Overview
This note outlines the clinical examination process for assessing finger flexor tendon injuries, focusing on the evaluation of the flexor digitorum profundus (FDP) and flexor digitorum superficialis (FDS).
Anatomy of Finger Flexors
- Each finger has two primary flexors: FDP (flexor digitorum profundus) and FDS (flexor digitorum superficialis).
- FDP is a mass action muscle with one muscle belly and four tendons serving all fingers.
- FDS acts individually on each finger, though there are anatomical variations, especially in the little finger.
General Examination Approach
- Assess for a normal cascade: at rest, each finger should be flexed to a different degree.
- In uncooperative patients (e.g., children, unconscious), gross composite flexion can be elicited by squeezing the forearm.
- Observing composite flexion helps determine continuity of the flexor tendons.
Specific Examination Techniques
- To test FDP: stabilize the middle phalanx and ask the patient to flex the fingertip at the distal interphalangeal joint.
- To test FDS: hold all other fingers in extension to "handicap" the FDP, then ask the patient to flex the target finger at the proximal interphalangeal joint.
- Isolating FDS ensures that flexion at the PIP joint is due to the FDS tendon only.
Considerations and Exceptions
- The little finger may not have an independent FDS tendon in all individuals.
- Failure to flex the PIP joint of the little finger could be due to either absence of FDS or tethering to the ring finger’s FDS.
- If releasing the ring finger enables flexion of both little and ring fingers simultaneously, tethering is likely present.