AN8.1-4 | Features of individual bones (Upper Limb) — Summary & Reflection

REFLECT

Return to the opening scenario: the cricket player with a normal X-ray and tenderness in the anatomical snuffbox.

Now answer:
1. What is the depression at the base of the thumb called when the thumb is extended? (Name the anatomical structure and its three boundaries)
2. Which bone lies in the floor of this depression?
3. Why was the X-ray normal despite a fracture?
4. What specific vessel supplies the proximal pole of this bone, and which direction does it enter?
5. If the fracture is at the waist, why will the proximal fragment likely die?

Write your answers before reading the discussion.

Discussion: The depression is the anatomical snuffbox, bounded by the EPL (medially) and APL + EPB (laterally), with the scaphoid (and trapezium + radial artery) in its floor. The X-ray is normal because the fracture line at the scaphoid waist is thin and not yet displaced enough to be visible on plain films in the first two weeks. The blood supply enters the scaphoid distally (through the dorsum, from the radial artery), so a fracture at the waist cuts off all blood to the proximal pole → avascular necrosis.

KEY TAKEAWAYS

Clavicle:
• Most commonly fractured bone; fracture at medial 2/3 — lateral 1/3 junction
• Inferior surface: subclavian groove (medial), conoid tubercle + trapezoid ridge (lateral)
• Sternocleidomastoid pulls medial fragment up in fractures

Scapula:
• Spine divides posterior surface into supraspinous and infraspinous fossae
• Costal (subscapular) fossa → subscapularis | Glenoid = shoulder joint socket
• Coracoid: short head biceps + coracobrachialis + pectoralis minor

Humerus:
• Surgical neck = proximal humeral fractures (axillary nerve at risk)
• Spiral groove = radial nerve + profunda brachii → mid-shaft fracture = wrist drop
• Medial epicondyle groove = ulnar nerve → medial epicondyle fracture = ulnar nerve palsy
• Greater tubercle: supraspinatus (sup), infraspinatus (mid), teres minor (inf) = rotator cuff

Radius: Head is disc-shaped (proximal); radial tuberosity = biceps attachment; Lister's tubercle dorsal at distal end; longer styloid than ulna
Ulna: Olecranon = triceps | Coronoid = brachialis | Head is distal (opposite of radius)

Carpal bones: "Some Lovers Try Positions That They Cannot Handle"
• Scaphoid: fractured at waist → avascular necrosis of proximal pole (blood supply enters distally)
• Pisiform: only sesamoid carpal bone; single facet (triquetrum); lever for FCU
• Lunate: most commonly dislocated (into carpal tunnel → acute median nerve compression)

Hand:
• 8 carpals + 5 metacarpals + 14 phalanges (2 in thumb, 3 in each finger)