Biomechanics hand

IP Joints

  • Single axis hinge joints. COR just anterior to collateral origin.
  • Static stability from:
    • Bony architecture (akin to the knee)
    • Collateral ligaments
    • Volar plate

MP Joints

  • Axes of movement: F/E & Abd/Add
    • Abduction/adduction only occurs if P1 is flexed 30 degrees
    • This results clinically in circumduction of the finger around the MC head
  • Static stability same as IPJ

CMC Joints of Fingers

  • Movement is progressively more restricted as we move ulnarward
  • 4th and 5th CMCJ very rigid and transmit forces from hand to wrist
  • 3rd MC is a cantilever supporting the flexor sheaths via a fibrous framework encompassing the collateral ligaments and transverse metacarpal ligaments

Thumb CMCJ

  • Saddle joint
    • Axes of movement like the MCPJs of fingers
    • Circumduction occurs as a consequence of the two movement axes with thumb MC flexed slightly
    • Trapezium and MC base have differing radii
    • This allows pronation of the thumb and increased ROM but is less stable
  • Stability is from volar oblique (beak) ligament and capsule
  • Loading of the thumb CMCJ is up to 120kg and mainly through the volar half
    • Therefore, the volar oblique ligament is prone to attenuation/rupture, which leads to joint instability – repair if possible or reconstruct with trapeziectomy
  • CMCJ and MCP of thumb work in unison as their motor tendons cross both

Motors Powering Movement

  • Flexors, extensors, and intrinsics
  • Tendon excursion is 30-50-70 mm
  • Wrist flexors and extensors 30mm

Extensor Mechanism

  • Long extensors (extrinsics) and intrinsics
  • Extensor hood made up of long extensor, lumbrical (radial side), and interossei
  • Lateral bands control distal phalanx and central slip controls middle phalanx

Flexor Mechanism

  • FDP is a mass action muscle, although index FDP has some independent action
  • Quadrigia effect: shortening or tethering of one FDP tendon results in slight flexion of the other DIPJs
  • Tension within FDP is constant when a contraction occurs, but torque is greater proximally as the moment arm between the COR of the MPJ is longer than that of the DIPJ
  • Therefore, proximally quite a high torque is generated even for a small load
  • The pulleys prevent the tendons from bowstringing and thus maintain their proximity to the COR of each joint in the hand, maximizing power
  • Annular pulleys are: A1-4 (2 and 4 most important)
  • Cruciate pulleys are C1-3 and lie between the annular pulleys supporting the tendon sheaths
  • Flexor retinaculum can also be considered to be a pulley

Intrinsic Muscles

  • Lumbricals:
    • Unique – arise from one tendon (FDP) and insert into another (extensor hood)
    • Fine-tune tension between the flexor and extensor systems as they connect them
  • Interossei:
    • Palmar adduct and dorsal abduct the fingers
    • Both arise from the MC shafts and contribute to the extensor hood
    • Together the intrinsics cause flexion at the MCPJ and extension at the PIPJ
    • This is mainly due to the lumbricals as they are further from the MCPJ COR (longer moment arm)
    • Intrinsics allow finger flexion without curling
    • This is vital to allow grasp of large objects
    • In intrinsic paralysis, the intrinsic minus hand occurs:
      • Hyperextension of the MCPJ due to long extensors and a flexed posture of the DIPJ, PIPJ due to long flexor tension
      • Grasping is not possible as the fingers curl to make a fist from distal (FDP) to more proximal (FDS)

Intrinsic Plus

  • The opposite – i.e., due to intrinsic tightness
    • The MCPJs are flexed and the PIPJs extended
    • Because the lumbricals bridge between the long flexors and extensors, paradoxical PIPJ extension may occur on attempted flexion

Lumbrical Plus Finger

  • An extreme example of this phenomenon
    • Occurs when FDS is sectioned distal to the lumbrical origin
    • All FDS muscle power is transmitted via the lumbrical to the long extensors causing PIPJ hyperextension when FDS fires

Oblique Retinacular Ligament of Landsmeer (ORL)

  • Arises from the proximal phalanx and A2 pulley
  • Runs diagonally in front of the PIPJ COR to attach to the lateral bands of the extensor tendon on the DIPJ
  • Thus, it causes passive extension of the DIPJ when the PIPJ is extended, allowing controlled extension of the whole finger rather than reliance on the long powerful extensors only

Thenar Muscles

  • Thumb intrinsics are:
    • ADP (ulna)
    • APB (median)
    • FPB, OP (variable)
  • The latter 3 arise from the carpal bones
  • APB palsy causes significant disability as it is required to abduct the thumb to allow opposition (e.g., severe CTS)
  • ADP palsy causes reduced pinch strength as ADP provides a stable post for strong pinch to occur – this occurs in intrinsic palsy (ulna nerve)
  • FPB palsy not too significant as can still pinch against an extended IP joint
  • Extrinsic thumb muscles are:
    • EPV, EPL, FPL, APL
    • Best individual test for EPL is lifting thumb off a flat surface (retropulsion)
    • EPL also acts as an extrinsic adductor when FPL flexes – tensions EPL
  • In intrinsic palsy, adduction of the thumb can be maintained by EPL despite loss of ADP
    • In this situation, the thumb posture is classic – IP and MP flexion due to long flexor moment arm and CMC extension due to favorable EPL moment arm
  • APL and EPV abduct and extend the thumb but together with ADP are important for stabilizing the thumb MC for strong pinch grip
    • The three muscles pull against each other, therefore stabilizing the thumb MC

Hypothenar Muscles

  • Mirror the thenar muscles (ABDm, FDM, ODM)
  • Functions are to:
    • Stabilize the little finger MC for opposition, cupping, and power grip
    • Provide a cushion mass which aids grip
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