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OBJECTIVES The shape of the aortic annulus is still under debate. Recent findings suggest a possible gradual spectrum of circularity from tricuspid aortic valves (TAVs), to type 1 bicuspid aortic valves (BAVs) to type 0 BAVs. BAVs have been recently classified in a symmetrical (type A), asymmetrical (type B) or very asymmetrical (type C) phenotype according to the commissural orientation (CO) (160°–180°, 140°–159° and 120°–139°, respectively). The aim of this study is to verify in BAVs the correlation between the aortic annular shape and the CO of valve cusps and to suggest a new anatomical and geometric classification of BAVs based on CO and annular shape. METHODS We retrospectively selected 191 consecutive patients who underwent both electrocardiography-gated computed tomography scan of the aortic root and transthoracic echocardiography between January 2016 and June 2019. The population was divided into 2 groups: 54 BAVs and 137 TAVs. We analysed the subgroup of BAV patients and divided them into group A, group B and group C depending on the CO. The shape of the aortic annulus was considered ‘circular’ or ‘elliptic’ according to the ellipticity index (EI).We studied the possible correlation between CO and annular shape in BAVs. We also analysed the subgroup of TAV patients studying their annular shape and EI. RESULTS After univariate linear regression, BAV patients showed a significant correlation between the CO and the EI (R = −0, 445, R2 = 0, 198, P = 0.001). As the CO decreases, the EI increases and approaches an elliptical shape. After grouping BAVs according to the CO, a mean EI of 1.10 ± 0.07 was found in group A, 1.13 ± 0.08 in group B, 1.18 ± 0.07 in group C, P = 0.0097 indicating a gradual spectrum of ellipticity with the decrease of CO. TAVs subgroup showed a mean EI of 1.27 ± 0.09, suggesting that the more the CO is reduced in BAVs, the more the annulus probably tends towards the very elliptical shape of TAVs. CONCLUSIONS This study shows a linear correlation between CO and annular shape in BAVs. In particular, the aortic annulus follows a continuous spectrum of ellipticity depending on the CO. These findings lay the groundwork for a new anatomical classification of BAVs based on CO and annular shape.

Background: Propensity score matching represents one of the most popular techniques to deal with treatment allocation bias in observational studies. However, when the number of enrolled patients is very low, the creation of matched set of subjects may highly depend on the model used to estimate individual propensity scores, undermining the stability of consequential clinical findings. In this study, we investigate the potential issues related to the stability of the matched sets created by different propensity score models and we propose some diagnostic tools to evaluate them. Methods: Matched groups of patients were created using five different methods: Logistic Regression, Classification and Regression Trees, Bagging, Random Forest and Generalized Boosted Model. Differences between subjects in the matched sets were evaluated by comparing both pre-treatment covariates and propensity score distributions. We applied our proposal to a cardio-surgical observational study that aims to compare two different procedures of cardiac valve replacement. Results: Both baseline characteristics and propensity score distributions were systematically different across matched samples of patients created with different models used to estimate propensity score. The most relevant differences were observed for the matched set created by estimating individual propensity scores with Classification and Regression Trees algorithm. Conclusion: Clinical stability of matched samples created with different statistical methods should always be evaluated to ensure reliability of final estimates. This work opens the door for future investigations that fully assess the implications of this finding.