of the shaft of the humerus.

The major function of the deltoid muscle is abduction of the arm beyond the initial 15° accomplished by the supraspinatus muscle.



The deltoid muscle is innervated by the axillary nerve, which is a branch of the posterior cord of the brachial plexus.



http://www.sciencelearn.org.nz/var/sciencelearn/storage/images/contexts/sporting-edge/sci-media/images/deltoid-muscles/14542-15-eng-NZ/Deltoid-muscles_full_size_portrait.jpg

Subscapularis

The subscapularis is a thick, triangular muscle that lies on the costal surface of the scapula and forms part of the posterior wall of the axilla. It originates from the subscapular fossa on the anterior surface of the scapula and attaches to the lesser tubercle of the humerus. The subscapularis is the primary medial rotator of the arm and also adducts it. It joins the other rotator cuff muscles in holding the head of the humerus in the glenoid cavity during all movements of the glenohumeral joint (i.e., it helps stabilize this joint during movements of the elbow, wrist, and hand). It is innervated by superior and inferior subscapular nerves of the brachial plexus.





http://www.coretherapy.com/images/fig_4.gif


POSTERIOR SCAPULAR REGION

The posterior scapular region occupies the posterior aspect of the scapula and is located deep to the trapezius and deltoid muscles. It contains four muscles, which pass between the scapula and proximal end

of the humerus: supraspinatus, infraspinatus, teres minor, and teres major muscles.

The posterior scapular region also contains part of one additional muscle, the long head of the triceps brachii, which passes between the scapula and the proximal end of the forearm. This muscle, along with other muscles of the region and the humerus, participates in forming a number of spaces through which nerves and vessels enter and leave the region.

The supraspinatus, infraspinatus, and teres minor muscles are components of the rotator cuff, which stabilizes the glenohumeral joint.


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