Dorsal Interossei (Hand): Anatomy, Action, DAB Mnemonic & Testing

The Dorsal Interossei of the hand are four bipennate muscles located in the voids between the metacarpal bones on the back of the hand. They are responsible for spreading the fingers apart (abduction) and are vital for strong grip and fine motor control.

[Image of Dorsal Interossei Hand anatomy]

Quick Anatomy Snapshot

Count	Four muscles (1st through 4th), numbered radial to ulnar.
Origin (Proximal)	Each muscle arises from the adjacent sides of two Metacarpal bones (Bipennate structure).
Insertion (Distal)	1st: Radial side of 2nd digit (Index). 2nd: Radial side of 3rd digit (Middle). 3rd: Ulnar side of 3rd digit (Middle). 4th: Ulnar side of 4th digit (Ring).
Nerve Supply	Deep Branch of Ulnar Nerve (C8, T1).
Blood Supply	Dorsal Metacarpal Arteries.
Primary Actions	Abduction: Spreads fingers away from the midline (3rd digit). MTP Flexion: Flexes the knuckles. IP Extension: Extends the interphalangeal joints.

Deep Dive: The "DAB" Concept

Just like in the foot, the mnemonic helps distinguish the Dorsal from the Plantar (Palmar) interossei.

1. The "DAB" Mnemonic

Dorsal interossei ABduct.
Palmar interossei ADduct (PAD).

2. The Midline Rule

In the hand, the 3rd Finger (Middle Finger) is the anatomical midline.
• The Middle finger cannot adduct; it can only abduct (move away from the midline) in either direction.
• Therefore, the middle finger has two Dorsal Interossei attached to it (the 2nd and 3rd DI), allowing it to waggle side-to-side.

Physio Corner: Clinical Relevance

💪 Functional Fact: The 1st Dorsal Interosseous is the largest and most powerful. It fills the web space between the thumb and index finger. You can see it bulge when you forcefully pinch your thumb and index finger together.

Palpation

The 1st Dorsal Interosseous is the easiest to palpate. Have the patient pinch their thumb and index finger together. Feel the large muscle belly on the back of the hand in the web space. The others can be palpated between the metacarpals while the patient spreads their fingers.

⚠️ Clinical Pathology: Ulnar Claw Hand
Paralysis of the interossei (Ulnar nerve injury) leads to a loss of the "intrinsic" position (MTP flexion + IP extension). Without them, the long extensors hyperextend the knuckles, and the long flexors curl the fingers, creating the "Claw Hand" deformity (specifically in the ring and little fingers).

Manual Muscle Testing (MMT)

Testing the ability to spread the fingers against resistance.

Testing Tip: The 3rd finger (middle) is the anchor. Test abduction of the Index away from the Middle, and Ring away from the Middle.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Hand pronated (palm down) on a table, fingers together. Action: Patient spreads the fingers apart. Resistance: 1st DI: Push Index finger toward Middle finger. 4th DI: Push Ring finger toward Middle finger. 2nd/3rd DI: Push Middle finger side-to-side.
Grade 0, 1 (Palpation)	Action: Palpate the dorsal web spaces. Cue: "Spread your fingers." Grade 1: Contraction felt between metacarpals. Grade 0: No activity.

Frequently Asked Questions

Why doesn't the Thumb have a Dorsal Interosseous?

The thumb has its own dedicated abductors: Abductor Pollicis Longus and Abductor Pollicis Brevis. It does not require an interosseous muscle for abduction.

Why doesn't the Pinky have a Dorsal Interosseous?

Similar to the thumb, the little finger has its own Abductor Digiti Minimi. Therefore, the Dorsal Interossei only insert on digits 2, 3, and 4.

What is "First Dorsal Interosseous Wasting"?

Visible hollowing of the skin between the thumb and index finger on the back of the hand. It is a sensitive clinical sign for Ulnar Nerve neuropathy (e.g., Cubital Tunnel Syndrome) or T1 nerve root compression.

Test Your Knowledge: Hand Dorsal Interossei Quiz

1. What is the primary action of the Dorsal Interossei?

2. How many Dorsal Interossei are in the hand?

3. Which finger defines the "midline" of the hand?

4. Which nerve supplies all the Dorsal Interossei?

5. The Middle Finger has how many Dorsal Interossei attached to it?

6. The 1st Dorsal Interosseous is located between which bones?

7. What effect do these muscles have on the IP joints?

8. Which muscle acts as an antagonist to the Dorsal Interossei?

9. Why does the thumb not have a dorsal interosseous?

10. Wasting of the 1st Dorsal Interosseous usually indicates:

References

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
• Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
• Kendall, F. P. (2005). Muscles: Testing and Function, with Posture and Pain. 5th ed. Lippincott Williams & Wilkins.