INTRODUCTION
The hypothenar eminence is formed by four muscles; the abductor digiti minimi muscle, the flexor digiti minimi brevis muscle, the opponens digiti minimi muscle, and the palmaris brevis muscle [10]. Many researchers have described abnormalities of the hypothenar muscles focusing mainly on their morphology and topography [1, 4, 6, 13, 17, 22]. The hypothenar muscle with the greatest frequency of variations appears to be the abductor digiti minimi manus and the second more variable muscle is the flexor digiti minimi manus [6]. Contrarywise, the opponens digiti minimi muscle mostly lacks anatomical variations [6]. To the best of our knowledge, the first description of an accessory abductor digiti minimi manus (AADM) was done by Wood in 1868 [23].
This small aberrant muscle usually originates either from the tendon of palmaris longus muscle [2, 7, 20, 21] or the tendon of flexor carpi radialis [1]. In addition, AADM origins may extend to the flexor retinaculum [3]. As for its insertion, the most common point is the proximal phalanx of the 5th finger [12].
The aim of this article is to present a rare case of an AADM muscle with an unusual origin, and further highlight the potential clinical implications regarding ulnar nerve and artery compression.
CASE REPORT
The reported case was identified during the dissection of the right wrist and hand of a male formalin-fixed (10% v/v solution) cadaver. The dissection was held for both educational and research purposes at the Dissection’s Hall of our Anatomy Department. The cadaver was of Greek origin and derived from body donation with the written and informed consent of the donor, according to the relevant legislation [16]. The specimen was properly cleaned and photographed. A WürthTM digital Vernier calliper (0.01 mm, accuracy) was used for the measurements of the distances and nerves’ diameters.
The case described refers to the existence of an AADM muscle identified in the right wrist of a male adult. The muscle originated from the tendons of the flexor digitorum superficialis and after following a course under the pisohamate ligament (Fig. 1) its tendon inserted with the tendon of AADM into the base of the 5th proximal phalanx beneath the flexor digiti minimi muscle (Fig. 2). The ulnar artery and ulnar nerve passed beneath the AADM.
The length (L) of the AADM was 53.54 mm. The width of the muscle was 7.98 mm at its origin (w1), 2.14 mm at its insertion (w2) and its belly was of maximum thickness 0.96 mm (Fig. 3). The muscle was innervated by minor motor branches arising from the ulnar nerve. This was a unilateral finding.
DISCUSSION
Supernumerary hypothenar muscles mostly involve the abductor and flexor digiti minimi muscles [6]. The existence of an AADM muscle has been previously described and May [15] recently proposed the use of term “long abductor digiti minimi” for this category of aberrant muscles. However, we describe a unique case of an AADM that originates from the tendons of the flexor digitorum superficialis and insert to the head of the 5th metacarpal.
Embryology
The development of the hand muscles has been thoroughly studied by Cihák back in 1970’s [5]. According to his study, there are six embryonic origins for these muscles. The surface layer differentiates first, resulting in the formation of three blastemas. The radial, middle, and ulnar blastemas are responsible for the development of the abductor pollicis brevis, flexor digitorum superficialis, and abductor digiti minimi muscles (in that order) [5]. Thus, the flexor digitorum superficialis originates in the carpal area, but its blastema migrates proximally [9], and the abductor digiti minimi and abductor pollicis brevis originate from where their blastema started [15].
Frequency
The existence of an AADM muscle is considered among the most common variations of the hypothenar muscles. Its frequency varies between 22% and 35%. [18].
Clinical considerations
Generally, the existence of an AADM muscle has been implicated with ulnar nerve compression at the wrist [6, 15]. Dimitriou and Natsis (2007) [8] reported a case of intraoperative identification of an AADM that was covering Guyon’s canal and creating apparent undue pressure on the ulnar nerve proximal to its bifurcation. In that case the neurological symptoms were relieved by the resection of the AADM. Moreover, AADM’s course via Guyon’s canal may induce compression of the deep branch of the ulnar nerve and the ulnar vessels, which consists a major structural cause of Guyon’s canal syndrome [14, 17]. Therefore, hand surgeons and orthopaedic surgeons in general, should consider the potential existence of an AADM muscle when diagnosing Guyon’s canal syndrome. Worth mentioning is that, when passing through the Guyon’s canal or in cases like the one we described, the AADM may compress not only the ulnar nerve but also the ulnar artery. In addition, the presence of an AADM muscle may complicate other common surgical procedures in hand, for instance during a surgical decompression of the carpal tunnel [19].
CONCLUSIONS
It is described a case of an aberrant AADM muscle found during routine dissection in the right hand of a male cadaver. This muscle took its origin from the tendons of the flexor digitorum superficialis and inserted into the head of the 5th metacarpal. Orthopaedic surgeons and especially hand surgeons should be aware of this anatomical variation that may cause Guyon’s canal syndrome or complicate common wrist and hand surgical procedures, for instance the carpal tunnel release.
Acknowledgements
Our sincere thanks to Emeritus Prof. of Anatomy Alexandros Paraschos for his quite useful assistance.
The authors also, sincerely thank those who donated their bodies to science so that anatomical research could be performed. Results from such research can potentially increase mankind’s overall knowledge that can then improve patient care. Therefore, these donors and their families deserve our highest gratitude [11].
