The circle of Willis (CoW), or circulus arteriosus cerebri, is a basal arterial anastomosis that links the internal carotid circulation with the vertebrobasilar system. Early anatomical series established that the “classical” CoW configuration is not universal; absence of segments, calibre reduction and side-to-side asymmetry are frequently encountered in brains without gross pathology [1,2]. Imaging-based descriptions have supported these observations, and time-of-flight MR angiography demonstrates substantial morphologic variation in living adults, including incomplete or markedly asymmetric circles [3].

This anatomical diversity is clinically important because collateral capacity depends on both continuity and lumen size. During carotid or basilar compromise, a complete circle can redistribute flow across territories, whereas an incomplete circle or severely hypoplastic communicating segments restrict cross-filling and can influence infarct distribution and clinical severity [3]. A meta-analysis focusing on posterior communicating artery (PCoA) variants also emphasizes that posterior segment calibre limitation is common and relevant to collateral pathways [4].

Variants of the posterior part of the circle deserve particular attention because the PCoA can provide a critical conduit between anterior and posterior territories. In the fetal-type configuration, the posterior cerebral artery (PCA) is supplied predominantly from the internal carotid artery, increasing carotid dependence of posterior territories and altering collateral routes [5]. Developmental and anatomical investigations further describe how posterior bifurcation patterns and PCoA–PCA relationships differ between fetal and adult life, providing a structural basis for these variants [6].

Cadaveric evaluation remains an important approach for documenting CoW morphology because direct visualization permits confident assessment of vessel presence, branching, and calibre. Comparative anatomical work suggests that the distribution of CoW variants differs across populations [7]. Large autopsy series applying external diameter criteria have reported high frequencies of hypoplasia and multiple coexisting anomalies, particularly involving the ACoA and PCoA [8].

Indian cadaveric studies also document substantial variability in CoW configurations [9,10], and international autopsy studies show that typical patterns can be relatively uncommon, with posterior communicating segments contributing disproportionately to atypical configurations [11,12]. In addition, A1 segment agenesis or hypoplasia has been linked to impaired anterior collateral supply and characteristic ischemic stroke patterns, underscoring the clinical relevance of anterior variants [13]. Given these considerations, institution- and region-specific morphological data remain useful for anatomists and clinicians.

Objectives: (i) to determine the overall morphological pattern of the CoW (complete symmetrical, complete asymmetrical, or incomplete), (ii) to document variations in anterior circulation components (ACoA and A1 segment), (iii) to document variations in posterior circulation components (PCoA and fetal-type PCA), and (iv) to describe laterality and multiplicity of observed variations.

Study design and setting: This observational cadaveric study was conducted in the Department of Anatomy, Rohilkhand Medical College and Hospital, Bareilly, Uttar Pradesh, India, over three years (October 2022 to September 2025).

Sample size and specimens: One hundred adult (≥18 years) formalin-fixed human brains available for routine teaching dissection were included. Specimens with gross damage to the basal surface, disrupted intracranial vessels, or distortion that interfered with reliable identification of circle components were excluded. Cadaveric identifiers were not uniformly available in the departmental collection; therefore, demographic variables and vascular risk histories were not analysed.

Dissection procedure: Each brain was examined from the basal aspect under adequate illumination. The arachnoid over the interpeduncular cistern, optic chiasma region and suprasellar area was gently removed to expose the CoW. Component arteries were identified systematically: bilateral internal carotid termini; precommunicating (A1) segments of the anterior cerebral arteries; the anterior communicating artery (ACoA); the posterior communicating arteries (PCoA); precommunicating (P1) segments of the posterior cerebral arteries (PCA); and the basilar artery bifurcation. Surrounding connective tissue was cleared to delineate vessel continuity and laterality. Representative photographs were taken for documentation.

Operational definitions and recording: Overall morphology was categorized as (a) classical complete and symmetrical circle, (b) complete but asymmetrical circle, or (c) incomplete circle, consistent with established anatomical descriptions [1,2]. Segmental variations were recorded as hypoplasia (markedly reduced calibre compared with the contralateral side and adjacent segments) or aplasia (absence of a segment). Where feasible, external diameters of ACoA, A1 and PCoA were measured with a digital vernier caliper. Hypoplasia was operationally considered when the external diameter was <1 mm, as applied in autopsy-based work [8], and this criterion was used to maintain comparability with published cadaveric series [9-11]. Fetal-type PCA was defined when the P1 segment was absent or markedly hypoplastic with a dominant PCoA supplying the PCA, consistent with established descriptions [5,14]. Laterality (unilateral/bilateral) and multiplicity (more than one variant in the same specimen) were recorded.

Data management and statistical analysis: Findings were recorded on a structured proforma and cross-checked by two observers for internal consistency. Data were entered into a spreadsheet and summarized using descriptive statistics. Categorical variables were expressed as number and percentage.

Ethical considerations: The study used preserved cadaveric specimens maintained for teaching and research. Institutional permission was obtained from the Department of Anatomy; informed consent was not applicable for the use of formalin-fixed teaching material.

A total of 100 adult cadaveric brains were examined. The overall morphological pattern of the circle of Willis is shown in Table 1. A classical complete and symmetrical circle was observed in 54 specimens, while 18 brains showed a complete but asymmetrical configuration. An incomplete circle with one or more missing segments was identified in 28 specimens.

Table 1. Overall Morphology of the Circle of Willis (n = 100)

Variations in the anterior circulation were identified in 34% of specimens (Table 2). Hypoplasia of the anterior communicating artery was the most frequent anterior variation (16%), followed by hypoplastic A1 segment of the anterior cerebral artery (10%). Absence of the anterior communicating artery (6%) and absence of the A1 segment (2%) were less common.

Table 2. Variations in the Anterior Circulation of the Circle of Willis (n = 100)

Posterior circulation variations were observed in 30% of brains (Table 3). Hypoplastic posterior communicating artery was the predominant posterior finding (18%), followed by absence of the posterior communicating artery (8%). Fetal-type posterior cerebral artery configuration was present in 4% of specimens (3% bilateral and 1% unilateral).

Table 3. Variations in the Posterior Circulation of the Circle of Willis (n = 100)

Figure 1: Posterior Circulation Variations in the Circle of Willis

With respect to laterality and multiplicity (Table 4), unilateral variations (31%) were more common than bilateral variations (15%). Multiple arterial variations occurring in the same specimen were documented in 14% of brains, indicating that combined anomalies form an important subset of circle morphology.

Table 4. Laterality and Multiplicity of Circle of Willis Variations (n = 100)

Figure 2: Laterality and Multiplicity of Circle of Willis Variations

In this cadaveric series from Bareilly, a classical complete and symmetrical circle of Willis was present in 54% of adult brains, while 46% demonstrated asymmetry or incompleteness. This aligns with foundational anatomical reports documenting frequent departures from the textbook configuration in structurally normal brains [1,2]. The relevance of such variation is supported by imaging evidence that circle morphology can be incomplete even in adults without overt neurological disease and that these differences influence available collateral routes during inflow compromise [3].

Published prevalence of “complete” circles varies because investigators use different operational definitions. Autopsy studies that apply diameter thresholds to define a “typical” circle tend to record higher rates of hypoplastic segments. De Silva et al. measured external diameters in 225 adult cadaveric brains and reported hypoplasia in 86%, with multiple anomalies in 56.4% [8]. In a cadaveric series emphasizing configuration patterns, Gunnal et al. reported a normal and complete circle in 60% of brains [10]. Klimek-Piotrowska et al. found that the typical literature pattern was present in 27%, while most specimens demonstrated complete but atypical configurations dominated by communicating segment changes [11]. Such heterogeneity in reported figures underscores the importance of stating definitions explicitly when comparing studies.

Anterior circulation variants in our study were dominated by ACoA and A1 calibre abnormalities. Reduced calibre of the ACoA can limit interhemispheric cross-filling when one internal carotid artery is compromised. De Silva et al. reported ACoA hypoplasia in 25% and unilateral A1 hypoplasia in 5% in their autopsy cohort [8]. Clinical work supports the importance of A1 variants; Chuang et al. observed a higher incidence of A1 agenesis/hypoplasia among acute ischemic stroke patients and described distinct patterns of infarction and collateral insufficiency [13].

Posterior circulation variability in our specimens (30%) was largely attributable to PCoA hypoplasia or aplasia, consistent with wider literature identifying the PCoA as the most variable component of the CoW [8,11,12]. Jones et al., in a meta-analysis of PCoA variations, emphasized the high prevalence of PCoA calibre limitation and its potential relevance to collateral capacity [4]. The fetal variant of the CoW has also been highlighted as haemodynamically meaningful because it increases carotid dependence of posterior territories [5]. Developmental anatomical observations from fetal–adult comparisons provide structural context for these posterior configurations [6], and CT angiography work also documents fetal PCA patterns in contemporary clinical cohorts [14].

Finally, our observation that unilateral variants outnumbered bilateral variants, and that multiple variants co-existed in 14% of brains, reinforces the need to evaluate the CoW as an integrated arterial network rather than as isolated segments. Population-comparative findings suggest that variant distributions differ across settings [7], supporting the value of local cadaveric data. For clinicians, systematic appraisal of both anterior and posterior connections can support safer skull-base approaches and more nuanced collateral assessment in cerebrovascular disease [9,12].

Limitations

This was a single-center cadaveric study using preserved teaching specimens, limiting external generalizability. Cadaver labels lacked uniform demographic and vascular risk information, preventing subgroup analyses. Formalin fixation and postmortem handling can alter vessel calibre, influencing hypoplasia categorization. The study assessed gross morphology without angiographic correlation or flow measurement, so functional collateral competence and clinical outcome linkage were not evaluated.

In this three-year cadaveric assessment of 100 adult brains, the classical complete and symmetrical circle of Willis was present in 54% of specimens, while 28% showed an incomplete circle. Variants predominantly involved the communicating arteries, with hypoplastic or absent ACoA and PCoA forming the commonest patterns. Unilateral changes exceeded bilateral changes, and multiple concurrent variants were identified in 14% of brains. These observations underscore substantial anatomic diversity at the skull base and support routine, segment-by-segment evaluation of the CoW on imaging. Preoperative consideration of common and rare cerebrovascular anatomical variants in surgical and endovascular planning can significantly reduce intraoperative uncertainty and improve procedural risk anticipation in aneurysm and ischemic cerebrovascular interventions.

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