Anatomical basis for the use of the fibularis tertius muscle in myocutaneous flaps Bases anatômicas para utilização do músculo fibular terceiro em retalhos miocutâneos

Background: Myocutaneous flaps have been increasingly used in surgical reconstruction of the lower limbs, requiring the use of muscles that result in less functional and esthetic damage as flaps. This study aimed to evaluate the use of the fibularis tertius muscle (in terms of frequency and anatomy) as flaps in this procedure. Methods: Sixty-four lower limbs from preserved cadavers were dissected and evaluated based on the following parameters: proximal insertion, distal insertion, syntopy, morphology, and morphometry. Results: The fibularis tertius muscle was detected in 96.9% of the study cases. Most pro ­ ximal insertions (96.8%) were found at the interosseous membrane, anterior border of the fibula, and anterior intermuscular septum. Most distal insertions (77.4%) were found at the lateral and dorsal sides of the 5th metatarsal. Mean value of muscle belly length was 17.89 cm and width was 1.95 cm. The mean length of the distal tendon with no muscle fibers was 1.2 cm, and the mean width was 0.45 cm. Conclusions: The fibularis tertius muscle is frequent and has a distinct morphology, making it a viable option for the repair of lower limb (distal segment) defects.


INTRODUCTION
Injuries of the leg and dorsal foot associated with soft tissue loss are common in emergency rooms 1 .Plastic surgeons use a range of surgical approaches to treat these lesions to ensure sufficient coverage and preservation of the structure, and to avoid complications and long hospital stays, especially in small or medium urban centers with a lack of skilled surgeons or the infrastructure for conducting microsurgery 2 .Soft tissue reconstruction of the lower limbs is particularly challenging with the small amount of subcutaneous tissue and the lack of muscle protection for the osteotendinous and neurovascular structures, so it often requires several surgical interventions using myocutaneous flaps 3 .
For moving tissue from the donor site to the grafting site, the following should be considered: distance between the donor site and the injured site, size of the flap available for the transplant conditions of the implant area, and presence of an adequate supply of tissue 1,46 .The ideal flap yields the best esthetic and functional results with minimal sequelae 1,4 .
Advantages of using myocutaneous flaps include the following: presence of various planes (skin, subcutaneous, fascia, and muscle) that provide volume, a donor site located far from the radiation field, and the reliability provided by the vascular pediculum 4,5 .Studies report the use of the soleus muscle 1 , anterior tibial muscle, gastrocnemius muscle 1 , and extensor digitorum muscle 5,7 as flaps for the distal third of the lower limb.However, using these muscles can be detrimental to the motor function of the foot and to patient quality of life 5,8,9 .
In this study, the fibularis tertius muscle was investigated as an alternative source of flaps for the repair of injuries of the leg and dorsal foot.This muscle has proximal insertion at the lower half of the anterior side of the fibula and distal insertion occurs at the bottom of the 5 th metatarsal 10 .Blood supply is provided by 1 to 3 branches of the anterior tibial artery, which irrigate the anterior section of the leg, and the deep fibular nerve provides innervation to the muscle 11 .The fibularis tertius muscle is an appendage of the extensor digitorum muscle, so it does not have an anatomical identity 12 .The negligible function of the fibularis tertius muscle means it can be used in myocutaneous flaps without compromising motor function and esthetics in the area.Thus, we analyzed the anatomy of the fibularis tertius muscle in terms of frequency, morphology, morphometry, proximal and distal insertion, and syntopy (position in relation to other muscles), and investigated the use of this muscle for reconstruction of the leg and dorsal foot.

METHODS
We received institutional approval and adhered with Brazilian Law 8501, of November 30, 1992, which rules on using nonreclaimed cadavers in scientific research and studies performed at medical schools.We dissected 64 connected and unconnected lower limbs (32 from the left side and 32 from the right side) from male and female cadavers.Cadavers, preserved in 5% formaldehyde, were of adult Brazilian subjects from the Sector of Human Anatomy, Institute of Health and Biological Sciences of the Federal University of Alagoas.
Dissections were performed using the naked eye, and incisions were made to expose the anterior section of the leg, and dorsum of the foot, while analyzing the muscles and tendons.The fibularis tertius muscle was identified in 62 of the dissected lower limbs and was analyzed for the following characteristics: proximal insertion, distal insertion, syntopy, morphology, and morphometry.
The muscle belly was evaluated both for total length and width (after calibration at the medial point of the muscle belly) -Figure 1.Three parameters were evaluated during tendon analysis: total extension, extension of the tendon section without muscle fibers, and width.

Frequency
Of the 64 dissected lower limbs, the fibularis tertius muscle was found in 62 (96.9%), and could not be found in both limbs of 1 cadaver (Figures 2 and 3).

Morphology
In all of the cases where it was located (62 limbs), the fibers of the fibularis tertius muscle were lying at an oblique angle to the insertion tendon in the craniocaudal direction.This muscle was characterized as unipennate with gradual broadening.

Morphometry
The mean length of the muscle belly was 17.89 cm, and the mean width was 1.95 cm.The total extension of the muscle with the tendons had a mean of 20 cm.The mean length of the distal tendon with no muscle fibers was 1.2 cm, and the mean width was 0.45 cm.

Muscle Insertion Proximal Insertion
The interosseous membrane, the anterior intermuscular septum, and the anterior border of the fibula were the proximal insertion points in 60 (96.8%) of the cases (Figure 4).For proximal insertion to the anterior border of the fibula, insertion to the distal third of this muscle occurred in 32 (51.6%) cases.In only 2 (3.2%) lower limbs, the insertion occurred at the entire border of the fibula including the distal, medial, and proximal thirds (Table 1).
The superior and medial thirds of the muscle belly were posterior and lateral to the anterior border of the fibula and the anterior intermuscular septum, and continuous with the lateral compartment of the leg.The posterior medial location of the fibularis tertius muscle set it near the lateral part of the interosseous membrane, the medial side of the fibula, and the extensor digitorum and extensor hallucis longus muscles.These muscles were intimately associated and often consisted of intercrossing fibers of both muscles with the fibularis tertius muscle at the proximal insertion level.
The fibularis tertius muscle and the extensor digitorum muscle were found adjacent to one another and located such that intercrossing of fibers of these muscles were observed.However, the direction of the cluster of muscle fibers from the fibularis tertius muscle formed an acute angle with the fiber path headed for the extensor digitorum muscle.

Distal Insertion
The following distal insertion patterns were detected: • model A -the tendon was inserted to the dorsal and lateral sides of the 5 th metatarsal in 48 (77.4%) cases; • model B (Figure 5) -the tendon was inserted to the dorsal and lateral sides of the 5 th metatarsal and to the dorsum of the base and proximal third of the 4 th metatarsal in 14 (22.6%)cases (Table 2).

Syntopy
The fibularis tertius muscle was found at the most lateral and inferior sections of the anterior compartment of the leg.Few studies have investigated the morphology of the fibularis tertius muscle, and it has been variably classified as long, thin 12 , flat 13,14 , shaped as a long band 15 , long and thin 16 , and stripshaped 17 .This classification does not concur with our findings, as it refers to muscles displaying longitudinal fiber orientation.In our study, we found that the fibularis tertius muscle broadened towards the craniocaudal direction and displayed oblique orientation of fibers to the tendon.Fibers were inserted to one side of the tendon, defining the muscle as unipennate.This morphological classification (unipennate) was not used by any previous authors.
Muscle's fibers become increasingly flat as they approach insertion so this favors use in myocutaneous flaps, because it permits the use of greater muscle thickness in the reconstruction of complex injuries of the lower limb 5 .
No consensus exists in the literature for proximal insertion with no previous description of simultaneous insertion at 3 points, particularly, insertion into the anterior border of the fibula.In a few studies, proximal insertion was to the distal third of the anterior surface of the fibula, adjacent to the interosseous membrane and the anterior intermuscular septum 1718 .In these cases, there was no detailed description of the point of insertion to the fibula.
Many studies report distal insertion at the dorsal side of the 5 th metatarsal 10,1216,18,19 .However, in this study, we found distal insertion to the base of the 4 th metatarsal, even though it occurred at a lower frequency (22.58% of the cases).
The length and width of the muscle belly of the fibularis tertius muscle was compatible with use in myocutaneous flaps, even though no previous research provides similar analysis.
The syntopy analysis of the fibularis tertius muscle showed that this muscle is independent of other leg muscles found in the anterior compartment of the leg.Although intimately associated with the extensor digitorum and extensor hallucis longus muscles, they are not continuous and their fibers display a completely different orientation, diverging towards their own tendons.Thus, we disagree with the theory that the fibularis tertius is a lateral section of the extensor digitorum muscle 15,17,18 .
Based on the results of this study and the ease of access by longitudinal incision on the anterior-lateral side of the leg, the fibularis tertius muscle can be used to prepare lower limbs grafts.

CONCLUSIONS
The fibularis tertius muscle is frequent, which is a fundamental attribute for its potential use as a source of local flaps, and it has distinctive features, including morphometric ones, which allows for its use in reconstruction surgery of the lower limb.In most cases, this muscle is of adequate length for use in a wide variety of grafts.The lower third of the muscle belly was posterior to the lateral malleolus, and medial to the tendons of the extensor digitorum and extensor hallucis longus muscles.The distal insertion tendon of the fibularis tertius muscle passed over the proximal end of the retinaculum of the extensor muscles and moved posterior over the short extensor finger muscle in the direction of the distal insertion point.

DISCUSSION
Reconstruction surgery of the distal extremities of the lower limbs is technically challenging due to the shortage of subcutaneous tissue, absence of intense vascularization, presence of bony prominences and, most importantly, the few options for generating myocutaneous flaps 9 .
The ideal muscle, with negligible function and minimal esthetic aspect, provides for use as a myocutaneous flap 10 .Moreover, our results confirmed a high frequency (96.9%) of the fibularis tertius muscle, demonstrating that these findings are statistically significant.Our results are in agreement with those from Le Double 11 , who reported a frequency of 91.2%, demonstrating that the presence of this muscle is almost constant.

Figure 1 -
Figure 1 -Morphometric analysis of the muscle.Black dotted line = length of the tendon without fibers; red dotted line = tendon width; blue dotted line = length of the tendon with muscle fibers; yellow dotted line = muscle length; and green dotted line = muscle width.

Figure 2 -
Figure 2 -Presence of the fibularis tertius muscle in the lower limb.

Figure 3 -
Figure 3 -Single case of absence of the fibularis tertius muscle.

Figure 4 -
Figure 4 -Anterior view of the proximal insertion.

Figure 5 -
Figure 5 -Model B of distal insertion.

Table 1 -
Distribution of the proximal insertion of the fibularis tertius muscle in the fibula.

Table 2 -
Frequency of the points of distal insertion of the fibularis tertius muscle according to the models described in this study.