The Circle of Willis (CoW) is a critical arterial anastomosis at the base of the brain, linking the internal carotid arteries (ICAs), anterior cerebral arteries (ACAs), anterior communicating artery (AComA), posterior cerebral arteries (PCAs), and posterior communicating arteries (PComAs) to ensure collateral blood flow between anterior and posterior cerebral circulations. This structure is vital for maintaining cerebral perfusion, especially during vascular occlusions or anomalies [1]. However, its anatomical variability is well-documented, with complete configurations reported in only 20–50% of individuals across diverse populations [2, 3]. Common variants include hypoplasia or aplasia of the PComA and AComA, as well as accessory vessels like fetal-type PCA or persistent embryonic arteries [4, 5].

Such variability significantly impacts cerebrovascular dynamics, potentially increasing risks of ischemic stroke, aneurysm formation, and inadequate collateral flow during surgical interventions [1, 3]. Morphometric analyses of the CoW, as conducted in cadaveric and imaging studies, provide critical insights into vessel diameters, lengths, and configurations, enhancing our understanding of its anatomical and clinical significance [2, 4]. Previous research, including Siddiqi et al. (2013) in Klimek-Piotrowska W et al. (2016) in Malawi, has highlighted frequent PComA hypoplasia, while Gunnal et al. (2014) and Saikia et al. (2014) noted AComA anomalies in Indian populations [1, 2, 3, 4]. Yet, inconsistencies in reported frequencies and measurements, as seen in CT angiography studies by Prasad et al. (2017), emphasize the need for standardized investigations [5].

The present study aims to address this gap by conducting a morphometric analysis of the CoW in 50 human cadaveric specimens. By quantifying vessel dimensions and cataloging anatomical variants, we seek to elucidate the prevalence of complete versus incomplete configurations, identify dominant patterns of asymmetry, and assess potential gender differences

This morphometric analysis of the Circle of Willis (CoW) was conducted on 50 human cadaveric specimens over a study period spanning from September 2019 to September 2020. The research was carried out at Siddhartha Medical College, Vijayawada, Andhra Pradesh, India, within the Department of Anatomy. Ethical approval was obtained from the Institutional Ethics Committee prior to commencement, ensuring compliance with guidelines for cadaveric research.

The specimens were sourced from donated bodies used for educational purposes, with inclusion criteria requiring intact cranial vaults and no visible evidence of head trauma, neurosurgery, or gross neuropathology. The sample consisted of 28 male and 22 female cadavers, with ages ranging from 42 to 78 years at the time of death (mean age: 61.3 ± 9.7 years). Demographic data were retrieved from autopsy records where available.

Dissection was performed using standard anatomical techniques. Following craniotomy, the dura mater was removed to expose the brain, which was carefully extracted to preserve the basal vasculature. The CoW was isolated by meticulous dissection of the ICAs, ACAs, AComA, PCAs, and PComAs. Vessel patency and configurations were assessed visually and classified as complete (all components present and patent) or incomplete (hypoplasia <1 mm diameter or aplasia of any segment). Hypoplasia was defined as a vessel diameter below 1 mm, measured using digital calipers (Mitutoyo, precision: 0.01 mm). Vessel diameters (ICA, ACA A1, PCA P1, PComA, AComA) and AComA length were recorded at their midpoints by two independent observers to minimize bias, with measurements averaged for analysis. Variants such as fetal-type PCA, duplicated AComA, or persistent trigeminal artery were noted.

Data were compiled in Microsoft Excel and analyzed using SPSS (version 25.0). Descriptive statistics (means, standard deviations, ranges) were calculated for morphometric parameters. Student’s t-tests assessed gender differences and side-to-side asymmetry, with a p-value <0.05 considered significant. Inter-observer variability was evaluated to ensure measurement reliability, targeting a threshold of less than 5%

In this morphometric analysis of the Circle of Willis (CoW), 50 human cadaveric specimens were examined to assess anatomical configurations and vessel measurements. The study population comprised 28 males (56%) and 22 females (44%), with an age range of 42 to 78 years and a mean age of 61.3 ± 9.7 years (Table 1). Dissections were performed to evaluate the completeness of the CoW, prevalence of anatomical variants, and morphometric characteristics of its major vessels.

Table 1: Demographic Characteristics of the Study Population

A complete CoW, defined by the presence and patency of all component vessels, was observed in 22 specimens (44%), while an incomplete configuration, marked by hypoplasia (<1 mm diameter) or absence of one or more vessels, was noted in 28 specimens (56%) (Table 2).

Table 2: Completeness of the Circle of Willis

Among the variants, hypoplasia of the posterior communicating artery (PComA) was the most frequent, occurring unilaterally in 16 cases (32%) and bilaterally in 8 cases (16%). PComA aplasia was identified in 6 specimens (12%). The anterior communicating artery (AComA) exhibited hypoplasia in 10 specimens (20%) and aplasia in 2 specimens (4%). Fetal-type posterior cerebral artery (PCA), where the PComA diameter exceeded that of the P1 segment, was present in 10 cases (20%), with 6 unilateral and 4 bilateral instances. Less common anomalies included duplicated AComA in 3 specimens (6%) and a persistent trigeminal artery in 1 specimen (2%) (Table 3).

Table 3: Prevalence of Anatomical Variants in the Circle of Willis

Morphometric analysis revealed the mean diameter of the internal carotid artery (ICA) to be 4.8 ± 0.6 mm (range: 3.9–6.2 mm), with no significant difference between left and right sides (p = 0.72). The anterior cerebral artery (ACA) A1 segment had a mean diameter of 2.3 ± 0.4 mm (range: 1.6–3.1 mm), while the PCA P1 segment measured 2.1 ± 0.5 mm (range: 1.4–3.0 mm). The PComA, when present, had a mean diameter of 1.8 ± 0.6 mm (range: 0.9–2.8 mm), and the AComA measured 1.9 ± 0.5 mm (range: 1.0–2.9 mm) (Table 4).

Table 4: Morphometric Measurements of Major Vessels

Additional measurements of the AComA showed a mean length of 2.5 ± 0.7 mm (range: 1.2–4.0 mm) (Table 5).

Table 5: Additional Measurements of the AComA

Asymmetry in vessel caliber, defined as a difference greater than 1 mm between left and right sides, was observed in 18 specimens (36%), most commonly in the ACA A1 segment (12 cases, 24%). Gender-based analysis indicated slightly larger ICA diameters in males (4.9 ± 0.6 mm) compared to females (4.7 ± 0.5 mm), though this difference was not statistically significant (p = 0.31). No significant gender differences were found in the prevalence of CoW variants (p = 0.58) (Table 6).

Table 6: Asymmetry and Gender Differences

Statistical consistency was maintained, with inter-observer variability below 5% and no significant side-to-side differences in ICA diameter (p = 0.72) (Table 7).

Table 7: Statistical Observations

This morphometric analysis of the Circle of Willis (CoW) in 50 cadaveric specimens, elucidates its anatomical variability. The observation of a complete CoW in 44% of specimens aligns with reported ranges of 20–50% across diverse populations [6, 7]. The predominance of incomplete configurations (56%) reflects the CoW’s structural heterogeneity, potentially due to embryological adaptations in cerebral vasculature [8].

The prevalent posterior communicating artery (PComA) hypoplasia (32% unilateral, 16% bilateral) and aplasia (12%) corroborate findings by Iqbal (2013) and Maaly & Ismail (2011), who identified PComA anomalies as frequent variants [6, 7]. These alterations may compromise posterior circulation collateral flow, elevating stroke risk in occlusive conditions [9]. Similarly, AComA hypoplasia (20%) and aplasia (4%) are consistent with Cui et al. (2015), suggesting links to altered hemodynamics and aneurysm formation [9]. The 20% incidence of fetal-type PCA aligns with Shaikh & Sohail (2018), indicating a reliance on ICA-derived posterior flow [8].

Mean vessel diameters (e.g., ICA: 4.8 ± 0.6 mm; ACA A1: 2.3 ± 0.4 mm) fall within ranges reported by Cui Y et al. (2015) and De Silva et al. (2009) [10, 11]. Asymmetry in 36% of specimens, primarily in ACA A1, supports Ansari et al. (2011), who noted its impact on collateral capacity [12]. The absence of significant gender differences (p = 0.58 for variants; p = 0.31 for ICA diameter) contrasts with Shaikh R, Sohail S et al. (2018), possibly due to sample size or regional factors [13].

Rare anomalies like duplicated AComA (6%) and persistent trigeminal artery (2%) carry clinical relevance for neurosurgery [10]. Robust measurement reliability (<5% inter-observer

variability) strengthens these findings, though the cadaveric approach limits functional insights

Limitations include the modest sample size and reliance on cadaveric rather than in vivo measurements, which may not fully reflect dynamic blood flow effects. Nonetheless, this study’s detailed morphometry and variant cataloging contribute to the anatomical knowledge base, offering implications for neurosurgery, stroke management, and vascular modeling. Future research could employ imaging modalities in larger cohorts to correlate these findings with functional outcomes.

This morphometric analysis of the Circle of Willis in 50 cadaveric specimens reveals significant anatomical variability. A complete CoW was present in 44% of cases, with incomplete configurations (56%) driven by frequent PComA hypoplasia (48% combined) and AComA anomalies (24% combined). Vessel diameters aligned with prior studies, and asymmetry was notable in 36% of specimens, primarily in ACA A1. No significant gender differences emerged. These findings highlight the CoW’s structural diversity, with implications for cerebrovascular risk and neurosurgical planning.

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