Elbow: Anatomy and Physiology
As with many human joints, the relatively simple anatomy of the elbow disguises a remarkable capability in both its range of motion as well as the forces it is capable of bearing. The elbow is composed of three bones, cartilage coverings at the end of each bone, ligaments, tendons, and companion larger muscles. The elbow is also the conduit to important nerve and blood vessel networks. Although popularly referred to as a single structure, anatomically the elbow is three separate and interrelated joints, each of which bears a name that essentially defines its position and function.
The humerus, ulnar, and radius bones are the skeletal composition of the elbow. The humerus, the large bone of the upper arm, forms the top of the elbow joint. The humerus is met by the larger of the forearm bones, the ulna, which is located on the opposite side of the forearm from the thumb. The position where these bones meet is the ulnohumeral joint, forming a hinge that possesses a range of motion of approximately 180°. The humerus bone is also joined by the radius at a point above the elbow hinge, forming the radiohumeral joint; the point where the radius and the ulnar bones meet is also capable of movement, as the superior radioulnar joint. All three joints are enclosed in a single enclosure named the synovial joint capsule, so named for the synovial fluid, a lubricating compound present in all human joints. The end, or head, of each of the three bones in the joint is smooth to facilitate the turning and rotation of the joint as it moves.
Ease of motion in the joint is further assisted through the presence of articular cartilage. This substance is found on the surface of all human bones that are capable of movement. The thickness of articular cartilage on the bone surface of some weight-bearing joints is as much as 0.25 in (0.5 cm) thick; at the elbow, the cartilage is very thin and slippery, performing the dual role of absorbing forces that are received in the joint, as well as reducing friction between the bones as the elbow joint moves through its entire range of movements: flexing (a movement that brings the hand closest to the shoulder), extending (where the elbow is fully straightened to a locked position), as well as the movements that cause the rotation of the hand palm upwards, referred to as supination, and downwards, known as pronation.
The elbow ligaments connect the three bones of the joint one to another, to permit stable and controlled movement. The ligaments are similar in both appearance and properties to a short, flexible piece of cord. The elbow has four primary ligaments that create its range of motion. On the medial (inside) of the elbow is the ulnar collateral ligament (UCL), which connects the ulnar bone to the humerus. On the opposite, or lateral, side of the elbow is the radial collateral ligament (RCL), which connects the radius to the humerus. These ligaments play a role in most movements performed by the entire elbow joint, and they are consequently the ligaments most prone to elbow injury.
The remaining two ligaments connect the radius to the ulnar bone, to ensure balance through the entire joint when it is flexed or rotated.
The muscles that power the flexing of the elbow are the biceps, brachialis, and brachioradialis muscles. The bicep is the largest and the most prominent muscle in the flexing action. Each bicep is connected to the skeleton by way of a tendon. To permit the elbow to perform the complimentary extension action, the tricep is the responsible muscle. It is also connected to the bones of the elbow by a strong tendon. The muscles of the wrist that enable extension and flexion are also connected to the elbow.
The anatomical partnership that exists between the wrist, hand, and elbow in a multitude of movements results from the tendons that connect the elbow to the various muscles of the wrist and the hand. The lateral epicondyle tendon functions as this connector, joining the elbow on the outside of the forearm immediately below the hinge of the ulnar and humerus bones. The medial epicondyle performs a similar function on the inside of the elbow. Like the UCL and RCL structures, these tendons are required to bear stresses in almost every form of elbow movement, making them vulnerable to injuries such as tendonitis.
To cushion the elbow from external forces, the joint contains a bursa, a sac of fluid that reduces friction and increases force absorption around the bones of the elbow. Of the three sacs of bursa that form a part of the joint, the olecranon bursa is the most important, as it is located above the point of the hinge formed between the ulnar and humerus bones. In circumstances in which steady or significant pressure is placed on this part of the elbow, the olecranon is vulnerable.
The elbow is also a pathway for three important components of the central nervous system, responsible for the control over movement. Each of the radial, ulnar, and median nerves runs from the shoulder to the hand of each arm by way of its own nerve tunnel. The constant bending of the elbow joint places a measure of pressure on these pathways.
The final anatomical component of the elbow joint is the blood vessels that both transport blood to the hand as well as the muscle requirements of the elbow. The brachial artery runs through the elbow to act as the sole supply of blood to the hand and the wrist.
The elbow was designed to carry out a number of functions, most of which are complimentary to the movements of the hand, wrist, and shoulder. As with so many other aspects of human physiology and the adaptations made for sport, there are a number of motions to which the elbow is subjected in athletic competition that present the risk of injury to the structure.
In terms of the physiology of the elbow and its components, both the ulnohumeral and the radio-humeral joints are described as modified hinge joints. The third elbow skeletal component, the superior radioulnar joint, is a pivot joint. The pivoting motion is that which creates supination and pronation of the forearm and hand. The combined presence of the biceps and the triceps muscles permit the elbow a movement that ranges between 135° and 160° in a normal person; women will sometimes have a greater degree of range of motion in the elbow.
The elbow is the subject of significant sport-generated force. Different sports impose widely variable kinds of stresses on the elbow. The physiology of the elbow can be examined in the context of different types of sport movement, subdivided as throwing mechanics, strength sports, and sports in which the elbow absorbs force that is not related to its own motion.
Throwing motion sports include baseball pitching, cricket bowling, American football quarterbacking, rugby, and basketball. In each activity, there is the common factor of a repeated action and stress on the elbow through delivery. Baseball is clearly the most significant sport for elbow injury, especially in the delivery of the curve ball and similar breaking ball pitches, as the pitcher must turn the elbow of the pitching hand with significant force prior to delivery of the pitch. The repetition of this action places significant stress on the UCL. In contrast, the motions of the cricket bowler (consistent with the rules of the game regarding delivery of the ball) limit the twisting forces being applied to the elbow. In American football, rugby, and basketball, the elbow is not required to receive large forces in the making of a pass, or in basketball, the taking of a shot.
Striking motion sports include golf, tennis, ice hockey shooting, and lacrosse. Each of these sports requires repetitive motions in which the elbow absorbs a degree of force to strike or send the ball forward. Golf and tennis strokes place the elbow in differing positions upon impact, whereas hockey and lacrosse, while repetitive, present little variation in the manner in which the elbow is required to move.
Strength sports, such as wrestling, boxing, and weightlifting, each requires the athlete to move very quickly and place the elbow in positions of bearing large forces. In weightlifting, if the athlete overestimates his or her ability to bear a particular weight, the elbow is exposed to a traumatic injury. In boxing matches or wrestling, the elbow will be engaged in striking or grappling movements where forces can be applied unexpectedly.
External force sports such as gymnastics, an athlete can miss a particular element of a routine, causing him or her to lose balance and to put out a hand to stabilize. Such sudden application of force to the arm will radiate to the elbow joint, often resulting in a partial dislocation (subluxation) of the elbow, a full dislocation, or a fracture of one or more of the elbow bones. The contact nature of sports such as American football, hockey, rugby, or Alpine skiing (in which the skier misses a gate or otherwise falls at a high rate of speed on a hard surface) also present a risk of fracture to the elbow on contact no matter what position it may occupy at the point of contact.