Opponens Digiti Minimi: Origin, Insertion, Nerve & Action

The Opponens Digiti Minimi (ODM) is a triangular muscle located deep within the hypothenar eminence (the fleshy pad on the pinky side of the palm). Unlike its neighbors that move the finger bones, this muscle acts directly on the metacarpal bone, allowing the hand to "cup" or oppose the little finger toward the thumb.

[Image of Opponens Digiti Minimi Hand anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Hook of Hamate and the Flexor Retinaculum (Transverse Carpal Ligament).
Insertion (Distal)	Medial border of the 5th Metacarpal bone (entire length).
Nerve Supply	Deep Branch of Ulnar Nerve (C8, T1).
Blood Supply	Deep Palmar Branch of Ulnar Artery.
Primary Actions	Opposition: Flexes and laterally rotates the 5th metacarpal at the CMC joint. Cupping: Deepens the hollow of the palm.

Deep Dive: The Deepest Hypothenar Muscle

The Hypothenar Eminence consists of three muscles (Abductor, Flexor, and Opponens). The Opponens Digiti Minimi is the deepest of the three.

1. The Metacarpal Mover

While the Abductor and Flexor muscles insert onto the Phalanx (finger bone), the Opponens inserts onto the Metacarpal (hand bone). Because it stops short of the finger, it cannot wiggle the finger itself. Instead, it moves the entire "ray" of the hand, lifting the ulnar border of the palm.

2. Opposition vs. Flexion

Opposition is a complex movement involving flexion, adduction, and rotation. The Opponens Digiti Minimi pulls the 5th metacarpal forward and rotates it radially, bringing the pinky pad to meet the thumb pad. This is essential for grasping spherical objects (like a ball or an apple).

Physio Corner: Clinical Relevance

💪 Functional Fact: Without the Opponens Digiti Minimi, you would lose the "Cylindrical Grip." This muscle locks the 5th metacarpal around handles (like a hammer or tennis racquet), preventing the object from slipping out of the ulnar side of the hand.

Palpation

Palpation is difficult because it lies deep to the Flexor Digiti Minimi Brevis and Abductor Digiti Minimi. To approximate it, press deeply into the hypothenar eminence while asking the patient to touch their pinky to their thumb. You are feeling for the deep tension that lifts the palm, not just the finger movement.

⚠️ Clinical Pathology: Guyon's Canal Compression
The Ulnar Nerve passes through Guyon's Canal (between the Pisiform and Hook of Hamate) just before supplying this muscle. Compression here (e.g., from handlebar palsy in cyclists) can lead to weakness in opposition and a flattened hypothenar eminence.

Manual Muscle Testing (MMT)

Testing the ability to cup the hand and oppose the 5th digit.

Testing Tip: You are testing the 5th Metacarpal, not the finger. Do not be fooled by the patient just curling their fingertip!

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Forearm supinated, wrist neutral. Action: Patient brings the 5th finger toward the thumb (Opposition/Cupping the palm). Resistance: Apply pressure to the palmar surface of the 5th Metacarpal (not the finger), pushing it flat down toward the table (Derotation/Extension). Grade 3: Completes movement. Grade 4/5: Holds against moderate/strong resistance.
Grade 0, 1 (Palpation)	Action: Palpate deep on the ulnar side of the palm, radial to the abductor muscle belly. Cue: "Try to touch your thumb with your pinky." Grade 1: Trace contraction felt deep in the pad. Grade 0: No activity.

Frequently Asked Questions

Does it flex the pinky finger?

No. It flexes the 5th Metacarpal at the Carpometacarpal (CMC) joint. It does not cross the MCP or IP joints, so it has no direct action on the finger bones themselves.

What is the difference between Opponens Pollicis and Opponens Digiti Minimi?

Opponens Pollicis: Thenar (thumb) side, Median Nerve, opposes the thumb.
Opponens Digiti Minimi: Hypothenar (pinky) side, Ulnar Nerve, opposes the little finger.

Is it a superficial muscle?

No. It lies deep to the Abductor Digiti Minimi and Flexor Digiti Minimi Brevis. You have to peel back those two muscles to see the Opponens.

Test Your Knowledge: Opponens Digiti Minimi Quiz

1. Where does the Opponens Digiti Minimi insert?

2. Which nerve supplies the Opponens Digiti Minimi?

3. What is the primary action of this muscle?

4. Where does the Opponens Digiti Minimi originate?

5. Is the Opponens Digiti Minimi superficial or deep?

6. Which joint does this muscle primarily act upon?

7. To apply resistance during MMT, you should push on the:

8. This muscle contributes to which functional grip?

9. The Opponens Digiti Minimi is part of the:

10. True or False: The Opponens Digiti Minimi rotates the 5th metacarpal.

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.