ANATOMY OF THE SHOULDER
The shoulder is not like the hip joint where just 2 bones connect. Instead, it is a complex where 5 joints or articulations have to work in harmony. Approximately 30 muscles control the movement of the shoulder complex. These muscles allow the shoulder to move almost 180° in 6 different directions. It is important to understand how these parts work and move to appreciate the problems that can develop to cause shoulder pain.
The primary joint of the shoulder complex is the ball and socket where the humeral head (the ball) attaches to the glenoid (the socket). This joint is not a deep socket like a hip rather a shallow socket like a golf ball on a tee. This allows the humeral head to rotate and move easily without constraint, but also makes it more susceptible to instability. Just as any golfer knows that a golf ball can easily fall off of a tee that is chipped or tilted, so too can the humeral head dislocate out of the glenoid if not aligned well or damaged.
The arm is attached to the rest of the body, called the axial skeleton, by a connection through the clavicle (collarbone) and the muscular attachments of the scapula (shoulder blade). The scapula is an odd-shaped bone that is only attached to the rest of her body by muscle attachments in the back and by ligament attachments to the clavicle in the front. It is a thin bone in the center but is surrounded by a harder edge around the perimeter. The glenoid is along the outside or lateral edge of the scapula. Thus the alignment of the scapula determines the position of the glenoid. If the scapula is not correctly positioned, it can tilt "the tee" and make the primary shoulder joint unstable. The top of the scapula has a bony ledge that projects over the top of the glenoid. It is the hard bone you feel when you press down on top of the shoulder. This bony ledge helps protect the rotator cuff tendons that are underneath it and also serves as the attachment point for the clavicle. The joint between the acromion and clavicle is called the acromioclavicular (AC) joint. This is a secondary joint in the shoulder complex, but a common location for arthritis.
The clavicle is the bony strut that connects the scapula to the rest of the axial skeleton at the sternum (breastbone). This junction between the clavicle and the sternum is called the sternoclavicular (SC) joint. The clavicle has hard, dense bone and is shaped like a lazy S, which is easily felt under the skin. Several muscles attach or originate on the clavicle but its primary function is to push the scapula and shoulder back and out where the arm can function better. People with a fractured clavicle have a shoulder that droops down and rotates inward because of the loss of this structural support.
There are 2 articulations in the shoulder where muscles and bones glide past each other. One of these is the junction between the scapula and the ribs. The scapula translates back and forth across the back of the ribs. It is controlled by muscles that attach to the ribs or the spine and pull the scapula back and forth. Other muscles also help hold the scapula onto the back of the ribs. Without these muscles, the scapula would displace outward and this is called winging. The scapular bone and the ribs do not come into contact with each other as there is an intervening layer of muscle between the two.
A second articulation is between the tendons of the rotator cuff and the undersurface of the acromion. The rotator cuff tendons encircle the humeral head and help move the shoulder in the glenoid. As the humeral head rotates and elevates, the rotator cuff tendons glide under the acromion. A thin fluid-filled sac, called the subacromial bursa (see diagram above), lubricates the tendons as they move underneath the acromion bone. The undersurface of the acromion is a common location for extra bone growth, called spurs, to develop. These can rub into the rotator cuff and cause problems with the tendons, which will be discussed later.
There are approximately 30 muscles that control the movement of the shoulder complex. These can be divided into 3 different groups. The more superficial group is the power muscles that are familiar to weightlifters. These are the ones that we can easily see and are usually strengthened while lifting weights in the gym. These muscles originate on the ribs, clavicle or spine and attach to the upper humerus or scapula to anchor the arm to the rest of the body. In the front is the large muscle covering the chest wall, called the pectoralis. Along the side is the muscle that gives the shoulder a round appearance, called the deltoid. In the back below the scapula is the latissimus dorsi and along the top from the neck to the top of the shoulder is the trapezius. These are large, powerful muscles that function well to stabilize the upper arm to the body. Because of their size, they are rarely injured.
Underneath the larger power muscles of the shoulder, is a layer of deep smaller muscles which help stabilize the scapula or fine tune the movement of the upper arm. Attaching along the inner or medial border of the scapula is the levator scapulae and the rhomboid major and minor. These muscles help connect the scapula to the spine and stabilize the scapula against the ribs. They can become easily fatigued and are a common source of muscle spasms. Arising from the ribs and extending along the side of the chest to attach to the bottom portion of the scapula is the serratus anterior. This helps pull the scapula forward against the chest wall to prevent the scapula from "winging ". Injury to the nerve to this muscle by a blow to the chest wall underneath the arm can cause the muscle to stop working. As a result, the scapula will move out away from the ribs and thus limit the movement of the shoulder. This is referred to as scapular winging. Additional deep muscles arise from the front of the ribs to attach to the scapula. There are several smaller muscles underneath the pectoralis major. These are the subclavius and pectoralis minor, which help stabilize the front part of the scapula. These deeper muscles, while not being very large in size, are very important for the movement of the scapula. During normal elevation of the arm overhead, approximately 1/3 of the overall movement is the rotation of the scapula away from the spine and out to the side. If the scapula does not move in rhythm with the upper arm, a greater strain is placed on the muscles that control movement of the humerus and they can become damaged.
The most important deep muscles of the shoulder are the muscles that originate on the scapula and cross the shoulder joint to attach to the upper humerus. Four of these muscles blend together where their tendons form a continuous attachment to two bony prominence along the side of the humeral head. These four muscles are called the rotator cuff. The bony prominence where they attach are the greater and lesser tuberosities. The rotator cuff muscles serve a very important role by stabilizing the humeral head in the glenoid socket. They are also involved in raising the arm from the side and rotating the shoulder. The rotator cuff muscle on the front of the scapula and shoulder is the subscapularis, which attaches to the lesser tuberosity. It helps rotate the shoulder inward (internal rotation). On the top of the rotator cuff is the supraspinatus. It passes directly under the acromion to attach to the top part of the greater tuberosity. Underneath the acromion, it is very susceptible to injury and is the most frequent portion of the rotator cuff that is torn. Directly behind the supraspinatus are the infraspinatus and teres minor. These rotator cuff muscles also attach to the greater tuberosity and help rotate the arm outward (external rotation). These four muscles serve such an important role in the use of the shoulder that they have earned their own name. Often incorrectly called the rotary cup or rotor cuff, there name stems from the fact that they form a continuous cuff around the humeral head. It was originally thought that they're primary function was rotation of the shoulder. Now, however, we realize their primary role is stabilizing the glenohumeral joint. If the rotator cuff is not functioning well, the humeral head can start to displace out of the glenoid socket.
The Rotator Cuff Muscles
The final group of muscles of the shoulder is the ones that originate around the shoulder, but whose muscle is primarily located in the upper arm. These muscles form much of the muscle bulk of the upper arm. In the front are the 2 tendons of the biceps (short head and long head) and the coracobrachialis. In the back of the upper arm is the triceps. These 3 muscles originate on the outer portion of the scapula and cross the shoulder joint to attach around the elbow. Because they cross the shoulder joint, they can serve a limited role in shoulder movement. The long head of the biceps tendon is susceptible to injury in the shoulder because it originates on the cartilage (labrum) attachment at the top of the glenoid. It then crosses through the glenohumeral joint to exit at an almost 90° angle between the greater and lesser tuberosities where it passes between the supraspinatus and the subscapularis rotator cuff tendons. It can be damaged at its weaker attachment point on the labrum, which is called a SLAP (Superior Labrum Anterior to Posterior) tear. It can also be damaged where it passes between the rotator cuff and is susceptible to any trauma that will also damage the rotator cuff.
Ligaments are soft tissue structures that connect bones to bones. There are several important ligaments in the shoulder to help stabilize the glenohumeral joint. These ligaments are much smaller than the ligaments found in the knee. They are not as tight as other joint ligaments to allow the glenohumeral joint such a large range of motion. However, damage to these ligaments can result in the shoulder being unstable. The glenohumeral ligaments (SGHL, MGHL and IGHL in the diagram) help form the water tight sac that surrounds the joint. They are thick areas of this joint sac, which is called a capsule. The most important ligaments are the ones at the bottom portion of the joint, the inferior glenohumeral ligaments. There is one in the front (anterior) and one in the back, (posterior) which form a hammock underneath the glenohumeral joint to help support it. They become tight as the arm is elevated away from the side and internally or externally rotates. If the anterior inferior glenohumeral ligament is damaged, the humeral head will displace anteriorly. It may tear on either side of the joint. The most common location for a tear is on the glenoid side and this is called a Bankart tear. The ligament can also be stretched out rather than torn, which will cause the joint to be loose.
A second important ligament connects one part of the scapula to another. This ligament is the coracoacromial ligament (CAL in the diagram) and is in the front of the shoulder from the coracoid to the acromion. This ligament can become thickened and cause an impingement syndrome where it rubs on the underlying supraspinatus tendon. Constant pulling of the ligament on the acromion and also result in an acromial bone spur developing.
The third major group of ligaments helps stabilize the end of the clavicle to the acromion. These ligaments are called the coracoclavicular ligaments (CCL in the diagram) and connect the clavicle to the coracoid portion of the scapula. Damage to these ligaments will cause the end of the clavicle to be displaced upward, known as an acromioclavicular joint or shoulder separation.
An important structure in the shoulder is the cartilage which attaches around the edge of the glenoid socket and is called the glenoid labrum. It is a rubbery structure which is similar to the meniscus of the knee. It serves several important functions. It helps deepen the shallow glenoid cavity by creating a thicker edge to the socket. In many ways, it functions like an O ring between the ball and socket. It is also the attachment point for the glenohumeral ligaments. Damage to the ligaments can cause the labrum to tear away from the glenoid. A tear at the front and bottom portion of the labrum is called a Bankart tear. A similar tear in the back bottom portion of the labrum is known as a reverse Bankart tear. The labrum can also tear at the top portion of the glenoid where the biceps tendon attaches. Excessive tension on the biceps can cause the superior labrum to tear away from the glenoid. This is called a SLAP tear which was noted earlier.
Bursae are found throughout the body. A bursa is a thin fluid filled sac located between 2 structures that move against one another where there is not a joint. The bursa has a small amount of fluid that helps lubricate this area. Located on top of the rotator cuff and underneath the acromion is the subacromial bursa. This bursa helps the rotator cuff glide underneath the acromion. Inflammation of the bursa is called bursitis. Injury to the rotator cuff will also cause the bursa to become inflamed. It can also be irritated from rubbing against a prominence, such as a spur, on the under-surface of the acromion. The bursa extends down the side of the upper arm underneath the deltoid for several inches. Inflammation of the bursa in this area from damage to the rotator cuff or impingement from a spur will call pain in the upper arm. This is why most people with rotator cuff problems or bursitis grab their upper arm when describing their shoulder pain. Most cortisone shots for rotator cuff problems are placed within the subacromial bursa. The medication will extend throughout the bursa to cover the rotator cuff. While the bursa can be a frequent source of shoulder pain, it does not seem to be an important structure for overall function of the shoulder. It is typically removed with any rotator cuff surgery.
Nerve injuries in the shoulder are rare. However, because the group of nerves that control the arm (brachial plexus) pass across the front of the shoulder, nerve pain can be felt in the shoulder area. The brachial plexus originates in the neck, crosses underneath the clavicle, and passes beneath the pectoralis and coracoid into the front of the armpit (axilla) where it begins to divide into the individual nerves. It is unusual for shoulder problems to cause numbness and tingling down the arm. The only exception is the so called "dead arm" that can occur after a shoulder dislocation where the nerves of the brachial plexus are damaged by the displaced joint. Inflammation of the brachial plexus nerves, known as a Parsonage Turner's syndrome, can also cause the shoulder to be weaker and muscles to atrophy.
There are several important individual nerves in the shoulder that can be damaged. The axillary nerve supplies the deltoid muscle and can be stretched during a shoulder dislocation or injured with surgery close to the deltoid. Lack of deltoid function caused by this nerve injury is a serious problem and makes it very difficult to lift the arm. The spinal accessory nerve supplies function to the trapezius muscle. It is located across the upper back portion of the neck and can be damaged during surgery at the base of the neck to remove a cyst or other lesions. Loss of the trapezius muscle will make it difficult to shrug your shoulders. The musculocutaneous nerve supplies the biceps tendon and rarely can be injured with open shoulder surgeries from the front where the biceps is retracted too vigorously. The long thoracic nerve supplies the serratus anterior muscle, which stabilizes the scapula against the ribs. This nerve is very superficial along the rib cage underneath the upper arm. A direct injury to the ribs or continual pressure to that area, such as sleeping on a firm surface with the arm extended, can cause the nerve to be damaged with resultant scapular winging. The suprascapular nerve supplies the supraspinatus and infraspinatus muscles of the rotator cuff. It can become entrapped as it passes around the scapula and results in weakness of the rotator cuff muscles.