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Design and durability test of Silastic trileaflet aortic valve prostheses
Design and durability test of Silastic trileaflet aortic valve prostheses Hitoshi Mohri, M.D., Dr. Med. Sci., * Eugene A. Hessel, II, M.D., ** Ronald J. Nelson, M.D.,*** Howard N. Anderson, M.D.,**** David H. Dillard, M.D.,***** and K. Alvin Merendino, M.D., Ph.D.,***** Seattle, Wash.
DesPite recent developments and improvements, currently available prosthetic valves are still prone to many problems and complications. These include thrombus formation, embolization, blood injury, tissue overgrowth, or material variance with resultant dysfunction of the prostheses. Some unfavorable reactions are closely related to valve design itself or to the material used. Valve prostheses that simulate human valves must have theoretical advantages, but, unfortunately, available materials have limited the development of such valves. In an attempt to develop a valve substitute which provides flow dynamic characteristics that are similar to those of the human aortic valve, trileaflet aortic valve prostheses have From the Department of Surgery and the First Surgical Service of the University Hospital, University of Washington School of Medicine, Seattle, Wash. 98105. This study was aided by funds from U.S. Public Health Service Grant No. 13517 and a Grant in Aid of the American Heart Association and; it was supported in part by the Washington and Idaho State Heart Associations. Received for publication Nov. 6, 1972. • Associate Professor of Surgery and Established Investigator of the American Heart Association. This work was done during the tenure of the investigatorship. •• Associate Professor of Surgery. "'Presently Assistant Professor of Surgery, University of California, Los Angeles, Calif. ,., 'Former National Institutes of Health Fellow, 19621963. It- *:4: *. Professor of Surgery.
576
been developed and tested in this institution since 1961. Among them, Teflon leaflet aortic valves were applied clinically.' In November, 1963, Silastic unistructural aortic valves were fabricated; various valve designs were studied, and the feasibility of various Silastic compounds was evaluated thereafter. This report is intended to depict the development of trileaflet valve design, to describe the durability tests of such valves made from various Silastic compounds, and to present the current model of the trileaflet aortic valve. Fabrication
A molding die consists of three major parts: (1) the core leaflet shaper which defines leaflet shape and insertion angle (Fig. 1, a); (2) three separate pieces of leaflet depressors (Fig. 1, b) which establish leaflet thickness and shape between the core leaflet shaper; and (3) a cylinder which defines valve framework (Fig. 1, c). A three-post metal valve shell (Fig. 1, d), which is the main supporting structure of the valve, is placed into a die so that the leaflet posts will be in proper relationship with assembled parts of the die. After the valve shell and die components are assembled, Silastic material is slowly injected into the lumen of the die through a 2 mm.
Volume 65
Silastic trileafiet aortic valve prostheses
N umber 4
577
April, 1973
c
a
d
b
Fig. 1. Valve molding die. a. Core leaflet shaper. b. Leaflet depressor. c. Cylinder. d. Valve shell.
Fig. 2. Various models of trileaflet Silastic aortic valves. a. An original valve with semidomeshaped leaflets. The valve is seen in a molding die. b, A valve with triangular-shaped leaflets and sinus of Valsalva. c, The latest model valve, in which the bottom of the valve frame is scalloped . It does not have a sinus wall.
hole located at the top of one post. While the die chamber is slowly pressurized at 4.14 x 10 dyne per square centimeter (6,000 p.s.i.) for 5 minutes, air and overflow Silastic are evacuated through a screw hole located at the base of the die opposite the injector hole. After the evacuation screw hole is closed, pressure is increased to 6.21 x 1O~ dyne per square centimeter (9,000 p.s.i.) for 1 minute. The filled die is then placed in an oven at 148.9° C. for 50 minutes for vulcan ization. The valve is next removed from the die and cured in an oven at 148.9° C. for 2 hours and 30 minutes. Since leaflets and framework are frabriQ
cated simultaneously , there is no discontinuity of material between the leaflets and valve frame. Valve designs Advantages of Silastic unistructural valves are satisfactory central flow, competence of the leaflet, and low vertical valve profile. Various models of trileaflet valves were developed and tested in a pulse duplicator. The location of leaflet tears or causes of unsatisfactory leaflet movement were examined, and various changes were made in leaflet thickness, leaflet insertion angles, or leaflet configurations. Semilunar leaflet valve. The original
The Journal of
578
Mohri et at.
Thoracic and Cardiovascular Surgery
valves were made to simulate the human aortic valve; thus the leaflet had a semidomeshaped surface (Fig. 2). Thirty-seven valves representing seven different models of various shapes and leaflet thicknesses (0.225 to 0.51 mm.) have been tested. However, the durability of these semilunar-shaped leaflet valves made of Silastic 372* was not satisfactory, as the average durability was 6.4 months. The dome-shaped leaflet base did not permit proper opening, and the leaflets usually tore from the base or free edge. Triangular leaflet valves. Short durability of semilunar leaflet valves stimulated development of the triangular-shaped leaflet valve. The triangular-shaped leaflet models differed from the previous semilunar valves primarily in configuration of the plane of the leaflet surface. Whereas the semilunar leaflet possessed a smooth, semidomed surface, the triangular leaflet simulated two flat triangular surfaces meeting in a median line equidistant from leaflet posts (Fig. 2, b). Eight models of various leaflet thicknesses and leaflet insertion angles were tested. Modifications of the valve were made by the trial and error method, with durability tests as the indicator. Slight changes of leaflet thickness at the flat surface, coaptation or median line, and leaflet insertion angles drastically altered durabilities. The most important developments in improvement of durability were thinning of the leaflet free edge and thickening of the median line of each leaflet at the junction of the two triangular planes. Five valves of an earlier model with 0.38 mm. leaflet thickness showed an average durability of 1.7 years (1.8 to 5.7 years). Finally, the current valve model (Fig. 2, c) was developed and leaflet thickness was reduced to 0.33 mm. The latest model prototype (No.8) was chosen as the most satisfactory valve, and durability tests were done with various Silastic compounds. Some minor alterations were made thereafter. In order to minimize the stress upon the leaflet at closure by allowing flexibility of the leaflet posts, two *The Silastic in this study was provided by Dow Corning
Corporation, Milland, Mioh.
modifications were made in the current model. Shortening the vertical struts of the stainless steel shell increased durability somewhat. However, the use of Lexan shells significantly shortened durability, presumably because of excess vibration of the Lexan posts in the accelerated test system. Detailed observation of leaflet movement and changes of leaflet structure in this highfrequency testing system were made possible by the use of a stroboscope. Durability tests (fatigue tests) Valve durability was examined in an accelerated fatigue testing system for which Merendino-Mueller pulse duplicators were used." Plain water was used as a testing fluid in most cases; however, in some, a glycerin-water solution of equi-blood viscosity was applied in order to approximate physiologic conditions closely. Loading pressure to the cusps was set at 80 mm. Hg at valve closure, and tests were done at accelerated speeds of either 1,000 or 2,000 cycles per minute. Time intervals from the start of the test to leaflet rupture were recorded by a built-in timer in the pulse duplicators. Tests were done continuously until the leaflets tore, except for occasional short stops for machine adjustment. Durability in equivalent cycle years was calculated by dividing total test cycles by 42 million, which reflects cycles for a year at a rate of 80 cycles per minute as normal. Results of durability tests are summarized in Table I. Valves that were tested at a rate of 1,000 cycles per minute lasted longer than those tested at 2,000 cycles per minute. Use of glycerin-water solution as the testing fluid also increased valve durability. For eleven valves made from Silastic 372, the average years equivalent cycles was 3.9, ranging from 1.2 to 8.2 years. Earlier model valves made of Silastic MDX4-4514 had an average durability of 10.2 years equivalent cycles, ranging from 6.7 to 16 years. However, the latest model of the same material demonstrated an aver-
Volume 65
Silastic trileafiet aortic valve prostheses
Number 4
579
April, 1973
Fig. 3. Leaflet tear due to accelerated fatigue test. a, A typical leaflet tear which runs longitudinally from the leaflet edge to the center of the leaflet is seen. There are multiple shallow fissures at leaflet coaptation areas. b, A tear that developed in an improperly fabricated valve. This valve was made at another institution by means of our valve molding die. Note the multiple horizontal leaflet tears due to improper injection of Silastic material into the die.
Table I. Summary of durability tests
I No.
Valve shell
Silastic
Stainless steel
372
II
MDX4-4031 MDX4-4514 MDX4-4514
3 5 6
MDX4-4059 372 MDX4-4514
4 2
Lexan
of valves
I
Durability (YE.C.)
1.2, 1.3, 2.2, 2.7, 3.6, 3.8, 4.1, 4.4, 4.6, 6.3, 8.2 1.2, 1.5, 2.0 6.7, 7.5, 8.5, 12.2, 16.0 17.7, 20.7, 22.6, 23.4, 23.8 18.0, 22.0, 24.0, 25.5 1.2, 3.0 1.3
Average durabillty (Y.E.C.)
3.9 1.6 10.2 21.9 22.4
Legend: Y.E.C., Years equivalent cycle.
age durability of 21.9 years equivalent cycles, ranging from 17.7 to 23.8 years. For three Silastic MDX4-4031 valves the average was 1.6 years. The most satisfactory durability was obtained in valves made of Silastic MDX4-4059: The average durability was 22.4 years, with a range of 18 to 25.5 years equivalent cycles. Leaflet tears or wear usually developed from the free edge of the coaptation area or the junction between coaptation and the flat triangular surface; then they split down
to the base of the leaflet (Fig. 3, a). Occasionally, a small slit or perforation developed at a coaptation area and did not progress to leaflet tear. These leaflets usually maintained their competence and lasted for 3 to 5 years equivalent cycles more before the slit became a larger tear. The technique of valve fabrication significantly affected valve durability. Non-uniform injection of the material into a die under low pressure caused uneven leaflet surfaces and resulted in multiple tears as
The Journal of
5 8 a Mohri et al.
Tho rac ic a nd Card iovascular
Surg e ry
veloped in valves made of Silastic MDX44514, but to a minor degree. However, valves made of Silastic MDX4-4059 remained nearly normal in appearance and thickness of the leaflet even after remaining in water more than 12 months. Flow visualization
Fig. 4. Flow visualization and flow dyn amic s study unit. There are three semi-dome-shaped spaces in the valve chamber simulating the sinus of Val salv a. The unit was placed vertically for the flow visualization study but horizontally for pressure mea surements. Pressure drop across the valves (P ~ - P 1 ) was recorded.
shown in Fig . 3, b. This valve was fabricated in another laboratory with the use of our die. Fatigue tests for 1 month in the pulse duplicator indicate a durability of approximately 1 year (1,000 test cycles) or 2 years (2,000 test cycles) equival ent cycles. Thus valves often remained in water for up to 12 to 15 months before a tear developed in the leaflet. During this period, valves made of Silastic compound 372 apparently absorbed water, and the leaflets became somewhat thickened and discolored. Changes also de-
A system used for flow visualization is depicted in Fig. 4. A water static pressure of 80 mm. Hg as the diastolic pressure, a flow of 6 L. per minute, and a pulse rate of 80 beats per minute were used in the testing system. Pressure drop across the valves was recorded. Flow patterns, including vortex movement of the sinus of Valsalva area, were visualized with the aid of bentonite clay in water and Polaroid glass. Detailed leaflet movements and flow patterns were analyzed by means of high-speed movie and still pictures. Various other valves such as Starr-Edwards, Gott, Wada-Cutter, and Edmark valves were also tested in the same system and compared with the trileaflet aortic valves. Wide central flow and competence of the trileaflet valves were noted. (Fig. 5, a and b). The area of highest shear rates was along the extension line of the leaflet margin, which was slightly smaller than the inside diameter of the valve. Some vortex movement at the sinuses of Valsalva during systole and washout at the diastolic period were visualized. Maximum pressure drop across the 18 mm. J.D. valves (24 mm. O.D.) in this setup was 1 to 2 mm. Hg. Ball valves, on the other hand , demonstrated turbulent flow in the central area and some regurgitation at early diastolic phases (Fig. 5, c) . Turbulence was often exaggerated by bouncing of the ball during the systolic period. Maximum pressure drop of the ball valves with the same inside diameter was 5 mm. Hg. All other types of valve demonstrated turbulent flow and higher pressure drop across the valves. Discussion
The present stud y has demonstrated satisfactory durability and function of unistruc-
Volume 6S Number 4 April, 1973
Silastic trileaflet aortic valve prostheses
581
Fig . 5. Comparison of flow char acteristics between a trileaflet Silastic valve and a ball valve (Sta rr-Edwa rds) . A pulsatile flow system was used a t 80 cycles per minute with a forw ard flow of 6 L. per minute. a and b show systolic and diastol ic phases in trile aflet aortic valves. and c demonstrates the systol ic pattern in ball valves. Centra l flow and competence of the tr ileaflet Silastic valve with sufficient washout flow at the sinus of Valsalva during the diastolic phase are observed.
tural Silastic trileaflet aortic valves . Accelerated fatigue tests are rather severe tests. Valves may last longer when tested in physiologic cycles, as current studies demonstrated slightly longer durability when valves were tested at 1,000 rather than 2,000 cycles per minute. However, absorption of water into some Silastic creates concern, since material varia nce of Silastic balls with resultant valve malfunction has been reported ,"- 4 Thus, when valves are implanted into the body, we should expect that the durability ma y be shorter than with fati gue tests. If the degree of water absorption directly reflects the degree of lipid infiltration into materials, Silastic MDX4-405 9 ma y have advantages over other materials tested. However , lipid absorption tests and tissue
compatibility of each material should be evaluated individually. The current valves are fabricated with the leaflets in closed position. As soon as forward flow decreases, leaflets snap back to the closed position . In this situation, it is difficult to judge how important the vortex movement in the sinus of Valsalva is to leaflet closure, although the vortex movements of the blood are said to be important for leaflet closure in human valves. G. G The presence of such a vortex in the sinus of Valsalva, however , ma y be important in preventing fibrin deposition or thrombus formation by eliminating stagnation of blood in the area between the leaflets and aortic wall. A series of animal experiments with the
582
The Journal of Thoracic and Cardiovascular
Mohri et al.
Surgery
current valves demonstrated the possibility of thrombus formation at the junction between the aortic wall, valve frame, and Dacron-suture ring. The cloth covering of the valve framework aggravated thrombus formation. Regional heparinization with an aminoethylcellulose-heparin coating over the valve frame was found effective.7 One of the most important factors in prosthetic valve production is control of the material. We have encountered problems in getting uniform durability among different lots. Valves made of the original shipment of Silastic MDX4-4059 showed satisfactory results as described in this paper. However, valves made of the same material but from a different shipment showed durabilities of 4 to 10 years equivalent cycles in the same testing system. The given reason for this was the existence of large variations in material characteristics when a small quantity of the testing material was produced.s Valves made from the material of large-scale productions seem to function satisfactorily and are being tested in fatigue machines that have run 17 and 14 years equivalent cycles to date.
Flow visualization studies with bentonite clay and Polaroid light demonstrated satisfactory central flow and valve movement with reasonable vortex flow in the sinus of Valsalva. Material variance and thrombus formation are discussed.
Summary
4
Unistructural Silastic trileaflet aortic valves have been developed. Various Silastic compounds and various valve models have been tested in an accelerated fatigue test system at a rate of 1,000 or 2,000 cycles per minute. Valves with semilunar-shaped leaflets which simulated two flat triangular surfaces meeting in a median line equidistant from leaflet posts were the most durable. Slight changes in leaflet shape, thickness, and insertion angles to the frame drastically changed durability. Valves made of Silastic MDX4-4514 and MDX4-4059 showed satisfactory durability, giving 21.9 to 22.4 years equivalent cycles.
The authors wish to thank Mr. Zake Dennett and Mr. J. L. Boone, of the Dow Corning Corporation, for their valuable assistance and the Dow Corning Corporation for supplying all testing materials. The authors are also indebted to Mr. Maxwell Kleinau and Mr. Gordon Kirkendall in the Medical Instrument Facility at the University of Washington School of Medicine. REFERENCES
2
3
5 6 7
8
Hessel, E. A., II, Karl, J., Jr., Steinmetz, G. P., Jr., Anderson, H. N., Dil\ard, D. H., and Merendino, K. A: A Prefrabricated Semirigid Tricusp Aortic Valve Prosthesis, 1. THORAc. CARDIOVASC. SURG. 54: 227, 1967. Steinmetz, G. P., May, K. J., Jr., Mueller, V., Anderson, H. N., and Merendino, K. A.: An Improved Accelerated Fatigue Machine and Pulse Simulator for Testing and Developing Prosthetic Cardiac Valves, J. THoRAc. CARDIOVASCo SURG. 47: 186, 1964. Laforet, E. G.: Death Due to Swelling of Ball Component of Aortic Ball-Valve Prosthesis, New Eng\. J. Med. 276: 1025, 1967. Collins, J. J., Jr., Berg, E. J., and Harken, D. E.: Case report: Silicone Ball Variance-A Clinical and Pulse Duplicator Study, Ann. Thorac. Surg. 6: 546, 1968. Bellhouse, B., and Bellhouse, F.: Fluid Mechanics of Model Normal and Stenosed Aortic Valves, Circ. Res. 25: 693, 1969. Bellhouse, B., and Talbot, L.: The Fluid Mechanics of Aortic Valve, J. Fluid Mech. 35: 721, 1969. Rittenhouse, E. A, Mohri, H., Reichenbach, D. D., and Merendino, K. A.: Heparin Bound Aminoethylcellulose as an Antithrombogenic Surface, Arch. Surg. 105: 752, 1972. Boone, J. L.: Medical Products Plant, Dow Corning Corporation. Personal communication.
DesPite recent developments and improvements, currently available prosthetic valves are still prone to many problems and complications. These include thrombus formation, embolization, blood injury, tissue overgrowth, or material variance with resultant dysfunction of the prostheses. Some unfavorable reactions are closely related to valve design itself or to the material used. Valve prostheses that simulate human valves must have theoretical advantages, but, unfortunately, available materials have limited the development of such valves. In an attempt to develop a valve substitute which provides flow dynamic characteristics that are similar to those of the human aortic valve, trileaflet aortic valve prostheses have From the Department of Surgery and the First Surgical Service of the University Hospital, University of Washington School of Medicine, Seattle, Wash. 98105. This study was aided by funds from U.S. Public Health Service Grant No. 13517 and a Grant in Aid of the American Heart Association and; it was supported in part by the Washington and Idaho State Heart Associations. Received for publication Nov. 6, 1972. • Associate Professor of Surgery and Established Investigator of the American Heart Association. This work was done during the tenure of the investigatorship. •• Associate Professor of Surgery. "'Presently Assistant Professor of Surgery, University of California, Los Angeles, Calif. ,., 'Former National Institutes of Health Fellow, 19621963. It- *:4: *. Professor of Surgery.
576
been developed and tested in this institution since 1961. Among them, Teflon leaflet aortic valves were applied clinically.' In November, 1963, Silastic unistructural aortic valves were fabricated; various valve designs were studied, and the feasibility of various Silastic compounds was evaluated thereafter. This report is intended to depict the development of trileaflet valve design, to describe the durability tests of such valves made from various Silastic compounds, and to present the current model of the trileaflet aortic valve. Fabrication
A molding die consists of three major parts: (1) the core leaflet shaper which defines leaflet shape and insertion angle (Fig. 1, a); (2) three separate pieces of leaflet depressors (Fig. 1, b) which establish leaflet thickness and shape between the core leaflet shaper; and (3) a cylinder which defines valve framework (Fig. 1, c). A three-post metal valve shell (Fig. 1, d), which is the main supporting structure of the valve, is placed into a die so that the leaflet posts will be in proper relationship with assembled parts of the die. After the valve shell and die components are assembled, Silastic material is slowly injected into the lumen of the die through a 2 mm.
Volume 65
Silastic trileafiet aortic valve prostheses
N umber 4
577
April, 1973
c
a
d
b
Fig. 1. Valve molding die. a. Core leaflet shaper. b. Leaflet depressor. c. Cylinder. d. Valve shell.
Fig. 2. Various models of trileaflet Silastic aortic valves. a. An original valve with semidomeshaped leaflets. The valve is seen in a molding die. b, A valve with triangular-shaped leaflets and sinus of Valsalva. c, The latest model valve, in which the bottom of the valve frame is scalloped . It does not have a sinus wall.
hole located at the top of one post. While the die chamber is slowly pressurized at 4.14 x 10 dyne per square centimeter (6,000 p.s.i.) for 5 minutes, air and overflow Silastic are evacuated through a screw hole located at the base of the die opposite the injector hole. After the evacuation screw hole is closed, pressure is increased to 6.21 x 1O~ dyne per square centimeter (9,000 p.s.i.) for 1 minute. The filled die is then placed in an oven at 148.9° C. for 50 minutes for vulcan ization. The valve is next removed from the die and cured in an oven at 148.9° C. for 2 hours and 30 minutes. Since leaflets and framework are frabriQ
cated simultaneously , there is no discontinuity of material between the leaflets and valve frame. Valve designs Advantages of Silastic unistructural valves are satisfactory central flow, competence of the leaflet, and low vertical valve profile. Various models of trileaflet valves were developed and tested in a pulse duplicator. The location of leaflet tears or causes of unsatisfactory leaflet movement were examined, and various changes were made in leaflet thickness, leaflet insertion angles, or leaflet configurations. Semilunar leaflet valve. The original
The Journal of
578
Mohri et at.
Thoracic and Cardiovascular Surgery
valves were made to simulate the human aortic valve; thus the leaflet had a semidomeshaped surface (Fig. 2). Thirty-seven valves representing seven different models of various shapes and leaflet thicknesses (0.225 to 0.51 mm.) have been tested. However, the durability of these semilunar-shaped leaflet valves made of Silastic 372* was not satisfactory, as the average durability was 6.4 months. The dome-shaped leaflet base did not permit proper opening, and the leaflets usually tore from the base or free edge. Triangular leaflet valves. Short durability of semilunar leaflet valves stimulated development of the triangular-shaped leaflet valve. The triangular-shaped leaflet models differed from the previous semilunar valves primarily in configuration of the plane of the leaflet surface. Whereas the semilunar leaflet possessed a smooth, semidomed surface, the triangular leaflet simulated two flat triangular surfaces meeting in a median line equidistant from leaflet posts (Fig. 2, b). Eight models of various leaflet thicknesses and leaflet insertion angles were tested. Modifications of the valve were made by the trial and error method, with durability tests as the indicator. Slight changes of leaflet thickness at the flat surface, coaptation or median line, and leaflet insertion angles drastically altered durabilities. The most important developments in improvement of durability were thinning of the leaflet free edge and thickening of the median line of each leaflet at the junction of the two triangular planes. Five valves of an earlier model with 0.38 mm. leaflet thickness showed an average durability of 1.7 years (1.8 to 5.7 years). Finally, the current valve model (Fig. 2, c) was developed and leaflet thickness was reduced to 0.33 mm. The latest model prototype (No.8) was chosen as the most satisfactory valve, and durability tests were done with various Silastic compounds. Some minor alterations were made thereafter. In order to minimize the stress upon the leaflet at closure by allowing flexibility of the leaflet posts, two *The Silastic in this study was provided by Dow Corning
Corporation, Milland, Mioh.
modifications were made in the current model. Shortening the vertical struts of the stainless steel shell increased durability somewhat. However, the use of Lexan shells significantly shortened durability, presumably because of excess vibration of the Lexan posts in the accelerated test system. Detailed observation of leaflet movement and changes of leaflet structure in this highfrequency testing system were made possible by the use of a stroboscope. Durability tests (fatigue tests) Valve durability was examined in an accelerated fatigue testing system for which Merendino-Mueller pulse duplicators were used." Plain water was used as a testing fluid in most cases; however, in some, a glycerin-water solution of equi-blood viscosity was applied in order to approximate physiologic conditions closely. Loading pressure to the cusps was set at 80 mm. Hg at valve closure, and tests were done at accelerated speeds of either 1,000 or 2,000 cycles per minute. Time intervals from the start of the test to leaflet rupture were recorded by a built-in timer in the pulse duplicators. Tests were done continuously until the leaflets tore, except for occasional short stops for machine adjustment. Durability in equivalent cycle years was calculated by dividing total test cycles by 42 million, which reflects cycles for a year at a rate of 80 cycles per minute as normal. Results of durability tests are summarized in Table I. Valves that were tested at a rate of 1,000 cycles per minute lasted longer than those tested at 2,000 cycles per minute. Use of glycerin-water solution as the testing fluid also increased valve durability. For eleven valves made from Silastic 372, the average years equivalent cycles was 3.9, ranging from 1.2 to 8.2 years. Earlier model valves made of Silastic MDX4-4514 had an average durability of 10.2 years equivalent cycles, ranging from 6.7 to 16 years. However, the latest model of the same material demonstrated an aver-
Volume 65
Silastic trileafiet aortic valve prostheses
Number 4
579
April, 1973
Fig. 3. Leaflet tear due to accelerated fatigue test. a, A typical leaflet tear which runs longitudinally from the leaflet edge to the center of the leaflet is seen. There are multiple shallow fissures at leaflet coaptation areas. b, A tear that developed in an improperly fabricated valve. This valve was made at another institution by means of our valve molding die. Note the multiple horizontal leaflet tears due to improper injection of Silastic material into the die.
Table I. Summary of durability tests
I No.
Valve shell
Silastic
Stainless steel
372
II
MDX4-4031 MDX4-4514 MDX4-4514
3 5 6
MDX4-4059 372 MDX4-4514
4 2
Lexan
of valves
I
Durability (YE.C.)
1.2, 1.3, 2.2, 2.7, 3.6, 3.8, 4.1, 4.4, 4.6, 6.3, 8.2 1.2, 1.5, 2.0 6.7, 7.5, 8.5, 12.2, 16.0 17.7, 20.7, 22.6, 23.4, 23.8 18.0, 22.0, 24.0, 25.5 1.2, 3.0 1.3
Average durabillty (Y.E.C.)
3.9 1.6 10.2 21.9 22.4
Legend: Y.E.C., Years equivalent cycle.
age durability of 21.9 years equivalent cycles, ranging from 17.7 to 23.8 years. For three Silastic MDX4-4031 valves the average was 1.6 years. The most satisfactory durability was obtained in valves made of Silastic MDX4-4059: The average durability was 22.4 years, with a range of 18 to 25.5 years equivalent cycles. Leaflet tears or wear usually developed from the free edge of the coaptation area or the junction between coaptation and the flat triangular surface; then they split down
to the base of the leaflet (Fig. 3, a). Occasionally, a small slit or perforation developed at a coaptation area and did not progress to leaflet tear. These leaflets usually maintained their competence and lasted for 3 to 5 years equivalent cycles more before the slit became a larger tear. The technique of valve fabrication significantly affected valve durability. Non-uniform injection of the material into a die under low pressure caused uneven leaflet surfaces and resulted in multiple tears as
The Journal of
5 8 a Mohri et al.
Tho rac ic a nd Card iovascular
Surg e ry
veloped in valves made of Silastic MDX44514, but to a minor degree. However, valves made of Silastic MDX4-4059 remained nearly normal in appearance and thickness of the leaflet even after remaining in water more than 12 months. Flow visualization
Fig. 4. Flow visualization and flow dyn amic s study unit. There are three semi-dome-shaped spaces in the valve chamber simulating the sinus of Val salv a. The unit was placed vertically for the flow visualization study but horizontally for pressure mea surements. Pressure drop across the valves (P ~ - P 1 ) was recorded.
shown in Fig . 3, b. This valve was fabricated in another laboratory with the use of our die. Fatigue tests for 1 month in the pulse duplicator indicate a durability of approximately 1 year (1,000 test cycles) or 2 years (2,000 test cycles) equival ent cycles. Thus valves often remained in water for up to 12 to 15 months before a tear developed in the leaflet. During this period, valves made of Silastic compound 372 apparently absorbed water, and the leaflets became somewhat thickened and discolored. Changes also de-
A system used for flow visualization is depicted in Fig. 4. A water static pressure of 80 mm. Hg as the diastolic pressure, a flow of 6 L. per minute, and a pulse rate of 80 beats per minute were used in the testing system. Pressure drop across the valves was recorded. Flow patterns, including vortex movement of the sinus of Valsalva area, were visualized with the aid of bentonite clay in water and Polaroid glass. Detailed leaflet movements and flow patterns were analyzed by means of high-speed movie and still pictures. Various other valves such as Starr-Edwards, Gott, Wada-Cutter, and Edmark valves were also tested in the same system and compared with the trileaflet aortic valves. Wide central flow and competence of the trileaflet valves were noted. (Fig. 5, a and b). The area of highest shear rates was along the extension line of the leaflet margin, which was slightly smaller than the inside diameter of the valve. Some vortex movement at the sinuses of Valsalva during systole and washout at the diastolic period were visualized. Maximum pressure drop across the 18 mm. J.D. valves (24 mm. O.D.) in this setup was 1 to 2 mm. Hg. Ball valves, on the other hand , demonstrated turbulent flow in the central area and some regurgitation at early diastolic phases (Fig. 5, c) . Turbulence was often exaggerated by bouncing of the ball during the systolic period. Maximum pressure drop of the ball valves with the same inside diameter was 5 mm. Hg. All other types of valve demonstrated turbulent flow and higher pressure drop across the valves. Discussion
The present stud y has demonstrated satisfactory durability and function of unistruc-
Volume 6S Number 4 April, 1973
Silastic trileaflet aortic valve prostheses
581
Fig . 5. Comparison of flow char acteristics between a trileaflet Silastic valve and a ball valve (Sta rr-Edwa rds) . A pulsatile flow system was used a t 80 cycles per minute with a forw ard flow of 6 L. per minute. a and b show systolic and diastol ic phases in trile aflet aortic valves. and c demonstrates the systol ic pattern in ball valves. Centra l flow and competence of the tr ileaflet Silastic valve with sufficient washout flow at the sinus of Valsalva during the diastolic phase are observed.
tural Silastic trileaflet aortic valves . Accelerated fatigue tests are rather severe tests. Valves may last longer when tested in physiologic cycles, as current studies demonstrated slightly longer durability when valves were tested at 1,000 rather than 2,000 cycles per minute. However, absorption of water into some Silastic creates concern, since material varia nce of Silastic balls with resultant valve malfunction has been reported ,"- 4 Thus, when valves are implanted into the body, we should expect that the durability ma y be shorter than with fati gue tests. If the degree of water absorption directly reflects the degree of lipid infiltration into materials, Silastic MDX4-405 9 ma y have advantages over other materials tested. However , lipid absorption tests and tissue
compatibility of each material should be evaluated individually. The current valves are fabricated with the leaflets in closed position. As soon as forward flow decreases, leaflets snap back to the closed position . In this situation, it is difficult to judge how important the vortex movement in the sinus of Valsalva is to leaflet closure, although the vortex movements of the blood are said to be important for leaflet closure in human valves. G. G The presence of such a vortex in the sinus of Valsalva, however , ma y be important in preventing fibrin deposition or thrombus formation by eliminating stagnation of blood in the area between the leaflets and aortic wall. A series of animal experiments with the
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Mohri et al.
Surgery
current valves demonstrated the possibility of thrombus formation at the junction between the aortic wall, valve frame, and Dacron-suture ring. The cloth covering of the valve framework aggravated thrombus formation. Regional heparinization with an aminoethylcellulose-heparin coating over the valve frame was found effective.7 One of the most important factors in prosthetic valve production is control of the material. We have encountered problems in getting uniform durability among different lots. Valves made of the original shipment of Silastic MDX4-4059 showed satisfactory results as described in this paper. However, valves made of the same material but from a different shipment showed durabilities of 4 to 10 years equivalent cycles in the same testing system. The given reason for this was the existence of large variations in material characteristics when a small quantity of the testing material was produced.s Valves made from the material of large-scale productions seem to function satisfactorily and are being tested in fatigue machines that have run 17 and 14 years equivalent cycles to date.
Flow visualization studies with bentonite clay and Polaroid light demonstrated satisfactory central flow and valve movement with reasonable vortex flow in the sinus of Valsalva. Material variance and thrombus formation are discussed.
Summary
4
Unistructural Silastic trileaflet aortic valves have been developed. Various Silastic compounds and various valve models have been tested in an accelerated fatigue test system at a rate of 1,000 or 2,000 cycles per minute. Valves with semilunar-shaped leaflets which simulated two flat triangular surfaces meeting in a median line equidistant from leaflet posts were the most durable. Slight changes in leaflet shape, thickness, and insertion angles to the frame drastically changed durability. Valves made of Silastic MDX4-4514 and MDX4-4059 showed satisfactory durability, giving 21.9 to 22.4 years equivalent cycles.
The authors wish to thank Mr. Zake Dennett and Mr. J. L. Boone, of the Dow Corning Corporation, for their valuable assistance and the Dow Corning Corporation for supplying all testing materials. The authors are also indebted to Mr. Maxwell Kleinau and Mr. Gordon Kirkendall in the Medical Instrument Facility at the University of Washington School of Medicine. REFERENCES
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Hessel, E. A., II, Karl, J., Jr., Steinmetz, G. P., Jr., Anderson, H. N., Dil\ard, D. H., and Merendino, K. A: A Prefrabricated Semirigid Tricusp Aortic Valve Prosthesis, 1. THORAc. CARDIOVASC. SURG. 54: 227, 1967. Steinmetz, G. P., May, K. J., Jr., Mueller, V., Anderson, H. N., and Merendino, K. A.: An Improved Accelerated Fatigue Machine and Pulse Simulator for Testing and Developing Prosthetic Cardiac Valves, J. THoRAc. CARDIOVASCo SURG. 47: 186, 1964. Laforet, E. G.: Death Due to Swelling of Ball Component of Aortic Ball-Valve Prosthesis, New Eng\. J. Med. 276: 1025, 1967. Collins, J. J., Jr., Berg, E. J., and Harken, D. E.: Case report: Silicone Ball Variance-A Clinical and Pulse Duplicator Study, Ann. Thorac. Surg. 6: 546, 1968. Bellhouse, B., and Bellhouse, F.: Fluid Mechanics of Model Normal and Stenosed Aortic Valves, Circ. Res. 25: 693, 1969. Bellhouse, B., and Talbot, L.: The Fluid Mechanics of Aortic Valve, J. Fluid Mech. 35: 721, 1969. Rittenhouse, E. A, Mohri, H., Reichenbach, D. D., and Merendino, K. A.: Heparin Bound Aminoethylcellulose as an Antithrombogenic Surface, Arch. Surg. 105: 752, 1972. Boone, J. L.: Medical Products Plant, Dow Corning Corporation. Personal communication.