Absence of fourth slip of flexor digitorum brevis in Nepalese population

Ajaya Jang Kunwar; Krishna Deo Sharma; Dhiraj Maskey

doi:10.4103/injms.injms_42_21

ORIGINAL ARTICLE

Year : 2021 | Volume : 12 | Issue : 4 | Page : 207-210

Absence of fourth slip of flexor digitorum brevis in Nepalese population

Ajaya Jang Kunwar¹, Krishna Deo Sharma², Dhiraj Maskey³
¹ Kathmandu Center for Genomics and Research Laboratory, Global Hospital, Gwarko, Lalitpur, Nepal
² Department of Biological Sciences, Arkansas Biosciences Institute, Arkansas State University, Jonesboro, AR, USA
³ NSW Brain Tissue Resource Centre, Charles Perkins Centre D17, The University of Sydney, Sydney, Australia

Date of Submission	04-Apr-2021
Date of Decision	20-Jun-2021
Date of Acceptance	27-Jun-2021
Date of Web Publication	26-Oct-2021

Correspondence Address:
Dr. Ajaya Jang Kunwar
Kathmandu Center for Genomics and Research Laboratory, Global Hospital, Gwarko, Lalitpur
Nepal

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/injms.injms_42_21

Abstract

Background: Flexor digitorum brevis (FDB), the most superficial muscle among the muscles of the sole, comprises four tendons running to the lateral four digits. Each tendon splits into two, providing the gap for the flexor digitorum longus to pass through and are inserted into the second to fifth middle phalanges. Several variations of FDB have been reported earlier around the world but only one study showing the absence of fourth tendon has been reported so far in Nepal that too has conflicting outcome compared to the present study. Aims and Objectives: The purpose of this study was to document the absence of the fourth tendon of FDB to the little toe in the Nepalese community, as well as to highlight the fact that there are variances within the Nepalese setting. Materials and Methods: The present study involved an examination of 30 cadavers [male], enabling us in a total of 60 intact feet collected over a 2-year period. Results: Out of 60 feet in toto, we found only two (3.33%) of those lacked the fourth tendon of Flexor Digitorum Brevis to the little toe, which was notably lesser than previously reported 100% absence in Nepalese population. Conclusion: The disparity in such data could be attributable to the Nepalese community's diversity, which could indicate the presence of ethnic significance. The probable difference due to ethnic diversity in the Nepalese population should be taken into account for the medical procedure

Keywords: Anatomy, flexor digitorum brevis, tendon, variation

How to cite this article:
Kunwar AJ, Sharma KD, Maskey D. Absence of fourth slip of flexor digitorum brevis in Nepalese population. Indian J Med Spec 2021;12:207-10

How to cite this URL:
Kunwar AJ, Sharma KD, Maskey D. Absence of fourth slip of flexor digitorum brevis in Nepalese population. Indian J Med Spec [serial online] 2021 [cited 2023 Mar 26];12:207-10. Available from: http://www.ijms.in/text.asp?2021/12/4/207/329298

Introduction

The flexor digitorum brevis (FDB) is the superficial most muscle of the sole lodged between abductor hallucis and abductor digiti minimi. It lies immediately deep to the plantar aponeurosis and arises from the medialis tuberis calcanei, intermuscular septum, and adjacent muscles. Diverging distally toward the phalanges, the FDB divides into four tendons inserting into the middle phalanx of the lateral four phalanges after splitting at the base of the proximal phalanx in each toe. The fourth tendon has been reported to be absent, which could be due to phylogenetic degeneration. Variations of FDB have been described in various anatomical textbooks and reported as well. A previous study conducted in the Nepalese cadavers has reported the absence of the fourth tendon of FDB to be 100% which is highly unusual.^[1] A study from Turkey has reported an 18% absence of FDB while 48% of the study cases from Ohio, USA, lacked the fourth tendon of FDB.^[2],[3] Similarly, it has been reported 83.3% from Tamil Nadu, India, and as a case study was documented from Kasturba Medical College, Manipal, India.^[4],[5]

Although the absence of the fourth tendon of FDB is generally regarded as a known anatomical variation, the knowledge of such anomaly would benefit the podiatric community taking into consideration its ethnic diversity.^[6] The fourth tendon of FDB muscle presence, according to ethnicity, in particular, plays a very significant clinical and surgical (surgical planning, tendon grafting, etc.) importance in medical practice. The aim of the present study was to investigate the absence of the fourth tendon of FDB in the Nepalese population.

Subjects and Methods

The present study involved an examination of thirty cadavers [male], enabling us in a total of 60 intact feet collected over a 2-year period. No history of foot muscle disorders was disclosed in the medical records of the cadavers, and the cause of death was not related to lower extremities displaying any trauma, tumors, or deformities. The foot dissections were performed on Nepalese Army Institute of Health Sciences (NAIHS), College of Medicine, Kathmandu, Nepal during the routine practical session for undergraduate medical students. The dissection was undertaken conforming to the provisions and approval of the Ethics Committee of the institute. Besides the head dissector, the identification of the FDB was further deemed to be necessary by two other anatomists for confirmation purposes. The tendon was noted to be absent only when it was not found but not dissected away. It was marked as normal when it was present during dissection irrespective of its size compared to other tendons.

Results

In the present study, few of the feet showed minute variations from the standard description given in various anatomical textbooks. Normally, the FDB muscle after its origin runs forward till the middle of the sole and divides into four slips/tendons going toward the second to fifth middle phalanges as shown in the diagrammatic picture [Figure 1]. They may differ in size, thickness, and length of tendons, however, as our study only wanted to comprehend the absolute presence/absence of the fourth tendon, such variations were not considered. Each of the four FDB tendons splits into two twigs around the proximal phalanx, allowing the flexor digitorum longus tendon to pass through the space between the two twigs, and then get inserted into the shaft of the middle phalanx of the 2nd to 5th toes [Figure 1].

Figure 1: The normal pattern of flexor digitorum brevis. The muscle after arising from the calcaneus and surrounding area divides into four slips and is inserted into second to fifth middle phalanges, respectively

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Of the 60 feet included in the study, the absence of the fourth tendon of FDB was observed in only two (3.3%) cases. Both the anomalies were noted to be present in the right foot. The FDB had three bellies, and its tendons were inserted into the second, third, and fourth toes; however, the fourth tendons supposedly for the fifth toe were completely missing [Figure 2].

Figure 2: The IV tendon of flexor digitorum brevis for the fifth toe is missing in 3.3% male population. The number I, II, and III in the given picture represent the respective flexor digitorum brevis tendons going toward the second, third, and fourth toes

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Discussion

As most of the classical anatomy books depict, the flexor digitorum brevis muscle after its origin crosses the middle of the sole and splits into four tendons and each tendon then splits into two twigs near the proximal phalanx which are finally inserted into the middle phalanx of the second to fifth toes. The innervation to the FDB is provided by medial plantar nerve – a branch of tibial nerve and blood supply comes from medial and lateral plantar arteries, plantar metatarsal arteries, and plantar digital arteries. Regardless of the position of the ankle joint, the function is to flex the proximal interphalangeal joints and metatarsophalangeal joints of four lateral toes. One of the important functions is also to maintain the longitudinal arch of the foot along with other muscles, which is peculiar to plantigrade bipedal organism like human. If the muscle gets paralyzed, it distorts the arches of the foot. Variation of this muscle is also clinically important as FDB musculocutaneous flap is used for the reconstruction of the heel pad and tendon transfer surgeries for claw and hammer toes.^[7],[8],[9]

The present study shows that although the absence of a fourth tendon of FDB does exist, it is not as prevalent in the Nepalese population as has been reported before^[1] or from other regions. Variations of FDB have been reported in 63% of the cases, with total absence reported to be 21.5%.^[10],[11] Total absence of FDB was also reported with an incidence of 83.3% by Gugapriya in South Indian Tamil Nadu population,^[4] 18.1% by Bulent and Hasan,^[12] 6.7% by Becerro de Bengoa Vallejo et al.,^[6] and 38.3% by Nathanial.^[13] Other anatomical variations of FDB have also been reported; like in Turkish population out of 33 feet examined, 18 differed variations were reported that were entirely different than description given in classical anatomical textbooks. In 12 feet, the muscle belly for the fifth toe was much smaller than the others, and in 6 feet, it was completely absent. Extreme variations occurred in two cases. The female cadaver on left FDB had a superficial and deep head with three muscle bellies and four tendons, whereas on her right foot, a separated muscle belly for the fifth toe was very thin and originated from the intermuscular septum as a flat fascia under the FDB. In another male case, the quadratus plantae had three heads (lateral, intermediate, and medial) and the lateral and intermediate heads fused with the FDB.^[2] Other common anatomical variations of FDB tendon to the fifth toe had a separate muscular slip arising from the flexor digitorum longus or from the intermuscular septum.^[12],[13] However, this study only included the total lack or presence of FDB and did not include the other type of variations.

We report the absence of the fourth tendon of FDB to be 3.3% which was strikingly similar to an earlier report.^[10] However, our study varied in sharp contrast to the previous study in the Nepalese population that reported a 100% absence of the fourth tendon of FDB.^[1] The Nepalese community is a mixture of Mongolian, Aryans as well as local ethnic communities making it harder to decipher the actual result, which we believe may be a contributing factor for such two diverse results from the same population. As both studies have not identified the ethnicity of the subjects, it is hard to decipher which community the result represents. This is a lacuna that has been hard to count for as the body donation or unclaimed body to the NAIHS comes from Institute of Medicine, Tribhuvan University, Kathmandu, without detailed information. These two differing incidences in the same Nepalese community could potentially be attributable to the fact that the absence of the fourth FDB slip has been seen more frequently in females than males in Caucasian and African American populations,^[3] and while Lobo et al. researched both sexes in the Nepalese community before (100% incidence), our study exclusively included males (3.3% incidence). However, we firmly believe that there cannot be a 100% absence of FDB in Nepalese population as our study showed only 3.3% incidence among Nepalese males. In future, larger sample sizes including both sexes and a range of ethnic groups may provide a more accurate picture.

With FDB's function accredited for dispersing plantar weight-bearing forces, the absence of its lateral most tendon could be due to steady bipedal evolution leading to the gradual reduction of little toe's usage.^[1] Compensation of its function by other surrounding muscles of the sole could result in it being redundant and disappearing. The decrease in the size of the fourth tendon of FDB could be a supporting point for such assumption.^[7] Consequently, the absence of the fourth tendon of FDB in our study and in various populations around the world may suggest that the rarely used fourth tendon has undergone phylogenetic variation in accordance with Darwin's theory of evolution, which states that acquired characters are passed down through generations through use or disuse. This opinion was also supported by Reeser et al.^[14] in their electromyographic study of human foot and Yammine^[15] in his systematic review and meta-analysis.

Such differences need to be addressed, and awareness should be generated about its importance to orthopedic surgeons and radiologists; equally, anatomists should also take it into account during the routine dissections. In conclusion, the study investigates the fourth tendon of FDB, which was found to be absent in 3.3% of cases, unlike the previous report of 100% absence from the Nepalese community. Such conflicting results point toward the ethnic diversity in the Nepalese community, suggesting the need to take ethnicity and possibly the sex into consideration during an emergency medical procedure for improving prognosis and performance.

Conclusion

Overall, the current study demonstrates that the 4th flexor digitorum brevis tendon travelling towards the little toe is absent in 3.33 % of Nepalese males, which is in stark contrast to the prior reported 100 % absence in Nepalese population. This discrepancy in results could be attributed to the Nepalese community's heterogeneous ethnic origin, which includes Aryans, Mongoloids, and various other mixed populations. The disparity could possibly be explained by the fact that the current study only included males, whereas the prior study included both sexes. As a result, ethnic differences and probably sexual differences can have a significant impact on the occurrence of flexor digitorum brevis muscle pattern, and this must be taken into account in the future when performing any medical or surgical procedure, at least in the Nepalese setting.

Financial support and sponsorship

None.

Conflicts of interest

There are no conflicts of interest.

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