Hypothenar Muscles and Guyon’s Canal

The increased number of articles in the last years about hypothenar variations and some misdescriptions and the role of the additional structures to ulnar nerve and artery compression, as well as my unostentatious contribution in the field, provoked me to write this chapter. The aim of it is to present in detail the usual hypothenar muscular anatomy, including the origins and insertions of the hypothenar muscles, their relations to each other, the vascular supply and innervation, the function of the muscles, the reported variations and their possible clinical implications. Herein, I also presented briefly the Guyon’s canal anatomy and some interesting comments about it. Presenting the compendium about hypothenar muscles and the canal to my opinion will help the anatomists and the clinicians to better understand the clinically oriented anatomy. They also will be more qualified in the anatomical dissection course as well as during the surgical interventions. The detailed knowledge of the anatomy in the region would be also useful to medical students in better understanding the hypothenar region.


Introduction
Muscular variations of the hypothenar have been well described in the medical literature [1][2][3]. In most cases, these muscles are asymptomatic and are found during anatomical dissections, surgical interventions or imaging modalities. In some cases, variant hypothenar muscles may cause ulnar nerve and artery compression, as presented in some surgical case reports [4][5][6][7][8][9][10]. Shea and McClain [11] reported that around 3% of compression neuropathies are due to variant muscle.
At the region of the wrist, the ulnar nerve passes through a fibro-osseous tunnel known as "Guyon's canal" or "distal ulnar tunnel", in which the ulnar nerve could be compressed [4,6,8,[12][13][14]. Different causes as trauma, lipoma, false aneurysm of the ulnar artery, ganglion cyst and rarely aberrant muscular slips have been reported [3,15]. In some surgical reports, different variant muscles, usually abductor digiti minimi, followed by flexor digiti minimi brevis have been reported, as a cause of ulnar nerve compression [4,6,8,[12][13][14]. In all cases the excision of the additional muscle was curative. However, it should be pointed out that the clinical appearance of the variant muscles should be related to two factors: the anatomical site and the muscle hypertrophy [4]. According to Turner and Caird [16], a provoking factor, such as acute injury or repetitive minor trauma, as well as the type of the work, could predispose to hypertrophy of the variant muscles. There are reports of ulnar nerve compression syndromes provoked by anomalous muscles in which additional factors exist [1,4,17]. In addition to nerve compression, a hypothenar muscle variation could be also associated with thrombosis of the ulnar artery in Guyon's canal [7,18].
In recent years the increased interest and numerous articles about hypothenar variants and Guyon's canal provoke me to make a brief review and make a compendium of normal anatomy of the muscles in the hypothenar region as well to present the reported variations and their possible clinical significance. I hope that this chapter will make future studies on this theme easier and help anatomist, hand surgeons and medical students for better knowledge of anatomy and for better clinical practice.

Palmaris brevis muscle (Pbm)
Pbm is a small cutaneous hand muscle, located superficially to the hypothenar eminence and considered to be atavistic remnant of the panniculus carnosus. The Pbm is usually presented by a thin, quadrilateral in form muscle body, lying beneath the skin of the ulnar aspect of the palm. It has been reported to start from the flexor retinaculum and palmar aponeurosis and insert in the skin and fascia of the medial palm (Figure 1) [19][20][21][22][23][24].

Variations
The variations of the Pbm are rarely reported in the literature. This muscle could vary in size and may be absent or duplicated; it could also insert to flexor digiti minimi brevis muscle or the pisiform bone [25,26].

Clinical application
This muscle and its variants could simulate soft tissue tumor [27] or provoke ulnar nerve compression at the wrist [17,28]; Pbm flap could be used in the treatment of recurrent carpal tunnel syndrome [29] and Pbm spasm syndrome [30].

Actions
Pbm deepens the hollow of the palm and presents muscular barrier of the ulnar neurovascular bundle at the wrist [21].

Innervation
The Pbm is innervated by the motor component of the superficial branch of the ulnar nerve [24].

Abductor digiti minimi muscle (ADMm)
The ADMm is situated more ulnarly of the hypothenar muscles and arises from the pisiform bone and from the tendon of the flexor carpi ulnaris muscle; it attaches as a flat tendon that finally divides into two parts: one that inserts to the ulnar side of the base of the proximal phalanx of the little finger and the other to the ulnar border of the aponeurosis of the extensor digiti quinti proprius (Figure 2) [20,21].

Actions
The ADMm abducts the little finger from the ring finger and contributes to the fifth metacarpophalangeal joint flexion and interphalangeal extension [20,21].

Flexor digiti minimi brevis muscle (FDMBm)
The FDMBm is situated more radially than the ADMm. It starts from the hamulus of the hamate bone, and the anterior surface of the flexor retinaculum, and inserts into the ulnar side of the base of the phalanx of the little finger (Figure 3) [20,21].

Variations
The reported variations of the FDMBm in the available literature are absence and presence of an accessory slip from the palmaris longus tendon, presence of a slip to the metacarpal, replacement by a tendinous band arising from the flexor carpi ulnaris muscle and inserting into the fifth proximal phalanx and the hamate bone, presence of accessory FDMBm, fusion with ADMm or opponens digiti minimi muscles, origin from the antebrachial fascia, deep abductor-flexor of little finger, FDMBm with triple origin and origin from flexor carpi radialis muscle [25,26,33,[40][41][42][43].

Actions
The FDBMm flexion of the proximal phalanx, also with some lateral rotation [20,21].

Opponens digiti minimi muscle (ODMm)
The ODMm has a triangular form, lying beneath the ADMm and FDMBm. It starts from the hamulus of the hamate bone and near part of the flexor retinaculum and attaches to the ulnar margin and palmar surface of the fifth metacarpal bone (Figure 4) [20,21]. The reported variations of the ODMm are rarely described in the literature and include absence, splitting into two parts and merging with ADMm [25,26].

Clinical application
There is no clinical application reported.

Actions
The ODMm flexes and laterally rotates the fifth metacarpal bone at the carpometacarpal joint, brings the fifth finger into opposition with the thumb and together with the ADMm and FDMBm absorbs forces on the ulnar border of the hand [20,21].

Anatomy of Guyon's canal
The Guyon's canal or ulnar canal is a fibro-osseous tunnel situated between the pisiform and the hook of the hamate and measured around 40-45 mm in length [45,46]. In 1861, it is first described by the French surgeon Jean Casimir Félix Guyon. He presented it as an intra-aponeurotic compartment which the anterior wall is constituted by a fibrous layer and its posterior wall by the anterior carpal ligament [47]. Guyon's canal is situated between the proximal edge of the palmar carpal ligament to the fibrous arch of the hypothenar muscles at the level of the hook of the hamate (Figure 5). Through this canal the ulnar nerve and artery pass from the forearm to the palm, as the nerve is lying deep and ulnar to the artery. Of course, the vena comitans and connective fatty tissue fill up this space [48, 49].
Guyon's canal has been described as a space with complex and variable anatomy [48]. It should be pointed out that the canal is not a rigid structure and varying in its dimensions [50]. According to Ombaba et al. [49], it has dynamic space, and its relationships are changed during wrist movement.
Precise knowledge of the anatomy of Guyon's canal is essential in understanding the diagnosis and treatment of ulnar tunnel syndrome [51]. This tunnel is a potential entrapment site that could provoke compression of the ulnar nerve, presented by paresthesia or numbness, or both, to the ulnar two digits, as well as hand weakness, atrophy and ungainliness [51]. The compression could be localized in three zones [11,52]: Zone I: compression occurs proximal to or within Guyon's canal, before the nerve bifurcation, and presents with combined motor and sensory deficits.
Zone II: compression involves only the deep motor branch and occurs as the ulnar nerve exits Guyon's canal or at the hook of the hamate level.
Zone III: compression, with isolated superficial sensory branch involvement, may also occur as the nerve exits Guyon's canal or at the hook of the hamate distal to the bifurcation.
Importantly during surgical interventions, all three compartments should be decompressed, including the pisohamate hiatus, by releasing the pisohamate arcade [49].

Conclusion
This chapter summarizes the existing data in the literature concerning the anatomy of the hypothenar muscles, as well as its variants and the anatomy of Guyon's canal. I hope that the presented literature data will help students in learning anatomy, help the anatomists and hand surgeons during their works, as well as for better scientific production and to better understand, describe and classify the muscular variations and Guyon's canal. Using the chapter, they will avoid mistakes in classifying different variations of the hypothenar muscles. I think that the knowledge of reported variants will ensure self-confidence and avoid confusions during wrist and palm procedures, especially during releasing Guyon's canal, extended carpal tunnel release, or palmar fasciectomy. Precise knowledge of the anatomy and its variant also would help radiologist in their routine work on ultrasonography and MRI, where different variations could be evaluated in cases of soft tissue tumor or in diagnosis of primary or recurrent ulnar nerve compression.

Conflict of interest
The author declares no conflict of interest.