A lumbrical plus finger is descibed as paradoxical extension of the IP joints while attempting to flex the fingers. In the case of a lumbrical plus finger secondary to a DIP amputation, the PIP will extend upon attempted finger flexion.
When the FDP is lacerated or amputated, FDP contraction leads to pull on the lumbricals. This leads to shortening of the lateral bands and paraodoxical PIP and DIP extension. There are several causes of lumbrical plus finger including (1) FDP laceration or rupture distal to the lumbrical origin, (2) amputation of the DIP distal to central slip insertion, and (3) excessively long flexor tendon graft. Treatment consists of lumbrical release at the level of the flexor sheath in the palm, which then prevents paradoxical PIP extension.
