The fourth lumbrical is the most laterally placed foot lumbrical, controlling the fifth toe MTP joint and extensor hood. It is the smallest of the four lumbricals. The fifth toe is served by the fourth lumbrical, the abductor digiti minimi, and the flexor digiti minimi brevis, making it the most complex of the lesser toes in terms of intrinsic balance.
| Origin | Adjacent borders of the third and fourth FDL tendon slips (bipennate) |
|---|---|
| Insertion | Medial base of the proximal phalanx of the fifth toe and extensor hood |
| Nerve Supply | Lateral plantar nerve (deep branch, S2, S3) |
| Blood Supply | Fourth plantar metatarsal artery |
| Actions | Flexes the MTP joint of the fifth toe; Extends the IP joints of the fifth toe |
|---|
The fourth lumbrical is the most variable in its anatomy and may be fused with adjacent intrinsics in some individuals. Its atrophy is a late sign in severe peripheral neuropathy affecting the lateral plantar deep branch. Curly fifth toe deformity in children involves imbalance between the fourth lumbrical and FDL, managed with FDL tenotomy or transfer in symptomatic cases.
Not directly palpable due to small size and deep plantar location adjacent to the fifth metatarsal.
Congenital or acquired flexion and medial rotation of the fifth toe from lumbrical-FDL imbalance producing toe-on-toe discomfort, managed with FDL tenotomy or derotation plasty.
