Title & Abstract

International Journal of Anatomy and Research

Sciatic Nerve Bifurcation Level and its Anatomical Relationship to the Piriformis Muscle: A Magnetic Resonance Imaging-Based Assessment in the Iraqi Population

DOI: 10.16965/ijar.2026.206

Mohammed, Mohammed Eimad ¹

Al-Yawer, Malak A. ¹

ABSTRACT

Background.

The sciatic nerve is the major nerve supply to the lower extremity and a critical structure in orthopedic surgery, radiologic evaluation, and regional anesthesia. Precise characterization of its relationship to the piriformis muscle is essential for safe procedural planning and for understanding anatomical variation.

Objectives.

This study aimed to determine the relationship between the piriformis muscle and the sciatic nerve with reference to sex and anatomical variations.

Methods.

This prospective cross-sectional observational study included 35 adults undergoing pelvic Magnetic Resonance Imaging (MRI) at Imam Al-Hussein Medical City, Karbala, Iraq, providing data from 70 lower limbs. MRI parameters such as the diameter and thickness of the sciatic nerve, its level of bifurcation, and its relationship to piriformis muscle according to the classical Beaton and Anson classification were recorded and statistically analyzed.

Results.

The study population included 18 men and 17 women (age mean 31.8 ± 6.2 years; mean BMI 24.6 ± 3.1 kg/m2). The mean sciatic nerve diameter and thickness were 11.62 ± 1.56 mm and 6.4 ± 1.2 mm, respectively and both measurements were significantly greater in males than females as p value less than 0.05. The typical piriformis morphology predominated (82.9%). Sciatic nerve bifurcation occurred most frequently at the popliteal fossa (78.6%), followed by pelvic (17.1%) and thigh (4.3%) levels, without significant association with sex or laterality (p > 0.05). Beaton and Anson Type I was observed in 78.6% of cases followed by Type II (10%), type III (5.7%), type IV (2.9%) and type V and VI each with 1.4%, without significant association with sex or laterality (p > 0.05).

Conclusion.

The present study demonstrated that the sciatic nerve diameter and thickness were significantly higher in males than females while no significant side-related differences were indicated. Furthermore, Piriformis muscle variation, level of sciatic nerve bifurcation and sciatic nerve-piriformis relationships showed no significant association with either gender or laterality. These findings highlight that sexual dimorphism may influence the size of sciatic nerve. In contrast, the anatomical patterns and relationships of the sciatic nerve remain comparable across sexes and sides.

Keywords: Sciatic nerve; Piriformis muscle; Anatomical variation; Magnetic Resonance Imaging (MRI)

INTRODUCTION

The sciatic nerve is the largest and longest peripheral nerve in the human body and represents the primary neural supply to the lower extremity. It originates from the sacral plexus, receiving contributions from the ventral rami of spinal nerves L4 to S3, and passes through the pelvis into the gluteal region before descending along the posterior compartment of the thigh [1,2]. Owing to its extensive anatomical course and its major motor and sensory functions, the sciatic nerve plays a fundamental role in lower limb movement and sensation. Consequently, detailed knowledge of its anatomy is essential for clinicians involved in orthopedic surgery, anesthesiology, neurology, and radiology [2].

The relationship between the sciatic nerve and the piriformis muscle within the gluteal region is of particular clinical significance. In the typical anatomical pattern, the sciatic nerve exits the pelvis inferior to the piriformis muscle through the greater sciatic foramen before continuing along the posterior thigh [3]. However, variations in this relationship may occur and can influence the course and branching pattern of the nerve. The sciatic nerve typically divides into its two terminal branches—the tibial nerve and the common peroneal (fibular) nerve—usually near the apex of the popliteal fossa, although higher divisions have also been documented [1,4]. These variations may contribute to several clinical conditions, including sciatica, piriformis syndrome, and complications during surgical or anesthetic procedures involving the gluteal region [5].

The classical anatomical descriptions are that the sciatic nerve originates from the pelvis as a solitary trunk beneath the piriformis muscle through the greater sciatic foramen and goes into inferiority along the back of the thigh. However, many anatomical studies have found that the relationship between the sciatic nerve and the piriformis muscle is not uniform [6]. These differences have been categorized in the well-known classification by Beaton and Anson [7]. Such course or early bifurcation of the sciatic nerve may lead to iatrogenic nerve damage during hip surgery, posterior approaches to the pelvis, and gluteal intramuscular injections [8]. For anesthesiologists, knowledge of these variations is particularly important when performing sciatic nerve blocks or other regional anesthesia techniques involving lower limb procedures [9].

The science of the sciatic nerve and different patterns has traditionally only been found in dissection studies. Although they have greatly broadened our understanding of anatomy, such investigations might not always reflect the typical anatomical structure seen in living people [10]. Recent developments in imaging technology—especially advances in magnetic resonance imaging (MRI)—have offered a non-invasive diagnostic tool to view and visualize peripheral nerves and their surrounding regions in vivo. MRI provides accurate analysis of sciatic nerve course, branching pattern and location relative to other muscles and bony sites [11]. This study was designed to evaluate the Piriformis Muscle - Sciatic nerve relationships and the level of sciatic nerve bifurcation with reference to gender and laterality in the Iraqi population.

METHODS

This descriptive observational cross-sectional study was conducted at the Department of Radiology, Imam Hussein Medical City Hospital, Karbala, Iraq. The study included 35 adult participants, yielding 70 lower limb observations because both right and left lower limbs were analyzed separately. Ethical approval was obtained from the Institutional Review Board (IRB) of the Anatomy Department, College of Medicine, University of Baghdad (Ref. No. 601; 30 June 2025). All procedures were performed in accordance with the ethical principles of the Declaration of Helsinki.

The study population consisted of healthy volunteers and patients referred for pelvic MRI for non-neurological indications. Inclusion criteria comprised age between 18 and 65 years, availability of high-quality pelvic MRI with clear sciatic nerve visualization, absence of previous pelvic trauma or surgery, and absence of known sciatic nerve pathology. Exclusion criteria included previous pelvic or hip surgery, sciatic nerve tumors or neuropathy, severe musculoskeletal deformities, or poor-quality MRI images.

All MRI examinations were performed using a 1.5-Tesla magnetic resonance imaging system (SIGNA Voyager; GE Healthcare, Chicago, Illinois, USA) equipped with a phased-array surface coil and integrated Picture Archiving and Communication System (PACS). Participants were positioned supine with both lower limbs maintained in neutral alignment. Imaging sequences included axial, coronal, and sagittal T1-weighted and T2-weighted images, with additional STIR sequences when required (slice thickness 3-4 mm, field of view 28-36 cm). Images were independently reviewed by two experienced radiologists.

Quantitative analysis included measurement of sciatic nerve diameter and thickness using electronic digital calipers on axial MRI images at the subgluteal region. Qualitative analysis included assessment of piriformis muscle shape variations and classification of the sciatic nerve-piriformis relationship according to Beaton and Anson (Types I-VI) [12]: Type I (undivided nerve below piriformis), Type II (common peroneal division through piriformis, tibial division below), Type III (common peroneal division above, tibial division below), Type IV (undivided nerve through piriformis), Type V (common peroneal division through, tibial division above), and Type VI (undivided nerve above piriformis). The bifurcation level was categorized as pelvic, gluteal, thigh, or popliteal.

Statistical analysis was performed using SPSS version 28 (IBM Corp.). Continuous variables were expressed as mean ± SD, and categorical variables as frequencies and percentages. Significance was set at p < 0.05.

RESULTS

Thirty-five participants (18 males [51.4%], 17 females [48.6%]; mean age 31.8 ± 6.2 years; mean BMI 24.6 ± 3.1 kg/m2) were evaluated across 70 lower limbs. The mean sciatic nerve diameter was 11.62 ± 1.56 mm and mean thickness was 6.4 ± 1.2 mm. Male participants demonstrated significantly greater sciatic nerve diameter (12.3 ± 1.5 mm vs. 10.9 ± 1.3 mm, p = 0.03) and thickness (7.1 ± 1.1 mm vs. 5.8 ± 0.9 mm, p = 0.02) than female participants, while side-to-side differences were non-significant (p > 0.05) (Table 1).

The typical pear-shaped morphology of the piriformis muscle (Variation I) was identified in 58 limbs (82.9%), followed by bipartite piriformis with CPN piercing (Variation II) in 7 limbs (10.0%) and piriformis-gluteus medius fusion (Variation III) in 5 limbs (7.1%), with no significant laterality (p = 0.41) or gender (p = 0.32) differences (Table 2).

Sciatic nerve bifurcation occurred most frequently at the popliteal fossa in 55 limbs (78.6%), followed by the pelvis in 12 limbs (17.1%) and the thigh in 3 limbs (4.3%), without significant differences by laterality (p = 0.74) or gender (p = 0.55) (Table 3).

According to the Beaton and Anson classification, Type I was the predominant pattern observed in 55 limbs (78.6%), followed by Type II in 7 limbs (10.0%), Type III in 4 limbs (5.7%), Type IV in 2 limbs (2.9%), and Types V and VI in 1 limb each (1.4%), without significant associations with laterality (p = 0.52) or gender (p = 0.89) (Table 4).

Table 1

MRI measurements of sciatic nerve diameter and thickness (Mean ± SD) according to gender and laterality.

Measurement (mm) | Total | Right Side | Left Side | Laterality p-value | Males | Females | Gender p-value
Sciatic nerve diameter | 11.62 ± 1.56 | 11.62 ± 1.56 | 11.62 ± 1.56 | > 0.05 | 12.3 ± 1.5 | 10.9 ± 1.3 | 0.03*
Sciatic nerve thickness | 6.4 ± 1.2 | 6.4 ± 1.2 | 6.4 ± 1.2 | > 0.05 | 7.1 ± 1.1 | 5.8 ± 0.9 | 0.02*

Table 2

Shape variation of piriformis muscle according to laterality and gender.

Shape variation of piriformis muscle | Total Number (%) | Right (n=35) (%) | Left (n=35) (%) | Laterality p-value | Male (n=36) (%) | Female (n=34) (%) | Gender p-value
Variation I: Typical pear-shaped Piriformis | 58 (82.9%) | 30 (85.7%) | 28 (80.0%) | 0.41 | 31 (86.1%) | 27 (79.4%) | 0.32
Variation II: Subdivision of piriformis by the CPN | 7 (10.0%) | 4 (11.4%) | 3 (8.6%) | - | 4 (11.1%) | 3 (8.8%) | -
Variation III: Piriformis - Gluteus Medius fusion | 5 (7.10%) | 1 (2.9%) | 4 (11.4%) | - | 1 (2.8%) | 4 (11.8%) | -

Table 3

Level of sciatic nerve bifurcation according to laterality and gender.

Level of bifurcation | Total n (%) | Right n (%) | Left n (%) | Laterality p-value | Male (n=36) (%) | Female (n=34) (%) | Gender p-value
Popliteal fossa | 55 (78.6%) | 27 (77.1%) | 28 (80.0%) | 0.74 | 30 (83.3%) | 25 (73.5%) | 0.55
Thigh | 3 (4.3%) | 1 (2.9%) | 2 (5.7%) | - | 1 (2.8%) | 2 (5.9%) | -
Pelvis | 12 (17.1%) | 7 (20.0%) | 5 (14.3%) | - | 5 (13.9%) | 7 (20.6%) | -
Total | 70 (100%) | 35 (100%) | 35 (100%) | - | 36 (100%) | 34 (100%) | -

Table 4

Beaton and Anson classification patterns according to laterality and gender.

Beaton & Anson Type | Right n (%) | Left n (%) | Laterality p-value | Male n (%) | Female n (%) | Gender p-value | Total n (%)
Type I | 25 (71.4%) | 30 (85.7%) | 0.52 | 29 (80.6%) | 26 (76.5%) | 0.89 | 55 (78.6%)
Type II | 4 (11.4%) | 3 (8.6%) | - | 4 (11.1%) | 3 (8.8%) | - | 7 (10.0%)
Type III | 2 (5.7%) | 2 (5.7%) | - | 2 (5.6%) | 2 (5.9%) | - | 4 (5.7%)
Type IV | 2 (5.7%) | 0 (0.0%) | - | 1 (2.8%) | 1 (2.9%) | - | 2 (2.9%)
Type V | 1 (2.9%) | 0 (0.0%) | - | 0 (0.0%) | 1 (2.9%) | - | 1 (1.4%)
Type VI | 1 (2.9%) | 0 (0.0%) | - | 0 (0.0%) | 1 (2.9%) | - | 1 (1.4%)
Total | 35 (100%) | 35 (100%) | - | 36 (100%) | 34 (100%) | - | 70 (100%)

Figure 1: Axial T2-weighted MRI images demonstrating levels of sciatic nerve bifurcation (A-C) at the upper border of popliteal fossa (A), upper thigh (B), sub gluteal region (C).

Figure 2: Coronal T2-weighted MRI of the pelvis demonstrating the sciatic nerve-piriformis muscle relationship according to the Beaton and Anson classification.

DISCUSSION

In the present study, sciatic nerve diameter and thickness at the subgluteal level were 11.62 mm and 6.4 mm, respectively, confirming an elliptical morphology where mediolateral width exceeds anteroposterior thickness [13]. These measurements are consistent with MRI data reported by Rusu et al. (10.4 mm and 5.5 mm) [13] and ultrasound ranges reported by Karmakar et al. and Vloka et al. (12-20 mm width, 4-8 mm thickness) [14,15]. Cadaveric dimensions are often larger due to tissue fixation and inclusion of connective tissue fascial sheaths [16,17].

Significant sexual dimorphism was noted in sciatic nerve dimensions, with males exhibiting greater diameter (12.3 mm vs. 10.9 mm) and thickness (7.1 mm vs. 5.8 mm) than females (p < 0.05), aligning with findings by Karmakar et al. and Rusu et al. [13,14].

Piriformis morphology was typical (single belly) in 82.9% of limbs, while 10% exhibited a bipartite muscle pierced by the common fibular nerve and 7.1% showed fusion with the gluteus medius, matching frequencies reported by Bergman, Haładaj et al., and Poutoglidou et al. [18-20].

Sciatic nerve bifurcation predominated at the apex of the popliteal fossa (78.6%), followed by pelvic (17.1%) and thigh (4.3%) divisions [21-24]. High divisions (pelvic/thigh) have clinical implications for failed popliteal sciatic blocks, piriformis entrapment syndrome, and iatrogenic injury during posterior hip approaches [24-26]. In the Beaton and Anson classification, Type I predominated (78.6%), while Types II-VI collectively accounted for 21.4% of limbs, without significant side or sex predilection, in agreement with major meta-analyses [27-29].

CONCLUSION

This MRI-based study in an Iraqi population demonstrated significant sexual dimorphism in sciatic nerve diameter and thickness (larger in males), whereas piriformis morphology, bifurcation level (predominantly popliteal at 78.6%), and Beaton and Anson nerve-muscle relationships (predominantly Type I at 78.6%) were independent of sex and laterality. Accurate in vivo awareness of these anatomical variations is critical for regional anesthesia, orthopedic surgery, and radiological assessment of gluteal entrapment syndromes.

Abbreviations & Declarations

DECLARATIONS

Ethical Approval: The project was approved by the Institutional Review Board of the Department of Anatomy, College of Medicine, University of Baghdad (Ref. No. 601; 30 June 2025).

Conflicts of Interest: The authors declare no conflicts of interest.

Author Contributions: Study conception and design: Malak A. Al-Yawer; literature search: Mohammed Eimad Mohammed and Malak A. Al-Yawer; data analysis and interpretation: Mohammed Eimad Mohammed and Malak A. Al-Yawer; manuscript preparation, editing, and review: Malak A. Al-Yawer and Mohammed Eimad Mohammed.

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