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Introduction
Foot drop, or the diminished ability to dorsiflex the foot, makes it necessary for patients to lift (and subsequently drop) the foot to properly ambulate [1-3]. Patients with foot drop face substantial difficulties in tasks such as walking or even balancing while standing upright [4]. Although patients experiencing foot drop can learn to manage their altered gaits, the condition is also associated with chronic pain and falls. It can have debilitating effects on quality of life [5, 6]. While foot drop can be associated with degenerative neurological disorders, including Charcot-Marie-Tooth (CMT) disorder and multiple sclerosis (MS), and with traumatic injury, compressive disorders, and autoimmune disorders, the vast majority of presentations are idiopathic, with deep fibular neuropathy at the top of the differential diagnosis [4, 5, 7].
Anatomy
The deep fibular nerve (DFN) and superficial fibular nerve (SFN) are the two terminal branches of the common fibular nerve (CFN) (Fig. 1) [8]. The CFN (anterior rami of spinal nerves L4–S1) is the lateral division of the sciatic nerve (anterior rami of spinal nerves L4–S3), which bifurcates into the CFN and tibial nerve, usually at the upper border of the popliteal fossa. The CFN then bifurcates into the SFN and DFN after wrapping around the lateral neck of the fibula passing through the fibular tunnel (FT) and posterior intermuscular septum (PIMS) [9-12]. The FT has been defined as a canal through which the CFN courses, bounded medially by the neck of the fibula [12, 13]. However, the distal boundary of the FT has been variously described as ending anywhere from the PIMS to the anterior intermuscular septum (AIMS) beneath the fibularis longus muscle [12-14]. Traveling distally, the SFN enters the lateral compartment of the leg near the anterior aspect of the AIMS. The course of the DFN has been described as descending along the anterior aspect of the interosseous membrane deep to the extensor digitorum longus muscle or descending by traversing the AIMS through a defect to innervate the muscles of the anterior compartment of the leg—the muscles primarily responsible for dorsiflexion of the foot [11, 12, 15, 16].
Moreover, the AIMS and PIMS can contribute to CFN compression when they cross underneath, increasing intramuscular pressure [9]. The previous study reported that the nerve branch from the DFN pierced through the AIMS, and the narrow portion in this area is associated with a higher incidence of nerve compression [9]. CFN compression and its branches can lead to nerve injury or neuropathy, with patients presenting with foot drop [17].
Deep fibular neuropathy affects the muscles of the anterior compartment of the leg; the tibialis anterior, extensor digitorum longus, and extensor hallucis longus [1, 11, 12]. Although the prevalence of deep fibular neuropathy as a result of neurodegenerative disorders, traumatic injury, compressive disorders, and autoimmune disorders has been examined, the prevalence of idiopathic deep fibular neuropathy, which is considered widespread, has yet to be investigated [3, 5]. Similarly, CFN entrapment has been extensively studied. It is posited to result from habitual lower limb “cross-legged” positioning, obstetric lower limb positioning, or compression of the FT [1, 2, 10, 12]. Surprisingly, there has been little research specifically about DFN entrapment; anterior compartment syndrome and case studies of osteochondromas of the fibular head dominate reports in the literature [18-21].
This cadaveric study aims to study the relationship of the DFN to adjacent bands in the deep leg. Secondary goals are to (1) provide clarity of the boundaries of the FT and (2) present likely cases of spared muscular involvement in foot drop cases, dependent on branching patterns of the terminal branches of the DFN.
Materials and Methods
Thirty formalin-fixed adult cadavers (15 female and 15 male) from the Tulane University School of Medicine were used in this study. The researchers will not need Institutional Review Board approval prior to performing research with cadavers; the donated bodies can be used by educators, students, researchers, and others. The age of death of the specimens ranged from 51 to 101 years, with a mean of 84 years. Specimens found to have been previously dissected or injured over the knee and leg region were excluded. The resulting pool of specimens included fifty-two sides, 27 right sides and 25 left sides. The right and left fibular regions were carefully dissected to reveal and identify the branching patterns of the CFN, SFN, and DFN, and their passage through the leg’s posterior and anterior intermuscular septa. The oval passageway of the CFN was classified as the superior fibular band, and the crescentic passageways of the DFN and SFN were classified as the middle and inferior fibular bands, respectively. The distance from each passageway’s inferior boundary to the fibular head’s lateral apex was documented, along with the width of each passageway at the most distal aspect. Additionally, the number of muscular branches the DFN gave was recorded and classified for each specimen presenting with a middle fibular band.
The fibular band types in relation to the DFN were classified into three categories.
1) Type I: superior fibular band
2) Type II: superior and inferior fibular bands
3) Type III: superior, middle, and inferior fibular bands
The DFN was classified based on the number of DFN branches before passing through the middle fibular band.
1) Type A: one DFN branch
2) Type B: two DFN branches
3) Type C: three DFN branches
4) Type D: four DFN branches
All data were collected and statistically analyzed using the t-test in IBM SPSS software version 23.0 (IBM Co.). Statistical analyses between sides and sexes were performed with significance set at P<0.05. The authors state that every effort was made to follow all local and international ethical guidelines and laws that pertain to the use of human cadaveric donors in anatomical research [22].
Results
Deep fibular nerve type
In type A (13.46%, n=7), the DFN continued through the middle fibular band as a single, unbranched structure. In type B (36.53%, n=19), it was divided into two branches before passing through the middle fibular band. In type C (42.31%, n=22), it divided into three branches before passing through it; in type D (3.85%, n=2), it divided into four branches before passing through it. There was no middle fibular band in the remaining 3.85% (n=2).
Moreover, in 19.23% of cases (n=10), the recurrent articular nerve (RAN) gave a single muscular branch to the tibialis anterior muscle (TAM) that did not pass through the middle fibular band.
Table 2 represent the measurement of distance from fibular head to the fibular bands (mean±SD). The inferior boundary of the oval-shaped superior fibular band of the PIMS was found at the lateral aspect of the superior most region of the fibular neck on all sides. This location was 16.2–30.3 mm from the lateral apex of the fibular head (mean 22.9±3.1 mm). The width of the most distal aspect of the superior fibular band ranged from 1.5 to 6.4 mm with a mean of 3.5±0.7 mm. The crescentic middle fibular band of the AIMS was present in 96.15% of cases (n=49), its inferior boundary being consistently positioned in 98% of them (n=48) with a mean distance from the lateral apex of the fibular head of 57.8±10.0 mm. The other 2% (n=1) presented with a “weak” middle fibular band, which was formed by a thin (2.4 mm wide) band of connective tissue in the absence of a distinguishable AIMS, 91.42 mm from the lateral apex of the fibular head. The width of the inferiormost aspect of the middle fibular bands ranged from 1.2 to 6.4 mm with a mean of 2.8±1.0 mm.
The crescentic inferior fibular band of the AIMS was present in 17.31% of cases (n=9). Its inferior boundary was variably positioned, ranging from 67.0 to 119.1 mm from the lateral apex of the fibular head, with a mean of 99.4±27.2 mm. The width of the inferior most aspect of the inferior fibular band ranged from 1.8 to 5.7 mm with a mean of 2.7±2.2 mm.
No statistical differences were identified between the measurements made or classifications of the left and right sides and between males and females (P>0.05).
Discussion
Compression of DFN is commonly known to occur underneath the extensor retinaculum [10]. A study by Nayak et al. [23], found variations in the anatomical pattern of nerve branches, including the SFN, DFN and sural nerve. Understanding these variations is crucial for surgical procedures and treatment, as it helps prevent nerve damage during operations [23]. Understanding the DFN’s branching pattern and the various potential constriction sites of the CFN, SFN, and DFN is necessary for identifying and treating potential causes of foot drop. We, therefore, used cadaveric dissection to investigate and classify the prevalence of perforations in the PIMS and AIMS through which the CFN and its terminal divisions travel. The consistently present (100%, n=52) oval PIMS passageway for the CFN was labeled the superior fibular band. The typically present (96.15%, n=50) crescentic AIMS passageway for the DFN was the middle fibular band. The inferior fibular band was the variably present (17.31%, n=9) crescentic AIMS passageway for the SFN.
Previously, knowledge of the intermuscular septum has been used in leg and heel flap reconstruction to avoid damage to nerves or vessels [24]. The CFN goes travel through the FT, where it divides into the DFN and SFN [12]. The boundaries of the FT have been variably described in the literature and ending anywhere between what we have termed the superior and middle fibular bands [12-14].
We, therefore, examined the prevalence of the superior fibular band of the PIMS in cadavers as a potentially defined distal boundary of the FT. Therefore, the location of the PIMS may affect the course of the FT and potentially cause issues for the DFN. Because of its constancy, the superior fibular band provides a reliable structural landmark for the distal boundary of the FT; previously defined distal boundaries were shown to be variable.
We found that the fibular band type with both superior and inferior fibular bands was present in 78.85% of cases. Additionally, our results indicated that there were no statistical differences in fibular distance and classifications between sides and sexes. This information could be useful in supporting treatment for leg or foot drop. Because branches of the DFN typically innervate the muscles associated with foot drop, it was also essential to document cases in which the TAM—the most powerful dorsiflexor of the ankle—received innervation from a branch of the RAN [3, 11, 15, 16]. In such cases, patients with deep fibular neuropathy can be spared the full expression of foot drop by maintaining the function of the TAM. In patients with foot drop, surgical release of the fibular bands, particularly the middle fibular band, could ameliorate the constriction of the DFN and potentially decrease foot drop symptoms.
In conclusion, connective tissue bands along the course of the fibular nerves are common and should be considered in cases of idiopathic palsy affecting these nerves. The information regarding the intermuscular septum, particularly in relation to the DFN, should be incorporated into the treatment of leg condition. Further clinical studies are now necessary to correlate the anatomical classification developed in the present study with patient findings.
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