49 - An Undescribed Origin of the Flexor Digit Minimi Brevis Muscle (FDMb). Anomalous Tendon Inside the Guyon’s Canal Forming Two Sub-compartments. Possible Etiology of Ulnar Tunnel Syndrome. a Case Report

Poster Board Number: 49
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Abstract Body : Introduction :
From centuries ago (Macalister in 1875 and Le Double in 1897) to nowadays (Saadeh & Bergman, 1988; Georgiev et al., 2007), numerous authors have described variations in the hypothenar region and stated that an aberrant muscle in the region, specifically in the Guyon's canal, may cause the ulnar tunnel syndrome (Sälgeback, 1977; Luethke & Dellon, 1992). The Ulnar canal is a space located at the anteromedial surface of the wrist, where the ulnar nerve, artery, and veins traverse to pass from the forearm to the wrist and hand region. Within the canal, the ulnar nerve can suffer entrapment from multiple etiologies causing signs and symptoms related to which part of the nerve is compressed or trapped. In 1861 Jean C. Felix Guyon described this space as an intra-aponeurotic tunnel.

Materials and Methods: During a routine prosection of the upper limbs, an anomalous origin related to the flexor digits minimi muscle was observed in the left hypothenar region of a female cadaver. Careful muscle dissection was performed using a Carl Zeiss loupe to recognize its attachment, function, and innervation.

Case Report:
An anomalous origin of the flexor digit minimi brevis muscle (FDM) with a fibrous band or flat tendon was found inside Guyon's canal during dissection in a female cadaver's left upper limb. The muscle originates from the pisiform bone and the tendon of the flexor carpi ulnaris muscle medially and laterally from a flat tendon or fibrous band attached to the flexor retinaculum close to the origins of the thenar muscles. The lateral attachment (fibrous band o flat tendon) of the FDMb muscle was superficial to the ulnar nerve and deep to the ulnar vessels. This flat tendon or band forms two compartments or spaces inside the ulnar canal (of Guyon), superficial and deep. The superficial sub-compartment or space contains the ulnar artery and veins. The deep space contains the ulnar nerve before its division and the deep palmar branch of the ulnar artery traveling with the nerve.

Conclusion
During the review of the pertinent literature, there is no report of the variant flexor digiti minimi brevis with the origins presented in this case report. It is a unique anatomical variation of the FDMb muscle. Because the origin flat tendon or band of this muscle crosses over the ulnar nerve in the ulnar canal of Guyon, the variant flexor digiti minimi brevis may result in ulnar nerve compression. Additionally, because of this anatomical variation of the FDMb muscle, the normal relationship between the ulnar nerve and the ulnar vessels inside Guyon's canal changed. The ulnar nerve is located deep in the posterior position. The vessels are anterior and superficially located to the nerve and separated by the anomalous tendon discovered inside the ulnar canal (Guyon’s). The knowledge of this variation is fundamental to all medical and surgical specialties to diagnose and treat problems related to the compression of the ulnar nerve in the canal of Guyon. It should be added in the differential diagnosis as a possible cause of the ulnar tunnel syndrome compressing the ulnar nerve before its division with sensory and motor clinical manifestations.