Morphological diversity of peroneus tertius: a cadaveric study

Objective The objective of this study was to demonstrate morphological anatomical variations of the peroneus tertius muscle which may have important clinical and surgical implications. Materials and methods Fifty four specimens of anterior compartment of the leg which were dissected routinely according to the undergraduate course were examined for variations of the above muscles and the anatomy was further defined using appropriate dissection methods. Results A wide variation was found with regard to the morphology of peroneus tertius and the extensor digitorum longus muscles with considerable differences between the legs of the same cadaver. When considering important variations deviating considerably from the normal structure, a variation of the distal attachment of peroneus tertius muscle in two specimens revealed that in addition to the normal attachment to the shaft of the fifth metatarsal, an additional slip inserted to the extensor expansion of the fifth toe. It apparently replaced the fourth tendon of extensor digitorum longus, which only had three divisions in these specimens.   Seventeen of the specimens had a single muscle belly giving rise to 5 tendons which had the usual distal attachments of the peroneus tertius and the extensor digitorum longus muscle as stated below. Absence of peroneus tertius was noted in one specimen. Conclusion The anatomical structure of the peroneus tertius vary substantially from individual to individual, inclusive of differences existing between the left and the right lower limbs of the same cadaver.


Introduction
Peroneus tertius or fibularis tertius and extensor digitorum longus are muscles in the anterior compartment of the leg. Although the peroneus tertius was once thought to be a muscle unique to humans (1) it has also been found in certain anthropoid apes (2,3). Sri Lanka Anatomy Journal (SLAJ), 5(I) 2021, [45][46][47][48][49][50][51][52] The earliest recorded description of these muscles is by Vesalius (6). Some contemporaries of Vesalius rejected peroneus tertius as a separate muscle and described it as a part of extensor digitorum longus.
Proximal insertion of the muscle is on the distal third or more of the fibula as well as on the interosseus membrane and anterior intermuscular septum (5,6,7). The distal attachment is classically described as to the base of fifth metatarsal bone (6,7). Evidence of variations of this insertion are described to be to shaft of the fifth metatarsal, fascia covering fourth interosseous space and base of fourth metatarsal (6,7,8,9). Though the tendon of peroneus tertius is described as usually one, work of Ericikt et al (8) has found that there can be 2 tendons with a slip.
Although interconnections between extensor digitorum longus muscle and peroneus tertius is mentioned in textbooks the precise manner of connection is not well documented. Stevens et al (9) however has documented intertendinous connections between peroneus tertius and extensor digitorum longus tendon.
Considering the action of this muscle, work of (15) has found that the muscle is important in the swing phase of the gait to level the foot and help toes clear the ground along with extensor digitorum longus and tibialis anterior. This muscle is implicated in the occurrence and management of Jones fracture (17). The muscle is additionally used in filling soft tissue defects in the limbs (16).
The extensor digitorum longus muscle arises from the inferior surface of the lateral condyle of tibia, proximal three quarters or upper half of the medial surface of fibula, adjacent anterior surface of interosseous membrane, deep fascia, anterior crural intermuscular septum and the fascial septum between itself and tibialis anterior. It descends to form a tendon which divides in to four on the dorsum of the foot each joined by a slip from the extensor digitorum brevis to form dorsal digital expansions through which they finally insert to the bases of distal and middle phalanges of lateral four toes. (6, 7) The action of this muscle is to dorsiflex the foot and extend the toes.

Method
Fifty four specimens received by the Department of Anatomy, Faculty of Medicine, University of Peradeniya for studying purpose of students were used for this study. Initially medical students dissected the specimens under the guidance of the academic staff members. Following completion of the relevant dissections, the lower limbs were thoroughly observed for variations of peroneus tertius muscle and its tendons and subsequent dissections carried out to clearly define the anatomy.

Results
Results showed a wide variation of morphology with remarkable differences noted between the legs of the same cadaver. Here are described five variations that significantly deviated from the normal text book description.
In the right leg of a female cadaver (figure 2) the peroneus tertius muscle gave rise to two tendons. The lateral tendon inserted into the base of the fifth metatarsal. The medial slip continued to the fifth toe beyond the fifth metatarsal, apparently replacing the fourth tendon of extensor digitorum longus. Extensor digitorum longus muscle gave rise to three tendons rather than the usual four, each heading towards second, third and fourth toes.
In the left leg of the same cadaver both extensor digitorum longus and peroneus tertius originated as a single muscle with 2 muscle bellies splitting high in the leg. Extensor In two male specimens on the left leg the two muscles had extensive intermuscular connections proximally apparently forming a single muscle which separated in to two muscle bellies distally. In both specimens the medial muscle belly representing the extensor digitorum longus gave rise to 3 tendons and the lateral muscle belly representing the peroneus tertius gave rise to 2 tendons as above (figure 5).
Seventeen of the specimens showed a single muscle belly arising from the anterior surface of the tibia giving rise to five tendons which had distal attachments to the usual distal attachments of the peroneus tertius and the extensor digitorum longus (figure 6).
Absence of Peroneus tertius was noted in one specimen, which was the left leg of a male cadaver (figure 7).   Figure 5. Proximally single muscle giving rise to two muscles representing peroneus tertius and extensor digitorum longus giving rise to two tendons and three tendons respectively.   upwards to give rise to the peroneus tertius (13). In our study as seventeen out of the fifty four specimens had a common muscle belly representing both muscles, origin of peroneus tertius by extensor digitorum longus may be more suggestive.