Vertebral Artery Entry-Level Variations: A Meta-analytic Anatomical Synthesis

The vertebral arteries supply the posterior circulation and ascend from the subclavian arteries through the cervical transverse foramina before entering the cranium. Classical descriptions place the entrance of the V2 segment at the C6 transverse foramen, with a straight vertical course up to C2, but both cadaveric and CT angiography (CTA) series consistently document higher (C3–C5) and lower (C7) entry levels in a minority of vessels.^{1, 2}

These entry-level variants have become clinically critical with the expansion of multilevel anterior cervical discectomy and fusion, anterior corpectomy and plating, and image-guided procedures such as selective nerve root and stellate ganglion blocks.^{4, 10} A vertebral artery entering at C5 or C4 can lie closer to commonly targeted disc spaces, while low entry at C7 shifts the artery into zones often regarded as relatively safe in standard C6-based approach maps.⁴

A 2023 systematic review and meta-analysis of 32,153 vertebral arteries by Tudose et al. provided the most robust pooled estimates for entry level and origin, showing that the vast majority of arteries enter at C6 but with a non-trivial burden of variants at other levels.¹ Magklara et al. synthesised the embryology and morphology of vertebral artery variants, emphasising that persistence of non-typical intersegmental arteries underpins both high and low entry patterns.²

This article adopts a meta-analytic mindset to summarise vertebral artery entry-level anatomy, focusing on four practical questions: (1) What is the global prevalence of C6 versus non-C6 entry? (2) How are high (C3–C5) and low (C7) entry levels distributed? (3) How strongly is atypical entry linked to aberrant origin, especially left arteries from the aortic arch? and (4) Do cadaveric versus CTA datasets show meaningful differences that matter for surgical planning?^{1, 3}

Rather than recalculating pooled effect sizes, this article uses the systematic review and meta-analysis by Tudose et al. as its quantitative backbone and cross-checks those estimates against large, clearly reported CTA and cadaveric series.¹

Key data sources include: (1) the global meta-analysis of 32,153 vertebral arteries reported by Tudose et al.; (2) the morphologic and embryologic review by Magklara et al.; (3) large CTA cohorts from East Asia and Europe, notably Yi et al., Hong et al., Lin et al. and Uchino et al.; and (4) cadaveric series such as that of Rawal et al., which provide selection-bias-free estimates of entry-level anatomy.^{1, 2}

For each study, reported percentages for entry at C3, C4, C5, C6 and C7 were extracted where available, along with information on arterial origin (subclavian versus aortic arch) and laterality. Studies were qualitatively grouped as cadaveric versus radiologic and as general versus variant-enriched clinical cohorts.^{3, 5}

This synthesis deliberately avoids generating new pooled prevalence estimates or confidence intervals; instead, it preserves the original numerical results of large, well-characterised cohorts and meta-analytic summaries, and uses simplified charts to visualise key patterns for applied anatomical interpretation.

Global prevalence of C6 versus non-C6 entry

In the meta-analysis by Tudose et al., 92.0% of vertebral arteries entered the transverse foramen at C6, based on 32,153 arteries from 62 studies, with the remaining 8.0% distributed—by decreasing frequency—across C5, C7, C4 and very rarely C3 (around 0.1%).¹ This pooled value is closely mirrored by individual CTA and cadaveric series, in which typical C6 entrance generally ranges from about 90% to 95%.^{3, 8}

Yi et al. provide one of the most granular CTA datasets: among 466 vertebral arteries, 91.7% entered at C6, while 8.3% entered at other levels, distributed as C3 0.2%, C4 2.5%, C5 4.7% and C7 0.9%.³ These figures closely match the meta-analytic pooled proportions, supporting the conclusion that approximately 1 in 12 vertebral arteries enter the cervical spine at a level other than C6.

Effect of Arterial Origin on Entry Level

The relationship between abnormal origin and entry level is best characterised in CTA studies that tracked both variables. In the cohort of Yi et al., the overall prevalence of abnormal (non-C6) entrance was 8.3%, but in the subgroup with aberrant origin—including left vertebral arteries arising directly from the aortic arch—non-C6 entry occurred in 50% of arteries, with a corresponding drop in C6 entry to 50%.³

The meta-analysis by Tudose et al. reported left vertebral artery origin from the aortic arch in 4.81% of arteries, and forest plots stratified by origin showed that this subgroup contributed disproportionately to high-entry (C4–C5) patterns.¹ Lin et al. and Uchino et al. similarly found that anomalous aortic-arch origin strongly co-segregated with C4 or C5 entry, consistent with the embryologic persistence of higher cervical intersegmental arteries.^{6, 7}

Modality and Population Differences

Hong et al. analysed 700 vertebral arteries on three-dimensional CTA and found C6 entry in 94.9% and abnormal entrance in 5.1%, with C4 1.6%, C5 3.3% and C7 0.3%.⁴ Their Korean cohort therefore showed slightly higher C6 predominance than the Chinese CTA dataset of Yi et al., but the same pattern of C5-dominant high entry.³

In contrast, Vujmilović et al. reported typical C6 entrance in approximately 80% of arteries and non-C6 entrance in 20% in a CT-based cohort, a higher variant burden likely reflecting inclusion of patients imaged for vascular pathology or degenerative cervical disease.⁵ Cadaveric work by Rawal et al. found C6 entry in 92% and C7 entry in 8% of arteries, suggesting that true population prevalence sits closer to the high-C6 range once clinical selection is removed.⁸

Taken together, these data show that cadaveric baselines and large CTA cohorts converge around roughly 90–95% C6 entry with 5–10% variants, whereas variant-enriched clinical imaging series can show non-C6 entrance in up to 1 in 5 arteries.

Meta-analytic and large-series evidence confirm that C6 is the overwhelmingly dominant entry level for the vertebral artery, with pooled estimates around 92% and tightly clustered CTA and cadaveric data in the 90–95% range.^{1, 3} High entry at C4–C5 and low entry at C7 are not curiosities; taken together, they occur in about 1 in 10 arteries overall and in up to 1 in 5 arteries in variant-enriched clinical cohorts.^{2, 5}

Embryologically, these patterns reflect persistence or regression of specific cervical intersegmental arteries. Magklara et al. linked high (C4–C5) entry to persistence of higher intersegmental channels, often accompanied by more medial or tortuous extraosseous loops, whereas low (C7) entry reflects persistence of lower channels supplying the V1 segment.^{2, 9} This framework explains the strong association between anomalous aortic-arch origin and high-entry patterns seen in CTA cohorts.^{1, 6}

From a surgical standpoint, assuming a textbook C6 entry without imaging is unsafe whenever procedures approach the lateral cervical column. Anterior cervical discectomy and fusion, corpectomy, and anterolateral screw fixation all rely on predictable distances between the uncinate process, longus colli, transverse foramen and vertebral artery.^{4, 10} High-entry arteries reduce the margin for error at upper cervical levels, while low entry at C7 undermines the assumption of a safe transverse process during stellate ganglion blocks.

Radiologically, Magklara et al. and others emphasise that vertebral artery variants—including high or low entry, anomalous origin and significant tortuosity—should be explicitly highlighted in reports because they materially alter interventional planning.^{2, 6} The convergence between cadaveric and CTA data supports a simple rule of thumb: most patients do have C6 entry, but clinicians should assume that roughly 1 in 10 do not and should insist on preoperative CTA or MRA whenever a deviation from midline anatomy is contemplated.

Limitations of the available literature include heterogeneity in inclusion criteria, imaging protocols and definitions of abnormal entry, as well as over-representation of East Asian cohorts in CTA datasets and selection bias in clinical imaging studies.^{1, 5} Nevertheless, the consistency of broad prevalence ranges across modalities and populations suggests that the key numerical messages are robust.

Across more than 32,000 vertebral arteries, current evidence shows that C6 entry into the transverse foramen is the rule, occurring in about 92% of vessels, but that 5–10% enter at other cervical levels, most often C5 and C4.^{1, 3} Variant-enriched clinical cohorts may show non-C6 entry in up to 20% of arteries, particularly when patients are imaged for vascular pathology or complex degenerative disease.⁵

Atypical entry is strongly associated with aberrant origin, especially left vertebral arteries arising directly from the aortic arch, in which half of arteries may enter above or below C6.^{1, 3} For surgeons, interventional radiologists and pain physicians, this mandates patient-specific vascular imaging rather than reliance on schematic C6-based safe zones when working around the lateral cervical column.^{4, 10}

Routine preoperative CTA or high-quality MRA, coupled with radiology reports that explicitly state vertebral artery origin, entry level and major tortuosity, can substantially reduce the risk of iatrogenic injury and help translate meta-analytic anatomical knowledge into safer cervical approaches.