Humerus, Radius and Ulna: The Bones That Reach
Hold your arm out and turn your palm from up to down. Nothing about that movement looks remarkable — you do it a hundred times a day, unscrewing a jar, turning a key, offering a hand. Yet inside your forearm one bone has just rolled bodily over another, pivoting on a ring of ligament at the elbow and swivelling around a notch at the wrist. Above it, a single long bone carries the largest nerves of the limb pressed hard against its own surface, so intimately that where the bone breaks tells a surgeon exactly which movement the patient will lose. Three bones, between shoulder and wrist, that turn a heavy limb into an instrument of extraordinary reach.
A young man comes off a motorbike and lands hard on the outside of his upper arm. The X-ray shows a clean break through the middle of the humerus — and when he is asked to lift his wrist, it hangs limp, dangling like a broken hinge. Down the corridor, a grandmother has tripped on a kerb and put out her hand to save herself; her wrist has swelled into the unmistakable curve of a dinner fork, the fractured end of her radius tipped backwards. In a third cubicle a five-year-old has fallen from the monkey bars onto a straight arm and cracked his humerus just above the elbow — and the doctors are not looking at the bone at all, but at the pulse in his wrist and the colour of his fingertips. Three falls, three bones, three different disasters. To understand any of them you have to know these bones not as shapes to memorize, but as landscapes over which nerves and arteries are draped.
The humerus from above: a ball, two tubercles and a groove
The longest bone of the upper limb begins as a smooth hemisphere and ends as a complex hinge. The proximal humerus presents a rounded head that faces medially, superiorly and slightly backwards to meet the shallow glenoid cavity of the scapula, described alongside the rest of the shoulder girdle. Immediately around the rim of the articular surface runs the anatomical neck, a narrow groove marking the old growth plate and the line of capsular attachment. Just distal to the head sit two bony prominences: the greater tubercle laterally, carrying three facets for the tendons of supraspinatus, infraspinatus and teres minor; and the lesser tubercle anteriorly, receiving subscapularis. Between them runs the intertubercular (bicipital) groove, a deep vertical gutter that carries the tendon of the long head of biceps brachii from its origin at the supraglenoid tubercle down into the arm, held in place by the transverse humeral ligament. Below the tubercles the bone tapers into the surgical neck — so named because it is where the bone actually breaks, unlike its anatomical namesake.
The shaft: a spiral track for the radial nerve
The humeral shaft is rounded above and triangular below. On its lateral surface, roughly halfway down, is the deltoid tuberosity — a roughened V-shaped ridge where the deltoid inserts and pulls the arm into abduction. Sweeping across the posterior surface, running downwards and laterally like a spiral staircase, is the radial (spiral) groove, which carries the radial nerve and the profunda brachii artery directly against naked bone. Further down, the shaft flares into the medial and lateral supracondylar ridges, sharp crests that give attachment to the intermuscular septa dividing the arm into anterior and posterior compartments — the flexor and extensor rooms explored in the muscles of the arm. On the medial ridge, roughly 5 cm above the elbow, a supracondylar process is occasionally present in about 1% of people, a small hook from which a fibrous band may arch to the medial epicondyle and trap the median nerve.
The distal humerus: capitulum, trochlea and three fossae
Two articular surfaces, shaped for two completely different jobs. Laterally the humerus ends in the capitulum, a smooth rounded knuckle that receives the dished head of the radius and allows it to both hinge and spin. Medially it ends in the trochlea, a grooved pulley shaped like a cotton reel, gripped by the trochlear notch of the ulna to form a true hinge. Flanking these are the two epicondyles: the small lateral epicondyle, origin of the common extensor tendon (and the site of pain in tennis elbow), and the much larger, more prominent medial epicondyle, origin of the common flexor tendon and — critically — the bony wall behind which the ulnar nerve runs in its own shallow groove. Three depressions accept the forearm bones in extreme positions: anteriorly the radial fossa above the capitulum and the coronoid fossa above the trochlea receive the radial head and the ulnar coronoid process in full flexion, while posteriorly the deep olecranon fossa swallows the olecranon in full extension. How these surfaces work together is the story of the elbow and radioulnar joints.
The nerve–fracture map: where it breaks, what it loses
No other bone in the body reports its injuries so precisely. Four levels, four nerves — this is the clinical spine of the humerus. A fracture at the surgical neck endangers the axillary nerve (C5–C6), which winds round the bone there with the posterior circumflex humeral artery; the patient cannot abduct the arm beyond the first few degrees because deltoid is paralysed, and loses sensation over a coin-sized patch on the lateral shoulder — the "regimental badge" area. A fracture of the mid-shaft, running through the radial groove, injures the radial nerve (C5–T1); the extensors of the wrist and fingers fall silent and the hand hangs in the classic wrist drop, with numbness over the first dorsal web space. A fracture or dislocation at the medial epicondyle catches the ulnar nerve (C8–T1) in its groove — the same nerve you assault when you knock your "funny bone" — producing weak finger abduction and numbness of the little finger. And a supracondylar fracture, the childhood injury from a fall onto an outstretched hand, drives the sharp proximal fragment forwards into the median nerve and the brachial artery, threatening both the hand's most important motor nerve and its blood supply. These nerves are followed in full in the radial, axillary and musculocutaneous nerves and the median and ulnar nerves.
Think of the humerus as a cable duct running down the side of a building. The cables — axillary, radial, median, ulnar — are not floating loosely in the middle of the shaft; they are clipped to its surface, pressed into named grooves and hooked around its corners. That is why breaking the duct at a given floor cuts a predictable set of services. An electrician reading a fault report can name the floor from the lights that went out; a clinician seeing wrist drop can name the level of the fracture before the X-ray arrives.
The radius: the bone that turns
Narrow at the elbow, broad at the wrist — the reverse of its partner. The radius lies on the lateral (thumb) side. Proximally it has a disc-shaped head with a shallow concave upper surface for the capitulum and a smooth circumference held inside the annular ligament, so that it can spin like a wheel in a collar. Below the head is the narrow neck, and just distal and medial to that is the radial tuberosity, into which the tendon of biceps brachii inserts — which is why biceps is not only a flexor but the body's most powerful supinator, unwinding the radius like a hand on a screwdriver. The shaft is gently curved and bears a sharp interosseous border facing the ulna, and laterally the roughened pronator tuberosity for pronator teres. Distally the radius expands: on its dorsal surface stands the dorsal (Lister's) tubercle, a small ridge you can feel through the skin that acts as a pulley for the tendon of extensor pollicis longus; medially there is the ulnar notch for the head of the ulna; laterally the pointed styloid process, which normally descends about 1 cm lower than the ulnar styloid. Its broad concave inferior surface is the true wrist joint's contribution from the forearm — it, and not the ulna, carries the bulk of the load coming up from the hand and the carpal bones.
The ulna: the stable hinge partner
The ulna is the mirror image in proportion: broad at the elbow, slender at the wrist. Its proximal end is dominated by two projections that grip the humeral trochlea like a wrench. The olecranon points upwards and backwards, forming the point of the elbow you lean on and receiving the tendon of triceps; the coronoid process points forwards from below. Between them lies the C-shaped trochlear notch, the deep articular jaw that makes the elbow a pure hinge and gives it remarkable stability. On the lateral side of the coronoid is the radial notch, which holds the circumference of the radial head; below the coronoid is the ulnar tuberosity for brachialis, the workhorse flexor of the elbow. The shaft has its own sharp interosseous border, and tapers distally to a small rounded head with an articular circumference, from whose posteromedial aspect projects the short ulnar styloid process. Crucially, the ulnar head does not touch the carpus directly — an articular disc of fibrocartilage separates them — which is why the ulna barely feels a fall while the radius takes the punishment.
The interosseous membrane: two bones acting as one
Stretched between the interosseous borders of radius and ulna is a strong sheet of fibrous tissue whose collagen fibres run obliquely downwards and medially — from the radius above to the ulna below. That direction is not decorative. When you land on your hand, force travels up through the carpus into the distal radius; the obliquely angled fibres catch that force and shunt part of it across to the ulna, and from the ulna it passes through the elbow into the humerus and on to the shoulder. Without the membrane, the radius alone would have to carry every impact into the arm. It also doubles as a broad attachment for the deep muscles of the forearm and, with the radioulnar joints at each end, binds the two bones into a single functional unit that can still rotate. Together, they are a textbook illustration of the principle described in the structure of living bone: shape follows the forces the tissue must survive.
Here is the elegance of pronation and supination: the ulna barely moves. The radius does all the work — its head spinning inside the annular ligament at the elbow while its distal end swings bodily around the head of the ulna at the wrist, carrying the whole hand with it. In supination the two bones lie parallel; in pronation the radius crosses over the ulna like a pair of scissors half-closed. That is why you can hold a full cup of tea and still turn your palm without moving your elbow — and why a badly healed forearm fracture that shortens or angulates the radius can silently steal 40 degrees of rotation, a loss the patient notices every time they turn a doorknob.
The single commonest mechanism in upper-limb injury has its own abbreviation: FOOSH. The energy enters at the palm and races up the forearm, and where it breaks depends largely on age. In an older adult with thinned bone, the distal radius gives way and the fragment tips backwards — a Colles' fracture, whose swollen, upward-angled profile is the famous "dinner-fork" deformity; if the fragment tips forwards instead, it is a Smith's fracture, the reverse. In a young adult the same fall more often snaps the scaphoid in the wrist, or dislocates the elbow. In a child, the force runs all the way to the supracondylar region of the humerus. And in every one of them the examination is the same discipline: feel the olecranon and the two epicondyles — in a normal extended elbow they sit in a straight line, and in flexion they form an equilateral triangle — then check the radial pulse and test the nerves before anyone touches the bone.
- Proximal humerus: head, anatomical neck (old growth plate), greater and lesser tubercles, intertubercular (bicipital) groove for the long head of biceps, then the surgical neck.
- Shaft: deltoid tuberosity laterally, radial (spiral) groove posteriorly for the radial nerve and profunda brachii artery, medial and lateral supracondylar ridges distally.
- Distal humerus: capitulum (for the radial head) laterally, trochlea (for the ulna) medially, lateral and medial epicondyles, plus radial, coronoid and olecranon fossae.
- Radius: head, neck, radial tuberosity (biceps), interosseous border, dorsal (Lister's) tubercle, ulnar notch, styloid process — broad distally, load-bearing at the wrist.
- Ulna: olecranon, coronoid process, trochlear notch, radial notch, ulnar tuberosity (brachialis), head and styloid — broad proximally, the stable hinge at the elbow.
- Surgical neck fracture → axillary nerve (C5–C6): deltoid paralysis, loss of abduction, "regimental badge" sensory loss.
- Mid-shaft / radial groove fracture → radial nerve: wrist drop and numbness of the first dorsal web space.
- Medial epicondyle injury → ulnar nerve: the "funny bone" nerve, weak finger abduction, little-finger numbness.
- Supracondylar fracture (children) → median nerve and brachial artery: check the pulse and hand perfusion first.
- Pronation/supination = the radius rotating about a nearly fixed ulna; biceps (radial tuberosity) is the strongest supinator.
- The interosseous membrane runs obliquely from radius down to ulna, transferring hand-to-humerus load and anchoring deep forearm muscles.
- Confusing the anatomical neck with the surgical neck. The anatomical neck is the groove around the head; the surgical neck is the narrowing below the tubercles — and it is the one that fractures and threatens the axillary nerve.
- Pairing the wrong bones at the elbow. The capitulum (lateral, rounded) articulates with the RADIUS; the trochlea (medial, grooved) articulates with the ULNA — never the reverse.
- Assuming the ulna bears the wrist's load. It does not reach the carpus at all — an articular disc intervenes, and the radius takes the force, which is why FOOSH injuries break the distal radius.
A man fractures the mid-shaft of his humerus. On examination he cannot extend his wrist or fingers and the hand hangs limply. Which nerve has been injured?
- The humerus runs from a rounded head and two tubercles (with the bicipital groove between them), down a shaft marked by the deltoid tuberosity and the radial groove, to a distal end of capitulum, trochlea, two epicondyles and three fossae.
- The nerve–fracture map is the clinical core: surgical neck → axillary; mid-shaft → radial (wrist drop); medial epicondyle → ulnar; supracondylar → median nerve and brachial artery.
- The radius is narrow above and broad below: it rotates around the ulna to give pronation and supination, and it — not the ulna — bears the load at the wrist, so it fractures in Colles' and Smith's injuries.
- The ulna is broad above and narrow below: its olecranon, coronoid process and trochlear notch make the elbow a stable hinge, while the interosseous membrane binds both bones and carries force from hand to humerus.
- Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students — Upper limb: bones of the arm and forearm.
- Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy — Humerus, radius and ulna; fractures and associated nerve injuries.
- Netter FH. Atlas of Human Anatomy — Humerus and scapula; radius and ulna, anterior and posterior views.
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- Last RJ. Anatomy: Regional and Applied — The arm and forearm skeleton.
- TeachMeAnatomy — The Humerus; The Ulna and Radius.
