Fibularis Tertius: Anatomical Study and Review of the Literature.

Although there is much morphological variation in the anterior compartment of the leg, there is little information about the morphological variability of the fibularis tertius muscle (FTM). The main aim of the present study was to characterize the morphology (origin and insertion) and frequency of occurrence of the FTM and to use these findings as the basis for a new classification of the fibularis tertius tendon. Classical anatomical dissection was performed on 106 lower limbs fixed in 10% formalin. The presence of the FTM and the morphology of both its origin and its insertion were described. The FTM was present in 91 limbs (85.8%). Three types of origin were observed: Type 1, the most common type, with its origin on the distal half fibula (67%); Type 2, with the origin on the distal third fibula (22%); and Type 3, with an origin from the tendon of the extensor digitorum longus (11%). In addition, six types of insertion were distinguished. The most common was Type I (45%), a single distal attachment where the tendon inserts into the shaft of the fifth metatarsal bone. The rarest was Type VI, characterized by fusion with an additional band of the fibularis brevis tendon, which gives rise to the fourth interosseus dorsalis muscle. Two morphological variants of insertion could be distinguished, fan-shaped and band-shaped. Both the origin and insertion of the FTM are very morphologically variable, with three types of origin (Types 1-3) and six types of insertion point (Types I-VI) observed. Knowledge of such variations is essential for both clinicians and anatomists. Clin. Anat. 32:1082-1093, 2019.