The elbow joint is a compound synovial joint, which means that it is a large working structure that is made up of several smaller moving parts, or separate articulations. A synovial joint, otherwise known as a diarthrosis, is the most flexible type of joint, seeing as it achieves its range of movement at the point of contact between the articulating bones. This mechanical area forms the meeting point between the radius and ulna of the forearm with the humerus of the brachial region. It is deemed a compound joint because the joint cavity is continuous with the radioulnar joint, as well as the contact points between these bones and the humerus respectively.
The humeroulnar joint is the contact point of the humerus and the ulna. It has a single axis and is therefore a synovial hinge joint that simply opens and closes. The articulating surfaces can be found between the trochlea of the humerus and the trochlear notch of the ulna.
The humeroradial joint is the contact point between the humerus and the radius. It is also a uniaxial synovial hinge joint that articulates at the capitulum of the humerus and the head of the radius.
The radioulnar joint has a proximal and a distal distinction, due to the fact that it is comprised of two separate joints at both the proximal and distal ends of the bones. The proximal part will be discussed in this article alone, since it is the joint that is encapsulated within the synovial tissue of the elbow, whereas the distal radioulnar joint can be found at the most distal point of the antebrachial region of the upper limb and is not part of the compound joint itself. As with the other two joints, the proximal radioulnar joint is also uniaxial, but functions as a pivot joint. Its articulating surfaces are the head of the radius and the radial notch of the ulna.
There are several ligaments which help reinforce the elbow joint. These include the collateral ligaments of the radius and ulna as well as the annular ligament.
The ulnar collateral ligament is a triangular ligament that has an anterior, a posterior and an oblique band. It extends from the medial epicondyle to the coronoid process and the olecranon.
The radial collateral ligament passes between the lateral epicondyle, the annular ligament and the anterior and posterior margins of the radial notch of the ulna.
The annular ligament is shaped like a strong fibrous circle that runs from the margins of the radial notch of the ulna and encircles the head of the radius. Its function is to maintain the position of the radial head during pronation and supination.
Blood Supply and Innervation
The humeroulnar and humeroradial joints have the same blood supply and innervation, because they are in such close proximity to one another. Around the elbow there is an arterial anastomosis between the brachial artery, the radial artery and the ulnar artery and this supplies two of the three parts of the elbow. The musculocutaneous nerve, the radial nerve and the ulnar nerve all contribute their fibers to the innervation of the humeroulnar and humeroradial joints.
The proximal radioulnar joint has its own blood supply and innervation due to the fact that although it sits within the synovial capsule, it is set slightly apart from the previously mentioned joints. Its blood supply arises from the anterior and posterior interosseous arteries. The musculocutaneous nerve, the median nerve and the ulnar nerve innervate this joint.
The humeroulnar and humeroradial joints are responsible for flexion which occurs at more than 180 degrees and extension. The proximal radioulnar joint and the annular ligament allow for pronation and supination with rotation of the head of the radius. The carrying angle is the angle created by the long axis of the humerus and the long axis of the ulna when the upper limb is fully extended and supinated in the anatomical position. The exact degree of this angle varies from ten to fifteen in males and over fifteen in females.
Tendonitis of the elbow is one of the most common forms of elbow pathology and occurs due to overuse of the joint. It is otherwise known as ‘tennis elbow’ or ‘golfer’s elbow’ due to the fact that these sports tend to encourage this type of injury. Inflammation of the tendon can only be treated by giving it a rest from the sport or continual movement that caused the overuse and in some cases anti-inflammatory drugs may be prescribed.