The shoulder is comprised of three bones view figure 1a and view figure 1b and several muscles. The bones include the scapula (shoulder blade), the humerus (upper arm bone), and the clavicle (collar bone).
The scapula is a unique bone of the human body as it has many nooks and crevices and prominences. The top of the scapula is called the acromion and can be flat or curved. The acromion resides above the rotator cuff tendons and if the acromion is curved it can impinge or pinch on the rotator cuff when the arm is elevated. This is a common cause of shoulder pain known as impingement syndrome.
The coracoid process comes off the front of the scapula and also can cause irritation of the rotator cuff. Unlike impingement from the acromion which causes pain down the side of the upper arm, coracoid impingement (also known as subcoracoid impingement) causes pain in the front of the shoulder view figure 1c. The glenoid or socket is the part of the scapula that articulates with the top of the humerus.
The shoulder joint is made up of a ball and socket. The top of the humerus is the ball. The ball is connected to the glenoid by flimsy ligaments that offer the shoulder tremendous flexibility as well as stability to carry out every day activities view figure 2.
When the ligaments are too loose, the shoulder may dislocate or slide too much. When the ligaments are too tight, the shoulder can be particularly painful as is seen in adhesive capsulitis(frozen shoulder)..
The rotator cuff muscles originate on the scapula and are connected to the top of the humerus via the rotator cuff tendons. The tendons have poor blood supply and are prone to developing inflammation (rotator cuff tendonitis) as well as tears (rotator cuff tears) view figure 3a, view figure 3b and view figure 3c.
The clavicle is the most commonly fractured bone in the shoulder. Clavicle fractures usually heal and rarely need surgery. It is uncommon to operate on a child with a clavicle fracture, as they invariably heal and remodel. In adults, clavicle fractures may be severe and can necessitate surgery if the bones are broken into several pieces or if the fracture ends are significantly displaced. view figure 5a, view figure 5b, view figure 5c and view figure 5d.
Articular cartilage is perhaps the most amazing structure around the joints. The surface of each bone is coated with articular cartilage at a joint. This material has no nerve fibers, a very low coefficient of friction, has no blood supply, and acts as a buffer for the underlying bone. Without nerves, there is no pain when two surfaces covered with articular cartilage rub against each other.
Articular cartilage is a very slippery surface with a coefficient of friction less than ice on ice. Coating the surface of our bones, articular cartilage enables our joints to move in a very efficient manner. Unfortunately, the lack of blood supply means that articular cartilage has a limited ability to heal itself. When the articular cartilage is damaged, the underlying bone may see excess pressure and since the bone has many nerve fibers, this condition presents with pain. The complete loss of articular cartilage is the definition of arthrosis.
Inflammation in the setting of arthrosis is called arthritis. view figure 6a, view figure 6b, view figure 6c and view figure 6d.
The humerus, scapula, and clavicle comprise the two major joints about the shoulder girdle. The ball and socket joint of the glenohumeral articulation is the major source of the shoulder's mobility. The small joint at the top of the shoulder between the acromion and the clavicle (the acromioclavicular joint) has a very small role with respect to the shoulder's mobility but can be a major source of pain from arthritis.
View Video of Normal Shoulder
View Video of Arthritis in the Glenohumeral Joint with a Biceps Tendon Tear
The shoulder joint is made up of a ball and socket. The top of the humerus is the ball. The ball is connected to the glenoid by flimsy ligaments. The ligaments offer the shoulder tremendous flexibility as well as stability to carry out every day activities