Extensor Indicis Brevis Muscle: A Case Report

Introduction

During routine cadaver dissection procedures, variations of hand muscles and tendons are commonly observed. Such variations can be discovered incidentally during surgical procedures or during routine dissection. However, these variations are rarely associated with clinical significance, especially when the variant muscle occupies the narrow dorsal compartments of the wrist. Hence, it is important to expect the existence of muscular variations of the hand. During reconstructive surgeries of the dorsum of the hand, it is mandatory to expect such muscular variant.¹

Extensor indicis brevis muscle (EIB) is variant muscle that is found on the dorsum of the hand and it is slightly dominant among males. This muscle is found in approximately 2% to 3% of the population which may be easily mistaken for other dorsal hand pathology.^2,3

Materials and Methods

The case report involved routine dissection of unknown age male cadaver. The dissection of upper limb carried out in the Department of Anatomy of Black Lion Specialized and Teaching Hospital, Addis Ababa, Ethiopia, 2018. Skin and superficial fascia was removed, the dorsum of the hand was exposed to look for variant muscle origin, insertion, action and its neuro vascular supply. The so named EIB muscle was followed from its origin to its insertion. Other nearby structures were thoroughly observed to detect variation. An elastic tape measure was used to measure the length and thickness of variant muscle. Fresh cadaver, which was preserved by injection of formalin (10% formalin) based preservative, was dissected during first year medical anatomy course.

Results

Routine anatomical dissection of the right upper extremity of an anonymous-aged and formaldehyde-fixed cadaver at Black Lion Specialized and Teaching Hospital for 1st year medical students (Figure 1).

Figure 1 Photograph of the extensor indicis brevis muscle taken from right hand of male cadaver at Black lion Hospital, Addis Ababa, Ethiopia, 2018.

Ethiopian male cadaver, from department of Anatomy at Black Lion Specialized and Teaching Hospital, muscular variation was observed (Figure 1). Skin and superficial fascia was removed, the dorsum of the hand was exposed to look for variant muscle origin, insertion, action and it neuro vascular supply. The variant muscle originates from ulnar styloid process and inserted as a single tendon to base of proximal phalanx of index finger. The action of the variant muscle is extension of index finger at metacarpo phalengeal joint (MPJ). Belly of the muscle was fusiform and it was innervated by small branches of radial nerve (posterior interosseous nerve). The blood supply was provided by the posterior interosseous artery.

Discussion

For interpretation of unusual presentation of musculoskeletal system, diagnosis and planning of surgical tendon transfer, it is important to understand anatomical variations.¹⁰ In our case report, which was found during routine cadaver dissection, we have found variant extensor indicis brevis (EIB) of single belly. The variant muscle measured about 4.1cm long and 0.4cm thick. Even though cases with two bellies of EIB have been reported,³ commonly the variant muscle has a single belly.^4–6 Based on findings of different authors, different origin and insertion of EIB has been explained (Table 1).

Table 1 Showing Comparison of Origin and Insertion of Variant Extensor Indicis Brevis Muscle of Different Authors

Based on its insertion, Ogura et al⁷ classified this variant muscle into three types which includes Type I – inserted onto the dorsal aponeurosis of index finger, Type II – inserted on the index finger and Type III – inserted on the long finger. In different clinical case reports, the EIB is commonly misdiagnosed as a dorsal wrist ganglion.^4,5 In the case of Murakami and Todani, together with a dorsal wrist ganglion an EIB was found lying over the ganglion.⁵

In conclusion, knowledge of muscle variation is not only important for differential diagnosis but also for elimination of bad outcomes of surgical procedure. Therefore, EIB is a muscle structure that can present as variant on the dorsum of the hand which needs consideration in case of surgical intervention and during routine dissection.