First Dorsal Interosseous

Origin, Insertion, Action, Innervation

Origin: Adjacent sides of the 1st and 2nd metacarpals (two heads filling the first dorsal web space)
Insertion: Lateral side of the proximal phalanx of the index finger and the extensor expansion of the index finger
Action:
Primary: Abduction of the index finger (away from the middle finger)
Flexion of the MCP joint of the index finger (via its attachment to the extensor expansion)
Extension of the IP joints of the index finger (via the lateral band of the extensor expansion)
Innervation: Ulnar nerve, deep branch (C8–T1)

Palpation Guide

Client position: Hand resting palm-down on the table with fingers relaxed.
Landmark sequence:

Locate the dorsal web space between the thumb and index finger — the first dorsal interosseous fills this space and is the most palpable intrinsic hand muscle.
Ask the client to abduct the index finger (spread it away from the middle finger) against resistance. The FDI contracts prominently and is easily visible and palpable in the web space.
The muscle has two heads originating from the adjacent surfaces of the 1st and 2nd metacarpals — you are palpating between these two bones.
The tendon inserts on the radial side of the index finger proximal phalanx — palpable at the lateral base of the index finger.

Tissue feel: Firm, rounded muscle belly that fills the web space. When contracted, it feels like a small, dense ball between the thumb and index metacarpals. When relaxed, the web space feels soft and yielding.
Confirmation test: Ask the client to abduct the index finger against resistance (press the index finger laterally away from the middle finger). The FDI contracts visibly in the web space. This is also the standard test for ulnar nerve integrity at the hand.
Common errors:
Confusing FDI contraction with adductor pollicis — the adductor pollicis lies palmar to the FDI and adducts the thumb. FDI abducts the index finger. The two muscles overlap in the web space but perform opposite movements on different digits.
Pressing too deep into the web space without awareness of the radial artery (first dorsal metacarpal artery branch) — vascular structures pass through the web space.

Trigger Point Referral

Common TrP locations: The TrP is in the belly of the muscle in the dorsal web space, midway between the 1st and 2nd metacarpals.
Referral pattern: Refers along the radial (lateral) side of the index finger and into the dorsal web space, sometimes extending to the palmar surface of the index finger.
Clinical significance: The referral along the index finger can be mistaken for C6 radiculopathy or digital nerve compression. If index finger pain does not correlate with cervical or wrist-level nerve findings, check the FDI in the web space.

Trigger point referral diagram — coming soon

Image coming soon. For visual reference, see [First Dorsal Interosseous at TriggerPoints.net](http://www.triggerpoints.net/muscle/first-dorsal-interosseous).

Clinical Notes

Innervation significance:

The FDI is innervated by the deep branch of the ulnar nerve. Wasting (atrophy) of the FDI is one of the earliest visible signs of ulnar nerve palsy — the web space between the thumb and index finger appears flattened or "scooped out" compared to the normal side. This finding on observation alone suggests ulnar nerve compromise and should prompt further testing.
FDI weakness (inability to abduct the index finger against resistance) combined with positive Froment's sign (thumb IP flexion during attempted key pinch, indicating adductor pollicis weakness) is the classic presentation of ulnar nerve palsy at the wrist (Guyon's canal) or elbow (cubital tunnel).

Common conditions:

The FDI is a key assessment muscle in conditions/cubital-tunnel-syndrome and Guyon's canal syndrome — both compress the ulnar nerve at different levels but produce FDI weakness and wasting.
Wartenberg's sign — inability to adduct the little finger (kept abducted at rest) indicates ulnar nerve palsy affecting the interossei. When the FDI and other interossei are weak, the hand loses fine motor control.
Overuse of the FDI occurs in occupations requiring sustained pinch grip (writing, pipetting, using scissors) — the muscle becomes hypertonic and tender in the web space.

What you'll typically find:

In clients with hand fatigue from writing, typing, or fine motor work, the FDI web space is tender on palpation. The muscle is small but works hard during pinch and grip activities.
In clients with suspected ulnar nerve palsy, compare the web space contour bilaterally — unilateral flattening or wasting is visible evidence of denervation.

Treatment effects:

Responds to sustained compression and gentle cross-fiber techniques in the web space. The muscle is small and sensitive — use moderate pressure.
Post-treatment, clients often report improved index finger dexterity and reduced aching in the web space.
Acupressure point LI-4 (Hegu) overlies the FDI — clients familiar with acupuncture may report characteristic sensations during treatment.

Cautions:

The radial artery passes through the web space (first dorsal metacarpal artery) — avoid sustained heavy compression in the deep web space.
In clients with known ulnar nerve palsy, the FDI may be denervated and atrophic — vigorous treatment of a denervated muscle is ineffective and may cause unnecessary soreness. Focus on nerve-level treatment instead.

Clinical pearl:

The FDI is the single fastest bedside test for ulnar nerve function. Ask the client to spread their fingers apart and resist you pushing the index finger toward the middle finger. If the FDI gives way or feels weak compared to the other side, the ulnar nerve is compromised. Combine with Froment's sign (weak key pinch) and you have a 10-second ulnar nerve screen.

Assessment

Manual muscle testing:

Index finger abduction: Client's hand flat on the table, palm down. Ask the client to spread the index finger away from the middle finger against resistance. Compare bilaterally — asymmetric weakness indicates ulnar nerve compromise.

Stretch test:

Index finger adduction: Gently adduct the index finger toward the middle finger with the MCP joint extended. Resistance in the web space suggests FDI shortening.

Related special orthopedic tests:

Froment's sign — weakness of key pinch (thumb adduction) indicates ulnar nerve palsy affecting adductor pollicis
Wartenberg's sign — little finger held in abduction at rest indicates interosseous weakness
Tinel's sign at Guyon's canal — tap over the pisiform/hook of hamate to test ulnar nerve at the wrist

Muscle Groups

Dorsal interossei (anatomical — hand):

First dorsal interosseous (this article) — index finger abduction
2nd dorsal interosseous — middle finger radial abduction
3rd dorsal interosseous — middle finger ulnar abduction
4th dorsal interosseous — ring finger abduction

Ulnar nerve — intrinsic hand muscles (innervation):

First dorsal interosseous (this article)
All interossei (dorsal and palmar)
Hypothenar muscles (abductor digiti minimi, flexor digiti minimi, opponens digiti minimi)
Adductor pollicis
3rd and 4th lumbricals

Related Muscles

Synergist for index finger MCP flexion with IP extension:

1st lumbrical — also flexes MCP and extends IP via the lateral band; innervated by the median nerve (unlike FDI)

Antagonist:

1st palmar interosseous — adducts the index finger toward the middle finger

Same innervation (ulnar nerve deep branch):

anatomy/muscles/flexor-carpi-ulnaris — wrist flexion with ulnar deviation (ulnar nerve, but proximal branch)
anatomy/muscles/flexor-digitorum-profundus — ring and little finger DIP flexion (ulnar nerve forearm branch)
Adductor pollicis — thumb adduction

Key Takeaways

FDI wasting (flattened web space) is the earliest visible sign of ulnar nerve palsy — observe the web space bilaterally during every hand assessment.
Resisted index finger abduction is the fastest bedside test for ulnar nerve function at the hand level.
The web space is small but clinically dense — the FDI, adductor pollicis, radial artery branch, and deep branch of the ulnar nerve all coexist in this confined area.