National Institute of Ayurveda, Jaipur. The cadaver was received through voluntary body donation program. The study adhered strictly to established ethical standards.

The dissection was carried out on the extensor compartment of forearm and dorsum of the hand. standard incisions were made and the skin was reflected. Following the removal of the superficial fascia, the extensor retinaculum, a specialized thickening of the deep fascia was exposed. The six osseofibrous compartments formed beneath the retinaculum were clearly identified.^[4,5]

All seven superficial extensor muscles were meticulously traced from their respective origins.

Case Report

During the routine dissection of the extensor compartment of the forearm and dorsum of the hand, in a formalin fixed male cadaver, standard incisions were made and the skin was reflected.

Following the removal of the superficial fascia, the extensor retinaculum, a specialized thickening of the deep fascia was exposed. The six osseofibrous compartments formed beneath the retinaculum were clearly identified.

All seven superficial extensor muscles were meticulously traced from their respective origins. We found these unusual findings and are reported below.

Upon proceeding to examine the deep muscles of the extensor compartment, only four of the five expected muscles were observed. The EI, typically found within the fourth extensor compartment, was notably absent. In this compartment, only the tendons of ED were present.

Upon careful dissection and clearance of the ED tendons, an unusual fusiform shaped muscle belly was identified on the dorsum of the hand. This muscle originated from the base of the fourth extensor compartment, with single belly measuring approximately 8.5 cm in length and width of 1.8 cm and gave rise to a long tendon having length of 7.2cm that inserted onto the middle phalanx of the index finger.

Based on its origin, insertion, and morphological characteristics, this accessory muscle was identified as an extensor indicis brevis (EIB).

Discussion

Eventhough the variations in muscles and tendinous distribution of the dorsum of the hand and wrist are common, the presence of EIB is relatively rare. One more point to be noted here is the absence of EI, where many of them reported the presence of EIB along with EI. del Índice, M. E. C. et al (2012) noted the origin of the muscle was noted be in the form of ligament and over the dorsum of scaphoid and trapezoid bones with insertion along the dorsal indices aponeurosis.^[6]

Esakkiammal et al. (2017) reported an additional fusiform muscle belly of the extensor indicis arising from the dorsal carpal ligament in the left hand. This accessory belly joined the main tendon distally and gave a small medial slip that merged with the dorsal hand fascia near the head of the third metacarpal.

In the right hand, the extensor indicis extended up to the dorsal carpal ligament, from which a slender additional belly originated and merged with its main tendon unlike in this case where extensor indices is found absent.^[7]

Hirai (2001) et al., Out of dissection of 548 cadavers only in 1 cadaver, the Extensor digitorum brevis manus (EDBM) as a variant of EIP was found in 1 specimen. The EDBM originated from the proximal portion of the radiocarpal ligament and ran ulnar to the EDC-index with the absence of EIP.^[8]

Solomon (2021) et al., reported single belly of EIB 4.1cm x 0.4cm in right extremity having originating from ulnar styloid process and inserted as a tendon to the proximal phalanx of index finger.^[9]