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Ann Anat 189 (2007) 276—282

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Undescribed variant muscle –

‘‘Deep abductor-flexor’’ of the little finger,in relation to ulnar nerve compressionat the wrist

G.P. Georgiev, L. Jelev, L. Surchev�

Department of Anatomy, Histology and Embryology, Medical University Sofia, blvd. Sv. Georgi Sofiiski 1,BG-1431 Sofia, Bulgaria

Received 21 August 2006; accepted 9 October 2006

KEYWORDSHypothenar muscles;Ulnar nerve;Variations;Human

ee front matter & 2006aanat.2006.11.003

ing author. Tel.: +359 287 83.ess: surchev@medfac.a

SummaryDuring routine anatomical dissection in the hypothenar region of the left hand of a64-year-old female cadaver, a number of variant structures were observed. The mostprominent finding in our case was a supernumerary muscle hitherto unknown in theanatomical literature. This variant muscle had a muscular body formed by theconnection of two deeply situated muscular bellies - medial and lateral. The lateralbelly originated from the flexor retinaculum, the medial one - from the hamatebone. The common muscular body inserted to the antero-lateral surface of the baseof the fifth proximal phalanx. Due to its location and possible function, we namedthe variant muscle ‘‘deep abductor-flexor’’ of the little finger. The flexor digitiminimi brevis muscle showed two proximal tendons – the medial tendon wasattached to the hamulus of the hamate bone while the aberrant lateral tendonoriginated from the lateral part of the flexor retinaculum. Both, the aberrant lateraltendon of the flexor digiti minimi brevis and the lateral belly of the ‘‘deep abductor-flexor’’, passed over the palmar branch of the ulnar nerve, which define theirpossible clinical significance in ulnar nerve compression. Therefore, the variations ofthe hypothenar muscles are reviewed and their relation to the compression of theulnar nerve is discussed.& 2006 Elsevier GmbH. All rights reserved.

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cad.bg (L. Surchev).

Introduction

The variations of the hypothenar muscles havebeen described by different authors (Frohse and

rved.

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Undescribed muscle of the little finger 277

Frankel, 1908; von Lanz and Wachsmuth, 1935;Gloobe and Pecket, 1973; Schmidt, 1988; Sanudoet al., 1993; Spinner et al., 1996; Olave et al.,1997; Schmidt and Lanz, 2004; Murata et al., 2004).In the literature there are reports of an aberrantpalmaris brevis muscle (Tonkin and Lister, 1985;Stark et al., 1979), abductor digiti minimi muscle(Bucher, 1943; Jeffery, 1971; Gloobe and Pecket,1973; Al-Qattan, 2004), flexor digiti minimi brevismuscle (Saadeh and Bergman, 1988; Spinner et al.,1996; Madhavi and Holla, 2003; Wingerter et al.,2003), and opponens digiti minimi muscle(Maca-lister, 1875; Le Double, 1897; Frohse andFrankel, 1908). Since the proximal attachments ofthe hypothenar muscles are closely related to theulnar nerve course at the wrist, some of thesemuscular variations might cause nerve compression(Schjelderup, 1964; Shea and McClain, 1969;Jeffery, 1971; Tonkin and Lister, 1985; Simodynesand Cochran, 1981; Soldado-Carrera et al., 2000;De Smet, 2002; Schmidt and Lanz, 2004; Al-Qattan,2004). Moreover, an aberrant muscle may coexistwith an ulnar nerve variation in that region(Bozkurt et al., 2004; Murata et al., 2004).

Here, a case of an unusual composition of thehypothenar muscles is reported. Together with thepalmaris brevis, abductor digiti minimi, flexor digitiminimi brevis and opponens digiti minimi, in ourcase, a well-defined but hitherto unknown in theanatomical literature deep aberrant muscle wasobserved. We compare our findings with theanatomical and clinical reports available up tonow on the variant hypothenar muscles. We alsoemphasize their potential role in ulnar nervecompression at the wrist.

Materials and methods

The muscular variations were found during aroutine anatomical dissection in the hypothenarregion of the left hand of a 64-year-old formol-carbol fixed female cadaver. It was one of a seriesof 108 dissected cadavers, examined over a 22-yr-period. All autopsy material was available at theDepartment of Anatomy, Histology and Embryologyof the Medical University of Sofia.

Results

During the dissection of the left hand, afterremoval of the palmar aponeurosis and the palmarislongus muscle, unusual muscular and tendinousstructures located between the ulnar artery and the

palmar branch of the ulnar nerve just distal to thecanal of Guyon were observed (Fig. 1). In order toexpose the aberrant structures, the ulnar artery wascut and retracted. Subsequent complete dissectionrevealed two proximal tendons of the flexor digitiminimi brevis muscle (Fig. 2). The medial shorttendon was attached to the common origin of themuscle from the hamulus of the hamate bone. Theaberrant lateral tendon (length – 1.5 cm, width –

0.3 cm) originated from the lateral part of the flexorretinaculum near the attachment of the abductorpollicis brevis muscle. This additional tendon archedover the attachment of an unknown muscle andjoined the medial tendon. To allow for a deeperapproach, the medial tendon of the flexor digitiminimi brevis was cut out and retracted laterally. Inaddition, the muscular body of the abductor digitiminimi was retracted medially. In that way anunusual muscle located deep to the flexor digitiminimi brevis and abductor digiti minimi wascompletely revealed (Fig. 3). This aberrant musclewas composed of two well-defined muscular bellies.The lateral one, approximately 3.3 cm long and0.7cm wide, originated from the lateral part of theflexor retinaculum, just medial to the origin of theabductor pollicis brevis. The muscular belly crossedthe ulnar nerve obliquely over the flexor retinaculumand passed in the same direction as the flexor digitiminimi brevis. The medial belly, approximately1.7 cm long and 0.5 cm wide, arose from the hamulusof the hamate bone. It was located under the flexordigiti minimi brevis and deep lateral to the abductordigiti minimi. The two aberrant bellies formed acommon muscular body, the lateral part of whichbecame tendinous. Distally, the muscular body wasattached to the antero-lateral surface of the base ofthe proximal phalanx of the little finger. The deepbranch of the ulnar nerve innervated the twomuscular bellies. The action of the identified muscleappeared to be flexion of the metacarpophalangealjoint and slight abduction of the little finger.

As was described previously, distal to the Guyon’scanal the ulnar nerve and artery were separated bytwo variant structures – the lateral belly of theaberrant muscle (proximally) and the additionallateral tendon of the flexor digiti minimi brevis(distally). Deep to these structures, the palmarbranch of the ulnar nerve divided into the super-ficial and the deep branches. Distal to the flexorretinaculum, a one-millimeter wide communicatingsensory branch between the superficial branch ofthe ulnar nerve and the third common palmardigital nerve of the median nerve was observed.According to Stancic et al. (1999) this commonbranch, also known as Berrettini branch, wasobserved in 81% of the examined hands.

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Figure 1. Photograph (a) and schematic en face view (b) of the wrist and hand. The abbreviations used in this and thesubsequent two figures are as follows: Muscles – FPB, flexor pollicis brevis; APB, abductor pollicis brevis; LT, lateraltendon of the flexor digiti minimi brevis; MT, medial tendon of the flexor digiti minimi brevis; FDMB, flexor digiti minimibrevis; LB, lateral belly of the ‘‘deep abductor-flexor’’ of the little finger; MB, medial belly of the ‘‘deep abductor-flexor’’ of the little finger; CB, common body of the ‘‘deep abductor-flexor’’ of the little finger; ADM, abductor digitiminimi. Arteries – RA, radial artery; MA, median artery; UA, ulnar artery. Nerves – MN, median nerve; PBUN, palmarbranch of the ulnar nerve.

G.P. Georgiev et al.278

Additionally in this case a variation in formationof the superficial palmar arch was observed. Duringthe dissection of the forearm, a persisting medianartery arising from the anterior interosseal arterywas established. The variant artery (1.8mm indiameter) passed on the lateral side of the mediannerve, entered the carpal tunnel of the hand andafter that descended under the palmar aponeurosisin the third interosseous space and joined the ulnarartery to form the superficial palmar arch. Accord-ing to Lippert and Pabst (1985) this type offormation of the superficial palmar arch is repre-sented in 4% of the cases.

Discussion

Variations of the hypothenar muscles, includingfusion with neighboring muscles, absence, orduplication, have previously been described(Macalister, 1875; Lipscomb, 1960; Hollinshead,

1967; Jeffery, 1971; Sanudo et al., 1993; Schmidtand Lanz, 2004; Al-Qattan, 2004; Murata et al.,2004). Different aberrant muscles have beenobserved during cadaver dissections (Macalister,1875; Bucher, 1943; Carr et al., 1977; Schmidt,1988; Wahba et al., 1998; Kobayashi et al., 2003;Madhavi and Holla, 2003; Bozkurt et al., 2004;Bakinde et al., 2005) and surgical interventions(Lipscomb, 1960; Roberts, 1972; Al-Qattan andDuerksen, 1992; Netscher and Cohen, 1998; Santoroet al., 2000; Al-Qattan, 2004). The unusual mus-cular structures described in our report seem to berelated to the flexor digiti minimi brevis andabductor digiti minimi and therefore we reviewthe reported variations of these two muscles.

Among the hypothenar muscles, the variations ofthe abductor digiti minimi are the most frequentlydescribed in the literature (Harvie et al., 2004).Macalister (1875) reported different variants of thismuscle - three origins, second head, fusion with theflexor digiti minimi brevis, origin only from the

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Figure 2. Photograph (a) and schematic en face view (b) of the wrist and hand. Ulnar artery is cut and retracted.Abbreviations are designated in Fig. 1.

Undescribed muscle of the little finger 279

pisiform bone and even absence. Other authorsdescribed an origin from the fascia of the forearm(Jeffery, 1971), palmaris longus muscle (Gloobe andPecket, 1973; Simodynes and Cochran, 1981;Soldado-Carrera et al., 2000), fascia of the flexorcarpi radialis muscle (Simodynes and Cochran,1981), intermuscular fascia (Bergman et al.,2004), flexor carpi ulnaris muscle (Bergman et al.,2004) and flexor retinaculum (Bergman et al.,2004). According to Le Double (1897) and Bucher(1943) this muscle occasionally divided into two orthree fascicles. Sanudo et al. (1993) also describedthe anomalous fascicles in abductor digiti minimioriginating both from the flexor retinaculum andantebrachial fascia. Although the aberrant muscu-lar structures belonging to the abductor digitiminimi have different origins, three main endingshave been reported – fusion with the muscularbelly of the abductor digiti minimi (Wood, 1867,1868; Jeffery, 1971; Sanudo et al., 1993; Al-Qattan,2004), attachment to the tendon of the abductordigiti minimi (Lipscomb, 1960) or attachment to thebase of the proximal phalanx of the little finger(Gloobe and Pecket, 1973; Sanudo et al., 1993).

Compared to the abductor digiti minimi, varia-tions related to the flexor digiti minimi brevis

muscle seem to occur less frequently (Jeffery,1971). Macalister (1875) reported different varia-tions of this muscle - absence, presence of anaccessory palmaris slip joined to the flexor digitiminimi brevis, an unciform origin, presence of a slipto the metacarpal bone, and fusion with theabductor. Other authors reported an origin fromthe antebrachial fascia (Saadeh and Bergman,1988; Madhavi and Holla, 2003; Wingerter et al.,2003). According to Le Double (1897) the flexordigiti minimi brevis may be fused with theopponens and abductor digiti minimi. This musclemay also be replaced by a tendinous band arisingfrom the flexor carpi ulnaris and inserting onto thebase of the fifth proximal phalanx and the hamulusof the hamate bone (Bergman et al., 2004).

After an extensive review of the literature, itseems that the variant muscle composed of twobellies situated deep to the abductor digiti minimiand flexor digiti minimi brevis, as presented here,is unknown in the anatomical descriptions. Aber-rant muscular structures similar in position to themedial (Macalister, 1875; Murata et al., 2004) or tothe lateral (Wood, 1868; Ruge, 1908; Sanudo et al.,1993; Bozkurt et al., 2004) bellies of the variantmuscle have been reported, however, we describe

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Figure 3. Photograph (a) and schematic en face view (b) of the wrist and hand. Ulnar artery is cut and retracted.Medial tendon of the flexor digiti minimi brevis is cut and retracted laterally. The abductor digiti minimi is retractedmedially. Abbreviations are designated in Fig. 1.

G.P. Georgiev et al.280

such a conjunction for the first time in theanatomical literature. Because of its morphologyand probable function we name the variant muscle‘‘deep abductor-flexor’’ of the little finger (m.abductor et flexor digiti minimi profundus).

With regard to embryological development, itshould be noted, that the hypothenar muscles arisefrom a mesenchimal blastema, dividing very earlyinto the superficial and deep layers. The superficiallayer gives rise to the abductor digiti minimi, whilethe flexor digiti minimi brevis and the opponensdigiti minimi develop from the deep layer (Cihak,1972). Due to their location and relation to theneighboring muscles, we suppose an origin of thebellies of the ‘‘deep abductor-flexor’’ of the littlefinger from the deep part of the muscular blas-tema.

Usually, the ulnar artery and the palmar branchof the ulnar nerve pass between the palmarisbrevis, composed of a thin muscular layer, and therest of the hypothenar muscles to reach the palm.Different relations might be observed in the casesof variations of the hypothenar muscles. Sanudoet al. (1993) and Bozkurt et al. (2004) describedcases, in which an aberrant muscle passed palmarto the ulnar nerve, between it and the ulnar artery.A similar constellation was observed in our case. In

the cases of Gloobe and Pecket (1973) and Sanudoet al. (1993) the aberrant muscles overlay both theulnar artery and nerve. So, the relations of thevariant muscles to the ulnar nerve defined theirrole in ulnar nerve compression.

In clinical practice, the variant muscles in theregion of the wrist could be incidentally foundduring surgical procedures without provoking clin-ical symptoms (Uriburu et al., 1976; Simodynes andCochran, 1981) or the presence of an accessorymuscle may simulate a soft-tissue tumor (Simo-dynes and Cochran, 1981). Nevertheless, accordingto Ruocco et al. (1998) there has been extensivesurgical documentation in the orthopedic literatureof anomalous muscles that induced ulnar nervecompression syndromes. Jeffery (1971) stated, thatwhen the anomalous muscles produced clinicalsymptoms, they appear to be related to twofactors: the anatomical site of the muscle and thepresence of muscle hypertrophy. Turner and Caird(1977) considered the fact that provoking factorscould be an injury or nature of the work, which maycause hypertrophy of the muscles or subject thehand to repetitive minor trauma. There are manyulnar nerve compression syndromes provoked byanomalous muscles in which additional factors playa role (Jeffery, 1971; Roberts, 1972; Tonkin and

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Lister, 1985; Al-Qattan, 2004). In addition to nervecompression, an aberrant muscle may be alsoassociated with thrombosis of the ulnar artery(Carneirio and Mann, 1979; Pribyl and Moneim,1994).

The ‘‘deep abductor-flexor’’ of the little finger,presented in our report, has a close relation to thepalmar branch of the ulnar nerve. Thus, this variantmuscle could be a cause of ulnar nerve compressionwith possible motor and sensory dysfunction andmust be borne in mind by clinicians.

Acknowledgements

The authors wish to thank Prof. Dr. Hans-MartinSchmidt (Anatomical Institute, University of Bonn,Germany) for his critical reading of an earlierversion of the manuscript and for many helpfulsuggestions.

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