Flexor Digiti Minimi Brevis

The flexor digiti minimi brevis (FDMB) is one of the three intrinsic hypothenar muscles of the hand. It functions to flex the little finger at the metacarpophalangeal joint and contributes to the bulk and mobility of the ulnar side of the palm. Despite its name, there is no "longus" equivalent in the human hand.

Location

The FDMB is situated in the hypothenar compartment of the palm, lying just lateral to the abductor digiti minimi and superficial to the opponens digiti minimi. It extends from the hamate bone to the proximal phalanx of the fifth digit.

Structure

• Origin: Hook of the hamate bone and the flexor retinaculum.
• Insertion: Medial side of the base of the proximal phalanx of the fifth digit (little finger).
• Fiber Orientation: Vertical, running from proximal-medial to distal-lateral within the hypothenar eminence.

Function

The flexor digiti minimi brevis performs the following actions:

• Flexion of the Little Finger: Flexes the proximal phalanx at the metacarpophalangeal (MCP) joint.
• Assists with Grip: Contributes to the power and coordination of the ulnar side of the hand during gripping tasks.
• Supports Hypothenar Eminence: Adds muscular bulk and definition to the medial palm.

Innervation

Supplied by the deep branch of the ulnar nerve (C8–T1).

Blood Supply

• Ulnar artery: Through branches of the superficial and deep palmar arches.

Relations

The FDMB lies medial to the flexor tendons of the fingers and lateral to the abductor digiti minimi. Deep to it is the opponens digiti minimi. The ulnar nerve and artery pass deep to the hypothenar muscles as they enter the palm.

Surface Anatomy

The FDMB contributes to the hypothenar bulge, becoming palpable during flexion of the little finger against resistance. It lies slightly lateral to the abductor digiti minimi.

Development

The muscle develops from the ulnar side of the hand’s mesenchymal muscle mass. It is homologous with the flexor pollicis brevis of the thenar compartment.

Anatomical Variations

• May be partially fused with the abductor digiti minimi.
• Sometimes absent or rudimentary.
• Occasionally confused with the rarely occurring flexor digiti minimi longus (a very rare anatomical variant).

Clinical Significance

• Ulnar Nerve Injury: Damage to the deep branch can lead to weakness or paralysis of the FDMB, impairing flexion and grip strength on the ulnar side.
• Thenar vs Hypothenar Atrophy: FDMB wasting may indicate a lesion of the ulnar nerve at or distal to Guyon’s canal.
• Surgical Landmark: FDMB may need to be retracted or exposed in surgical procedures addressing ulnar nerve decompression or deep palmar mass removal.

Comparative Anatomy

In lower primates and quadrupeds, the hypothenar muscles are generally less developed due to limited use of the fifth digit. In humans, the FDMB is part of a specialized muscle group supporting fine motor control and dexterous grip on the ulnar side of the hand.