Variations of the flexor digitorum superficialis as determined by an expanded clinical examination

Current examination techniques do not detect the wide clinical variability of the flexor digitorum superficialis (FDS). Modification of current examination techniques may be necessary to detect anomalies and lead to more accurate diagnosis. We examined 500 subjects using an expansion of current techniques to elicit the range of variations in FDS function discernable on clinical examination.

FDS function was evaluated by asking subjects to flex the finger of interest while all other fingers were held fully extended at the metacarpophalangeal and interphalangeal joints. Isolated flexion at the proximal interphalangeal (PIP) joint indicated independent FDS function. Obligatory flexion at the distal interphalangeal (DIP) joint indicated flexor digitorum profundus (FDP) activity. Because FDS activity could not be evaluated if concurrent FDP activity was present, these fingers were designated as having FDP substitution. Difficulty isolating PIP joint flexion suggested connections to adjacent FDS or FDP, prompting the examiner to serially release adjacent fingers while observing for improvement in PIP or DIP range of motion.

Independent FDS function was most consistently present in the ring and middle fingers, less so in the index finger, and least in the small finger. Variations included FDP substitution or connections to flexor tendons in 1 or 2 adjacent fingers with or without evidence of FDP substitution. Absent FDS function cannot be presumed in any subject based solely on clinical examination of a single digit.

Current examination techniques are inadequate to discern among the multiple variations of FDS function. An expanded examination technique is recommended for accurate diagnosis of FDS function following flexor tendon injury.