Courtesy: Rishi Dhir, FRCS Orth, Harlow, UK

Overview

• Understanding normal biomechanics is essential before analyzing pathology of the hand and wrist.
• For conceptual learning, biomechanics can be divided into two major components: finger biomechanics and wrist and carpal biomechanics.
• Finger movement involves coordinated interaction between joints and muscular mechanisms.

Joints of the Fingers

• Three major joints are involved in finger movement.
• Interphalangeal joints include the proximal interphalangeal joint and distal interphalangeal joint.
• Interphalangeal joints are hinge joints allowing movement in a single plane: flexion and extension.
• Metacarpophalangeal joints are ellipsoid joints allowing flexion, extension, abduction, and adduction.
• Carpometacarpal joints of the fingers behave as saddle joints with two axes of movement.

Muscle Systems Controlling Finger Motion

• Finger movement is produced by two functional motor groups.
• Extrinsic muscles originate in the forearm and provide power for flexion and extension.
• Intrinsic muscles originate within the hand and provide precision and fine control of finger movement.

Extensor Mechanism

• The extensor mechanism begins over the proximal phalanx.
• Lumbricals and interossei contribute to the extensor hood on the radial side.
• The extensor hood divides into a central slip and two lateral bands.
• The central slip inserts into the middle phalanx and extends the proximal interphalangeal joint.
• The lateral bands unite distally to form the terminal extensor tendon that inserts into the distal phalanx and extends the distal interphalangeal joint.

Sagittal Bands

• Sagittal bands stabilize the extensor tendon over the metacarpophalangeal joint.
• They maintain alignment of the extensor mechanism during finger flexion and extension.
• Rupture of sagittal bands may cause subluxation or dislocation of the extensor tendon.
• This injury is commonly associated with rheumatoid arthritis.

Retinacular Ligaments

• Triangular retinacular ligament prevents volar displacement of the lateral bands and is injured in boutonniere deformity.
• Transverse retinacular ligament prevents dorsal displacement of the lateral bands and is injured in swan neck deformity.
• Oblique retinacular ligament connects motion between proximal and distal interphalangeal joints.
• When the proximal interphalangeal joint extends, the distal interphalangeal joint also extends due to this ligament.

Volar Plate

• The volar plate is a thickened capsular structure located on the palmar side of finger joints.
• It prevents hyperextension of the metacarpophalangeal and interphalangeal joints.
• Injury to the volar plate can occur in hyperextension injuries or joint dislocations.

Flexor Mechanism

• Flexor digitorum superficialis splits at the proximal phalanx forming the chiasm known as Camper chiasm.
• Flexor digitorum profundus passes through this split to insert into the distal phalanx.
• Flexor digitorum superficialis flexes the proximal interphalangeal joint.
• Flexor digitorum profundus flexes the distal interphalangeal joint.

Mass Action of Flexor Digitorum Profundus

• The tendon to the index finger acts independently.
• Tendons to the middle, ring, and little fingers often function together as a mass action muscle.
• Shortening of one tendon may restrict excursion of the others.
• This phenomenon is known as the quadriga effect.

Intrinsic Muscles

• Intrinsic muscles include lumbricals and interossei.
• These muscles primarily control fine motor function.
• They flex the metacarpophalangeal joints and extend the interphalangeal joints.

Lumbricals

• Lumbricals originate from the radial side of the flexor digitorum profundus tendon.
• They insert into the radial portion of the extensor hood.
• They coordinate activity between flexor and extensor systems.
• They flex metacarpophalangeal joints and extend interphalangeal joints.

Interossei

• Interossei also flex metacarpophalangeal joints and extend interphalangeal joints.
• They assist in finger abduction and adduction.
• A useful mnemonic is PAD: palmar interossei adduct and dorsal interossei abduct.

Intrinsic Plus Hand

• Intrinsic plus hand results from relatively stronger intrinsic muscles compared with extrinsic muscles.
• The metacarpophalangeal joints are flexed.
• The interphalangeal joints are extended.
• This posture is commonly used as a safe position during splinting.

Intrinsic Minus (Claw Hand)

• This deformity results from weakness or paralysis of intrinsic muscles.
• The metacarpophalangeal joints become hyperextended.
• The proximal and distal interphalangeal joints become flexed.
• It is commonly associated with ulnar nerve palsy.

Lumbrical Plus Finger

• A lumbrical plus finger shows paradoxical extension of the interphalangeal joints during attempted finger flexion.
• It occurs when the flexor digitorum profundus tendon is disrupted distal to the origin of the lumbricals.
• Contraction of the flexor tendon causes the lumbricals to extend the interphalangeal joints.

Carpal Anatomy and Biomechanics

• The carpus consists of two functional rows.
• The distal row is relatively rigid.
• The proximal row is more mobile and acts as an intercalated segment between the forearm and distal carpal row.

Theories of Carpal Motion

• Link theory describes the radius, lunate, and capitate forming a mechanical chain.
• Row theory describes a rigid distal row and a mobile proximal row connected by the scaphoid.
• Column theory divides the wrist into radial, central, and ulnar columns.

Ligaments of the Wrist

• Wrist stability is maintained by extrinsic and intrinsic ligaments.
• Extrinsic ligaments connect the radius or ulna to carpal bones.
• Intrinsic ligaments connect the carpal bones to each other.

Important Intrinsic Ligaments

• The scapholunate ligament connects the scaphoid and lunate.
• The lunotriquetral ligament connects the lunate and triquetrum.
• Injury to these ligaments leads to carpal instability.

Carpal Instability

• Carpal instability dissociative occurs when instability develops between bones within the same row.
• Carpal instability non-dissociative occurs between proximal and distal rows.
• Carpal instability adaptive occurs due to deformity outside the wrist such as distal radius malunion.
• Carpal instability complex includes combined patterns such as perilunate dislocations.

Functional Wrist Motion

• Wrist movement includes flexion, extension, radial deviation, and ulnar deviation.
• The most functional movement of the wrist is dart thrower’s motion.

This movement occurs from radial extension to ulnar flexion