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a The Harrison Department of Surgical Research, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania
b Department of Medicine, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania
Accepted for publication March 31, 2008.
* Address correspondence to Dr Robert C. Gorman, Department of Surgery, 313 Stemmler Hall, 36th and Hamilton Walk, University of Pennsylvania, Philadelphia, PA 19104-4283 (Email: gormanr{at}uphs.upenn.edu).
Presented at the Forty-fourth Annual Meeting of The Society of Thoracic Surgeons, Fort Lauderdale, FL, Jan 28–30, 2008.
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Abstract |
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 Top Abstract IntroductionMaterial and MethodsResultsCommentDiscussionAcknowledgmentsReferences |
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Methods: Multiple mitral annular and leaflet geometric variables were calculated for 8 naïve adult male sheep using real-time three-dimensional echocardiographic images. These indexes were recalculated after annuloplasty using a 30-mm Carpentier-Edward Physio ring (n = 4; Edwards Lifesciences, Irvine, CA) or a 30-mm saddle ring (n = 4).
Results: After implantation of the Physio ring, the annular height to commissural width ratio (AHCWR) decreased from 19.4% ± 2.3% to 11.1% ± 2.5% (p = 0.06). After implantation of the saddle ring, AHCWR increased from 19.6% ± 1.3% to 24.3% ± 1.3% (p < 0.05). Statistically significant increases in three-dimensional Gaussian curvature occurred after implantation within six defined leaflet regions (A1 to A3, P1 to P3) of the saddle ring but only within the P1 and P3 leaflet regions with the Physio ring.
Conclusions: Annuloplasty ring shape affects leaflet curvature. Implantation of a saddle ring reflecting normal human annular geometry augmented ovine annular nonplanarity and increased three-dimensional leaflet curvature across the entire mitral valve surface. The Physio ring decreased annular nonplanarity and increased leaflet curvature only across limited regions of the posterior leaflet. These findings confirm the hypothesis that ring design influences leaflet curvature.
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Introduction |
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TopAbstractIntroductionMaterial and MethodsResultsCommentDiscussionAcknowledgmentsReferences |
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A significant number of repair failures result from chordal, leaflet, and suture line disruption, which suggest that mechanical leaflet stress plays a significant role [14]. Leaflet curvature is an important determinant of valve stress [15]. As such, repair devices and techniques that optimize leaflet curvature will reduce valvular stress and may increase repair durability.
In a previous work, we presented a finite element model describing the synergistic contributions of annular nonplanarity [16, 17] and leaflet billowing to leaflet curvature and the relative effect of each on leaflet stress [18]. These results led us to hypothesize that annuloplasty ring shape could directly affect leaflet curvature and potentially repair durability.
To evaluate this hypothesis, we describe a novel method using real-time three-dimensional (rt-3D) echocardiography for quantifying mitral valve geometry and apply this technique to the characterization of ovine mitral valve geometry before and after implantation of either the Carpentier-Edwards Physio ring (Edwards Lifesciences, Irvine, CA) or a custom-built rigid saddle annuloplasty ring that conforms to normal human annular geometry.
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Material and Methods |
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TopAbstractIntroductionMaterial and MethodsResultsCommentDiscussionAcknowledgmentsReferences |
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| (2) |
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| (3) |
varies from 0 to 2
, a is a nominal scaling constant (2a is defined as the intercommissural width), b() is an empiric function derived from our imaging data, and h is the vertical height of the annulus where the annular height to commisural width ratio (AHCWR) = h/2a.
For the ring used in this study, b() and AHCWR were defined by normal human data obtained using rt-3D echocardiography. The parameter a was chosen to make the ring similar in area to a 30-mm Carpentier-Edwards Physio ring. Using these specifications, prototype rings were generously created by Medtronic (Medtronic Inc, Minneapolis, MN).
Surgical Protocol
The animals used in this study were cared for in compliance with Guide for the Care and Use of Laboratory Animals (National Institutes of Health Publication No. 85-23, revised 1996). The 8 adult male sheep were pretreated with buprenorphine (2 µg/kg) and then induced with intravenous (IV) sodium thiopental (10 to 15 mg/kg), intubated, and anesthetized with isoflurane (1.5% to 2.0%) and oxygen. All animals received glycopyrrolate (0.02 mg/kg IV) and cefazolin (1.0 g IV). After left thoracotomy and pericardiotomy, epicardial echocardiography was performed in all sheep.
The 8 animals were randomized to undergo placement of a 30-mm Physio ring (n = 4) or a 30-mm saddle ring (n = 4). Annuloplasty was performed on cardiopulmonary bypass (CPB) through a left atriotomy using standard techniques. After separation from CPB, epicardial echocardiography was repeated.
Image Acquisition
An epicardial rt-3D echocardiogram was performed at baseline and was then repeated 1 hour post-CPB separation after annuloplasty. In each case, electrocardiogram-gated, full-volume data sets were acquired by a single operator using a Sonos 7500 (Philips Medical Systems, Andover, MA) platform equipped with a 2- to 4-MHz X4 handheld transducer. Each full-volume data set was exported to a dedicated Cardio-View (Tomtec Imaging Systems, Munich, Germany) software workstation for image manipulation and analysis.
Image Analysis
Image analysis was performed in Cardio-View by visual inspection. Cardio-View allows the interactive manipulation—including rotation, translation, surface rendering, and measurement—of fully 3D ultrasound data sets. All analysis was performed at end-systole, which was defined as the first frame demonstrating closure of the aortic valve. In Cardio-View, the plane of the mitral valve orifice was rotated into a short-axis view. The geometric center of the mitral valve was then translated to the intersection of the two corresponding long-axis planes, which then corresponded to the intercommissural and septolateral axes of the mitral valve orifice. A rotational template consisting of 18 long-axis cross-sectional planes separated by 10° increments was superimposed on the 3D echocardiogram. The two annular points intersecting each of the 18 long-axis rotational planes were identified by orthogonal visualization of each plane; the two points were marked interactively in Cardio-View (Fig 1A and B).
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Continuous free-hand curves, each consisting of between 5 and 40 data points, connecting the juxtaposed anterior and posterior annulus across the surfaces of the anterior and posterior leaflet, were constructed in each long-axis cross section (Fig 1C), resulting in a 600- to 1200-point data set for each mitral valve (Fig 2). Each data point fit within a given measurement plane was then assigned to the anterior mitral leaflet (AML), the posterior mitral leaflet (PML), or the coaptation point (CP).
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Calculation of Leaflet Surface Area and Curvature
Subsequently, each point along the anterior mitral annulus (as determined by visual inspection) was merged with the anterior mitral leaflet data set for a given subject, and each point along the posterior mitral annulus was merged with the corresponding posterior mitral leaflet data set. All coaptation points were subsequently added to both data sets. Separate meshed grids with boundary conditions determined by the given data set extremes were then created for the anterior leaflet and posterior leaflet data sets.
Smoothing splines were constructed separately for anterior and posterior leaflet data sets using the Matlab TPAPS function. The smoothing parameter (P), which varies between 0 for a least squares approximation and 1 for a thin-plate spline interpolant, was assigned automatically for each data set using the Matlab default. The z-coordinates of the spline surface at each point of the rectangular mesh grid were calculated separately over each leaflet. An iterative Delaunay triangulation was used to exclude mesh points that fell outside of the borders described by annular data points.
The area of each triangle described by three adjacent mesh points was then calculated continuously across both anterior and posterior leaflets. The anterior leaflet surface area (SAAL) was then defined the as the sum of triangular areas across the anterior leaflet. The posterior leaflet surface area (SAPL) was calculated analogously. The total mitral valve surface area (SATotal) was then defined as the sum of SAAL and SAPL. The ratio of total leaflet surface area to annular area, which reflects relative leaflet redundancy, was subsequently defined as SATotal:MAA.
The coapting regions of the leaflets are not measured using this technique. As a result, SAAL, SAPL, and SATotal are underestimates and represent the leaflet area of the closed valve as viewed from the left atrium.
The 3D leaflet Gaussian curvature (K)
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| (4) |
Septolateral and intercommissural 2D leaflet curvatures (KSL, KIC) were then calculated as follows. Let a, b, and c be the distances between each of three sequential mesh points along either the x (septolateral) or y (intercommissural) axis. Then the 2D curvature along the selected axis at a given mesh point is defined as:
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| (5) |
For purposes of subsequent data analyses, the intercommissural axis of each mitral valve data set was then subdivided into equal thirds. Each mesh point in a given data set was subsequently assigned to one of six leaflet zones—A1, A2, A3, P1, P2, P3—as illustrated in Figure 3. Values of KXYZ, KSL and KIC were subsequently grouped on the basis of their assigned position on the mitral valve. In order to negate the influence of curvature directionality, the absolute values of KXYZ, KSL and KIC were used for the calculation of regional mean values.
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The hybrid anterior and posterior leaflets were modelled independently. The individual geometric centers of each anterior and posterior leaflet mesh within a given cohort were translated to their averaged geometric centers, scaled to their averaged area in 2D (xy) space, and then overlapped. Those areas of the overlapped individual leaflet meshes in which all data sets—or all data sets except one—coincided were included in the hybrid mesh. All other points were excluded. The z-coordinate at each point along the hybrid anterior and posterior leaflet meshes was determined by the arithmetic mean of the compiled z-coordinates of the individual overlapped models. Finally, the point in 2D (xz) space at which anterior and posterior leaflet meshes intersected was defined for each mesh interval along the intercommissural (y) axis. The resultant series of coaptation points was then defined as the line of leaflet coaptation for the 3D rendering.
Visualization and Statistics
Three distinct shading techniques, in which interpolated color contouring was determined by KXYZ, KSL, and KIC at a given mesh point, were then applied to each hybrid rendering to facilitate visualization of each of these geometric indexes in 3D space. All graphics were produced using Matlab.
Comparisons between values at baseline and after annuloplasty were made using the Student paired t test. All statistical analysis was performed using SPSS software (SPSS Inc, Chicago, IL). The level of significance selected for all variables was p < 0.05. Numerical results are presented as mean ± standard error of the mean (SEM).
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Results |
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TopAbstractIntroductionMaterial and MethodsResultsCommentDiscussionAcknowledgmentsReferences |
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Mitral Leaflet Geometry
Regional mean values ± SEM are presented for |K|, |KSL| and |KIC| in Figure 4
for both animal cohorts. Baseline values were derived collectively from all 8 animals. However, the statistical significance of changes in regional curvature was determined with respect to the baseline values for the 4 animals comprising each cohort (using the paired Student t test).
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After implantation of the saddle ring, statistically significant increases in |KIC| were observed in the A1, A3, P1 and P3 leaflet regions, whereas statistically significant increases in |KSL| occurred in both the A2 and P3 leaflet regions. Statistically significant increases in |K| occurred in all six of the defined leaflet regions.
Figure 5 illustrates the changes in global valvular geometry as well as the changes in both the magnitude and spatial distribution of 3D Gaussian curvature that accompanied placement of both the saddle ring (Fig 5B) and the Physio ring (Fig 5C) when compared with baseline (Fig 5A). A marked reduction in mitral annular area is apparent in both cases, but the observed patterns of geometric change are otherwise dissimilar.
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Figures 6 and 7
illustrate the relationship between 3D valvular geometry and orthogonal 2D curvature (KSL and KIC). Note that in both cases, positive curvature indicates atrially oriented surface concavity, whereas negative curvature indicates apically oriented surface concavity.
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Comment |
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TopAbstractIntroductionMaterial and MethodsResultsCommentDiscussionAcknowledgmentsReferences |
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Levine and colleagues [16, 21] first described the nonplanar saddle shape of the mitral annulus nearly two decades ago. Subsequently, our group used a combination of sonomicrometry and early stage rotational 3D echocardiography to quantify the maximal nonplanarity of the mitral annulus by describing the AHCWR [17, 18].
Using a quantitative knowledge of annular geometry, numeric simulation, and simplified leaflet models, we have previously demonstrated that alterations in annular saddle shape are capable of affecting both leaflet geometry and stress distribution. In that theoretic analysis, we also demonstrated that leaflet stress reaches a minimum at AHCWR values between 15% and 30% (normal anatomic range) and increases exponentially as ACHWR approaches zero or as the annulus approaches planarity. This work has led us to the general hypothesis that annuloplasty ring shape may influence leaflet curvature and, more specifically, to postulate that a saddle ring designed to approximate normal annular shape may maintain leaflet curvature whereas standard flat rings would diminish it. Although theoretic work supports these hypotheses, experimental conformation has been absent due to a relative inability to describe mitral valve geometry precisely and in reproducible terms.
To provide such experimental confirmation, we have developed an imaging methodology utilizing rt-3D echocardiography in conjunction with commercially available image-processing software and a series of custom mathematic algorithms to examine the impact of annuloplasty ring shape on leaflet geometry. Geometric modeling and graphic rendering techniques have been used to characterize the direction, orientation, and magnitude of both 2D and 3D curvature across the entire ovine mitral valve.
We have previously derived a series of parametric equations based on imaging data obtained using both sonomicrometry and rt-3D echocardiography [17, 18, 22] that can be used to describe the 3D shape of the normal mitral annulus where the only independent variable is intercommissural width. The geometry of the saddle ring used in the current study was derived from these equations.
The current study used these tools to corroborate several previous studies and demonstrates the effect of annuloplasty ring shape on the entire mitral valve, including all surfaces of the mitral leaflets.
Miller and colleagues [23, 24] have previously described substantial reduction in both annular height and AHCWR after implantation of a Physio ring in naïve sheep. The current study confirms their findings. Implantation of a 30-mm Physio ring into naïve sheep decreased AH by 49.0% and reduced CW by 15.8%, thus effecting a 43.0% decrease in AHCWR. Conversely, implantation of a 30- mm saddle ring into naïve sheep increased AH by 18.9% and reduced CW by 4.4, thus effecting a 23.8% increase in AHCWR. Both ring types reduced SL and MAA by similar degrees.
It is interesting to note that the saddle ring augmented normal ovine mitral annular nonplanarity at end systole. This is because the saddle ring geometry was derived from measurements of the normal human mitral annulus, which is markedly more nonplanar than its ovine counterpart [18]. Although the effect of ring geometry on annular and mitral leaflet geometry was not studied in pathologic states in the current study, this finding supports the conclusion that a saddle shaped ring will not only maintain normal annular geometry but may also be efficacious in restoring annular geometry when used in the context of ischemic mitral regurgitation, a disease process known to flatten the mitral annular shape [25–27]. In fact, the AHCWR of 24.3% observed after implantation of the saddle ring was nearly identical to that of the ring itself (25%). The relative geometries of the Physio and saddle rings can be appreciated in Figure 10.
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Multiple previous studies involving invasive imaging techniques have also documented compound curvature along the septolateral meridian of the anterior leaflet as well as hyperbolic surface geometry within the belly of the midanterior (A2) leaflet due to the presence of directionally opposed, orthogonally oriented intercommissural curvature in this region [29–31]. The current results both confirm the presence of compound meridional curvature and indicate that compound septolateral curvature is in fact present along nearly the entire surface of the anterior leaflet.
Our findings indicate that posterior leaflet septolateral curvature is also compound rather than simple and is far more spatially heterogeneous than previously believed [29].
The most compelling finding of this study is that annular geometry influences leaflet curvature to a substantial degree. Implantation of a 30-mm saddle ring in naïve sheep affected increases in both annular nonplanarity and 3D Gaussian curvature in all leaflet zones as well as marked augmentation of both septolateral and intercommissural curvature in multiple leaflet zones. It follows from the law of Laplace that these observed increases in leaflet (membrane) curvature would tend to reduce leaflet stress. Furthermore, the preservation of normal spatial patterns of leaflet curvature (Figs 5, 6, and 7) indicates that optimal valvular loading patterns are maintained in this geometric configuration.
Conversely, implantation of 30-mm Physio ring markedly reduced annular nonplanarity. Although substantial foreshortening of the septolateral axis of the mitral annulus was achieved, no appreciable increase in septolateral curvature occurred in any leaflet zone, although intercommissural curvature and 3D Gaussian curvature increased within multiple leaflet zones. Normal patterns of 2D and 3D curvature were obliterated in several leaflet regions with implantation of the Physio ring (Fig 5, Fig 6, and Fig 7), indicating that optimal valvular loading patterns are disrupted in this configuration.
The current study establishes the relationship between annular nonplanarity and mitral leaflet geometry and also provides a series of quantitative indexes with which to describe this relationship. The imaging and analytic techniques we describe can be used to study and describe detailed geometric features in patients with various valvular and subvalvular pathologies. Furthermore, as this technique is applied to patients with MR, preoperative and postoperative geometric predictors of surgical outcomes may begin to emerge. This work represents an initial foray into the realm of rational mitral prosthesis design. The imaging, visualization, modeling, and simulation techniques described here will in the near future constitute the basis for both cardiac valvular prosthetic design and patient-specific customized valvular repair and prostheses.
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Discussion |
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TopAbstractIntroductionMaterial and MethodsResultsCommentDiscussionAcknowledgmentsReferences |
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DR JOSEPH H. GORMAN III: The next step is to develop a finite element stress analysis based on the data obtained from our 3D echo analysis of leaflet and annular geometry. In order to do this, we need to define realistic boundary conditions. This ultimately requires the ability to image the papillary muscles and chordae. While the papillary muscles are relatively easy to image, the chordae are currently much more difficult. But based on postmortem observation of the chordal origins and insertion we are progressing with the model now. We believe that 3D-TEE [transesophageal echocardography] resolution will progress to eventually allow the identification of chordae in vivo.
While finite element stress analysis has been used to assess mitral valve function for almost 20 years, all previous models were based on assumed leaflet geometries. The models we are developing are truly novel in that they are based on precise in vivo measurements of the geometry of the mitral annulus and leaflets. The data sets we use have thousands of data points for each leaflet. As a result, even with chordal insertion assumptions, our new analysis will provide much more refined information than has been available previously.
DR SOON J. PARK (Rochester, MN): Great study! I think we can expect a lot of good things out of this kind of animal research and fine data analyses. However, I think we have to look at more than just the mitral valve itself. Mitral valve incompetence often has a lot more to do with the poorly functioning ventricle. Remodeling process of the ventricle can adversely affect the geometry of the ventricle that supports the mitral valve for its competency. So, I am not too optimistic that newly designed rings would be able to solve the problems of mitral regurgitation associated with the failing heart.
DR GORMAN: I make the points about repair outcome to focus attention on the fact that mitral valve repair techniques are not as mature as was previously thought. I agree with you that repair durability is influenced by many factors but I also believe strongly that subtle alterations in valve geometry are important. I think our work to date supports the concept that repair techniques designed to restore more normal leaflet geometry will reduce stress on the repair and increase its durability.
DR SIMON MOTEN (Heidelberg, Australia): I've followed your work very closely and enjoyed the results you present and thank you for your presentation today. I have had a lot of experience with using the St. Jude Rigid Saddle Ring and been impressed with the results so far, both in the appearance of the repaired valve at the time of surgery, in particular the depth of leaflet coaptation and doming of the annulus and leaflets, and in the excellent midterm results achieved for degenerative, myxomatous, and ischemic mitral valve repairs, as a result of reading some of your work.
My only concern relates to fixing the annulus in the 3D saddle position throughout the cardiac cycle. I am not sure how real this concern is, and this is my question. What do you think the effect is, if any, of fixing the mitral annulus in a domed position, normally seen in systole, on valve ring function in diastole, in particular the effect on transmitral gradients. I wonder whether you've had any experience in that area and maybe could comment on it.
DR GORMAN: Remember that for years we have been fixing the mitral annulus in an "abnormal" position with flat rings, without any significant influence on diastolic mitral flow. In reality the annulus is saddle shaped in diastole as well as systole so the application of a saddle ring maintains a more normal diastolic geometry than a flat ring and potentially a more normal diastolic flow pattern.
DR GUS J. VLAHAKES (Boston, MA): A question about the experimental model. Is the planar shape in the ovine model a result of an anatomic detail relative to the position and the size of the aorta?
DR GORMAN: I'm not sure. I think the saddle shape of the annulus has a lot to do with the geometry of the left ventricle. The geometry of the sheep left ventricle is somewhat different than the human left ventricle. Despite being somewhat flatter than the human the sheep annulus is still saddle-shaped. It has an annular height to commissural width ratio of about 15%, while the human has an annular height to commissural width ratio of about 25%.
DR ROBERT S. D. HIGGINS (Chicago, IL): Have you tried to extrapolate 3D echo information before and after repair in humans?
DR GORMAN: We're in the process of doing that now. The new Philips 3D-TEE probe is really facilitating that work. We are in the process of developing software packages that will allow for the assessment of human leaflet geometry in vivo.
DR HIGGINS: So we will have real-time assessment of stress, theoretically?
DR GORMAN: We are currently working on developing software that will assist in the preoperative planning of mitral valve procedures as well as the intraoperative assessment of valve geometry and stress. These computer programs will allow the surgeon to individualize and optimize each repair to minimize systolic valve stress prior to entering the operating room.
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Acknowledgments |
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TopAbstractIntroductionMaterial and MethodsResultsCommentDiscussionAcknowledgmentsReferences |
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References |
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TopAbstractIntroductionMaterial and MethodsResultsCommentDiscussionAcknowledgmentsReferences |
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