Discussion
There are several advantages of using coronary CTA in morphologic aortic valve analysis. Aortic valves are observed in vivo in diastole, pressurized with physiologic diastolic pressure with all the surrounding structures intact. Unlike in anatomical studies where tissues are fixed with glutaraldehyde5,6 the mechanical properties of leaflets and root in our study were not altered. Measurements performed on the segmented root in 3D space allow definition of different points, lines, planes, angles and relationships, some of which cannot be done on anatomical models.
The mean age of patients was 53 years, which reflects the age of patients referred to coronary CTA, as coronary artery disease is rare in younger patients. However, another reason to preferentially choose older patients is visibility of aortic leaflets. Some of the CTAs of young patients that were provided by our radiology service were rejected simply because aortic leaflets were very thin and hardly visible on the CTA. It seems that with aging leaflets thicken, which increases visibility on CTA.
The patients in our study had normal tricuspid aortic valves and normal root dimensions as in the study on homografts by De Kerchove et al6. In both studies the right-noncoronary commissure was the highest (20.6 ± 3.0 mm in our study vs. 21.8 ± 2.1 mm), although intraoperatively it appears to be the lowest one as external root dissection is limited in this area due to right atrial attachment and membranous septum. Mean free margin length was almost identical (34.0 ± 4.0 mm in our study vs 34.4 ± 3.1 mm), longest in the right aortic leaflet and shortest in the left aortic leaflet. Similar relationship was found in intercommissural distances. Mean geometric leaflet height was shorter in our study (15.9 ± 1.6 mm vs 18.9 ± 1.5 mm), with both studies showing longest geometric height in noncoronary leaflet, similar to study by Schafers et al7. The reason for this difference may be different body size of patients and different methodology of measurement. In a Japanese study8 with similar methodology mean geometric height was 14.7 mm with mean body surface area of 1.7 m2, whereas mean body surface area in our study was 1.9 m2. On CTA, geometric height is measured in diastole with aortic valve closed, whereas in homografts or intraoperatively the leaflet is placed under tension during geometric height measurement. A difference of 1.7 mm between preoperative CTA and intraoperative measurements of geometric size was found in a study by Komiya T et al9. Longer intraoperative geometric leaflet height was also found in a study by Schäfers et al7, however, some of the difference may be attributed to the fact that measurements were performed on patients with aortic valve and root pathology, where leaflet distension may occur10. Coaptation surfaces were larger in our study, with the largest surface on the right coronary leaflet, a finding similar to De Kerchove et al6. However, as coaptation surface is often difficult to delineate on CTA as it may not lie in a single plane, this measurement should be interpreted carefully as it is prone to error. Central coaptation length was also similar (3.7 ± 0.6 mm in our study vs 3.3 ± 0.8 mm). When normalized to geometric height we found all dimensions longer than data published by De Kerchove et al6, which probably reflects the relatively shorter mean geometric height.
Normalizing root and leaflet dimensions to annulus diameter gave results comparable to the study by Swanson and Clark5 with aortic valve casts made at pressure of 100 mmHg. Some dimensions such as geometric height and central coaptation length were somewhat smaller, however normalized sinutubular diameter was larger (1.13 vs 1.0) and one half of free margin length was longer (0.69 vs 0.62) in our study. Leaflet belly to annulus angle was similar (24 degrees vs 22 degrees) as well as free margin to sinutubular plane angle (37 degrees vs 32 degrees). One explanation could be the higher age of our patients (53 ± 11 years vs 35 ± 6 years) causing the increase in sinutubular junction diameter and longer free margin length.
All aortic valves displayed some degree of asymmetry. Absolute differences between the three leaflets of individual patients were very similar to data published by De Kerchove et al6. Absolute difference in free margin length 3.2 ± 1.4 mm vs 3.1 ± 1.4 mm, in intercommissural distance 2.6 ± 1.1 mm vs 3.6 ± 1.7 mm, in geometric height 2.5 ± 1.3 mm vs 1.9 ± 1.0, and in leaflet attachment length 4.0 ± 2.1 mm vs 5.0 ± 2.3 mm. Using predefined cut-off values for absolute differences in leaflet dimensions and observing a single leaflet dimension at once, on average 42% of valves in our study were classified as asymmetric, whereas approximately 54% were asymmetric in the study by De Kerchove et al6. However, when observing all four leaflet dimensions at once, 86% of valves were asymmetric in at least one of the four selected dimensions. In a study by Vollebergh et al11 on 200 cadavers with normal tricuspid aortic valves they found that only 5 had leaflets of equal geometric size and intercommissural distance. Similarly, asymmetry of the leaflet surface areas, intercommissural distances and sinus of Valsalva volumes was found in a computed tomography study by Yang DH et al12. In a completely symmetric aortic valve, all leaflet measurements would be identical, however the two anatomical facts, that annulus plane and commissural plane are not parallel and that left ventricular outflow is elliptical result in small differences in commissural heights, leaflet dimensions and angles8. Effective height, however, showed the least amount of asymmetry – on average only 6% (0.5 mm), which explains its importance in keeping the aortic valve competent.
The main limitations of CTA were contrast and resolution. Ideally, there was a lot of contrast in the aorta and left ventricle but much less in the right side of the heart and pulmonary circulation. In these cases, the aortic root segmentation was fast and straightforward. When this was not the case, segmentation was prolonged because the structures surrounding the root needed to be removed manually. The second issue was visibility of aortic leaflets. Particularly in young patients the leaflets are normally very thin and may not be clearly visible on CTA. Consequently, the least reliable measurement in our study were the coaptation areas between the leaflets, which may not even lie in a single plane and are sometimes difficult to visualize. Another limitation is a relatively small sample size of the study.