The Elbow Joint - Bones
What is the elbow?
The elbow is the joint connecting the upper arm to the forearm. It is classed as a hinge-type synovial joint.
It consists of two main articulations:
Trochlear notch of the ulna and the trochlea of the humerus
Head of the radius and the capitulum of the humerus
The proximal radioulnar joint is found within same joint capsule of the elbow, but most resources consider it as a separate articulation.
Like all synovial joints, the elbow joint has a capsule enclosing the joint. This in itself is strong and fibrous, strengthening the joint. The joint capsule is thickened medially and laterally to form collateral ligaments, which stabilise the flexing and extending motion of the arm.
What is the humerus?
The humerus is a long bone of the upper limb, which extends from the shoulder to the elbow.
Distally, at the elbow joint, the humerus articulates with the head of the radius and trochlear notch of the ulna.
The lateral and medial borders of the distal humerus form medial and lateral supraepicondylar ridges. The lateral supraepicondylar ridge is more roughened, providing the site of common origin of the forearm extensor muscles.
Immediately distal to the supraepicondylar ridges are extracapsular projections of bone, the lateral and medial epicondyles. Both can be palpated at the elbow. The medial is the larger of the two and extends more distally. The ulnar nerve passes in a groove on the posterior aspect of the medial epicondyle where it is palpable.
Distally, the trochlea is located medially, and extends onto the posterior aspect of the bone. Lateral to the trochlea is the capitulum, which articulates with the radius.
Also located on the distal portion of the humerus are three depressions, known as the coronoid, radial and olecranon fossae. They accommodate the forearm bones during flexion or extension at the elbow.
(For the purpose of this article about the elbow, we will not be discussing the anatomy of the proximal humerus or shaft, as it will be covered in the article about the shoulder)
What is the radius?
The radius is a long bone in the forearm. It lies laterally and parallel to ulna, the second of the forearm bones.
The radius pivots around the ulna to produce movement at the proximal and distal radio-ulnar joints.
The proximal end of the radius articulates in both the elbow and proximal radioulnar joints. Important bony landmarks include the head, neck and radial tuberosity:
The head of the radius is a disc shaped structure, with a concave articulating surface. It is thicker medially, where it takes part in the proximal radioulnar joint.
The neck is a narrow area of bone, which lies between the radial head and radial tuberosity.
The radial tuberosity is a bony projection, which serves as the place of attachment of the biceps brachii muscle.
The radial shaft expands in diameter as it moves distally. Much like the ulna, it is triangular in shape, with three borders and three surfaces. In the middle of the lateral surface, there is a small roughening for the attachment of the pronator teres muscle.
What is the ulna?
The ulna is a long bone in the forearm. It lies medially and parallel to the radius, the second of the forearm bones. The ulna acts as the stabilising bone, with the radius pivoting to produce movement.
Proximally, the ulna articulates with the humerus at the elbow joint.
Important landmarks of the proximal ulna are the olecranon, coronoid process, trochlear notch, radial notch and the tuberosity of ulna:
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A large projection of bone that extends proximally, forming part of the trochlear notch. It can be palpated as the ‘tip’ of the elbow. The triceps brachii muscle attaches to its superior surface.
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This ridge of bone projects outwards anteriorly, forming part of the trochlear notch
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Formed by the olecranon and coronoid process. It is wrench shaped, and articulates with the trochlea of the humerus.
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Located on the lateral surface of the trochlear notch, this area articulates with the head of the radius.
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A roughening immediately distal to the coronoid process. It is where the brachialis muscle attaches.
The ulnar shaft is triangular in shape, with three borders and three surfaces. As it moves distally, it decreases in width. The three surfaces are as follows:
Anterior – site of attachment for the pronator quadratus muscle distally.
Posterior – site of attachment for many muscles.
Medial – unremarkable.
The three borders of the ulnar shaft are as follows:
Posterior – palpable along the entire length of the forearm posteriorly
Interosseous – site of attachment for the interosseous membrane, which spans the distance between the two forearm bones.
Anterior – unremarkable.
(For the purpose of this article about the elbow, we will not be discussing the anatomy of the distal radius or ulna, as it will be covered in the article about the wrist & hand)
What is the bursa?
A bursa is a membranous sac filled with synovial fluid. It acts as a cushion to reduce friction between the moving parts of a joint, limiting degenerative damage.
There are many bursae in the elbow, but only a few have clinical importance:
Intratendinous – located within the tendon of the triceps brachii.
Subtendinous – between the olecranon and the tendon of the triceps brachii, reducing friction between the two structures during extension and flexion of the arm.
Subcutaneous (olecranon) bursa – between the olecranon and the overlying connective tissue (implicated in olecranon bursitis).
The bicipitoradial bursa surrounds the biceps tendon in supination. In pronation, the radial tuberosity rotates posteriorly, which compresses the bicipitoradial bursa between the biceps tendon and the radial cortex, which consequently increases the pressure within the bursa.
