Review Question - QID 389

The 1st and 2nd lumbricals are unipennate and originate on the flexor digitorum profundus (FDP) to these fingers. The 3rd and 4th lumbricals are bipennate and each have one head that originates from the FDP from the respective finger and a second head that originates from the FDP of the middle and ring finger, respectively (Illustration A).

Lumbrical insertion is classically thought to be on the radial lateral band of the extensor expansion, but Eladoumikdachi et al performed a cadaveric dissection on 14 hands and found that the insertions were variable and included proximal phalanx and volar plate. The lumbricals extend the PIP and DIP joints while also flexing the MCP joint.

Their mechanism is pathologically exemplified in the intrinsic-plus hand with associated intrinsic tightness, where the contracted intrinsic muscles (lumbricals and interosseous) as shown in (Illustration B) will prevent supple DIP/PIP flexion. The 1st and 4th dorsal interosseous muscles are bipennate muscles originating from the adjacent metacarpals of each web space and causes abduction of the fingers. The 3 palmar interosseous muscles are located on the ulnar side of the index metacarpal and on the radial aspect of the ring and small metacarpals. They insert on to the lateral bands of their respective digits and cause adduction of the fingers.