Vol.:(0123456789) 1 3 Surgical and Radiologic Anatomy https://doi.org/10.1007/s00276-020-02439-1 ANATOMIC VARIATIONS Complete extensor digitorum profundus complex: a deep hand extensor muscle to the medial four digits Athikhun Suwannakhan 1  · Nittiya Nontunha 1  · Krai Meemon 1 Received: 29 November 2019 / Accepted: 10 February 2020 © Springer-Verlag France SAS, part of Springer Nature 2020 Abstract Purpose Anatomical variants in the posterior compartment of forearm and hand are not uncommon. Physicians should be aware of variations in this region for correct diagnosis and treatment of diseased hands. Methods During a routine dissection at our department, an extremely rare case of deep hand extensor muscle was discovered. Results A complete extensor digitorum profundus complex was found in the fourth extensor compartment in addition to the extensor indicis proprius. The complex consisted of two muscle bellies originating from the lateral aspect of distal ulna and the adjacent interosseous membrane. The frst belly resembled the conventional extensor indicis proprius. The second belly gave of two tendon slips: one inserted to the index and middle fngers and the other formed aponeurosis before inserting to the ring and little fngers. Conclusion To our knowledge, a complete extensor digitorum profundus complex which inserts to all medial four digits has never been reported in humans. Awareness of variations in this region is critical for surgeons operating in the forearm and hand. The present case also provides insights into the evolutionary and developmental origin of these structures. Keywords Anatomical variation · Extensor digitorum profundus · Extensor indicis proprius · Forearm extensor · Hand Introduction The “extensor digitorum profundus complex” (EDPC) refers to the group of muscles in the ulno-carpal series group in primates which lies deep to the superfcial extensor mus- cles including the extensor digitorum and the extensor digiti minimi [2]. In humans, the only muscle belonging to the deep group is the extensor indicis proprius (EIP), the sole extensor of the index. In many species of primates, however, the “profundus complex” exhibits variable insertion ranging from the index to little fnger. In catarrhine and platyrrhine monkeys, the muscle typically supplies the index and middle fngers [2]. In lemurs, Tarsius, and Cebidae monkeys, inser- tion of the EDPC to the index, middle, and ring fngers are frequently observed [11]. Insertion to the little fnger is phe- notypically normal in Cercopithecus, Pan, and Gorilla [2]. While these structures are considered “normal” in the aforementioned non-human primate species, they are atavistic and are regarded as variations in humans. Numer- ous studies and meta-analysis have been conducted to study the anatomy and prevalence of variations in the deep exten- sor region [12, 14], and a classifcation system was recently proposed by Georgiev et al. [6]. The extensor medii proprius (EMP) is a muscular variant analogous to the extensor indi- cis, but with the insertion to the middle fnger. The EMP has a prevalence of 3.7% [14]. The extensor indicis et medii communis (EIMC) is another variant with a prevalence of 1.6% across the populations. The EIMC, a muscular variant with insertions to the index and middle fnger, has a preva- lence of 1.6% [14]. The extensor policies et indicis (EPI), a common thumb and index extensor, is the rarest variant in this region having a prevalence of 0.75% [14]. Despite a predictable occurrence of muscular variants to the thumb, index and middle fngers, variations of the EDPC to the ring and little fngers are extremely rare, and only a few cases have been reported in humans. Herein, we report a case of complete EDPC, which extends to all medial four digits. This is the frst case report in human and no similar case has ever been found in the lit- erature. The understanding and awareness of such variation is important for hand surgeons, especially when recognizing * Krai Meemon krai.mee@mahidol.edu; krai.mee@mahidol.ac.th 1 Department of Anatomy, Faculty of Science, Mahidol University, Bangkok 10400, Thailand