Shoulder Structure, Function and Common Problems
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The shoulder has a wider and more varied range of motion than any other joint in the body. Our shoulder allows us to do everything from paint to play basketball, but this flexibility also makes the shoulder one of the most unstable joints in the body. The shoulder joint is not held in place with bones, but rather an elaborate system of muscles, tendons and ligaments. Those most at risk for shoulder problems are athletes or workers with “overhead” movements—swimmers, throwers, painters and construction workers. The older we get, the more vulnerable we all are.
The shoulder is not a single joint, but a complex arrangement of bones, ligaments, muscles, and tendons that is better called the shoulder girdle. The primary function of the shoulder girdle is to give strength and range of motion to the arm. The shoulder girdle includes three bones—the scapula, clavicle and humerus. There are three joints in the shoulder girdle. One joint is where the head of the humerus articulates inside the glenoid cavity of the scapula, called the glenohumeral joint which includes the ligaments, tendons and muscles attached to these two bones. The acromioclavicular joint (A/C Joint) includes the ligaments, tendons, and bones where the acromion (on the shoulder blade) joins at the clavicle (collar bone). The third joint is the sternoclavicular joint which forms where the sternum (breastbone) joins the clavicle (collar bone).
Anatomy terms allow us to describe the body clearly and precisely using planes, areas and lines. Instead of your doctor saying “his knee hurts” she can say “his knee hurts in the anterolateral region” and another doctor will know exactly what is meant. Below are some anatomic terms surgeons use as these terms apply to the shoulder:
- Anterior — the abdominal side of the shoulder
- Posterior — the back side of the shoulder
- Medial — the side of the shoulder closest to the spine
- Lateral — the side of the shoulder farthest from the spine
- Abduction — move away from the body (raising the arm)
- Adduction — move toward the body (lowering the arm)
- Proximal — located nearest to the point of attachment or reference, or center of the body
- Distal — located farthest from the point of attachment or reference, or center of the body
- Inferior — located beneath, under or below; under-surface
If you think of the shoulder in layers, the deepest layer is bone, then ligaments of the joint capsule, and the tendons and muscles are on top. Nerves and blood vessels supply the muscles and bones of the shoulder. Nerves carry signals from the brain to the muscles to move the shoulder and carries signals from the muscles back to the brain about pain, pressure and temperature.
The Shoulder Joints
A joint is formed where two or more bones meet. There are three joints in the shoulder girdle:
- the glenohumeral joint (GH) is a ball-and-socket joint where the humerus meets the glenoid on the scapula; this joint is informally known as the shoulder joint.
- the acromioclavicular joint (AC) is a gliding joint where the acromial process on the scapula links to the collar bone (clavicle)
- the sternoclavicular joint (SC) is a double gliding joint between the sternum (breastbone) and the clavicle (collar bone)
The glenohumeral (GH) joint forms what people commonly think of as the shoulder joint and is the most important of the shoulder joints. The GH joint links the humerus (arm) with the thorax (chest). The stability of the GH joint depends on keeping the humeral head centered in the glenoid fossa (socket) on the scapula. The humerus is held in place with ligaments, tendons and anterior muscles, mainly the muscles and tendons of the rotator cuff.
The Glenohumeral Joint (GH)
The extremely mobile shoulder joint is naturally unstable and not surprisingly, the most commonly dislocated joint in the body.The glenohumeral joint provides most of the motion in the shoulder girdle. The GH joint allows us to move our arm forward, backward and side-to-side, to rotate it inward and outward, move it across the body in front and behind, and make a circle with our arm both clockwise and counterclockwise. The glenohumeral joint has the greatest mobility of any joint in the body and it seems as if movement would be possible in all directions—but certain structures limit raising the arm straight out behind us to about 60 degrees. This great range of motion can lead to several common problems and injuries affecting the shoulder girdle.
The glenohumeral joint is a ball-and-socket joint like the hip joint. The shoulder is different from the hip in that the hip is a weight-bearing joint and the shoulder is a suspension joint. The large, almost perfectly round head of the humerus (ball) fits into the small, shallow glenoid fossa (socket) on the lateral side of the scapula. The shoulder socket is very shallow and comes in very little contact with the round head of the humerus—similar to a golf ball on a tee. Also, the cup of the socket is much lager than the ball that fits into it. At any point in the shoulder’s arc of motion this poor fit and contact of the bones make the glenohumeral joint unstable. Therefore, it is the soft tissues in the joint that maintain stability and mobility.
The soft tissues of the glenohumeral joint include:
- the joint capsule
- the glenohumeral ligaments – function as static stabilizers of the GH joint
- the glenoid labrum – this is a ring of fibrocartilaginous tissue structure that attaches to the rim of the glenoid cavity on the scapula; it increases the depth of the glenoid “socket” by 50%. Its function is to increase the surface contact area for the ball on the humerus to create a better fit. The glenoid labrum also serves as an attachment point for the shoulder capsule, glenohumeral ligaments, and the long head of the biceps tendon.
- the long head of the biceps tendon
- the rotator cuff tendons and muscles
These soft tissues are where most degenerative (wear and tear) and traumatic conditions of the shoulder occur. A smooth cushion of shiny white articular cartilage less than 1/4 inch thick covers the end of the humerus and in the glenoid socket of the scapula. The articular cartilage is kept slippery by fluid made in the synovial membrane (joint lining). Since the cartilage is smooth and slippery, the bones of the joint move against each other easily and without pain.
The adult skeleton is mainly made of bone and a little cartilage in places. Bone and cartilage are both connective tissues, with specialized cells called chondrocytes embedded in a gel-like matrix of collagen and elastin fibers. Cartilage can be hyaline, fibrocartilage and elastic and differ based on the proportions of collagen and elastin. Cartilage is a stiff but flexible tissue that is good with weight-bearing which is why it is found in our joints. Cartilage has almost no blood vessels and is very bad at repairing itself. Bone is full of blood vessels and is very good at self repair. It is the high water content that makes cartilage flexible.
The muscles on the lateral side of the shoulder allow movement and stabilize the joint. These muscles are strong on the upper and back sides of the arm, but not on the underside. A strong outside force in this area can cause the head of the humerus to slip out of the glenoid socket, called dislocation.
The Acromioclavicular Joint (AC)
The AC joint helps link the arm to the body at the chest. Since there is little bony stability in this joint, a number of ligaments and other soft tissues stabilize this joint. The superior AC ligament is the most important horizontal stabilizer. The coracoclavicular ligaments help stabilize the clavicle vertically. A significant amount of rotation occurs in the clavicle and about 10% occurs at the AC joint.
The Sternoclavicular Joint (SC)
Most of the rotation occurs at the sternoclavicular joint and joint stability comes from the soft tissues. The posterior sternoclavicular joint capsule is the most important structure for preventing forward and backward displacement of the medial clavicle.
The Rotator Cuff
The rotator cuff consists of four muscle-tendon units that originate on the scapula and attach to the tuberosities of the humerus. The role of the rotator cuff is to keep the head of the humerus centered in the glenoid fossa throughout the shoulder’s range of motion and when raising the arm. The rotator cuff is the primary stabilizer during movement of the GH joint. Both overuse and traumatic injuries to the rotator cuff are the most common problems in the shoulder girdle.
The Subacromial Space
The subacromial space is beneath the acromion and above the rotator cuff. The subacromial bursa outlines this space and provides frictionless gliding of the rotator cuff beneath the arch formed by the acromion and coracoacromion. Bone spurs on the underside of the acromion narrow this space, irritate the bursa and contribute to tears in the rotator cuff.
Bones of the Shoulder Girdle
The bones of the shoulder girdle include the humerus, the scapula, and the clavicle. There are four articulations (movements) in the shoulder named for their anatomic locations:
- glenohumeral (the only true synovial joint)
Scapula (shoulder blade). The scapula is the most complex of the bones in the shoulder and is part of the shoulder girdle. The scapula floats on the rib cage, and is attached to it only with muscles. There are three landmarks on the scapula; the spine, acromion and coracoid processes. The roof of the glenohumeral joint is formed by the acromion. The acromion articulates with the clavicle forming the acromioclavicular (AC) joint. A spine divides the back of the scapula into two sections. The muscles that attach below this spine are called infraspinatus muscles; the ones that attach above this spine are called supraspinatus muscles.
Humerus (upper arm). The humerus is the ball part of the ball-and-socket joint. The head (ball) of the humerus articulates within the glenoid fossa. Below the humeral head is the anatomic neck which separates the head (ball) from the tuberosities. Each tuberosity provides a place for the attachment for the anterior muscles of the rotator cuff—the 4 rotator cuff muscles originate from the scapula and their tendons attach at the humerus. The bicipital groove separates the tuberosities. Just below the tuberosities is the surgical neck of the humerus and is the most common area for fractures of the proximal humerus.
Clavicle (collar bone) . The clavicle originates at the sternum (breastbone) just above the first rib, and is held in place by the acromioclavicular ligament, several muscles and the coracoclavicular ligament. The clavicle helps hold the shoulder out to the side while allowing the scapula to move around.
There are several important ligaments about the shoulder girdle. Ligaments are soft tissue structures that connect bones to bones. Ligaments are strong, tough bands that are not particularly flexible. Once stretched, they tend to stay stretched and if stretched too far, they snap.
Ligaments, along with muscles and tendons, are the main source of stability for the shoulder. Shoulder ligaments also form the joint capsule that surround the glenohumeral joint. These passive stabilizers serve to keep the joints of the shoulder from dislocating. Some of the main ligaments are the acromioclavicular, coracoclavicular and the coracoacromial.
When injured, the ligament that attaches the clavicle to the acromion—the acromioclavicular ligament—is called a separated shoulder. Two ligaments connect the clavicle to the scapula by attaching to the coracoid process—the coracoclavicular and the coracoacromial ligaments. The coracoacromial prevents upward dislocation of the shoulder. When the head of the humerus dislocates, it’s usually in a downward direction.
- Ligaments of the AC joint: Capsular; superior and inferior acromioclavicular; articular disk; coracoclavicular (trapezoid and conoid)
- Ligaments of the Sternoclavicular joint: Capsular; anterior and posterior sternoclavicular; inter- and costo- clavicular; articular disk
- Ligaments of the GH joint: Capsular; coracohumeral; glenohumeral; transverse humeral; glenoid of humerus
Shoulder Muscles and Shoulder Tendons
Muscles in the back, neck, shoulder, chest and upper arm all work together to support and move the shoulder. One of the most important is the deltoid.
Tendons are elastic, soft, connective tissue structures that attach muscles to bone. Muscles move the bones by pulling on the tendons. The rotator cuff tendons are a group of tendons that connect the deepest layer of muscles to the humerus. As they form their tendinous attachment to the humerus, they become a fibrous capsule. The rotator cuff muscles and tendons control our ability to raise the arm from our side (abduction).
Each muscle of the shoulder assists with specific movements. The deep muscle group that moves the shoulder are the rotator cuff muscles and tendons. Keeping the head of the humerus inside the glenoid fossa is the primary function of the rotator cuff muscles. This important group of muscles lies just outside the glenohumeral joint and helps rotate the shoulder in the many directions. The rotator cuff muscles include the:
- teres minor muscles (SIT)
The rotator cuff muscles are the muscles most often involved in shoulder rehab.
The biceps tendon is attached to the biceps muscle and runs from the humerus across the front of the shoulder to the glenoid. The biceps tendon attaches to the glenoid and becomes part of the labrum. The biceps tendon can cause problems if it’s damaged and pulled away from its attachment to the glenoid.
The outer muscle layer is formed by the large deltoid muscle which overlies the SIT muscles. This is probably the largest, strongest muscle of the shoulder. The deltoid takes over lifting the arm once the arm is away from the side. Other muscles include the biceps.
Sandwiched between the rotator cuff muscle layer and the outer layer of large bulky muscles is the large subacromial bursa, called the subdeltoid bursa. Bursae are everywhere in the body. A bursa is simply a padlike sac found between two moving surfaces that is lined with synovial membrane and contains a small amount of lubricating fluid inside—synovial fluid is similar in consistency to raw egg white—to reduce friction and aid movement. Bursae occur in connective tissue wherever two body parts—other than joints—move against each other. Their function is to lessen the friction between tendon and bone, ligament and bone, tendons and ligaments, and between muscles. Inflammation or infection of the bursa is called bursitis.
Blood Vessel and Nerves in the Shoulder
There are several important nerves in the shoulder including the radial nerve, ulnar nerve, and median nerve. But the most important is the brachial plexus nerve which supplies all the muscles that contribute to the function of the arm and shoulder girdle. The most vulnerable to direct injury are the brachial plexus and its nerve branches, the spinal accessory nerve and the long thoracic nerve. Shoulder dislocation is most often responsible for damage to the brachial plexus. Direct trauma to the scapula that causes fracture or dislocation can damage the spinal accessory or thoracic nerves. Injury to spinal nerves can result in the alteration of movement and sensation in the shoulder.
The subclavian artery and vein are the two main vascular structures in the shoulder and are part of the thoracic outlet. Trauma and fractures to the clavicle can injure these vessels. There are many blood vessels that supply the rotator cuff. The large axillary artery can be felt pulsing in your armpit. The axillary artery branches into many smaller vessels that supply blood to different parts of the shoulder.
Problems of the Shoulder
- Acromioclavicular degeneration
- Acromioclavicular joint separation
- Adhesive capsulitis (Frozen Shoulder)
- Arthritis—rheumatoid, traumatic
- Baseball shoulder
- Calcific Tendonitis
- Cervicobrachial syndrome
- Impingement Syndrome (Bursitis)
- Labral tear
- Growths or Tumors, benign or malignant (Neoplasm)
- Muscle spasm
- Necrosis (cell or tissue death)
- Painful Arc Syndrome
- Rotator cuff injury or disease
- Shoulder instability
- Supraspinatus syndrome
- Sprengel’s deformity
Diagnosis, Treatment, and Surgery for Shoulder Problems
The goals of shoulder surgery are to reduce pain, increase function, mobility and stability of the joint, and correct deformities or injuries.
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