Orthopaedic Anatomy Reference
A guide to the principal structures of the elbow joint and the mechanical functions they perform.
Fig. 1 — Principal structures of the human elbow joint
Key Structures
01
The single bone of the upper arm, the humerus articulates with the shoulder above and the radius and ulna below. Its distal end broadens into the condyle region, bearing the trochlea and capitulum that form the joint surfaces of the elbow.
02
The shorter of the two forearm bones, the radius runs along the thumb side of the forearm. Its flattened, disc-shaped head articulates with the capitulum of the humerus and rotates against the ulna, enabling the forearm's pronation and supination movements.
03
The longer and more stable forearm bone, the ulna forms the primary hinge of the elbow. Its deep trochlear notch wraps around the humerus's trochlea and, together with the olecranon and coronoid process, provides the core mechanical stability of the joint.
04
The bony prominence on the inner side of the humerus, easily felt as the "funny bone." It serves as the attachment point for the flexor muscles of the forearm and wrist, and for the ulnar collateral ligament. The ulnar nerve passes directly behind it, which is why a knock there causes a distinctive electric sensation.
05
The bony prominence on the outer side of the humerus and the origin of the extensor muscles of the forearm and wrist. Overuse of these muscles — in racket sports, for example — causes inflammation at this attachment point, the condition widely known as tennis elbow (lateral epicondylitis).
06
The spool-shaped medial articular surface of the humerus, the trochlea articulates with the trochlear notch of the ulna to form the primary hinge joint of the elbow. Its pulley-like shape guides the path of flexion and extension, constraining movement to a single plane.
07
The smooth, spherical lateral articular surface of the humerus, the capitulum articulates with the concave superior surface of the radial head. This joint allows both the flexion-extension of the elbow and the rotational movements of the forearm (pronation and supination).
08
The disc-shaped top of the radius, the radial head articulates with the capitulum above and the radial notch of the ulna on its side. It is held in place by the annular ligament and is the pivot point for forearm rotation. Radial head fractures are among the most common elbow injuries in adults.
09
The prominent bony tip of the ulna that forms the point of the elbow. It fits into the olecranon fossa of the humerus when the arm is straightened, acting as a bony stop to prevent hyperextension. The triceps tendon inserts directly onto its posterior surface.
10
The primary stabiliser of the medial elbow, the UCL runs from the medial epicondyle to the coronoid process and olecranon of the ulna. It resists valgus forces — those that push the forearm outward — and is the ligament most commonly torn in overhead-throwing athletes.
11
Running from the lateral epicondyle to the annular ligament and ulna, the RCL resists varus forces that push the forearm inward. It contributes to lateral elbow stability and blends with the lateral ulnar collateral ligament, which specifically resists rotational instability of the elbow.
12
A strong ring-like ligament that encircles the radial head and attaches to the ulna on both sides, holding the radial head snugly against the ulna while still allowing it to spin freely. In young children, a sudden pull on the arm can dislocate the radial head through this ring — a condition called "nursemaid's elbow."
13
The distal biceps tendon inserts onto the radial tuberosity just below the radial head. When the biceps contracts, it flexes the elbow and powerfully supinates the forearm (rotates the palm upward). Rupture of this tendon, often during a sudden eccentric load, causes the classic "Popeye" deformity as the muscle retracts up the arm.
14
The triceps tendon inserts onto the posterior surface of the olecranon, making it the sole extensor of the elbow. Contraction of the triceps straightens the arm against resistance — essential for pushing movements. It is the largest tendon crossing the elbow and the least commonly ruptured.
15
Smooth hyaline cartilage covers the articulating surfaces of the humerus, radius, and ulna within the elbow joint. It reduces friction to near zero, distributes compressive loads, and allows the joint to move through its full arc without wear on the underlying bone. Loss of cartilage leads to elbow osteoarthritis.
16
A thin fluid-filled sac located directly over the olecranon, between the skin and the bone. It allows the skin to glide freely over the bony point during elbow movement. Repeated pressure or a single direct blow can cause bursitis — a painful, sometimes dramatically swollen lump at the tip of the elbow.