Fig. 4.

The proximal interphalangeal (PIP) joint of the little finger could only be flexed in conjunction with the distal interphalangeal (DIP) joint flexion of the same finger, this was designated as flexor digitorum profundus (FDP) substitution (flexor digitorum superficialis [FDS] deficiency), including three types. 1) A&E showed the PIP joint of the little finger of the right hand flexed dependent on the FDP action of the same finger, but the FDP tendon of the little finger has no anatomical connection with the other fingers. 2) B&F (loose) and C&G (close) showed the FDS deficiency of the little finger of the right hand, and the FDP tendon of the little finger has connection with the FDS or FDP tendons of the adjacent ring finger. 3) D&H showed the FDS deficiency of the little finger of the left hand, and the FDP tendon of the little finger has connection with the flexor tendons of the ring and middle fingers. (A-D) were examined by the expanded method, and (E-H) by our new test.