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Dorsal Interossei (DI)

OVERVIEW
Notations:
I – First Dorsal Interosseous
II – Second Dorsal Interosseous
III – Third Dorsal Interosseous
IV – Fourth Dorsal Interosseous
Key Points:
• Abduct fingers away from midline (which is the axis of the long finger metacarpal).
• Assists in metacarpal-phalangeal joint flexion and interphalangeal joint extension.
• Intrinsic hand muscles.

EXAMINATION
Muscle Function:
Primary: Abducts index, long, and ring fingers away from the midline of the hand. 

• First dorsal interosseous abducts index finger radially towards thumb.
• Second dorsal interosseous abducts the long finger radially towards index finger.
• Third dorsal interossei abducts the long finger ulnarly towards ring finger.
• Fourth dorsal interossei abducts the ring finger ulnarly towards small finger.

Secondary:

• Assists in flexion of the metacarpal-phalangeal joints and extension of the interphalangeal joints of the respective finger.
• In addition, first dorsal interosseous assists in adduction of the thumb.

Palpation: Too deep to palpate.

Strength Testing: Position – the metacarpal-phalangeal and interphalangeal joints in extension. Stabilize – the hand. Resist – abduction of each finger away from midline of hand (in the direction of adduction back towards the midline).

Possible Substitutions: Extensor digitorum communis.

Relevant Anatomy:
Innervation:

• Roots: C8, T1.
• Nerve: Ulnar, deep motor branch.
• Innervation Route: C8, T1 → ulnar nerve → deep motor branch → dorsal interossei branches.

Origins: All muscles are bipennate (two heads).

• First DI:

• Lateral head origin – ulnar aspect of thumb metacarpal.
• Medial head origin – radial aspect of the index metacarpal.

• Second, third and fourth DI

• Origin at adjacent sides of the metacarpal bones in each interspace. 

Insertions: The muscles pass dorsal to deep transverse metacarpal ligament before reaching its insertion which is located as follows:

• First DI – radial proximal phalanx base and extensor apparatus of index finger.
• Second DI – radial proximal phalanx base and extensor apparatus of long finger.
• Third DI – ulnar extensor apparatus of long finger.
• Fourth DI – ulnar proximal phalanx base and extensor apparatus of ring finger.