Flexor Pollicis Longus Function
Primary Function
The flexor pollicis longus (FPL) is the sole extrinsic flexor of the thumb's interphalangeal (IP) joint and provides critical stabilization of the terminal thumb phalanx during forceful gripping and tool use activities. 1
Anatomical Actions
The FPL produces the following movements:
- Primary action: Flexion of the thumb interphalangeal joint 2
- Secondary action: Flexion of the thumb metacarpophalangeal joint 3
- Wrist-level action: Acts as a wrist flexor throughout the entire range of wrist motion and functions as a weak radial deviator when the wrist is in ulnar deviation 4
Functional Characteristics
Force Generation and Distribution
- FPL motor units generate forces highly focused on the thumb with minimal peripheral transfer to other digits 5
- During multi-digit grasping, approximately 42% of FPL motor units produce small loading forces on the index finger (averaging 5.3% of thumb force), representing a neural coupling between thumb and index finger flexors 5
- Forces on other fingers (middle, ring, small) are negligible and not statistically significant 5
Grip and Tool Use Applications
FPL is recruited most heavily during forceful tool-using behaviors and stone tool making, regardless of whether power or precision grips are employed. 1
- Consistently high FPL activity occurs during three-jaw and four-jaw chuck precision grips used in hammering, cutting, and knapping behaviors 1
- FPL activity increases maximally when resistance is applied to the thumb's volar pad during these forceful activities 1
- Relatively low FPL activity occurs during fine manipulation of food items and delicate probe-making tasks 1
Clinical Implications
Tenodesis Effect
The FPL's moment arm at the wrist provides a mechanistic rationale for passive IP joint motion therapy when treating thumb flexor tendon injuries, with benefits seen primarily during wrist extension movements 4
Injury and Repair Outcomes
- Zone II FPL lacerations require primary repair with both core and epitendinous sutures, as epitendinous suture use is associated with 10° greater active IP joint flexion postoperatively 2
- Approximately 17% of patients require reoperation following Zone II FPL repair, primarily for adhesion formation (56%) or re-rupture (28%) 2
- Solitary thumb injuries achieve 17.9° greater IP flexion compared to multi-digit injuries 2
Fluoroquinolone Risk
FPL tendinopathy can occur as a rare complication of fluoroquinolone antibiotic use, with symptoms appearing as early as 2 hours after initial exposure or as late as 6 months after discontinuation 6