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Open in new tab Download slide OBJECTIVES This study aimed to analyse and determine the role of aortic length and curvature in the pathogenesis of acute type A aortic dissection (ATAAD) with ascending aortic diameters (AADs) <5 cm. METHODS We reviewed the clinical and imaging data of patients with ATAAD (n  = 201) and ascending aortic dilation (n  = 83). Thoracic aortic bending index (TABI) was used to quantify aortic curvature and analyse its role in ATAAD below the diameter risk threshold. RESULTS The AAD was <5.0 and <4.0 cm in 78% and 37% of patients with ATAAD, respectively. The median ascending aortic length (AAL) was 104.6 mm (Q1–Q3, 96.5–113.6 mm), and in 62.7% of patients, it was <11 cm. The median TABI was 14.99 mm/cm (Q1–Q3, 14.18–15.86 mm/cm). Patients with ATAAD and those with aortic dilation were matched for AAD, age, sex, height and other clinical factors. After matched, the dissection group had higher AALs (median, 102.9 mm; Q1–Q3, 96.0–112.5 mm vs median, 88.2 mm; Q1–Q3, 83.7–95.9 mm; P  < 0.001) and TABI (median, 14.84 mm/cm; Q1–Q3, 14.06–15.83 mm/cm vs median, 13.55 mm/cm; Q1–Q3, 13.03–14.28 mm/cm; P  < 0.001). According to the regression analysis, the area under the curve required to distinguish patients with ATAAD from those with aortic dilation was 0.831 in AAL, 0.837 in TABI and 0.907 when AAL was combined with TABI. CONCLUSIONS The patients with ATAAD had higher AAL and TABI than those with aortic dilation. The combination of TABI and AAL might be a potential morphological marker for determining ATAAD risk below the current aortic diameter risk threshold. [ABSTRACT FROM AUTHOR]