Bones of Upper Limb
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Direct answer
Clavicle to fingertip, thirty-two bones build each upper limb: one clavicle, one scapula, one humerus, one radius, one ulna, eight carpals, five metacarpals and fourteen phalanges. The clavicle is the first bone in the body to begin ossifying (fifth week intrauterine, by both membranous and endochondral centres) and the last to finish (around the twenty-fifth year); the humerus is the longest bone of the upper limb and carries its three nerve traps — the axillary nerve at the surgical neck, the radial nerve in the spiral groove, and the ulnar nerve behind the medial epicondyle. The carpals ossify in a predictable circle beginning with the capitate, ending with the pisiform, a sequence that radiologists use to read a child's bone age.
What you must remember
- Clavicle rules: first to ossify (two primary centres, fifth to sixth week), the only long bone ossified partly in membrane, the only long bone lying horizontally, and the commonest fractured bone in the body — usually at the junction of its middle and lateral thirds.
- Humerus nerve map: axillary nerve with the posterior circumflex humeral artery at the surgical neck (through the quadrangular space); radial nerve with the profunda brachii in the spiral groove; ulnar nerve behind the medial epicondyle; anterior dislocation and each fracture site have their matching palsy.
- Head of humerus supply: ascending branch of the anterior circumflex humeral artery (the arcuate artery) — its disruption in proximal fractures causes avascular necrosis of the head.
- Radius and ulna: the radius rotates (its head disc-shaped in the annular ligament, its styloid longer than the ulnar's by a centimetre — shortened in Colles fracture); the ulna is the stable hinge bone with the olecranon, coronoid and trochlear notch.
- Carpals: eight in two rows — scaphoid (largest of the proximal row, fractured), lunate (most dislocated), triquetral, pisiform (a sesamoid in flexor carpi ulnaris) then trapezium (thumb's saddle partner), trapezoid, capitate (largest carpal, first to ossify), hamate (with its hook).
- Ossification order to quote: capitate, hamate, triquetral, lunate, scaphoid, trapezium, trapezoid, pisiform (last, around 8–12 years) — bone age arithmetic on a wrist radiograph.
- Metacarpals and phalanges: five and fourteen respectively; the thumb has two phalanges; the first metacarpal is the shortest and stoutest, and its saddle joint permits opposition.
Reading a bone age film
A child referred for short stature has a left-hand radiograph, and the carpals count out the years. The capitate and hamate appear within the first six months, the triquetral around three years, lunate around four, scaphoid, trapezium and trapezoid between five and seven, and the pisiform — the only carpal with a single ossification centre arising in a tendon — last, near ten. The skeletal age read off this sequence is compared with chronological age in growth clinics, from thyroid dysgenesis to growth hormone deficiency. The same geography explains a scaphoid fracture's danger: its retrograde supply from distal to proximal underlies the dictum that a snuffbox-tender wrist is fractured until proved otherwise.
How the exam frames it
Viva examiners personalise three questions on these bones. "Which is the commonest fractured bone of the body, and at what site?" — the clavicle, at the middle-lateral third junction, from the pull of sternocleidomastoid elevating the medial fragment. "Why does a midshaft humeral fracture cause wrist drop?" — the radial nerve lies in the spiral groove in direct contact with the humerus, so the fragments injure it, paralysing the extensors of the wrist and fingers (triceps is usually spared, its branches leaving proximal to the groove). "Why is the ulnar nerve injured so easily at the elbow?" — it lies in the bony groove behind the medial epicondyle with only skin and fascia over it. Candidates who add that the clavicle subcutaneous course makes it the injection-free, fracture-visible bone — and that its fracture overlaps because sternocleidomastoid pulls the medial fragment up — close the topic with distinction.
Frequently asked questions
How many bones form one upper limb, and what is the breakdown?
Thirty-two: clavicle, scapula, humerus, radius, ulna (one each), eight carpals, five metacarpals and fourteen phalanges.
Why is the clavicle unique among long bones?
It is the first bone to begin ossification, the only long bone ossified partly intramembranously, the last to complete ossification, and the only one lying horizontally across the root of the neck.
Which nerves lie in direct contact with the humerus, and where?
The axillary nerve at the surgical neck, the radial nerve in the spiral (radial) groove, and the ulnar nerve behind the medial epicondyle — each injured by its neighbouring fracture.
Which carpal ossifies first and which last?
The capitate ossifies first (within the first six months of life) and the pisiform last (around 8–12 years), the pisiform being a sesamoid developing within flexor carpi ulnaris.
Why does the thumb have only two phalanges?
The pollex is developmentally a two-phalanx digit with a saddle-shaped first carpometacarpal joint, trading an extra phalanx for the rotatory freedom of opposition.