A unique bilateral anatomical variation of the extensor digitorum longus and fibularis tertius muscles.

Purpose: The extensor digitorum longus (EDL) and fibularis (peroneus) tertius (FT), muscles of the anterior compartment of the leg, synergistically work to dorsiflex and evert the foot. The EDL additionally extends the lateral four toes, while the FT normally lacks this ability. Anatomical variations of the variably present FT are common, but infrequently involve the EDL. The FT has been known to give off additional accessory tendons which may alter its role in ankle and foot movement. Documentation of unique anatomical variations of the FT and EDL can assist not only anatomists, but clinicians and surgeons as well.

Methods: During routine cadaveric dissection, a rare and unique bilateral presentation of the extensor digitorum longus and fibularis tertius was discovered.

Results: The extensor digitorum longus was found to be missing its lateral-most tendon that would normally insert on the 5th digit. A large, overriding FT split into three tendons, with the medial-most tendon inserting to the phalanges of the 5th digit, effectively replacing the missing EDL tendon. All findings were present bilaterally.

Conclusion: To our knowledge, despite previous literature of unilateral presentations like this case, a bilateral EDL and FT variation as seen here has never been reported in humans. The use of accessory fibularis muscles in tendon graft procedures, especially the FT, makes awareness of leg anatomical variations critical for surgeons operating in the area. The present case is discussed alongside other documented anatomical variations, highlighting potential clinical implications for tendon grafting and preoperative imaging.