When the flexor digitorum profundus (FDP) is completely lacerated distal to the origin of the lumbrical muscle, the lumbrical muscle remains connected to both the FDP origin and its insertion into the dorsal apparatus. Normally contraction of the FDP pulls the FDP proximally but since it is disconnected, its proximal movement cannot flex the finger. Instead, the FDP pulls through the still-connected lumbrical muscle to flex the MP joints and extend the IP joints of the finger. This is called lumbrical-plus.(1) A lumbrical-plus finger results from abnormal biomechanics and only surgery to reconnect the profundus can resolve this paradoxical extension.
Parkes, A. “The “lumbricalis plus” finger.” Journal of Bone Joint Surgery 53B (1971): 236.
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