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WRIST AND HAND COMPLEX - Comprehensive Notes (15 Marks Each)

Reference: Norkin & Levangie - Joint Structure and Function (supplemented by Rockwood & Green's Fractures, Imaging Anatomy Vol. 3)

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QUESTION 1: Anatomy of the Wrist and Hand Complex (15 Marks)

A. Bony Anatomy of the Wrist

• Scaphoid - largest bone of proximal row; bridges both rows; acts as a mechanical link
• Lunate - crescent-shaped; sits in center; key intercalated segment
• Triquetrum - pyramidal; medial most
• Pisiform - sesamoid bone; lies anterior to triquetrum; site of flexor carpi ulnaris insertion

• Trapezium - articulates with 1st metacarpal (saddle joint)
• Trapezoid - smallest of distal row
• Capitate - largest carpal bone; occupies central position
• Hamate - has distinctive hook (hamulus) on palmar surface

B. Joints of the Wrist Complex

C. Ligaments of the Wrist

• Palmar radioscaphocapitate - strongest; prevents ulnar translation
• Palmar radiolunate (long and short)
• Palmar ulnocarpal - ulnolunate, ulnotriquetral
• Dorsal radiocarpal - weak; provides secondary constraint
• Dorsal intercarpal ligament (DICL) - runs from triquetrum to trapezoid/scaphoid

• Scapholunate interosseous ligament (SLIL) - most important; prevents carpal dissociation
• Lunotriquetral ligament
• Distal row ligaments (very stiff; allow minimal motion)

• Comprises articular disc, meniscus homologue, ECU subsheath, ulnocarpal ligaments, and dorsal/volar radioulnar ligaments
• Functions: (1) load transmission across ulnar wrist, (2) DRUJ stability, (3) ulnocarpal stability
• Attached radially to sigmoid notch; ulnarly to fovea of ulnar styloid

D. Joints of the Hand

• Condyloid joints (digits 2-5); saddle at thumb
• Allow flexion/extension + abduction/adduction
• Stabilized by collateral ligaments (taut in flexion) and volar plate

• Hinge joints; allow flexion/extension only
• Most stable joint in the hand (bicondylar head + volar plate + proper/accessory collaterals)
• Normal ROM: 0-110 degrees

• Hinge joints
• Normal ROM: 0-80 degrees

• Hinge joint; 0-80 degrees

E. Muscles of the Wrist and Hand

• Flexor carpi radialis (FCR) - flexion + radial deviation
• Flexor carpi ulnaris (FCU) - flexion + ulnar deviation
• Palmaris longus - tenses palmar aponeurosis

• Compartment 1: APL, EPB
• Compartment 2: ECRL, ECRB - primary wrist extensors + radial deviation
• Compartment 3: EPL
• Compartment 4: EDC, EIP
• Compartment 5: EDM
• Compartment 6: ECU - extension + ulnar deviation

• FDS - inserts middle phalanx after splitting at A2 pulley
• FDP - inserts distal phalanx (deep to FDS)
• FPL - flexor of thumb IP joint

• Abductor pollicis brevis (APB)
• Flexor pollicis brevis (FPB)
• Opponens pollicis (OP) - the most important for opposition
• Adductor pollicis (ulnar nerve)

• Abductor digiti minimi
• Flexor digiti minimi
• Opponens digiti minimi

• Flex MCPs while extending IPs

• Abduct fingers from middle finger axis
• Also flex MCPs, extend IPs

• Adduct fingers toward middle finger

F. Neurovascular Supply

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QUESTION 2: Basic Joint Biomechanics of the Wrist and Hand (15 Marks)

A. Principles of Joint Biomechanics

• Radiocarpal + midcarpal together = 2 DOF (flexion-extension; radial-ulnar deviation)
• DRUJ = 1 DOF (pronation-supination)
• MCP joint digits 2-5 = 2 DOF
• PIP/DIP = 1 DOF

• Radiocarpal joint: shallow concave distal radius + convex proximal carpal row = inherently unstable; depends on ligaments
• DRUJ: stability provided by TFCC (primary) + DRUJ capsule + interosseous membrane

• Wrist close-packed: extension with radial deviation (maximal ligament tension)
• Wrist resting position: 0-10° extension, neutral deviation

• Proximal carpal row (convex) moves on fixed distal radius (concave) - roll and slide are in OPPOSITE directions
• E.g., in wrist flexion: proximal row rolls palmarward but glides dorsally

• Load passes from hand through carpus to forearm:
  • ~80% through radiocarpal joint (radius)
  • ~20% through ulnocarpal joint (via TFCC)
• The radial inclination (~23°) and palmar tilt (~11°) of the distal radius are important for normal load distribution

B. Carpal Kinematics - Theories

• Lateral (mobile) column: Scaphoid, trapezium, trapezoid - allows radial deviation
• Central (flexion-extension) column: Lunate, capitate, hamate - primary flexion/extension axis
• Medial (rotational) column: Triquetrum, pisiform - ulnar rotation

• Proximal intercalated segment: Scaphoid, lunate, triquetrum move as an intercalated unit
• Variable geometry: The proximal carpal row changes shape/contour dynamically to allow extreme motion while maintaining stability about the longitudinal axis
• Synchronous angulation: During flexion-extension, each carpal bone angulates in the same direction with nearly equal amplitude (Rockwood & Green, p. 1869)

C. Range of Motion (Norkin)

D. Radiocarpal vs. Midcarpal Contribution to Motion

• In flexion: 62-75% at radioscaphoid joint; 31-50% at radiolunate
• In extension: 87-99% at radioscaphoid; 52-68% at radiolunate

E. Dart-Throwing Motion

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QUESTION 3: Kinematics and Kinetics of the Wrist and Hand (15 Marks)

A. Wrist Kinematics - Radial/Ulnar Deviation

• Proximal carpal row flexes (palmar angulation)
• Capitate extends - reciprocal motion between rows
• Scaphoid flexes (reduces its radiodorsal clearance); tuberosity approaches radial styloid

• Proximal row extends (dorsal angulation)
• Triquetrum pivots, acts as rotational fulcrum
• Scaphoid extends (elongates palmarly)
• Greater range than radial deviation (40° vs 20°) due to radial styloid bony block

B. Kinematics of the Fingers

• Flexion: collateral ligaments tighten (cam effect) - "intrinsic plus" position prevents MCP hyperextension
• Extension: collateral ligaments slacken - abduction/adduction possible only in extension
• Lateral stability greater in flexion

• Lateral band mechanism: central slip (extends PIP) + lateral bands (extend DIP)
• In flexion: lateral bands shift palmarward
• Boutonniere deformity: central slip rupture → lateral bands shift volar → PIP flexion + DIP hyperextension

• Controlled by FDP (flexion) and terminal extensor tendon (extension)
• Mallet finger: terminal tendon rupture → DIP droop

C. Thumb Kinematics

• Flexion/extension: in plane of palm
• Abduction/adduction: perpendicular to palm
• Opposition: composite motion = abduction + flexion + internal rotation (pronation) of 1st metacarpal
  • Mediated by: APB, OP, FPB (median), and Adductor pollicis (ulnar)
• Conjunct rotation occurs at CMC during abduction - thumb pronates automatically

D. Kinetics - Force Transmission

• Compressive forces at radiocarpal joint = 0.8 x body weight during light activities
• Peak grip generates >900 N at wrist
• In neutral: 80% to radius, 20% to ulna
• Ulnar variance affects load distribution: positive ulnar variance increases ulnar load

• FDP generates 3-10x greater force than FDS
• During power grip, finger flexor tendons generate forces up to 300 N each
• Pulleys (A2 and A4 most important) prevent bowstringing - direct force along bone axis

• Pinch force is approximately 1/3 of grip force
• Lateral (key) pinch: highest force; adductor pollicis is primary
• Tip pinch: FPL + FDP of index + APB

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QUESTION 4: Prehensile Function of the Hand (15 Marks)

A. Definition

B. Classification of Prehensile Grips

1. POWER GRIP

• Fingers are flexed around an object; thumb acts as reinforcer
• Object is held between fingers and palm
• Intrinsic muscles stabilize joints; extrinsic muscles generate force
• Used for: hammering, holding a rod, turning a key in a lock

2. PRECISION GRIP

• Object is held between fingertip and thumb pulp
• Object manipulated in fingers, not held in palm
• Requires sensory feedback; intrinsics are critical

C. Muscles Active During Prehension

• Extrinsic flexors (FDS, FDP): generate finger flexion force
• Wrist extensors (ECRL, ECRB): stabilize wrist in extension (~30°) to optimize finger flexor length-tension
• Intrinsics: stabilize MCP joints
• Thumb: adductor pollicis (key grip) or entire thenar group (cylindrical)

• Intrinsic muscles dominate (lumbricals, interossei, thenar)
• FPL for fine thumb flexion
• Fine motor control from median nerve
• Sensory feedback from digital nerves essential

D. Role of the Wrist Position in Grip

• Maximum grip strength occurs with wrist in 15-30° extension and slight ulnar deviation
• Wrist flexion dramatically reduces grip strength (finger flexors shorten, lose length-tension advantage)
• Norkin: wrist extension pre-tensions the extrinsic finger flexors via the tenodesis effect

E. Tenodesis Effect

• Wrist extension → passive finger flexion (flexor tendons become taut)
• Wrist flexion → passive finger extension

• Rehabilitation of tetraplegic/quadriplegic patients (tenodesis grasp)
• Assessment of FDS/FDP integrity (the "tenodesis test")

F. Opposition of the Thumb

1. Abduction of thumb (APB)
2. Flexion at CMC joint (FPB, OP)
3. Internal rotation/pronation of 1st metacarpal (OP)
4. Final contact: thumb pad meets finger pad

• Injury at carpal tunnel → loss of opposition (ape thumb deformity/simian hand)

G. Non-Prehensile Movements

• Pushing, lifting, pressing, tapping
• Do not involve grasping
• Important in daily tasks and occupational performance

H. Functional Significance

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QUESTION 5: Integration - Joint Biomechanics Applied to Wrist and Hand (Comprehensive 15-Mark Integrative Answer)

A. Open and Close-Packed Positions

B. Axes of Motion

• Wrist flexion-extension: transverse axis through capitate head
• Radial-ulnar deviation: AP axis through capitate head
• All wrist motion thus has a single functional axis through the head of the capitate (Norkin)

C. Muscular Force Couples

• Wrist flexion: FCR + FCU (balanced)
• Wrist extension: ECRL + ECRB + ECU (balanced)
• Radial deviation: FCR + ECRL + ECRB (radial side muscles)
• Ulnar deviation: FCU + ECU (ulnar side muscles)

D. Clinical Applications

1. Colles' fracture: disrupts radial inclination and palmar tilt → alters load distribution; rehabilitation must restore these parameters
2. Carpal tunnel syndrome: wrist held in flexion compresses median nerve → impairs precision grip and opposition
3. Scaphoid fracture: disrupts scapholunate axis → DISI deformity (dorsal intercalated segment instability); flexion at radiocarpal + extension at midcarpal
4. Rheumatoid arthritis: synovitis stretches SLIL + collaterals → radial deviation of wrist + ulnar drift of fingers
5. Trigger finger: nodule on FDP/FDS catches at A1 pulley → altered kinematics of MCP flexion

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SUMMARY TABLE: Quick Review for Exams

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Primary Reference: Norkin CC, Levangie PK. Joint Structure and Function: A Comprehensive Analysis, 5th ed. FA Davis. Supplementary: Rockwood & Green's Fractures in Adults, 10th ed., 2025 (Carpal Kinematics chapter); Imaging Anatomy: Bones, Joints, Vessels and Nerves, Vol. 3 (Wrist biomechanics, extensor compartments).