Anatomy and Physiology of the Hands, Arms, Feet, and Legs
This domain tests the structure and growth of the nail unit, the bones, muscles, nerves, and vessels of the hands and feet, and your ability to recognize conditions that must be referred rather than serviced.
The nail unit
- Nail plate: keratin, produced by the matrix; the plate itself has no nerve endings.
- Matrix: extends under the proximal nail fold and determines thickness and shape; injury there causes a permanent groove or ridge.
- Lunula: the visible portion of the matrix, the whitish half-moon.
- Cuticle (eponychium): dead-tissue seal at the base; the hyponychium seals under the free edge.
- Also: nail bed (pink from capillaries), sidewalls and nail grooves, free edge, nail mantle, nail root, and the ligament anchoring the plate to bone.
Growth
Fingernails grow about 3.5 mm (1/8 inch) per month and are fully replaced in 4-6 months; toenails grow about 1.5 mm per month and take 9-12 months. Growth is faster in summer, in children, and on the dominant hand, and slower with poor circulation, illness, or malnutrition.
Bones, muscles, vessels
Hand and wrist contain 27 bones: 8 carpals, 5 metacarpals, 14 phalanges. The forearm is radius (thumb side) and ulna. The foot has 26 bones (7 tarsals, 5 metatarsals, 14 phalanges), plus tibia, fibula, and patella. Flexors bend, extensors straighten. Key nerves are the median, ulnar, and radial; key vessels are the radial and ulnar arteries.
Conditions mean referral
Onychocryptosis (ingrown nail), onychomycosis (fungal infection), paronychia (bacterial fold infection), onycholysis, leukonychia, Beau's lines, melanonychia, pterygium, callus, and corn.
Worked example
Green-black discoloration under an acrylic is typically Pseudomonas. Remove the product, do not reapply, and refer the client. Do not call it mold and do not diagnose.
Traps that cost marks
Confusing cuticle with eponychium, saying nails grow from the tip, cutting cuticle, diagnosing fungus, and blaming white spots on calcium deficiency.