Opponens Pollicis: Origin, Insertion, Nerve & Action

The Opponens Pollicis is the deep muscle of the Thenar Eminence (the fleshy mound at the base of the thumb). It is arguably the most important muscle for human dexterity. Its primary function is Opposition, which allows the thumb to swing across the palm to touch the fingertips.

[Image of Opponens Pollicis Hand anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Tubercle of the Trapezium and the Flexor Retinaculum (Transverse Carpal Ligament).
Insertion (Distal)	Lateral (Radial) border of the 1st Metacarpal bone (entire length).
Nerve Supply	Median Nerve (Recurrent Branch) - C8, T1.
Blood Supply	Superficial Palmar Branch of the Radial Artery.
Primary Actions	Opposition: A combination of flexion, abduction, and medial rotation of the 1st Metacarpal at the CMC joint.

Deep Dive: The Metacarpal Mover

The Opponens Pollicis is unique because it does not attach to a phalanx (finger bone). It acts exclusively on the Metacarpal.

1. Deep Location

It lies deep to the Abductor Pollicis Brevis and lateral to the Flexor Pollicis Brevis. You essentially have to peel back the superficial thenar muscles to see the Opponens wrapping around the metacarpal shaft.

2. The Complex Motion of Opposition

Opposition isn't a simple hinge movement. The saddle shape of the Carpometacarpal (CMC) joint allows the Opponens Pollicis to perform three actions simultaneously:
1. Abduction: Lifts the thumb away from the palm.
2. Flexion: Moves the thumb across the palm.
3. Medial Rotation: Spins the thumb so the pad faces the fingers.

Physio Corner: Clinical Relevance

💪 Functional Fact: Without the Opponens Pollicis, you cannot perform a "Pincer Grasp" (picking up a coin) or a precision grip. Losing this muscle reduces the hand to a simple hook (simian hand).

Palpation

Palpation is difficult because it is covered by the Abductor Pollicis Brevis. However, you can feel its action. Place your finger on the lateral shaft of the 1st metacarpal. Ask the patient to touch their pinky with their thumb. You will feel the metacarpal bone rotate and flex inward under the superficial muscle mass.

⚠️ Clinical Pathology: Carpal Tunnel Syndrome
The Recurrent Branch of the Median Nerve (the "Millionaire Nerve") supplies the Opponens Pollicis. Severe Carpal Tunnel Syndrome compresses this nerve, leading to atrophy (wasting) of the thenar eminence. The thumb falls back into line with the fingers, creating an "Ape Hand Deformity."

Manual Muscle Testing (MMT)

We test the ability to rotate and flex the metacarpal.

Testing Tip: Ensure the patient is rotating the thumb, not just flexing it. The thumbnail should turn to face the ceiling (if hand is palm up) or face the other fingers.

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 thumb pad to the little finger pad (Opposition). Resistance: Applied to the 1st Metacarpal head (not the tip of the thumb). The therapist tries to "peel" the thumb away by extending, adducting, and laterally rotating it. Grade 3: Completes range. Grade 4/5: Holds against resistance.
Grade 0, 1 (Palpation)	Action: Palpate deep along the radial shaft of the 1st metacarpal. Cue: "Try to touch your pinky." Grade 1: Trace contraction felt along the bone. Grade 0: No activity.

Frequently Asked Questions

Why is it called "Opponens"?

It is named after its primary action: Opposition. This is the movement that places the thumb "opposite" to the fingers, enabling grasping.

Does it flex the thumb tip?

No. It inserts on the Metacarpal, so it only moves the CMC joint. Flexion of the thumb tip is done by the Flexor Pollicis Longus (FPL).

Which nerve injury paralyzes this muscle?

Injury to the Median Nerve (specifically the Recurrent Branch). This commonly happens in Carpal Tunnel Syndrome or lacerations to the base of the palm.

Test Your Knowledge: Opponens Pollicis Quiz

1. Where does the Opponens Pollicis insert?

2. Which nerve supplies the Opponens Pollicis?

3. What is the primary action of this muscle?

4. Where does the Opponens Pollicis originate?

5. Is the Opponens Pollicis deep or superficial?

6. Atrophy of this muscle leads to which deformity?

7. Opposition involves which rotation of the metacarpal?

8. Which joint does the Opponens Pollicis act upon?

9. To MMT the Opponens, you resist the thumb moving:

10. Which muscle lies lateral to the Opponens Pollicis?

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.