I – First Dorsal Interosseous
II – Second Dorsal Interosseous
III – Third Dorsal Interosseous
IV – Fourth Dorsal Interosseous
• 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.
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.
• 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.
• 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.