At the PIP joint, the biconcave base of the middle phalanx articulates with the convex head of the proximal phalanx. These joints are stabilized by a volar plate, extensor apparatus, capsule, and collateral ligaments. Just proximal to the PIP joint, the flexor digitorum superficialis tendon splits to allow for the flexor digitorum profundus (FDP) to travel through its center (figure 3). The FDP then passes along the palmar surface of the middle phalanx and attaches to the distal phalanx. The FDP enables DIP joint flexion.
The flexor digitorum superficialis (FDS) attaches to the palmar surface of the middle phalanx and is the primary flexor of the PIP joint. The deforming forces that act on the middle phalanx fractures are the FDS and the intrinsic tendons. Middle phalangeal fractures proximal to the FDS insertion have an apex dorsal angulation, whereas fractures distal to the FDS insertion have an apex volar angulation as shown in the figure (figure 4). Deep to the flexor tendons the volar plate provides stability against hyperextension.