Introduction

The existing anatomical literature indicates that there are six compartments for tendons dorsal to the wrist separated by six longitudinal vertical septa. Among them, the extensor pollicis brevis (EPB) tendon passes via the first compartment and enters the base of the proximal phalanx of the thumb [9]. Conversely, the extensor pollicis longus (EPL) passes via the third compartment and enters the base of the first distal phalanx of the thumb [9]. With respect to the anatomical variation and compartments, there have been reports on different types of supernumerary and the absence of EPB tendons [1, 2]. Many studies have reported that the presence of the septum in the first compartment is observed in cadavers and de Quervain patients [3,4,5,6]. Regarding EPL variation outside of the third compartment, Yoshida et al. reported that although most EPL tendons are single tendons, duplicated tendons represented 3.6% of subjects in their cadaver study. However, variation in the third compartment is extremely rare [10]. Furthermore, to the best of our knowledge, the variation of the EPB tendon passing through the third compartment has not been reported. In this study, we describe a case of a variant course of the EPB tendon appearing in the third extensor compartment of the wrist.

Materials and methods

During routine cadaveric dissection in the Department of Gross Anatomy, we detected an variant EPB muscle on left side of a 96-year-old fresh female cadaver. No upper limbs were damaged by injury or modified by surgery. The extensor and extensor compartments of the hand were dissected, and the EPL, EPB, and abductor pollicis longus (APL) tendons were displayed. The length and diameter of each tendon at distal dorsal compartment were measured.

Results

The EPB muscle originated from the posterior surface of the radius and interosseous membrane. However, the EPB tendon passed through the third compartment instead of the first compartment. The EPB remained parallel and radial to the EPL tendon and entered the base of the first proximal phalanx (Fig. 1). The EPL tendon was attached to the base of the first distal phalanx, as observed in the normal cases. Both EPB and EPL muscles were innervated by the posterior interosseous nerve (Fig. 2). The APL was the only tendon with no variation in the first dorsal compartment without septum and entered the base of the first metacarpal, as observed in normal cases.

Fig. 1
figure 1

a The dorsal side of the wrist. EPL extensor pollicis longus, EPB extensor pollicis brevis, APL abductor pollicis longus. b The radial side of the wrists. EPL extensor pollicis longus, EPB extensor pollicis brevis, APL abductor pollicis longus

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Fig. 2
figure 2

The dorsal side of the forearm. A Posterior interosseous nerve, EPL extensor pollicis longus, EPB extensor pollicis brevis

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The lengths of the EPL, EPB, and APL tendons were 128.9, 133.6, and 104.5 mm, respectively. The diameters of the EPL, EPB, and APL at the distal dorsal compartment were 2.9, 1.9, and 6.54 mm, respectively.

This variation was only noted on the left side of the cadaver. No other associated anomalies were observed. In addition, no anomalies were found in the right hand.

Discussion

The anatomical literature reported that there are six extensor compartments comprising the extensor retinaculum in the dorsal wrist, and each extensor tendon passes through its corresponding compartment, among them, the EPB tendon is one of the thumb extensors in the first extensor compartment of the wrist. The EPB originates from the posterior surface of the radius and interosseous membrane and enters the first proximal phalanx. The EPL tendon is also a thumb extensor in the third extensor compartment of the wrist [9].

Regarding the variants of the EPL and EPB tendons, Yoshida reported that the EPL tendon is the most consistent extensor tendon of the upper limb [10]. Conversely, a number of studies on the EPB have reported on accessory tendons and their insertions [1, 2, 7].

In this study, both EPB and EPL were observed in the third compartment. The EPL tendon entered the base of the first distal phalanx. The EPB tendon entered the first proximal phalanx. Such a variant course exhibited by a single EPB tendon outside the third compartment is extremely rare.

Brunelli GA et al. reported the absence of the EPB in two of 52 examined hands [1]. Therefore, if the EPB tendon is absent in the first compartment during surgery, it may be possible that its route differs from that normally followed and that it may be present in the third compartment. Conversely, if two tendons are present in the third compartment, one may be the EPB tendon. Rousset at al. reported that ultrasonography accurately assesses variations of APL and EPB tendons in the first extensor compartment [8]. Therefore, this method is useful for reviewing each compartment using ultrasonography prior to surgery.

The knowledge of this anatomic variant will be helpful for accurate diagnosis and surgical planning.