Anomalous bilateral contribution of extensor pollicis longus and muscle fusion of the first compartment of the wrist.

Knowledge of the anatomical variations of the muscles of the first dorsal compartments of the wrist is clinically relevant to De Quervain's tenosynovitis and to reconstructive surgeries. In the literature, there are many reports of the presence of multiple insertion tendons in the first dorsal compartment of the wrist, but few reports describe occurrences of fusion and muscle contributions. This case report describes an anomalous bilateral contribution of the extensor pollicis longus. This anomalous contribution was found through a slender auxiliary tendon that crossed laterally under the extensor retinaculum, entered the first dorsal compartment of the wrist and merged with the tendon of the extensor pollicis brevis muscle. In the same cadaver in which this contribution was present, there was atypical muscle fusion of the abductor pollicis longus muscle and extensor pollicis brevis muscle. In conclusion, anomalous bilateral contribution of the extensor pollicis longus muscle and atypical muscle fusion, concomitant with a variant insertion pattern, are the highlight of this case report. Furthermore, it is concluded that additional tendons may be effectively used in reconstructive surgeries, but that there is a need for knowledge of the possible numerical and positional variations of these tendons, with a view to making more effective surgical plans.

Introduction

The tendons of the abductor pollicis longus (APL) muscle and extensor pollicis brevis (EPB) muscle are located in the first dorsal compartment of the wrist. The APL originates in the proximal region of the dorsal surface of the radius, ulna and interosseous membrane, and it follows an inferolateral path and becomes superficial in the distal region of the forearm. There, it divides into two portions, which are inserted in the base of the first metacarpal and the base of the trapezium. The EPB originates in the distal region of the dorsal surface of the radius and the adjacent interosseous membrane and inserts in the base of the proximal phalanx of the thumb.

De Quervain's syndrome is characterized by pain in the region of the styloid process of the radius, coming from stenosing tenosynovitis of the tendons of the APL and EPB, in the region of the first dorsal compartment of the wrist. Variations in the number, length, thickness and insertion pattern of the tendons have been well described in the literature and have an important role in comprehension of the etiology of De Quervain's stenosing tenosynovitis. Therefore, knowledge of the anatomical variations of the muscles of this region is important for surgeons during reconstructive surgery. This case report had the objective of presenting an occurrence of a fused muscle belly of the first dorsal compartment of the wrist and the presence of an anomalous contribution from the extensor pollicis longus muscle with the extensor pollicis brevis muscle.

Case report

During an investigation on occurrences of abnormal muscle fusions of the first dorsal compartment of the wrist, an anomalous bilateral contribution from the extensor pollicis longus muscle was found (Fig. 1). There was an unusual bilateral contribution from a slender auxiliary tendon that crossed laterally under the extensor retinaculum, entered the first dorsal compartment of the wrist and merged with the tendon of the extensor pollicis brevis muscle. In the same cadaver in which this contribution was present, there was anomalous muscle fusion of the APL and EPB (Fig. 1, Fig. 2).

Fig. 1

Photo showing the left forearm in pronated position. * denotes fused muscle belly of the abductor pollicis longus and extensor pollicis brevis. EPL, extensor pollicis longus; EPB, extensor pollicis brevis; #, auxiliary tendon; APL, abductor pollicis longus.

Fig. 2

Photo showing details of the anomalous muscles of the first dorsal compartment of the wrist and the auxiliary tendon of the extensor pollicis longus muscle. * denotes fused muscle belly of the abductor pollicis longus and extensor pollicis brevis. EPL, extensor pollicis longus; EPB, extensor pollicis brevis; #, auxiliary tendon; APL, abductor pollicis longus.

Morphological characteristics such as length and width were described by using the reference points of the standard muscle origin and insertion. Anthropometric measurements were made by positioning a string passively on the tendons and muscle bellies, and the dimensions were confirmed by means of universal digital calipers (Mitutoyo®), with precision of 0.05 mm. The measurement technique was carried out by the same evaluator, who made three consecutive measurements. The mean of the values obtained was used.

The right APL was 11.5 cm in length and 2.7 cm in width. The ulnar origin (ulnar head) was located 20.5 cm from the insertion (base of the first metacarpal). The distance between the radial origin (radial head) and its insertion (base of the first metacarpal) was 18.2 cm. The right APL had a since tendon of approximately 9 cm in length and it had two insertion fascicles: one to the abductor pollicis brevis (APB) muscle and the other to the opponens pollicis (OP) muscle.

The left APL was 15.7 cm in length and 2.5 cm in width. The ulnar origin (ulnar head) was located 20 cm from the insertion (base of the first metacarpal). The distance from the radial origin (radial head) and the insertion (base of the first metacarpal) was 17.5 cm. The tendon of the left APL had three branches: intermediate tendon (I), lateral tendon (L) and medial tendon (M) (Fig. 3).

Fig. 3

Photo showing details of the insertion tendons of the abductor pollicis longus of the left arm. * denotes fused muscle belly of the abductor pollicis longus and extensor pollicis brevis. EPL, extensor pollicis longus; EPB, extensor pollicis brevis; #, auxiliary tendon; APL, abductor pollicis longus; M, medial tendon; I, intermediary tendon; L, lateral tendon of the fused muscle belly.

Tendon I was 7.5 cm in length, while tendons M and L were 7 cm. Tendons M and I were inserted in the base of the first metacarpal, while tendon M presented three insertion fascicles: APB, OP and the anteromedial region of the base of the first metacarpal. Both of the EPB muscles received anomalous donation from the EPL, through a slender auxiliary tendon of mean length 9.2 cm, which crossed obliquely and laterally under the extensor retinaculum, entered the first dorsal compartment of the wrist and merged with the tendon of the EPB.

The right EPB was 7.2 cm in length and 1.2 cm in width. Its radial origin was located 14.5 cm from the insertion. It was inserted at the level of the extensor aponeurosis of the metacarpophalangeal joint of the thumb. However, the left EPB was 8.5 cm in length and 1.1 cm in width, and its radial origin was located 17 cm from the insertion.

The innervation of the fused bellies was, as usual, through the posterior interosseous nerve. No neurovascular variation was observed in the region of the wrist and hand.

Discussion

There are several reports on multiple insertion tendons of the APL in the literature. These are presented as an atavism,4, 5 with reports of three, four, five, seven and nine insertion tendons.8, 9

The presence of supernumerary tendons of the APL is of great clinical importance, particularly during reconstructive surgery. Accessory tendons of the APL can be used for tendon transfer, tendon translocation and interposition arthroplasty.9, 10, 11, 12

Prior knowledge of the existence and importance of these accessory tendons may be useful in surgical procedures, in view of their use as replacements for the extensor pollicis longus (EPL) and in reconstructions of the first dorsal compartment of the wrist.14, 15 However, the possibility that the auxiliary extensor tendon might contribute toward the action of the extensor pollicis brevis muscle is very interesting and unexpected. There are no reports of this in the literature.

According to the literature, the presence of two or more tendons of the APL is observed in 94% of the cases. The EPL presents two tendons in 3% of these cases. The presence of multiple tendons represents a type of atavism. It is generally reported in primates such as chimpanzees, gorillas and gibbons. These anomalous tendons in humans are of anthropological and phylogenetic importance, given that their presence constitutes a reminiscence of ancestral forms of muscle insertion.2, 3, 6, 8, 17

From a phylogenetic point of view, the APL and EPB are known to develop through differentiation from a common muscle mass. Complete separation of the EPB is only observed in humans. Reports of its absence or of fusion with the EPL have only rarely been described in the literature. However, further studies are needed in order to elucidate the factors relating to fusion of the APL and EPB. The incidence of doubling of the EPB is unclear, given that the rates reported in different studies have been 1%, 2% and 8.3%. However, multiple tendons may constitute a reminiscence of the pattern of fetal tendon development.

The most important clinical condition involving anomalies of the APL and EPB is De Quervain's syndrome. This condition arises due to thickening and inflammation of the tendons of the APL and EPB and results in pain and edema in the first dorsal compartment of the wrist. The pain becomes exacerbated during movements of the thumb, which makes the tendons more liable to degenerative processes and lesions. Surgical release of the tendons inside the dorsal compartment of the wrist is a curative measure for De Quervain's syndrome. Thus, deeper knowledge of the possibilities for tendon anomalies is needed before undertaking any reconstruction or in planning for surgical correction of tendinopathy.

In conclusion, the anomalous bilateral contribution of the EPL and atypical fusion of the APL and EPB, concomitantly with a variant insertion pattern, are the highlight of this case report. Furthermore, it is concluded that additional tendons may be effectively used in reconstructive surgeries, but that there is a need for knowledge of the possible numerical and positional variations of these tendons, with a view to making more effective surgical plans.

Conflicts of interest

The authors declare that there were no conflicts of interest.