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AN8.1-4 | Features of individual bones (Upper Limb) — Part 2

Radius and Ulna — The Forearm Bones

The radius (lateral = thumb side) and ulna (medial = little finger side) together form the forearm skeleton, connected by the interosseous membrane — a strong fibrous sheet that also transmits compressive forces from the radius to the ulna.

Radius:
• Head (proximal) — disc-shaped, rotates within the annular ligament (at the proximal radioulnar joint) to produce pronation and supination; articulates with the capitulum of the humerus
• Neck — just distal to the head
• Radial tuberosity — medial surface just distal to the neck; attachment of biceps brachii (the main supinator of the forearm when elbow is flexed)
• Shaft — lateral convexity; attachment of pronator teres at its mid-point (lateral surface); anterior surface has the anterior interosseous artery running along it
• Lower end — wider than upper; has the carpal articular surface (for scaphoid and lunate), ulnar notch (for the head of ulna), dorsal tubercle (Lister's tubercle) — a dorsal ridge that the tendon of extensor pollicis longus winds around; and the styloid process projecting inferolaterally

Ulna:
• Olecranon — the large proximal projection forming the point of the elbow; attachment of triceps brachii (posterior) and anconeus; the bursa overlying it can become inflamed (olecranon bursitis — "student's elbow")
• Trochlear (semilunar) notch — the C-shaped articular surface that articulates with the trochlea of the humerus; this is the primary hinge joint of the elbow
• Coronoid process — anterior projection of the upper ulna; attachment of brachialis (main elbow flexor, attaches here unlike biceps which attaches to the radius)
• Radial notch — lateral surface just distal to the coronoid; for the head of the radius (proximal radioulnar joint)
• Shaft — subcutaneous medial border throughout its length (easy to palpate); attachment of many forearm muscles
• Head of ulna — distal end (opposite to radius — remember: head of radius is proximal, head of ulna is distal); articulates with the ulnar notch of the radius
• Styloid process — at the very tip of the head of ulna (posteromedial)

How to determine side (radius): Lateral bone, head is at proximal end and is disc-shaped; styloid process is at distal end and is longer than the ulna styloid. The dorsal tubercle (Lister's) is on the dorsal surface at the lower end. Hold with the styloid pointing downwards — the dorsal tubercle should be on the back, the head should be at the top.

Carpal Bones, Metacarpals, and Phalanges

The Carpal Bones (AN8.3)

The wrist contains 8 carpal bones arranged in two rows of four, from lateral (thumb side) to medial:

Proximal row (articulates with the radius and articular disc of the inferior radioulnar joint):
1. Scaphoid (navicular-shaped; boat-shaped) — largest; has a tubercle on its palmar surface; covered mostly by articular cartilage, limiting its blood supply
2. Lunate (moon-shaped) — most commonly dislocated carpal bone (anteriorly, into the carpal tunnel, compressing the median nerve)
3. Triquetrum (triangular)
4. Pisiform — a sesamoid bone within the tendon of flexor carpi ulnaris; it is the only carpal bone with a single articular surface (with the triquetrum); it is small, pea-shaped, and palpable on the medial aspect of the wrist

Distal row (articulates with the metacarpals):
5. Trapezium — articulates with the 1st metacarpal (the saddle joint of the thumb); has the ridge of the trapezium on its palmar surface (attachment point for the flexor retinaculum)
6. Trapezoid — smallest bone of the distal row
7. Capitate — largest carpal bone; central position; the "keystone" of the wrist
8. Hamate — has a hook (hamulus) on its palmar surface (palpable in Guyon's canal); the flexor retinaculum attaches to it

Mnemonic (lateral to medial, proximal then distal):
"Some Lovers Try Positions That They Cannot Handle"
= Scaphoid, Lunate, Triquetrum, Pisiform | Trapezium, Trapezoid, Capitate, Hamate

Peculiarities of the pisiform:
• Only carpal bone that is a sesamoid (develops in a tendon — FCU)
• Only one articular facet (on its dorsal surface, for the triquetrum)
• Gives attachment to: flexor retinaculum, abductor digiti minimi, flexor carpi ulnaris
• Acts as a lever arm to improve the mechanical advantage of the FCU tendon

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Metacarpals and Phalanges:

Each hand has 5 metacarpals (numbered 1–5, thumb to little finger). Each metacarpal has a:
• Base (proximal, articulates with the distal carpal row at carpometacarpal joints)
• Shaft (body — convex dorsally)
• Head (distal, rounded; articulates with the proximal phalanx at the metacarpophalangeal = MCPJ)

The 1st metacarpal (thumb) is shortest and broadest; its carpometacarpal joint is a saddle joint (the most mobile joint in the hand — allows opposition of the thumb).

There are 14 phalanges in each hand: the thumb has 2 (proximal and distal), each finger has 3 (proximal, middle, and distal). Each phalanx has a base, body, and head (except the distal phalanx, which has a base, body, and ungual tuberosity distally — the roughened area under the fingernail).