Hand Anatomy and Biomechanics

Hand Anatomy & Biomechanics - Skeleton Scaffolding

• Carpal Bones (8): Two rows. 📌 So Long To Pinky, Here Comes The Thumb.

  Row	Bones (Proximal First)	Key Features
  Proximal	Scaphoid, Lunate, Triquetrum, Pisiform	Scaphoid: Fx, AVN. Lunate: Kienböck's. Pisiform: Sesamoid.
  Distal	Trapezium, Trapezoid, Capitate, Hamate	Trapezium: CMC OA. Capitate: Largest. Hamate: Hook fx, ulnar n.

• Metacarpals (5), Phalanges (14): Palm & digits. Thumb: 2 phalanges; Fingers: 3.
• Key Joints: Radiocarpal, Midcarpal, CMC, MCP, IP (PIP, DIP).

⭐ Scaphoid: most common carpal fracture; proximal pole AVN risk (retrograde blood supply).

Hand Anatomy & Biomechanics - Movers & Shakers

• Flexor Tendons:
  • FDS (PIP flexion), FDP (DIP flexion).
  • Verdan's zones 1-5 for injury classification.
  • Pulleys (Annular A1-5, Cruciform C1-3) prevent bowstringing.
• Extensor Tendons:
  • Organized into 6 dorsal compartments at the wrist.
  • Extensor mechanism (hood): complex structure with central slip (PIP extension) & lateral bands (DIP extension).
• Intrinsic Muscles: For fine motor control & grip strength.
  • 📌 Lumbrical action: 'Lumbricals flex MCP, extend IP' joints (form an 'L' shape). | Muscle Group | Key Muscles | Innervation | Main Action(s) | |--------------|----------------------|-----------------------------|-----------------------------------------| | Thenar | APB, OP, FPB (part) | Median N. | Thumb opposition, abduction, flexion | | Hypothenar | ADM, ODM, FDM | Ulnar N. | Little finger abduction, opposition | | Lumbricals | L1-4 | Median (L1,2), Ulnar (L3,4) | Flex MCPs, Extend IPs | | Interossei | Palmar (PAD), Dorsal (DAB) | Ulnar N. | Adduct (PAD), Abduct (DAB) fingers |

⭐ The FDP tendons to the ring and little fingers often share a common muscle belly, which can limit independent flexion of these digits (Quadriga effect).

Hand Anatomy & Biomechanics - Wiring Wonders

• Hand's intricate function relies on Median, Ulnar, & Radial nerve integrity.
• Key Nerve Injuries Summary:

  Nerve	Motor Loss	Sensory Loss	Deformity(ies)
  Median	Thenar, Lumbricals 1&2. 📌 LOAF*	Palmar thumb, index, middle	Ape/Benediction hand
  Ulnar	Intrinsics, Adductor Pollicis	Ulnar 1.5 fingers	Claw hand, Froment's (+)
  Radial	Wrist/finger/thumb extensors	Dorsal 1st web space	Wrist/Finger drop
  *📌 LOAF: Lumbricals 1&2, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis.

• Clinical Nerve Testing Algorithm:

⭐ Froment's sign indicates ulnar nerve palsy: weakness of Adductor Pollicis causes thumb IP joint flexion (FPL-median nerve) when gripping paper.

Hand Anatomy & Biomechanics - Life Lines

• Arterial supply: Ulnar & Radial arteries.
• Superficial Palmar Arch (SPA):
  • Main: Ulnar artery.
  • Completion: Superficial palmar branch of Radial artery.
  • Branches: Common & proper digital arteries.
• Deep Palmar Arch (DPA):
  • Main: Radial artery.
  • Completion: Deep palmar branch of Ulnar artery.
  • Branches: Palmar metacarpal arteries.

⭐ Allen's test is used to assess the patency of the radial and ulnar arteries supplying the hand.

Hand Anatomy & Biomechanics - Grip & Go

• Functional position: Wrist 20-30° extension, MCPs 45-50° flexion, IPs slight flexion, thumb opposition.
• Hand Grips:

  Type	Key Features / Examples
  Power	Full hand, static; Cylindrical (hammer), Spherical (ball), Hook (bag)
  Precision	Fine motor, dynamic; Pinch (key), Tripod (pen), Tip-to-tip (needle)

⭐ The cascade sign, where fingers point to the scaphoid tubercle upon flexion, indicates normal rotational alignment of metacarpals/phalanges.

High‑Yield Points - ⚡ Biggest Takeaways

• Median nerve is crucial for thenar muscle function and thumb opposition.
• Scaphoid is the most fractured carpal bone, prone to avascular necrosis (AVN).
• Flexor Digitorum Profundus (FDP) flexes DIP joints; Flexor Digitorum Superficialis (FDS) flexes PIP joints.
• Ulnar nerve injury leads to claw hand deformity and a positive Froment's sign.
• Radial nerve injury typically causes wrist drop due to loss of extensor muscle function.
• Intrinsic hand muscles (lumbricals, interossei) are essential for fine motor skills and grip strength.