The Anatomy of the Shoulder.
The following brief description is designed to provide some background information to help understand what may be wrong with your shoulder or the treatment that has been advised.
Introduction:
The shoulder has the greatest range of movement of any joint in the body. The shoulder can be considered to consist of 4 separate individual joints that function as one unit to allow the arm to move freely. The four joints are the Gleno-Humeral Joint, the Scapulo-Thoracic Joint, the Acromio-Clavicular Joint and the Sterno-Clavicular Joint. The principal articulation is the Ball and Socket joint between the Humerus (the arm bone) and the Glenoid of the Scapula (shoulder blade), The Gleno-Humeral Joint (GHJ).
The Bones:
The Humerus:
The Humerus can be divided into a number of different parts. The head (or ball of the ball-and–socket joint) provides the articular (or gliding) surface for the main shoulder joint with the Scapula (shoulder blade). Anteriorly (or at the front) there is the channel or Biciptal Groove (which contains the Long Head Biceps or LHB). There are prominences on either side of this groove. The Greater Tuberosity lies on the lateral (or outside) surface and the Lesser Tuberosity lies anteriorly (or on the front). These prominences act as a point of insertion (or attachment) for an important group of muscles called the Rotator Cuff muscles. The shaft of the Humerus extends down the arm to the elbow.
The Scapula:
The Scapula (or shoulders blade) has several parts. The blade of the scapula lies on the back of the chest wall. This provides a surface for the attachment of many muscles. Posteriorly (at the back) there is a ridge called the Spine of the Scapula. The Acromion is a continuation of this spine and forms the lateral (or outer) bony tip of the shoulder. The Acromion lies over the top of the shoulder joint, covering the rotator cuff muscles. Part of the Deltoid muscle takes its origin from this bone. The Acromion forms a joint with the Clavicle (or collar bone) at the front of the shoulder. This joint is called the Acromioclavicular joint (or ACJ). The Coracoid is a small forward protrusion of the scapula and is the origin of a number of muscles including part of the biceps muscle (the Short Head Biceps). The final part of the scapula is the Glenoid. This is a flattened area facing laterally (or outwards) which serves as the articulating surface (or socket of the ball-and-socket joint) with the Humerus.
The Clavicle:
The Clavicle (or collar bone) lies anteriorly (at the front). It acts to strut the shoulder out to the side away from the midline. It has a joint at either end. Laterally (at the outer end) it articulates (or forms a joint) with the Acromion (or tip of the shoulder blade) to form the Acromioclavicular joint (ACJ). Medially (or at the inner end) the Clavicle articulates (or forms a joint) with the Sternum (or breastbone) and the first rib to form the Sternoclavicular joint (SCJ).
The Joints:
The 4 components of the shoulder joint are the Gleno-Humeral Joint (GHJ) (or ball-and-socket) joint, the Acromio-Clavicular joint (ACJ), the Sterno-Clavicular joint (SCJ) and the Scapulo-Thoracic Joint.
Gleno-Humeral Joint:
The humeral head (or ball) articulates with the Glenoid (or socket). Both are covered with articular cartilage that allows low friction movement. One side (the humerus) is convex and the other (the Glenoid) is flat. As this is an inherently unstable arrangement. A rim of tissue, the labrum, deepens the Glenoid. If this rim becomes damaged or detached the shoulder joint can become unstable or dislocate. At the top of the Glenoid (the 12 o'clock position) the long head of the biceps tendon attaches and blends with the Labrum.
There are a number of important ligaments that stabilise the Gleno-Humeral (ball and socket) Joint.
Acromio-Clavicular Joint:
The distal or lateral end of the collarbone (Clavicle) articulates with the Acromion of the Shoulder blade (Scapula). This joint is stabilised by the Acromio-Clavicular ligaments which are essentially thickening of the joint capsule above (superior) and below (inferior). The main stabilisers of the Joint are the two Coraco-Clavicular ligaments which bind the clavicle to the Scapula. The Trapezoid lies laterally and the Conoid medially.
Sterno-Clavicular Joint:
The proximal or medial end of the collarbone (Clavicle) articulates with the Sternum (breast bone) and first rib.
The Muscles:
The shoulder joint is capable of a large range of motion and requires several muscles to do this. These muscles are in 2 layers. An outer layer provides power of movement and an inner layer, or rotator cuff, acts principally to stabilize the Gleno-Humeral (shoulder) joint.
External Muscles:
The main external muscles include the Deltoid, Pectoralis Major and Coracobrachialis.
Deltoid:
The deltoid is the largest muscle of the shoulder and takes its origin from the clavicle (collar bone), the Acromion (tip of shoulder blade) and the scapula spine. It inserts into the Humerus about 8cm down on the outer side. This is a very powerful muscle, which allows the shoulder to flex forward, abduct (to lift to the side) and to extend (push the elbow backwards).
Pectoralis Major:
The Pectoralis Major originates from the chest wall and clavicle and attaches onto the front of the humerus. It acts to adduct and internally rotate the arm.
Coarcobrachialis:
The Coracobrachialis and Short Head of Biceps originate from the coracoid and inserts separately into the anterior humerus and the Biciptal tuberosity of the ulna and Lacertus Fibrosis of the Forearm. The muscles form a conjoint tendon and Flex the shoulder as well as the elbow.
Biceps:
Biceps as the name implies originates from two points. The smaller tendon or Long Head starts from within the Gleno-Humeral Joint, from the bicipital tubercle and the labrum (or lip) at the top of the Glenoid. The Short head is larger and comes from the Coracoid as part of the Conjoint tendon with the Coracobrachialis. The Biceps muscle belly extends to the elbow where it attaches to the Biciptal tuberosity of the Radius and the Lacertus Fibrosis. The Biceps acts to flex the shoulder and elbow. It is the principal supinator of the forearm (rotation of the hand to place the palm upwards).
Rotator cuff muscles:
The Rotator Cuff is a cuff of 4 muscles which surround the Gleno-Humeral (ball and socket) joint. From front to back the muscles are the Subscapularis, the Supraspinatus, the Infraspinatus and the Teres Minor.
Subscapularis:
The Subscapularis originates from the under surface of the Scapula and inserts into the Lesser Tuberosity at the front of the Humerus.
Supraspinatus:
The Supraspinatus originate from above the spine of the Scapula on the superficial or outer surface of the Scapula. It passes under the Acromion and inserts onto the Greater Tuberosity at the top, outer edge of the Humerus just lateral to the articular or joint surface.
Infraspinatus:
The Infraspinatus originates from below the spine of the Scapula on the superficial or outer surface of the scapula. It passes laterally and inserts onto the Greater Tuberosity at the top, outer edge of the Humerus just lateral and behind the articular or joint surface.
Teres Minor:
The Teres Minor originates from the lateral or outside edge of the Scapula and again inserts on to the Greater Tuberosity. When these muscles contract they act to center the Humeral head in the Glenoid, facilitating the action of the deltoid. If they are weak or torn (rotator cuff rupture) shoulder function may be significantly compromised.
Long Head Biceps:
The Biceps originates from two tendons. The main tendon or Short Head is attached to the Coracoid and lies away from the Gleno-Humeral joint itself. The smaller tendon or Long Head starts from within the Gleno-Humeral Joint, from the bicipital tubercle and the labrum (or lip) at the top of the Glenoid.
The Long Head of Biceps is not a part of the rotator cuff but does penetrate the cuff between Subscapularis and Supraspinatus at the front of the joint. It lies between the 2 muscles in the Inter-Tubercular Groove. The origin is from the Biceps Tubercle and is continuous with the Labrum of the Glenoid. The tendon lies within the Gleno-Humeral for a significant length before becoming extra articular as it leaves the Inter-tubercular Groove.
The Bursae:
A bursa is a fluid filled sac, which allows two surfaces to glide over each other more easily. Several bursae exist in the shoulder. The Subacromial Bursa is the one most often associated with problems. It is the one that lies between the undersurface of the acromion and the upper surface of the rotator cuff tendons. In some conditions the bursa can become irritated or inflamed and contributes to the pain of subacromial impingement.