INTRODUCTION

In the classic or standard description of extensor pollicis long (EPL) muscle in most of the anatomical references, it is considered with the supinator, abductor pollicis longus, extensor pollicis brevis and extensor indicis proprius as the muscles of deep group of dorsal antibrachial muscles of the forearm (^{Austin, 2005}).

The extensor pollicis longus (also called Extensor secundi internodii pollicis) takes a proximal attachment or origin from the lateral aspect of the middle third of posterior surface of the shaft of ulna (^{Beatty et al., 2000}). The fleshy part, then gets converted into a tendon which is about 6.7 to 9.7 cm. in length (^{Christ & Brand-Saberi, 2002}). It then passes through dorsal radiocarpal ligament in the detached third compartment (Beatty et al.). The tendon of EPL then forms the posterior boundary of anatomical snuff box. Finally, it passes into the posterior aspect of the base of distal phalanx of thumb (^{Leaver & Chatterjee, 2013}).

The action of EPL can be easily understood from the tendon insertion that it makes as extension for the thumb at interphalangeal joint at about 100 (^{Lee & Jung, 2014}). It is the only tendon that can achieve the full hyperextension in this joint (^{Moore et al., 2010}). It shares in the extension of metacarpophalangeal and carpometacarpal joints of the same digit (Austin).

The nerve supply of EPL is by the motor branch of radial nerve i.e. posterior interosseous nerve (or dorsal interosseous nerve) which also supplies to the most of the muscles on the lateral side and posterior aspect of the forearm (^{Papaloïzos, 2004}).

The aim of this work is to highlight the unusual morphology of the distal part of the extensor polices longus muscle, and to test the role that the muscle might play in the range of the thumb movement. The incidence of this variation is to be discussed because of its great importance in the diagnoses and in medical and surgical reconstruction treatments of hand and wrist diseases such as arthritis at the base of the thumb, de Quervain syndrome, extensor tendons injuries and in many other cases of the same category.

CASE REPORT

A-38-years old right-handed Sudanese male showed an abnormal painless hyperextension in the interphalangeal (IP) joint of the right-hand thumb. On examination by goniometry (measure the angle of joint) of the thumb, we found out that he can reach about 70º hyperextension (Fig. 1). While Checking the surface anatomy of the thumb, we noticed that there is an abnormal lateral division in the extensor pollicis longus tendon at the dorsal aspect of the proximal phalanx. Both of the tendons inserted into the sides of the base of the distal phalanx (Fig. 2). Assuming that this might play a role in the range of extension in the interphalangeal (IP) joint up to 70o.

Multisequence MRI study was conducted after 3plane axial localizer, coronal T1 weighted image, tSE revealed splitting of the pre-insertional portion of Extensor pollicis longus tendon which was seen just distal to the carpometacarpal joint of thumb. Each tendon has its own sheeth. The described splitted tendon appear intact with normal signal void nature, the surrounding osseous and soft tissue structures appear intact (Fig. 3).

Fig. 1 Range of the hyperextension in the interphalangeal joint of the thumb. 

Fig. 2 Dorsal aspect of the thumb shows the division from the main EPL muscle which runs laterally. 

Fig. 3 T1 weighted MRI photograph shows revealed splitting of the pre_insertional portion of extensor pollicis longus tendon. 

DISCUSSION

^{Türker et al. (2010}) has categorized a classification system to the anomalies of the extensor policis longus muscle. They mention that the muscle might have double tendons but in a different compartment or double tendons but in the same compartment (Fig. 4).

Fig 4 photograph for two anatomical variants for the extensor policis longus (8). 

It has also been reported by ^{Sevivas et al. (2009}), that there was a double extensor pollicis longus tendon found in a rheumatoid arthritis patient during a dorsal approach to his wrist. The tendon runs in an additional separate wrist dorsal compartment (^{Snell, 2012}).

This is approved by Beatty et al. who reported a patient who presented with dorsal wrist pain. The investigations showed an accessory extensor pollicis longus muscle which was also found in a third extensor compartment. The pain was resolved following excision of the accessory muscle (^{Standring, 2008}).

Türker et al. added that duplicate extensor pollicis longus tendons might cause inability to extend the thumb at full capacity. This due to an impingement of the musculotendinous junction of the duplicate tendons. They conducted an axial T1-weighted MRI at the level of the distal radioulnar joint which showed two tendons in a third extensor compartment.

In this study, we observed a rare case of the muscle where some of the tendon fibers split. We can also put a hypothesis of proportionality between the angle of motion and number of tendons depending on the side of attachment.

CONCLUSION

Variation of extensor pollicis longus muscle is a rare anatomical anomaly (^{Wang & Weiss, 2013}) but knowledge of it, is immense significant because it plays an important role especially during radiological diagnosis and surgical procedures such as reconstructive surgery. Furthermore, it gives us a lot of insight about limb and joint movements. Finally we must keep in mind that hand and thumb tendons variations are usually asymptomatic.

The developmental justification of muscle and/or tendon duplication is due to migration of myoblast into distal of the limb or due to changes that occur in the muscles pattern by the influence of somatopleure mesoderm (^{Zbrodowski et al., 1982}). The division of the tendon of extensor pollicis longus occurs due to signaling from the mesenchymal cells.