A unique Anatomical variation of flexor digitorum superficialis muscle: a cadaveric case report

Anatomical variations of the flexor digitorum superficialis (FDS) muscle are uncommon and is related to evolution of muscle from amphibians to human. These variations are of academic interest and should be considered when facing variations in clinical examination, interpreting radiological images and in surgical procedures such as tendon graft. We report a case of unilateral variation of FDS muscle in the forearm with two muscle bellies: superficial and deep which has not been reported before. The humeroulnar and radial origins of the superficial belly united to form a single muscle belly, which divided in the upper part of forearm into two tendons, that passed through carpal tunnel and inserted into the middle phalanges of third and fourth digits. The deep belly originated from medial epicondyle of the humerus next to flexor carpi ulnaris, continued as a short tendon in the upper part of forearm and then formed a second muscle belly in the middle third of the forearm (two muscle bellies separated by an intermediate tendon). In the lower part of forearm deep muscle belly divided into two tendons which traversed the carpal tunnel and was inserted into middle phalanx of second and fifth digits. The report enhances the knowledge on variation of FDS and add a note to understand atypical findings on physical examination and during surgery.


Introduction
The flexor digitorum superficialis (FDS) is an extrinsic muscle of the hand. It arises from two headshumeroulnar and radial, which unite to form a singular muscle belly in the forearm between the superficial and deep muscle groups. In the lower part of antebrachium, it splits into four tendon. The tendons pass deep to the flexor retinaculum within the carpal tunnel in 2 rows, with the tendons for middle and ring finger being superficial to that of index and little finger. The tendons diverge towards the medial four digits and enters the digital flexor sheath. At the base of the proximal phalanx each tendon splits into 2 slips to allow the passage of flexor digitorum profundus (FDP) and are inserted into the sides of the shaft of middle phalanx of the Sri Lanka Anatomy Journal (SLAJ), 5(I) 2021,[62][63][64][65][66][67][68] corresponding digit. The FDS is a prime flexor of proximal interphalangeal joint (PIPJ) (1).
Anatomical discrepancy of FDS muscle belly, abnormal connections of muscle and tendon and absence of the tendinous part have been described in anatomical and clinical studies.
The present case describes a unique type V variant observed during cadaveric dissection and this finding will add to note to our knowledge of FDS muscle disparity

Case report
During undergraduate dissection of a 65-yearold male preserved cadaver by medical students of Faculty of Medicine, University of Jaffna, morphological variation in the FDS was observed on left upper limb. All other forearm muscles were anatomically normal.
The FDS had 2 bellies, superficial and deep. The superficial belly had the usual humeroulnar and radial origins, which united to form a single muscle belly. It bifurcated at the region of the upper and middle third of forearm, passed below the flexor retinaculum ( Fig 1) and inserted into middle phalanx of 3 rd and 4 th finger (Fig 3). The deep belly originated from medial epicondyle of the humerus (common flexor origin), next to flexor carpi ulnaris muscle. It had double belly appearance (digastric) in upper part of forearm; two muscle bellies connected by an intermediate tendon (Fig 2). The deep belly bifurcated at middle and lower third of forearm. These tendons passed deep to the tendons of superficial belly through carpal tunnel and insert into 2 nd and 5 th fingers ( Fig   3). The contralateral forearm does not show any anatomical variation. The median nerve was found deep to the deep belly of FDS.  interventions. These anomalous muscles can produce soft tissue masses and can compress vessels and nerves leading to compression symptoms. Therefore, knowledge of these variation helps the clinicians to detect he etiology, interpret symptoms and to manage the patients with such anomalous presentation.
Brandsma et al (15) pointed out another importance of knowing the FDS variant. Following injury to ulnar and radial nerve, flexor digitorum superficialis tendons are used in tendon transfer to correct claw hand deformity and loss of opposition of the thumb respectively. Variations in the origin and course of the tendon may cause difficulty in identifying these tendons during surgical procedures, especially when window incisions are made over the normal orientation of these tendons.
Identifying these variations will add knowledge on morphological variation of FDS and knowledge of the anatomical variations help to understand atypical findings on physical examination and during surgery and help to interpret abnormal radiological findings.