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Flexor Digitorum Brevis Fourth Slip

musculus flexor digitorum brevis (fasciculus quartus)

The fourth FDB slip is smaller than the second and third slips, reflecting the reduced load-bearing role of the fourth ray. It inserts into the fourth middle phalanx after splitting for FDL passage. Its loss contributes to fourth toe claw deformity in intrinsic minus foot.

Nerve: Medial plantar nerve (S1, S2) Blood Supply: Lateral plantar artery (distal branches) Region: Foot & Ankle

Anatomical Data

Origin, Insertion & Supply

Origin	Medial process of the calcaneal tuberosity and plantar fascia
Insertion	Middle phalanx of the fourth toe via two slips
Nerve Supply	Medial plantar nerve (S1, S2)
Blood Supply	Lateral plantar artery (distal branches)

Biomechanics

Function & Actions

Actions	Flexes the PIP joint of the fourth toe; Assists MTP flexion of the fourth toe

Clinical Relevance

Clinical Notes

The FDB fourth slip is atrophied in peripheral neuropathy affecting the medial plantar nerve. Fourth toe deformities are less common than second and third toe problems. The FDB fourth slip may be absent in individuals with congenital absence of the fourth toe middle phalanx (brachymetatarsia). Evaluation of fourth toe intrinsic strength requires specific testing with the foot in neutral.

Palpation

Palpated plantarly over the fourth proximal phalanx during resisted PIP flexion of the fourth toe.

Pathology

Common Injuries & Conditions

Fourth Toe Intrinsic Insufficiency

Loss of FDB fourth slip function in neuropathy or fibrosis producing fourth toe MTP instability and crossover tendency managed with orthotic support and toe-stabilising exercises.