Asian Cardiovasc Thorac Ann 2001;9:14-18
© 2001 Asia Publishing EXchange Pte Ltd
Development of In-Vitro Evaluation System for Annuloplasty Rings
Makoto Arita, MS,
Hitoshi Kasegawa, MD,
Mitsuo Umezu, MD, PhD
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Department of Mechanical Engineering Umezu Biomedical Engineering Laboratory Waseda University Tokyo, Japan
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For reprint information contact: Mitsuo Umezu, MD, PhD Tel: 81 3 5286 3256 Fax: 81 3 3200 2516 email: umezu{at}mn.waseda.ac.jp Department of Mechanical Engineering, Umezu Biomedical Engineering Laboratory, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan.
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Abstract
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An in-vitro system was devised to study the performance of newly developed annuloplasty rings as well as various conventional rings. The Duran Flexible Annuloplasty Ring was tested to define its characteristics and to validate the system for comparative testing. It was possible to obtain quantitative data using a microscope and a load cell to respectively measure valve orifice area and tensile load on the valve annulus. The results suggest that this apparatus could be employed to characterize the features and functional performance of other types of annuloplasty ring.
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Introduction
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Both replacement and repair of the mitral valve are established surgical treatments for mitral insufficiency. Mitral valve repair limits the risk of heart failure by remodeling the valve. It has recently been performed more aggressively in many centers as the first choice of surgical treatment, and a number of studies have confirmed its clinical benefits. Presently, several types of rigid or flexible ring are commercially available and various novel annuloplasty rings are being developed. However, the functional characteristics of these devices have mainly been considered only subjectively, based on clinical data using transesophageal echocardiography or other methods.1–4 If the basic characteristics of annuloplasty rings could be obtained under more controlled and reproducible circumstances, such data might provide surgeons with guidelines for selection of the most appropriate device. The purposes of this study were to identify certain performance characteristics of flexible annuloplasty rings and to develop a quantitative and reproducible evaluation system to compare the characteristics of currently available annuloplasty rings.
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Materials and Methods
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Figure 1
is a schematic drawing of the test apparatus developed at the Umezu Biomedical Engineering Laboratory of Waseda University. The mitral valve orifice model was made from a sheet of latex rubber (Kyowa Ltd, Osaka, Japan) of 1 mm thickness. It mimicked the anatomical shape of the natural mitral valve orifice with reference to porcine heart, and realistically accommodated suturing of the annuloplasty ring to the annulus, as shown in Figure 2. This was just a flat sheet cut to conform to the anatomical crescent shape of the cuspal coaptation zone developed during mitral valve closure. Hence, it did not have any leaflet contact area or function as a valve. We adopted a mitral valve orifice of 5 cm2 or larger for the model as this accommodated suturing of annuloplasty rings with the nominal diameters of 35 to 36 mm currently offered by most manufacturers. A Duran Flexible Annuloplasty Ring (Medtronic, Inc., Minneapolis, MN, USA) with a nominal diameter of 35 mm was sutured onto the model by an experienced cardiac surgeon using multiple mattress stitches.
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Figure 1. Schematic drawing of evaluation system for annuloplasty rings developed by Waseda University. The circle in the upper right corner shows the enlarged illustration of a part of the flat-head screw that permits adjustment of the zero load point.
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Figure 2. Valve configuration adopted in this study. Four dots show the location of the suturing points of the chordae tendineae-papillary muscle model. All measurements are in millimeters. R = radius.
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To develop a valve orifice, an element for applying load to the leaflet free margins was needed. Surgical suture was used to represent chords and papillary muscles at the free margin of the latex rubber orifice. There were 4 sutures, 2 for the anterior and 2 for the posterior leaflet, as shown in Figure 2
. This was a lumped parameter model of the natural chords and papillary muscles that represented the valve orifice only. Hence, physiological chords, setting, or the actual number of chords were irrelevant. Size 2/0 Ethibond polyester suture (Ethicon, Inc., Somerville, NJ, USA) that has a Young's modulus greater than natural chordae, was used to improve the orifice area response relative to the applied load.5 An aluminium rod (Nippon Light Metal Company Ltd, Tokyo, Japan) with an inner diameter of 2 mm and an outer diameter of 3 mm was used as the connection element between the load cell and the test sample, to improve the reproducibility and reliability of the data acquired. This lightweight aluminium rod insignificantly influenced tare weight to the load cell because the weight was approximately 1.4 g and the resolution of the load cell was approximately 5 g. The rod was provided with a fine thread to permit adjustment of the zero load point, as shown in Figure 1.
The test apparatus consisted of a testing component (load cell and amplifier for measuring annular load) and a microscope for measuring valve orifice area (Figure 1). To study annuloplasty ring performance, the latex rubber mitral valve orifice model with the annuloplasty ring was mounted on a set of flat acrylic plates and installed in the test apparatus. Two 2/0 polyester sutures were sewn to the free margins of each leaflet coaptation area. The other ends of both sutures on the anterior leaflet side were tied just before a pulley, and a dead weight was hooked on the free end. The 2 sutures on the posterior side were also tied and hooked to a dead weight. Loading was applied to these sutures in sequence. Initially, loads of 50 g and 100 g were applied and then augmented (with increments of 100 g) to a maximum of 600 g. During this process of valve opening, the valve orifice area and annular load were sequentially and simultaneously measured at 8 points from the anterior to the posterior part of the orifice, as shown in Figure 3. Eight hooks were attached to these measuring points to connect the aluminium rod. To observe and measure the orifice area during loading, a digital microscope (Model VH-6300; Keyence Corp., Osaka, Japan) was installed above the test area. A micro-load load cell (Model LVS-1KA; Kyowa, Tokyo, Japan) was also fixed above the apparatus to measure the load on the annulus of the simulated mitral valve.
The annuloplasty ring performance criteria were valve orifice area and annular load. Valve orifice area, that is the facility to allow development of a large valve orifice area, is considered one of the major indices of satisfactory diastolic performance of an annuloplasty ring. Differences in annular load could be expected from the variation in material properties of different annuloplasty rings. Therefore, the tensile load detected at the valve annulus as the orifice developed was used as another index of performance of the annuloplasty ring.
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Results
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Figure 4 shows the changes in valve orifice area relative to the load applied to the chordae; as a control, the same tests were carried out on a latex model of the mitral valve orifice without attachment of an annuloplasty ring. Note that valve orifice area increased linearly with increasing load on the suture attachment (r2 = 0.9704, p < 0.0001). The samples mounted with a flexible annuloplasty ring showed an average increase of approximately 14.5% in load, compared to the control, to develop the same orifice area.
Figure 5 shows the relationship between annular load and valve orifice area obtained with this test system. In every case, throughout the opening process, the posterior portion of the annulus experienced a higher load than that measured at the anterior annulus. An almost constant load from the commissure to the posterior annulus was noted, which was also greater than that seen at the anterior portion of the annulus. At maximum orifice area, the posterior part of the annulus showed a tensile load 5.55 times larger than that of the anterior annulus.
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Discussion
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Larger valve orifice areas were obtained with the flexible ring than with the control (no ring) when the same loads were applied. It was inferred from this finding that suturing of the annuloplasty ring to the annulus resulted in shrinkage, as shown in Figure 6. Although this experiment did not aim to determine the effects of flexible ring placement in cases of dilated annuli, this result indicates that application of such a ring to a normal annulus may cause a Òshrink by sutureÓ effect that is one of the expected outcomes of valve remodeling by annuloplasty rings. On the other hand, annular load tended to increase significantly as the orifice area became larger. In this experiment, overly large orifice areas were obtained because the objective was to compare currently used annuloplasty rings. However, significant differences in load at each region of the annuloplasty ring were confirmed even with orifice areas of 3 to 4 cm2 and a 35-mm diameter flexible ring. According to these results, the anterior region of the annulus had a low load compared to the posterior annulus. This correlates well with the fact that the inter-trigonal area of the mitral annulus consists largely of tough fibrous tissue and undergoes little circumferential displacement during the cardiac cycle, in contrast to the posterior part. Although it is a simple evaluation system, the apparatus is believed to be capable of determining the response of the mitral orifice and annulus to diastolic loading of the cusps after application of various types of annuloplasty ring.
The mitral orifice model was devised to compare differences in the diastolic performance inherent in the material properties or shape of the annuloplasty devices available, during various annular loads. The simplicity of the model was not considered to detract from the validity of the results. The preliminary findings indicate that this model could adequately evaluate ring performance even though it did not duplicate the natural mitral valve configuration or function. Tensile testing was performed to characterize the mechanical response of the latex rubber and to compare it with data derived from natural tissue. A model AG-25TB autograph (Shimadzu Corp., Kyoto, Japan) was used and all testing was conducted according to Japanese Industrial Standard K6301.6 Figure 7 shows a typical stress-strain curve obtained from the tensile tests. Previous testing demonstrated that a strain zone of less than 0.05 was needed for substantial evaluation of annuloplasty rings. Figure 8 is an enlarged diagram of portion A in the curve shown in Figure 7. These data represent the mean values and 95% confidence intervals of 5 test results. The Young's modulus of the latex rubber sheet (calculated by the least-squares method) was 1206 kPa, which is 36.2% lower than the Young's modulus of 1890 kPa for natural mitral valve tissue.7 Consequently, the tensile response was considered fairly equivalent to natural tissue, thus the latex rubber sheet adopted in this study was valid as a model of the mitral orifice.
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Figure 7. Stress-strain curve of the latex rubber mitral orifice model. Section A is expanded in Figure 8.
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Figure 8. Comparison of elastic modulus of the natural tissue and the latex rubber sheet (expanded section A from Figure 7).
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Basic studies using the mitral valve orifice model were conducted to investigate whether the characteristics of flexible annuloplasty rings could be evaluated in terms of valve orifice area and annular load. Review of the valve orifice area measurements showed that area tended to increase linearly as load was applied to the chordaepapillary muscle model. This was expected to result from a change in the elastic properties of the latex rubber sheet. Measurement of orifice area was performed using a microscope and an error of ± 0.05 cm2 during area measurements was determined. However, the linearity of area enlargement with increasing load was still noted after taking into account this experimental error. There was some concern regarding the effect that variations in suture technique might have on measurement of the orifice area. As mentioned, the measurement errors of the loads and valve orifice areas were quite small, thus it was concluded that this method could reliably characterize the performance of various rings even if only small differences in orifice area were obtained.
Regarding the influence of the annuloplasty ring on annular load, the loads acting on the coaptation edges of the orifice and on the sutured part of the annuloplasty ring were directly measured with a load cell. Consequently, differences in annular load were recorded reproducibly, according to the region of the annuloplasty ring. Although quantifying the load and interpreting the results were accomplished with some difficulty (due to the simplicity of the evaluation system), it was found to be sufficiently useful for comparative tests such as investigating differences in regions of the same sample, or comparing different samples. Therefore, it was concluded that the application of this evaluation system for load measurement was feasible for obtaining objective and reproducible data.
The purpose of this study was to investigate whether reliable measurements could be made of the orifice area and tensile load characteristics imposed by various types of annuloplasty ring on the annulus of a simulated mitral valve. Although these experiments were conducted with the Duran Flexible Annuloplasty Ring, the system was considered to be sufficiently applicable to the evaluation of various types of annuloplasty ring.
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Acknowledgments
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This research was carried out with the aid of the following research funds: the Program for Promotion of Fundamental Studies in Health Science of the Organization for Drug ADR Relief, R&D Promotion and Product Review of Japan (no. 96-12), Grant-in-aid for Scientific Research of Japan (nos. 09557112 and 09470288), and Health Sciences Research Grants (no. H-11-Drug-006). The authors would like to express their gratitude to Medtronic Japan Co. Inc., for providing the Duran Flexible Annuloplasty Rings and to Mr. Dave Myers, Medtronic Technical Fellow, for reviewing the manuscript. Also, we are very thankful to Mr. Kenta Iwadate and Mr. Akira Nakayama, undergraduate students in Prof. Umezu's Laboratory, who provided much valuable assistance in the performance of these experiments.
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References
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[Abstract]
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Abstract
Introduction
