A case of double Gantzer's muscle and its possible role in nerve entrapment

Department of Anatomy and Biology, Osaka Medical College, Takatsuki, Osaka, JapanAnomalous variants are essential for teaching bothanatomists and surgeons. Muscle anomalies of theupper extremity are recognized causes of periph-eral nerve disorder (Jones et al., 1997; Pai et al.,2008). Gantzer’s muscle (GM) or accessory head ofﬂexor pollicis longus (FPL) has been implicated inKiloh-Nevin (i.e., ‘‘anterior interosseous’’ nerve)syndrome (Rask, 1979; Oh et al., 2000; Mahakka-nukrauh et al., 2004; Pai et al., 2008). Moreover,both GM and ﬁbrous arch of ﬂexor digitorum sub-limes (FDS) are considered as possible anatomicalfactors for median nerve (MN) entrapment neurop-athy (Lee and LaStayo, 2004; Bilecenoglu et al.,2005). Two GMs (Fig. 1) were detected in the rightforearm of an old Japanese male cadaver duringpractical anatomy course for undergraduate medi-cal students at Osaka medical college. Thesemuscles were present at two different anatomicalplanes. One of them, the deep GM (DGM), waslarger, voluminous in shape, and originated fromthe medial epicondyle of humerus and inserted intothe ulnar side of FPL muscle at the junctionbetween proximal and middle thirds of forearm.The second one or superﬁcial GM (SGM) wassmaller and fusiform in shape and arose from theundersurface of humeroulnar head of FDS muscleand terminated distally into the FPL close and lat-eral to insertion of DGM. The MN passed superﬁcialto the deep head of pronator teres (PT) musclewhere it gave the anterior interosseous nerve(AIN) just distal to the lower border of the crossedmuscle. The AIN arose from the lateral side of MNand both nerves coursed distally between twomyo-ﬁbro-tendinous layers; the ﬁbrous arch of FDSand SGM superﬁcially and the DGM deeply. It hasbeen reported that the arcuate ligament of Fearnand Goodfellow is the ﬁbrous tissue that blends to-gether from the PT and FDS muscles and can catchboth the AIN and MN in some patients sufferingMN compressive neuropathy (Rask, 1979). In ourcase, the origin of AIN from the radial side of MNwas reported by Ashworth et al. (1997). The AINdescended radial to the DGM, gave branches toboth SGM and FPL, sank deep to the tendon ofSGM and then passed deep to FPL where it reap-peared again medial to the latter muscle on theinterossous membrane. The manner of proximaland distal attachments, the shape as well as theinnervation of GMs, have been reported by others(Oh et al., 2000; Mahakkanukrauh et al., 2004;Pai et al., 2008). It seems that both MN and AINwere caught in intermuscular tunnel roofed by theSGM and FDS and ﬂoored by the DGM. The pres-ence of both DGM and SGM in addition to ﬁbrousarch of FDS may be involved in MN and AIN com-pressive neuropathy. Moreover, the presence oftwo GMs may result in restricted movement of FPLand subsequent pain in lower forearm as reportedby others (Ryu and Watson, 1987; Pai et al.,2008). On the other hand, the GMs may be impor-tant in local transfers for restoring function in mul-tiple nerve palsies (Pai et al., 2008). The accessoryheads of the ﬂexor muscles have been described inprimates and other mammals (pigs, foxes, andmarmots) as a muscle belly that connects themedial epicondyle origin of the FDS with the moreor less differentiated deep ﬂexors muscle. Theﬂexor muscles of the forearm that develop fromthe ﬂexor mass divide into two layers, superﬁcialand deep. The FDS, FPL, and ﬂexor digitorum pro-fundus originate from the deep layer. The existenceof accessory muscles connecting the ﬂexor musclescould be explained by the incomplete cleavage ofthe deep layer of the ﬂexor mass during develop-ment (Jones et al., 1997). This is the ﬁrst report inthe literature regarding the presence of two GMsarising from different proximal attachments at two