Entrapment of the Proximal Phalangeal Head by the Flexor Digitorum Superficialis Tendon as a Mechanical Block to Reduction in Open Dorsal Proximal Interphalangeal Joint Dislocations: Two Case Reports.

Most dorsal proximal interphalangeal (PIP) joint dislocations can be reduced closed, but soft-tissue entrapment may prevent reduction. We report 2 open dorsal PIP joint dislocations in which the flexor digitorum profundus (FDP) tendon was displaced ulnarly. In both cases, the joint remained irreducible after the FDP tendon was restored to its normal volar position. In case 1, the ulnar condyle of the proximal phalangeal head was entrapped by the flexor digitorum superficialis (FDS) tendon. In case 2, the entire proximal phalangeal head had passed between the 2 FDS slips and become entrapped. Releasing the entrapment allowed reduction in both cases, after which the volar plate was repaired. At final follow-up, active PIP motion was 0° to 100° at 12 months in case 1 and -10° to 100° at 6 months in case 2, without lateral instability. Persistent irreducibility after restoration of the FDP tendon to its normal volar position should prompt direct inspection for entrapment of the proximal phalangeal head by the FDS tendon.