BASIC ELBOW BIOMECHANICS
Biomechanics is the term used to describe movement of the body. This section is a review of basic elbow biomechanics. In order to understand the biomechanics of the elbow it is important to understand its anatomy. Please read the section on basic elbow anatomy before reading this section. It discusses the bones, ligaments, muscles and other structures that make up the elbow.
The elbow joint is actually three separate joints; the ulnohumeral joint, the radiohumeral joint and the superior radioulnar joint. All three joints are enclosed by a single joint capsule. Movement between the ulna and the humerus occurs at the ulnohumeral joint. Movement between the radius and the humerus occurs at the radiohumeral joint and movement between the radius and the ulna occurs at the superior radioulnar joint.
The ulnohumeral and radiohumeral joints are modified hinge joints. The biceps, brachialis, and brachioradialis muscles bend (flex) these two joints. The triceps muscles on the back of the arm straighten (extend) these two joints. Normal elbow flexion varies between 135 degrees to 155 degrees. The superior radioulnar joint is a pivot joint. This joint allows supination and pronation of the forearm and wrist to occur. Supination is rotation of the forearm so that the palm is turned up. Pronation is rotation of the forearm so that the palm is turned down. The biceps and supinator muscles supinate the elbow. The pronator quadratus, pronator teres and flexor carpi radialis muscles pronate the elbow.
When the elbow is fully extended and supinated, the forearm is angled slightly away from the long axis of the humerus. This angle is called the "carrying angle". In men this angle ranges between 10 to 15 degrees and in women this angle ranges between 15 to 20 degrees. Muscle weakness or ligament injury can lead to abnormal biomechanics of the elbow that can result in abnormal forces in the elbow. Over time these abnormal forces can cause the articular cartilage of the elbow to wear out prematurely. Finally, injury to the bones, ligaments and other support structures of the elbow can occur when excessive extension, flexion, rotational or side to side forces are applied to the elbow.
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