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.