The deep head is ulnar-innervated (unlike the superficial head which is median), making FPB the only intrinsic muscle with dual innervation. Inserts via the ulnar sesamoid.
| Origin | Ulnar aspect of the first metacarpal, trapezoid and capitate |
|---|---|
| Insertion | Ulnar sesamoid at the MCP joint |
| Nerve Supply | Deep branch of ulnar nerve (C8, T1) |
| Blood Supply | Princeps pollicis artery |
| Actions | Flexes the thumb MCP joint; Provides medial MCP stability via ulnar sesamoid |
|---|
The dual innervation explains why thumb MCP flexion may be partially preserved in both median and ulnar palsies. The deep head's ulnar sesamoid insertion contributes to MCP volar plate integrity.
Not testable separately from the superficial head clinically.
Loss of deep FPB head producing ulnar sesamoid instability and MCP ulnar deviation.
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