Hand biomechanics

Biomechanics of the Hand

Interphalangeal (IP) Joints

  • Single axis hinge joints allowing flexion/extension.
  • Center of rotation (COR) is slightly anterior to the collateral ligament origin.
  • Static stability is provided by:
    • Bony architecture (similar to the knee joint)
    • Collateral ligaments
    • Volar plate

Metacarpophalangeal (MP) Joints

  • These joints have two axes of movement: flexion/extension and abduction/adduction.
  • Abduction/adduction only occurs when the proximal phalanx (P1) is flexed to 30 degrees.
  • This leads to circumduction around the metacarpal head.
  • Static stability components are the same as in IP joints (bony structure, collateral ligaments, volar plate).

Carpometacarpal (CMC) Joints of the Fingers

  • Ulnarward movement restricts mobility progressively, with the 4th and 5th CMC joints being highly rigid, transmitting forces from hand to wrist.
  • The third metacarpal (MC) acts as a cantilever to support flexor sheaths, and the fibrous framework includes collateral ligaments and transverse metacarpal ligaments.

Thumb CMC Joint

  • A saddle joint with two axes of movement similar to the MP joints of the fingers.
  • Circumduction is a consequence of the two axes of movement when the thumb metacarpal is slightly flexed.
  • Trapezium and metacarpal base have differing radii of curvature, allowing thumb pronation and increased range of motion, though at the expense of stability.
  • Stability comes from the volar oblique (beak) ligament and the capsule, but this ligament is prone to rupture, leading to joint instability. It can be repaired or reconstructed with a trapeziectomy if needed.
  • Loading of the thumb CMCJ can reach up to 120 kg, with most force transmitted through the volar half, making the volar oblique ligament particularly vulnerable to attenuation or rupture.
  • The thumb CMCJ and MPJ work in unison, as motor tendons cross both joints.

Motor Systems and Tendon Excursion

  • The hand’s movement is powered by flexors, extensors, and intrinsic muscles.
  • Tendon excursion lengths:
    • Flexor and extensor tendons: 30-50-70 mm.
    • Wrist flexors and extensors: 30 mm.

Extensor Mechanism

  • Composed of long extensors (extrinsics) and intrinsic muscles.
  • The extensor hood is made up of:
    • Long extensor tendons
    • Lumbricals (on the radial side)
    • Interossei
  • Lateral bands control the distal phalanx, and the central slip controls the middle phalanx.

Flexor Mechanism

  • Flexor Digitorum Profundus (FDP) is a mass-action muscle with some independent action in the index finger.
  • The quadrigia effect occurs when shortening or tethering of one FDP tendon causes slight flexion in the other DIP joints.
  • Tension in the FDP is constant during contraction, with greater torque proximally because the moment arm between the MPJ COR and DIPJ COR is longer at the MPJ. Thus, higher torque is generated at the MPJ even for small loads.
  • Annular pulleys (A1-4) prevent bowstringing and maintain tendon proximity to the COR of each joint, maximizing power. A2 and A4 are the most important.
  • Cruciate pulleys (C1-3) lie between annular pulleys, supporting tendon sheaths.
  • The flexor retinaculum also acts as a pulley.

Intrinsic Muscles

Lumbricals

  • Unique muscles: they originate from the FDP tendons and insert into the extensor hood.
  • Lumbricals fine-tune the tension between the flexor and extensor systems.

Interossei

  • Palmar interossei adduct the fingers, while dorsal interossei abduct them.
  • Both types arise from the metacarpal shafts and contribute to the extensor hood.

Combined Function of Intrinsics

  • Together, the intrinsics flex the MPJ and extend the PIPJ, with the lumbricals playing a larger role due to their distance from the MPJ COR, which gives them a longer moment arm.
  • Intrinsics allow the fingers to flex without curling, which is essential for grasping large objects.

Clinical Conditions of Intrinsic Muscle Imbalance

  • Intrinsic paralysis leads to the intrinsic-minus hand, characterized by hyperextension of the MPJ due to the long extensors and a flexed posture of the DIPJ and PIPJ due to long flexor tension, creating a claw-hand deformity.

    • Grasping is impossible in this condition, and fingers curl into a fist from the distal (FDP) to proximal (FDS) tendons.

  • Intrinsic-plus hand is the opposite, due to intrinsic tightness. The MPJs are flexed, and the PIPJs extended.

  • Lumbrical paradox: Because the lumbricals bridge between the long flexors and extensors, paradoxical PIPJ extension can occur when attempting to flex the fingers.

  • Lumbrical-plus finger: An extreme example of this paradox, where FDS is sectioned distal to the lumbrical origin. All FDS power is transmitted via the lumbrical to the long extensors, causing PIPJ hyperextension when the FDS contracts.

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