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Effect of Annular Shape on Leaflet Curvature in Reducing
Mitral Leaflet Stress
Ivan S. Salgo, MS, MD; Joseph H. Gorman III, MD; Robert C. Gorman, MD;
Benjamin M. Jackson, MD; Frank W. Bowen, MD; Theodore Plappert, CVT;
Martin G. St John Sutton, MBBS, FRCP; L. Henry Edmunds, Jr, MD
Background—Leaflet curvature is known to reduce mechanical stress. There are 2 major components that contribute to this
curvature. Leaflet billowing introduces the most obvious form of leaflet curvature. The saddle shape of the mitral
annulus imparts a more subtle form of leaflet curvature. This study explores the relative contributions of leaflet
billowing and annular shape on leaflet curvature and stress distribution.
Methods and Results—Both numerical simulation and experimental data were used. The simulation consisted of an array
of numerically generated mitral annular phantoms encompassing flat to markedly saddle-shaped annular heights.
Highest peak leaflet stresses occurred for the flat annulus. As saddle height increased, peak stresses decreased. The
minimum peak leaflet stress occurred at an annular height to commissural width ratio of 15% to 25%. The second phase
involved data acquisition for the annulus from 3 humans by 3D echocardiography, 3 sheep by sonomicrometry array
localization, 2 sheep by 3D echocardiography, and 2 baboons by 3D echocardiography. All 3 species imaged had annuli
of a similar shape, with an annular height to commissural width ratio of 10% to 15%.
Conclusion—The saddle shape of the mitral annulus confers a mechanical advantage to the leaflets by adding curvature.
This may be valuable when leaflet curvature becomes reduced due to diminished leaflet billowing caused by annular
dilatation. The fact that the saddle shape is conserved across mammalian species provides indirect evidence of the
advantages it confers. This analysis of mitral annular contour may prove applicable in developing the next generation
of mitral annular prostheses. (Circulation. 2002;106:711-717.)
Key Words: finite element analysis 䡲echocardiography 䡲ultrasonics 䡲valves
The shape of the mitral annulus conforms to a saddle configu-
ration.
1–5
Sonomicrometry array localization, marker angiogra-
phy, and 3D echocardiography confirm both the saddle shape and
dynamic changes in mitral annular geometry during the cardiac
cycle.
1,6–9
However, a teleologic reason for nature’s selection of this
shape has not been proposed quantitatively. The effects of leaflet
curvature on stress reduction have been described and are generally
accepted as an important mechanism in the efficiency of valve
function.
10
Until now, the only component of leaflet curvature
analyzed has been leaflet billowing. The contribution of annular
shape on leaflet curvature and stress has not been described. The
present study tests the hypothesis that the annular saddle shape
imposes another form of leaflet curvature, which acts independently
although synergistically with leaflet billowing to minimize leaflet
stress.
Methods
Overview
The methods consist of a numerical simulation and experimental
measurements. The first phase of the numerical simulation is
designed to isolate the relative contributions of leaflet billowing
versus annular nonplanarity on leaflet stress reduction in a basic
model. The next numerical phase is to study the synergistic effect of
these 2 shape factors together in further reducing peak leaflet stress
in an anatomically realistic design. Finally, the degree of annular
nonplanarity predicted to minimize peak leaflet stress by the numer-
ical simulation is compared with experimental data.
Numerical Simulation (Phase 1): Isolating the
Effect of Leaflet Billowing Versus Annular
Nonplanarity in Peak Stress Reduction
Two fundamental shapes (not leaflet designs) are generated: an
elliptic paraboloid to model leaflet billowing with a flat annulus and
a hyperbolic paraboloid to model annular nonplanarity with minimal
leaflet billowing. These shapes are shown in Figure 1. The parabo-
loid with flat annulus (billowing leaflet model) is generated accord-
ing to the following parametric formula in cylindrical coordinates:
(1) ar cos
,br sin
,⫺hbr2
The “pure” saddle phantom (nonplanar annulus model) is gener-
ated according to the following formula:
(2) ar cos
,br sin
,har2cos 2
Received January 10, 2002; revision received May 23, 2002; accepted May 24, 2002.
From Philips Medical Systems (I.S.S.), Andover, Mass, and the Departments of Surgery (J.H.G., R.C.G., B.M.J., F.W.B,. L.H.E.), Medicine (T.P.,
M.G.S.J.S.), and Anesthesia (I.S.S.), School of Medicine, University of Pennsylvania, Philadelphia, Pa.
Dr Salgo is an employee of Philips Medical Systems.
Presented in part at the 73rd Scientific Sessions of the American Heart Association, New Orleans, La, November 12–15, 2000, and published in abstract
form (Circulation. 2000;102(suppl II):II-631).
Correspondence to Ivan S. Salgo, MS, MD, 500 Brookside Drive, Andover, MA 01810. E-mail ivan_salgo@attbi.com
© 2002 American Heart Association, Inc.
Circulation is available at http://www.circulationaha.org DOI: 10.1161/01.CIR.0000025426.39426.83
711
In both formulas, The length 2arepresents the commissural
diameter, 2bis the posterior-anterior diameter, and r is radius.
Therefore, these parameters aand bcontrol transverse annular
dimension and hence flow area. The parametric equations 1 and 2
describe both an annulus and a surface. The parameter hbcontrols
dome height (degree of bulging) for the billowing leaflet model
(equation 1), whereas annular half-height is controlled by hafor the
nonplanar annulus model (equation 2). To allow a comparison of
theoretical results with experimental findings, 2 normalized param-
eters are defined. For the billowing leaflet model, normalized leaflet
bulge (or billow) is defined as hb/2a. For the nonplanar annulus
model, the annular height to commissural width ratio (AHCWR) is
computed as ha/a. Using dimensions from published anatomic
drawings, the parameter ais set at 19.4 mm and bat 15 mm.
11
The
values hband 2ha, respectively, varied from 0 mm (a flat annulus and
leaflets) to 38.75 mm, which corresponded to a dome height
(billowing leaflet model) and AHCWR (nonplanar annulus model) of
100%. Using custom utilities and scripts written in C⫹⫹ and
Mathematica 4.0 (Wolfram Research), surfaces are plotted to dis-
crete points and transferred to a commercial mesh generator and
finite element solver (described below).
Numerical Simulation (Phase 2): Quantifying the
Synergistic Effect of Leaflet Billowing and
Annular Nonplanarity in Peak Stress Reduction in
an Anatomically More Pragmatic Model
Two families of mitral phantoms are generated. Both families of
leaflet design have constant flow orifice areas, but their annular
height varies from 0 to 9.6875 mm or an AHCWR of 0% to 25%.
The 2 families are distinct on the basis of the curvature of their
leaflets: one has anterior and posterior leaflets of lesser curvature,
whereas the other family has leaflets with more pronounced curva-
ture as defined below. The mitral annular saddle is defined by the
following parametric equation:
(3) acos
,bsin
,hacos 2
Equation 3 represents an annulus without leaflets because the
variable r is removed. It defines a 3D space curve (ie, one wire
frame), not a surface. Figure 2A displays a typical annular curve
generated by the equation. For all phantoms, the parameter ais set to
19.4 mm and bto 15 mm. Therefore, the commissural diameter is
38.75 mm. The parameter hais varied to yield AHCWRs of 0%, 3%,
7%, 15%, 20%, and 25%. Two families of wire frames are con-
structed and transferred to a computer-aided design application
(Superdraw and Supersurf, Algor, Inc). For each wire frame, a
connecting line denoting the apposition of the anterior and posterior
leaflets is interposed into the flat and saddle-shaped annuli as shown
in Figure 2A. The center of this apposition line is connected to the
aortic portion of the anterior annulus by either a curved or flat line
in 3D space. Moreover, another line is connected to the midpoint of
the posterior leaflet (centered within the potential medial scallop).
For the flat family of phantoms, these are straight lines. For the
leaflets with curved domes, they are circular arcs with tangential
angles fixed at 35 and 70 degrees. Therefore, all leaflet phantoms
have known curvature as described by computer-aided design
techniques. Finally, the leaflets themselves are placed onto the
phantom wire frames with nonuniform rational B-spline surfaces.
These surfaces and general anatomic orientations are shown in
Figure 2. There are 6 phantoms in each of the 2 families.
Finite Element Analysis of Mechanical Stress
Given a specific fundamental shape (Phase I) or mitral phantom
(Phase II), the next step in the analysis is to solve the differential
equations of continuum mechanics to compute leaflet stress. A
commercial finite element solver was used to compute stresses
(Algor, Inc). A specific material model and property is used for all
shapes. This is a linear, static, orthotropic material model chosen to
simulate the effect of predominant collagen orientation within the
leaflets.
12,13
The major material axis is oriented with respect to the
commissure-to-commissure orientation. Table 1 shows the values
used for the analysis. Leaflet thickness is kept constant over the
entire valve and to simulate end systole, a peak pressure of 16 kPa
(120 torr) is used for all calculations. Therefore, the computed leaflet
stresses are derived from data designed to simulate peak load. The
leaflets are bound to the annulus and chordal attachments by fixed
boundary conditions because the leaflets themselves flex and do not
have joints. Moreover, no attempt is made to simulate chordal
structures. The leaflet models are first refined using a mesh generator
to space the finite element nodes optimally. Orthotropic plate
elements in 3D are set for the phantoms. Peak leaflet stresses using
von Mises maximum distortion energy criterion are then collected
from the analysis.
14,15
Figure 1. Plot of an elliptic (A) and hyperbolic (B) paraboloid as fundamental shapes, not leaflets. The annular dimensions are
38.75 mm across the major axis and 30 mm on the minor axis. The height is 15 mm, which corresponds to a dome-height ratio of 39%
(hb/2a) for A (billowing leaflet model) and an AHCWR of 39% (ha/a) for B (nonplanar annulus model). The hue of the shapes varies by
curvature using differential geometry. The configuration in B represents minimal surface area.
712 Circulation August 6, 2002
Human Imaging Protocol
After approval from the Institutional Review Board, 3 people were
scanned using 5 to 6.2 MHz multiplane transesophageal echocardi-
ography (Omniplane II, Philips Medical Systems). Only subjects
scheduled for first-time coronary revascularization and transesoph-
ageal echocardiography who gave consent were selected. The anes-
thetic included narcotic and inhalational agents (fentanyl 20 to 40
g/kg and isoflurane 1% vol). All patients had normal ejection
fractions and ventricular size. Data acquisition was gated to the ECG
and respiration. Images were then centered about the mitral annulus
and obtained every 10 degrees at end-systole and stored onto a
magneto-optical disk for offline analysis. Images were gated to
end-systole for all studies.
Ovine Surgical and Echocardiographic
Imaging Protocol
Two Dorsett hybrid sheep (40 to 50 kg) were given sodium
thiopental (7 mg/kg IV), intubated, anesthetized with isoflurane
(1.5% to 2%), and ventilated with oxygen (Narkomed, North
American Drager). All animals received glycopyrrolate (0.01 to 0.02
mg/kg IV), cefazolin (1 g IV) and gentamicin (80 mg IV). A
thoracotomy incision was performed, and the heart was supported in
a pericardial cradle. Three-dimensional echocardiographic scanning
was performed at the cardiac apex, and 3D data were acquired every
5 degrees using a handheld 5 MHz rotational probe (Transthoracic
Omniplane, Philips Medical Systems) and stored onto a magneto-
optical disk. Images were gated to end-systole for all studies. Sheep
were treated in compliance with NIH publication No. 85-23 as
revised in 1985.
Sonomicrometry Array Localization
Three separate sheep underwent general anesthesia as described
above and were placed on cardiopulmonary bypass. Through a left
atriotomy, 12 sonomicrometry transducers were placed equidistant
around the annulus. The animals were weaned from bypass, the
incision was closed, and the animals were allowed to recover. Seven
days later, intratransducer distance measurements were taken every 5
Figure 2. A,Figure of wire frame used to construct the mitral annulus and leaflets. The AHCWR was 20% for this case. The annulus
shown here is saddle-shaped. The apposition of the anterior and posterior leaflet is shown across the major axis of the annulus. The
central struts of the anterior and posterior leaflets are shown. These were either curved for domed leaflets or flat for flattened leaflets.
B, Demonstration of nonuniform rational B-spline surfaces used to fit phantom leaflets. Collagen orientation is not shown. C, Model of
phantom after mesh generation for optimal alignment of computational elements. Anatomic landmarks are labeled (LAA indicates loca-
tion of left atrial appendage; Ao, aorta; A, anterior leaflet; P, posterior leaflet; and 1, 2, and 3, lateral, middle, and medial areas of the
leaflets). The mesh shown was used numerically for the finite element solutions. D, Stress plot of linear static analysis.
Salgo et al Mitral Leaflet Stress and Annular Shape 713
ms throughout the cardiac cycle. The 3D coordinates of each
transducer were then derived from the distance data using a multi-
dimensional scaling technique.
Baboon Echocardiographic Imaging Protocol
Two adult baboons (13 and 18 kg) were given ketamine (10 mg/kg
IV), intubated, anesthetized with isoflurane (1.5% to 2%), and
ventilated with oxygen (Narkomed, North American Drager). All
animals received atropine (0.01 to 0.02 mg/kg IV), cefazolin (1 g
IV), and gentamicin (80 mg IV). Baboons were treated in compliance
with NIH publication No. 85-23 as revised in 1985.
The baboons were imaged from the chest wall by 3D multiplane
echocardiography. Data were acquired from 2 baboons every 5
degrees using a handheld rotational probe (Transthoracic Om-
niplane) and stored onto a magneto-optical disk for offline analysis.
Images were gated to end-systole for all studies.
3D Echo Image Analysis
Using custom image manipulation software written for this work, the
mitral annulus was identified offline from the 3D echocardiographic
studies. Annular points (2 for each corresponding angle from the
representative image) were transformed in 3D space. These points
were calibrated to the depth setting of the imaging probe, and the
aspect ratio was accounted for by a constant in the software. The
mitral annulus was fit to a plane using the following formula:
(4) x
a⫹y
b⫹z
c⫽1
Its orientation minimizes the least squares error. The xy plane is
oriented orthogonal to flow. Using a quaternion geometric transfor-
mation, the maximum and minimum annular heights (zaxis) are
compared with the maximum cross-sectional diameter. Data ac-
quired from echocardiography or sonomicrometry are interpolated
and analyzed using a discrete Fourier transform with a Gaussian
convolution kernel. This is used for both visualization and quantita-
tion. Figure 3 demonstrates one of the echocardiographic reconstruc-
tions done on a human subject. The AHCWR was computed for the
3 humans, 5 sheep, and 2 baboons.
Results
Phase I: Finite Element Analysis of the Effect of
Leaflet Billowing Versus Annular Nonplanarity in
Peak Stress Reduction
Peak stresses are affected by intrinsic shape changes for a
constant flow orifice area. The peak stress for a flat disc is
335 MPa (where 1 Pascal⫽1 Newton/m2⫽10 dyne/cm2).
Figure 4A demonstrates that leaflet billowing reduces peak
leaflet stress relative to the flat annulus. Small changes in the
shape of the leaflet (ie, the introduction of leaflet curvature)
result in marked reductions of peak leaflet stresses. Even for
a normalized height of 3%, leaflet stress is reduced 28-fold.
An approximate minimum occurs at a normalized height of
30%. However, there is no substantial reduction between
10% and 40%. Figure 4B demonstrates that the fundamental
saddle shape has a significantly lower peak leaflet stress than
the flat disc. As the AHCWR increases away from zero, peak
stresses in the leaflets diminish. There is a 37-fold reduction
in peak leaflet stress at an AHCWR of 20%. Reductions in
peak stress reach a minimum at 25% AHCWR and higher.
The plate model indicates that peak stresses increase for both
shapes after inordinate increases in normalized dome height
or an AHCWR ⬎60% (Figure 4).
TABLE 1. Material Properties Used for Finite Element Analysis
Loaded at Peak Systole
Thickness, mm 1.31
Young’s modulus, major axis (Ex), kPa 6230.0
Young’s modulus, minor axis (Ey), kPa 2350.0
Poisson’s ratio, major axis,
10.45
Poisson’s ratio, minor axis,
20.17
Bulk modulus, Gxy 1370.0
Peak systolic pressure, P (torr) 120.0
This material represents a composite type with increased stiffness across
one dimension over its orthogonal dimension. This represents a predominance
of the orientation of collagen within the leaflet. Most of these data were derived
experimentally and are reported in the literature.
Figure 3. Fourier smoothed mitral annulus reconstruction from a
human subject. The data used to acquire this image was gath-
ered by transesophageal echocardiography. The commissures
are located at the low points of the saddle.
Figure 4. Plot of peak stress versus percent height. The elliptic
paraboloid (A) and the hyperbolic paraboloid (B; saddle) are
shown. A, Peak stress for the billowing leaflet model. B, Peak
stress for the nonplanar annulus model. A flat disc results when
the parameters for each model, hband harespectively, are set
to 0. The peak stress of the flat disc is 335 MPa in A and B.
714 Circulation August 6, 2002
Phase 2: Finite Element Analysis of the Synergistic
Effect of Leaflet Bulging and Annular
Nonplanarity in Peak Stress Reduction
Figure 5 shows peak stresses for the mitral phantoms.
Because the center of the leaflet is supported by chordal
boundaries, peak leaflet stress is lower than that of the flat
annulus presented in the prior section: 110 MPa (Figure 5A)
versus 336 MPa (Figure 4), respectively. Figure 5 shows that
for both families of mitral phantoms (flattened and curved
leaflets), introduction of the saddle-shaped annulus reduces
peak leaflet stress. Increasing AHCWR to 20% with flat
leaflets reduces peak leaflet stress 3-fold from 110 MPa to
⬍40 MPa. The addition of curved leaflets (bulging) to the
20% AHCWR model decreases the total leaflet stress another
5-fold to 7 MPa. Figure 6 shows stress distribution for a valve
with an AHCWR of 20%. Figures 5 and 6 illustrate the
synergistic nature of combined leaflet bulging and the annular
saddle shape on peak mitral valve leaflet stress. With or
without leaflet billowing, the stress reduction associated
with annular saddle shape reaches a maximum effect at an
AHCWR value between 15% and 20% (Figure 5).
Experimental Determination of Mitral
Annular Height
Table 2 displays the annular height ratios found in 3 species.
All have a saddle-shaped mitral annuli. Three-dimensional
echocardiographic determinations of the 3 human patients
yield an AHCWR of 14% to 16%. The 2 sheep studied by
echocardiography have AHCWRs of 12% and 17%. Sonomi-
crometry results yield AHCWRs of 13%, 19%, and 7% for
different sheep. Finally, the 2 baboons have AHCWRs of
14% and 17%.
Figure 5. Plot of peak leaflet stress (A, flat leaflets; B, curved
leaflets) versus AHCWR for the mitral phantoms (phase II)
described in this article.
Figure 6. A, Plot of mitral phantom leaflets without billowing showing stresses by von Mises distortion energy theory. B, Plot of mitral
phantom leaflet with billowing showing stresses by von Mises distortion energy theory. The AHCWR was 15% for both cases. Note the
decrease in stress for the curved leaflets in B.
TABLE 2. Results of 3D Transesophageal Echocardiography
and Transthoracic Omniplane Echocardiography for 3 Humans,
2 Sheep, and 2 Baboons
Subject Acquisition AHCWR
Subject 1 3D TEE 16
Subject 2 3D TEE 14
Subject 3 3D TEE 14
Sheep 1 SAL 13
Sheep 2 SAL 19
Sheep 3 SAL 7
Sheep 4 3D TTO 17
Sheep 5 3D TTO 12
Baboon 1 3D TTO 14
Baboon 2 3D TTO 17
Results of 3 sheep using sonomicrometry array localization (SAL) are also
presented. AHCWR is shown as a percentage. TEE indicates transesophageal
echocardiography; TTO, transthoracic omniplane echocardiography.
Salgo et al Mitral Leaflet Stress and Annular Shape 715
Discussion
This study corroborates the conclusion that leaflet billowing
reduces peak leaflet stress considerably.
10
It also supports the
hypothesis that mitral annular nonplanarity further reduces
peak leaflet stress. The synergistic combination of leaflet
billowing and annular nonplanarity act together to optimize
leaflet curvature, thereby minimizing peak mitral leaflet
stress. These results, coupled with general principles of
mechanics, show that mitral leaflet stress is determined by
load (the pressure differential between the left ventricle and
atrium), leaflet area, and leaflet curvature.
16–19
Finite element analysis is a tool used to solve the differ-
ential equations of mechanical stress. This computer-based
technique is used so solve problems of arbitrarily defined
geometry according to a specific design because only the
most simple of shapes can be solved analytically by hand. For
example, linear static finite element computer modeling of
the mitral valve has been used to examine general mechanical
properties. For static linear models, computation of stress
(although not strain) yields the same final result independent
of material modulus.
The simulation for phase II was designed to extend the
hypothesis to a more realistic geometric shape, one that
specifically uses chordal support. An early study using a
Laplace’s law model indicated that curvature of the leaflets is
beneficial. For the current study, the leaflets were designed to
satisfy the following design constraints. First, symmetry of
leaflets was preserved with reference to the commissures.
This assumes that peak stresses of equal magnitude occur
bilaterally. Second, the posterior leaflets were not scalloped
(ie, sharply marginalized) to study the effect of the annulus on
leaflets that may lose curvature in vivo. The design, although
more realistic than the first part, is intentionally kept con-
strained to understand effects of stress by controlling pertur-
bations of leaflet design. Under these conditions, the data for
the mitral phantom simulation demonstrate that (1) leaflet
curvature is a major determinant of leaflet stress and (2) the
saddle shape of the annulus contributes to reductions in
leaflet stress by providing another mechanism of leaflet
curvature in addition to leaflet billowing.
Two different imaging techniques that were performed to
verify these predictions show that the saddle shape of the
mitral annulus is preserved across 3 mammalian species. All
subjects had an AHCWR of ⬇15% (with one outlier), and
none of the subject hearts was dilated. The finite element
analyses consistently demonstrate that leaflet stress reduc-
tions occur in the range of AHCWRs of 15% to 20%. These
data strongly suggest that nature conserves the saddle-shaped
annulus for a mechanical benefit.
Analysis of actual leaflet shapes by imaging studies was
not a goal of this study because of errors created by
spatiotemporal shifting in gated 3D echocardiographic tech-
niques.
20–23
Medical imaging techniques are only beginning
to acquire data (of the fidelity needed for this type of analysis)
of structures that move as quickly as leaflets. Because the
annulus has much slower velocity components than the
leaflets during the cardiac cycle, gated 3D echocardiography
was used to image the annulus. However, optimal annulus
filtering techniques in the Fourier (spectral) domain are not
well established. Excessive low pass spatial filtering yields an
annulus that is flatter than actual. Without spatial filtering, all
AHCWRs would be higher, and thus our data report the lower
bounds. This may explain why our experimental results
tended toward the low end (15% AHCWR) of the predicted
stress-minimizing AHCWR range of 15% to 20%. As real
time magnetic resonance and matrix array 3D echocardio-
graphic techniques evolve, imaging of the entire leaflets
themselves with subsequent finite element analysis will
become feasible. These results, taken in conjunction with the
theoretical calculations presented above, provide compelling
evidence that the saddle shape of the mitral annulus signifi-
cantly reduces leaflet stress at AHCRW ratios between 15%
to 20%.
The analysis presented in this study provides insight into
potential methods for improving current mitral valve repair
techniques. Over the past 20 years, Carpentier has pioneered
and standardized surgical techniques that allow reliable repair
of valves with leaflet, chordal, and annular deformities.
24
The
widespread use of these techniques has produced notable
results at centers all over the world. Although the durability
of these repairs has been relatively impressive, there is a
long-term failure rate of between 5% and 12% (return of
significant mitral regurgitation) at 10 years, depending on the
type of initial leaflet or annular pathology.
25–27
Twenty year
recurrence rates for repaired rheumatic valves may be as high
as 50%.
28
In most cases, these long-term failures are the result
of disruption at leaflet, chordal, or annular suture lines,
suggesting mechanical stress as an etiological factor.
29
Most surgeons with experience in mitral valve repair agree
that restoration of normal annular geometry with ring annu-
loplasty is an essential component to all mitral valve repairs.
In fact, it has been shown that leaflet repairs without ring
annuloplasty are less durable than those that include it.
26
All
currently available annuloplasty rings are flat. When im-
planted, these devices only restore the annulus to a more
normal size in 2 dimensions. The height of the annulus is
totally flattened by these devices, potentially placing undue
stress and subsequent strain on the leaflet and chordal suture
lines of the repair by diminishing leaflet curvature. Moreover,
using larger saddle-shaped annular prostheses instead of their
flat counterparts could allow increases in flow area while
keeping peak leaflet stresses reduced. With this understand-
ing, it seems reasonable to hypothesize that a saddle-shaped
annuloplasty ring may increase mitral valve repair durability
by reducing leaflet and chordal strain.
Acknowledgments
Supported by a beginning Grant-in-Aid from the American Heart
Association, Pennsylvania-Delaware Affiliate (to I.S.S.); grants
HL63594 (to R.C.G.) and HL 36308 (to L.H.E.) from the National
Heart, Lung, and Blood Institute of the National Institutes of Health;
and a grant from the Mary L. Smith Charitable Trust, Philadelphia
Pa. The authors gratefully thank Hubert Yeung and Michael Peszyn-
ski for a discussion of the finite element findings.
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