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Palmar Interossei (PI)

OVERVIEW
Notations:

I – First Palmar Interosseous (to the index finger).
II – Second Palmar Interosseous (to the ring finger).
III – Third Palmar Interosseous (to the small finger).

Key Points:

• Adducts fingers towards midline (which is the axis of the long finger metacarpal).
• Assists in metacarpal-phalangeal joint flexion and interphalangeal joint extension.
• Intrinsic hand muscles.

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EXAMINATION
Muscle Function:
Primary: Adducts index, ring, and small finger towards the midline of the hand (axis of long finger metacarpal).
Secondary: Assists in flexion of the metacarpal-phalangeal joints and extension of the interphalangeal joints of the index, ring, and small fingers respectively.
Palpation: Too deep to palpate.
 
Strength Testing: Position – the metacarpal-phalangeal and interphalangeal joints in extension.  Stabilize – the hand.  Resist – adduction of each finger towards midline of hand (in the direction of abduction).  Can test muscles as a group or individually.
 
Possible Substitutions: Muscles for finger flexion – to prevent this, test palmar interossei with fingers in full extension or in neutral position.
 
Relevant Anatomy:
Innervation:
• Roots: C8, T1.
• Nerve: Ulnar, deep motor branch.
• Innervation Route: C8, T1 → ulnar nerve → deep motor branch → palmar interossei branches.
Origin:
• I – ulnar palmar side of index finger metacarpal.
• II – radial palmar side of ring finger metacarpal.
• III – radial palmar side of small finger metacarpal.
Insertion: Mainly to the extensor apparatus (of the respective finger) and also to the base of the proximal phalanx as follows:
• I – ulnar aspect of index finger.
• II – radial aspect of ring finger.
• III – radial aspect of small finger.