Courtesy: Prof Nabil Ebraheim, University of Toledo, Ohio, USA

 

Intrinsic Minus and Intrinsic Plus Hand

Intrinsic Minus Hand (Claw Hand)

Definition

Intrinsic minus hand is characterized by:

• Hyperextension at the metacarpophalangeal (MCP) joints
• Flexion at the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints

This produces the classic claw hand deformity.

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Mechanism

The deformity results from imbalance between:

• Strong extrinsic muscles
• Weak intrinsic muscles

The intrinsic muscles include:

• Lumbricals
• Interossei

Loss of intrinsic muscle function removes:

• MCP joint flexion
• Interphalangeal joint extension

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Muscle Actions Involved

Extensor Digitorum Communis (EDC)

• Produces hyperextension at the MCP joints

Flexor Digitorum Superficialis (FDS) and Flexor Digitorum Profundus (FDP)

• Produce flexion at the PIP and DIP joints

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Resulting Deformity

The imbalance leads to:

• MCP hyperextension
• IP joint flexion
• Clawing of the fingers

The deformity is especially prominent in:

• Ulnar nerve palsy
• Particularly involving the ring and little fingers

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Causes of Intrinsic Minus Hand

Common causes include:

• Ulnar nerve palsy
• Volkmann ischemic contracture
• Neglected compartment syndrome
• Charcot–Marie–Tooth disease
• Leprosy
• Improper splinting after hand trauma

Failure to splint the hand in the intrinsic plus position after injury may contribute to clawing.

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Intrinsic Plus Hand (Safe Position of the Hand)

Definition

Intrinsic plus hand is characterized by:

• Flexion at the MCP joints
• Extension at the PIP and DIP joints

This is also called:

• Safe position
• Writing position

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Mechanism

The deformity occurs due to imbalance between:

• Strong intrinsic muscles
• Weak extrinsic muscles

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Muscle Actions Involved

Intrinsic Muscles (Lumbricals and Interossei)

These muscles:

• Flex the MCP joints
• Extend the PIP and DIP joints

Weak Extrinsic Muscles

Weakness of the following contributes to the posture:

• Extensor Digitorum Communis
• FDP
• FDS

This leads to:

• Reduced MCP extension
• Reduced interphalangeal flexion

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Functional Effect

Patients may have difficulty with:

• Power grip
• Grasping large objects

The hand tends to assume a writing posture.

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Clinical Sign: Bunnell Test

Purpose

Used to assess intrinsic muscle tightness.

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Positive Test

• PIP joint flexion is limited when the MCP joint is extended
• PIP flexion improves when the MCP joint is flexed

This indicates intrinsic tightness.

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Causes of Intrinsic Plus Hand

Common causes include:

• Trauma
• Stroke
• Brain injury
• Neurological spasticity involving intrinsic muscles

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Comparison Between Intrinsic Minus and Intrinsic Plus Hand

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Key Clinical Points

• Intrinsic muscles flex the MCP joints and extend the IP joints.
• Loss of intrinsic muscle function produces claw hand deformity.
• The intrinsic plus position is considered the safe position for splinting the hand.
• Clinical assessment should include:
  • Nerve involvement
  • Muscle balance
  • Passive versus active deformity evaluation