The fifth slip of flexor digitorum brevis to the little toe is inconstant, absent in approximately 20-25% of individuals. When present, it inserts into the middle phalanx of the fifth toe alongside the FDL. Its absence explains why the little toe has a higher incidence of PIP joint stiffness and is less amenable to FDB transfer procedures.
| Origin | Medial process of the calcaneal tuberosity and plantar fascia |
|---|---|
| Insertion | Middle phalanx of the fifth toe via two slips (when present) |
| Nerve Supply | Lateral plantar nerve (S1, S2) |
| Blood Supply | Lateral plantar artery |
| Actions | Flexes the PIP joint of the fifth toe when present |
|---|
The inconstant FDB fifth slip should be assessed during preoperative planning for fifth toe PIP joint surgery and flexor tendon transfer procedures. Its absence means the FDL is the only extrinsic PIP flexor, making transfers to the fifth toe less predictable than to the other toes. MRI can confirm the presence or absence of the fifth FDB slip before intrinsic rebalancing surgery.
May be palpated on the plantar fifth toe proximal phalanx level if present; absent in approximately 20-25% of feet.
Congenital or acquired deformity of the fifth toe involving PIP and MTP joint flexion from FDB fifth slip contracture or FDL dominance, managed with FDL tenotomy or PIP release.
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