Patent Publication Number: US-2002005073-A1

Title: Method and apparatus for testing the strength of autologous tissue

Description:
PRIORITY CLAIM  
     [0001] This application claims the benefit under 35 U.S.C. §119(e) of provisional application No. 60/198,650, filed Apr. 20, 2000. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] This invention relates to improvements in constructing heart valves using autologous tissue.  
       BACKGROUND OF THE INVENTION  
       [0003] Several types of heart valves are presently available for use in replacing diseased or malfunctioning heart valves in humans.  
       [0004] One form of heart valve is constructed from animal tissue, typically from bovine or porcine aortic valve tissue. These valves must typically be constructed in a laboratory well in advance of when they will be needed and then stored in an aldehyde solution. Skilled technicians are required to assemble these valves. The valves constructed from animal tissue have relatively short lifetimes. The short lifetimes are caused by two factors. First, there is an antigenic reaction by the body to the animal tissue which causes the tissue to calcify, making it inflexible and more susceptible to failure with time. Second, the tissue is often stored in glutaraldehyde before implantation to try to decrease the antigenic reaction. The aldehyde tends to tan the tissue to a leather-like consistency. The repeated stress of opening and closing tends to cause the tissue to wear out.  
       [0005] Mechanical heart valves are also available. These valves are made from hard, non-biological materials such as metals or ceramics. Although the mechanical heart valves are durable, the hard, non-biological surfaces on the valves tend to cause blood clots. The blood clots can cause heart attacks or strokes, and, as a result, patients with mechanical heart valves must take anticoagulant drugs. These drugs can lead to hemorrhagic complications. Also, patients who take these drugs require frequent and life-long laboratory tests of their clotting time.  
       [0006] Another type of heart valve, the autologous tissue valve, is constructed with the patient&#39;s own tissue. A number of patents for autologous tissue heart valves and methods of making autologous tissue heart valves have issued to Autogenics, assignee of this application, including U.S. Pat. Nos. 5,161,955 and 5,326,371 and pending U.S. application Ser. No. 09/161,809, hereby incorporated herein by reference.  
       SUMMARY OF THE INVENTION  
       [0007] One aspect of the invention provides an improved method for constructing an autologous heart valve. During construction of this type of valve, the individual or individuals building the valve currently rely upon their judgement and experience as to whether the harvested tissue is of adequate quality to allow a durable valve to be built. Because these valves are built during open heart surgery, there is only a limited amount of time for testing the mechanical properties of the available tissue.  
       [0008] As described below, the embodiments of the invention provide a simple go/no go test of tissue strength using a strip of tissue cut from the valve material adjacent to the edge of the tissue leaflet subject to the greatest stress when the tissue leaflet is mounted in the artificial heart valve. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009]FIG. 1 is a perspective view of a preferred embodiment of an assembled autologous heart valve;  
     [0010]FIG. 2A is a front view of an autologous tissue leaflet cut so as to include a test strip portion;  
     [0011]FIG. 2B is a front view of the autologous tissue leaflet of FIG. 2A, after the test strip portion is cut off for testing; and  
     [0012]FIG. 3 is a plan view of a tissue loading device constructed in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0013]FIG. 1 illustrates an exemplary embodiment of an assembled heart valve  9 . This valve uses the patient&#39;s own tissue and is constructed intraoperatively from several factory manufactured components. These components include a tissue mounting frame that mounts three individual autologous tissue leaflets  10 , one such leaflet being shown in FIG. 2B. The final assembled configuration of the three leaflets is shown at  15  in FIG. 1. This type of valve is designed to be intraoperatively assembled by the surgeon during an open heart procedure. Typical assembly times are of the order of 10 minutes.  
     [0014] During construction of the valve, the individual or individuals building the valve currently rely upon their judgement and experience as to whether the harvested tissue is of adequate quality to allow a durable valve to be built. Because the valves are built during open heart surgery, there is only a limited amount of time for testing the mechanical properties of the available tissue. Among the many factors influencing the strength of the autologous tissue are the collagen quality, its cross linking (the effect of glutaraldehyde treatment), the direction of the main mass of collagen fibers, and the proportion of collagen within the tissue mass. The methods which are currently available to determine these parameters are time consuming.  
     [0015] As described below, an embodiment of the method provides a simple go/no go test of tissue strength using a strip of tissue cut from the valve material adjacent to the edge of the tissue leaflet subject to the greatest stress when the tissue leaflet is mounted in the artificial heart valve.  
     [0016] Early Finite Element Analysis (FEA) work on the stresses within a leaflet indicates that the highest stress points occur at points close to the coaptation line near the anchor positions at the top of the tissue leaflet. These highest stresses can be related to what is known as the “membrane” stress. The membrane stress can be visualized as the stress that is found in the rubber of an inflated balloon. The highest stress in the tissue leaflet is always higher than the membrane stress, and the quality of the design is related to how close the maximum stress in the tissue leaflet is to the membrane stress.  
     [0017] However, in the balloon and the valve leaflet, the important strength factor is not the ultimate allowable stress but the ultimate allowable load per unit width of the material. Thus, if a constant width of tissue is tested, there will be a minimum load which the strip of tissue must be able to withstand. This minimum load is independent of the tissue thickness. Thus, a thick tissue with a low ultimate tensile stress can be matched in strength by a thin tissue with a high ultimate tensile stress.  
     [0018] If the test sample is taken from a position close to the leaflet&#39;s highest stress point, the differences due to possible collagen alignment are minimized. The close coupling of the sample to the portion of the leaflet subject to the highest stress also minimizes the effects of any other limiting factors such as inadequate fixing.  
     [0019]FIG. 2A shows a test tissue leaflet  20  cut to include a test strip portion  30  adjacent the portion of the tissue leaflet subject to the highest amount of stress, the top anchor points  58 . The test tissue leaflet  20  of FIG. 2A includes the test strip portion  30  and the tissue leaflet  10  of FIG. 2B. In an exemplary embodiment, the test strip portion  30  includes a test strip tissue hole  34  near each end of the test strip portion  30 . In a series of laboratory tests to evaluate pericardium, a standard strip width of 5 mm has been generally accepted as a reasonable compromise between minimizing the amount of tissue and what would provide an ideal test sample shape. In an exemplary embodiment, the test strip portion  30  is therefore selected to be approximately 5 mm wide.  
     [0020] Tissue leaflets are typically cut with a tissue cutting die. Examples of cutting dies suitable for cutting predetermined shapes in autologous tissue are shown and described in U.S. Pat. Nos. 5,163,955 and 5,425,741, hereby incorporated herein by reference. In order to produce a test tissue leaflet  20  with the shape shown in FIG. 2A with the test strip portion  30 , the cutting die can be modified to provide the test strip portion  30  with the desired qualities.  
     [0021] Another form of cutting die which is suitable for cutting tissue leaflets is the rotatable tissue die shown and described in U.S. Pat. No. 5,609,600, hereby incorporated herein by reference. Although the shape of the tissue leaflet produced by the cutting die of U.S. Pat. Nos. 5,163,955; 5,425,741; and 5,609,600 is different than the shape of the test tissue leaflet  20  shown in FIG. 2A, the cutting dies in the above-referenced patents can be modified to produce the test tissue leaflet  20  with the test strip portion  30  as shown in FIG. 2A.  
     [0022] In an exemplary embodiment, the location of the test strip portion  30  is selected so that it is close to the portion of the tissue leaflet  10  subject to the highest load, the top anchor points  58 . This location is also the best compromise for the alignment with the direction of these loads. In an embodiment, a coaptation line  36  can be used as one of the sides of the test strip portion  30  in order to minimize the tissue usage.  
     [0023]FIG. 2B shows the autologous test tissue leaflet  20  of FIG. 2A after the test strip portion  30  has been cut off for testing. The test strip portion  30  can be cut off from the test tissue leaflet  20  by any suitable means, for example, with a scalpel, a cutting die, or a laser cutting device. After the test strip portion  30  has been evaluated to assess the strength of the tissue, the remaining autologous tissue leaflet  10  as shown in FIG. 2B can be used in the fabrication of an assembled autologous heart valve  9  such as shown in FIG. 1.  
     [0024] In an exemplary alternative embodiment, the cutting die produces the autologous tissue leaflet  10  and the test strip portion  30  in a single operation without the need to separate the test strip portion  30  from the test tissue leaflet  20  in a second cutting step. In an embodiment, the coaptation line  36  is used as one of the sides of the test strip portion  30  when the tissue leaflet  10  and the separate test strip portion  30  are simultaneously produced in a single operation.  
     [0025]FIG. 3 shows an embodiment of a tissue testing device  40  which is suitable for testing the strength of the test strip portion  30  of the test tissue leaflet  20  of FIG. 2A. The tissue testing device  40  of FIG. 3 has a generally X-shaped frame having a generally linear first piece  44 , a generally V-shaped second piece  46 , and a generally V-shaped third piece  48 . The generally linear first piece  44 , the generally V-shaped second piece  46 , and the generally V-shaped third piece  48  are pivotally joined to one another by a pivot  50  at or near the center of the generally linear first piece  44 , the generally V-shaped second piece  46 , and the generally V-shaped third piece  48 .  
     [0026] The generally linear first piece  44  has a top handle  52  and an upper arm  54 . The generally V-shaped second piece  46  includes a second piece loading lever  56  and a lower arm  58 . The third piece has an actuating handle  60  and a third piece loading lever  62 .  
     [0027] In an exemplary embodiment, the ends of both the upper arm  54  and the lower arm  58  are curved. In another exemplary embodiment, a projection  64  is attached to each of the curved ends. The second piece loading lever  56  and the third piece loading lever  62  are joined by a load spring  66  having a known load.  
     [0028] The test strip portion  30  of the test tissue leaflet  20  may be tested for strength as follows. The test strip portion  30  is attached to the tissue testing device  40  by placing the test strip tissue holes  34  over the projections  64  on the upper arm  54  and the lower arm  58  so that the length of the test strip portion  30  extends over the ends of the upper arm  54  and the lower arm  58 . The top handle  52  of the generally linear first piece  44  and the actuating handle  60  of the generally V-shaped third piece  48  are squeezed together to place the top handle  52  in contact with the actuating handle  60 . The load spring  66  pulls the first piece loading lever  56  and the second piece loading lever  62  toward one another, exerting the known load of the load spring  66  on the test strip portion  30  by pulling apart the ends of the upper arm  54  and the lower arm  58 .  
     [0029] If the test strip portion  30  breaks under the known load of the load spring  66 , the tensile strength of the autologous tissue forming the test tissue leaflet  20  is considered to be unsuitable for forming a tissue leaflet  10  for use in the autologous tissue valve  9  of FIG. 1. A different portion of tissue is then chosen for forming a new test tissue leaflet  20 . If the test strip portion  30  does not break under the known load of the load spring  66 , the tensile strength of the autologous tissue forming the test tissue leaflet  20  is judged to be suitable, and the tissue leaflet  10  remaining after the test strip portion  30  is removed from the test tissue leaflet  20  can be used in the preparation of an autologous tissue valve such as shown in FIG. 1.  
     [0030] The tissue testing device  40  is therefore a suitable device for testing the tensile strength of autologous tissue to determine the suitability of the autologous tissue for use in a tissue leaflet to be mounted in an artificial heart valve.  
     [0031] Although described in the context of testing autologous tissue, the embodiments of the method and the apparatus may be used in testing autogenous tissue, porcine tissue, or bovine tissue. The tissue may be fixed, partially fixed, or nonfixed.  
     [0032] Because the load which is applied to the test strip portion  30  is a specific load which depends on the strength of the load spring  66 , the determination of the strength of the load spring  66  is an important parameter. The determination of the appropriate strength of the load spring  66  is determined by one of ordinary skill in the art. The strength of the load spring  66  may be reduced as the size of the heart valve and the tissue leaflets  10  is reduced.  
     [0033] The tissue testing device  40  shown in FIG. 3 is to be considered only as illustrative of a suitable apparatus and method. The tissue testing device  40  may be modified in various manners. For example, an elastic rubber strip could be substituted for the load spring  66  to exert the known load on the test strip portion  30 .  
     [0034] Although the top handle  52  and the actuating handle  60  provide a convenient way to allow the force of the load spring  66  to be exerted on the test strip portion  30 , in an alternative embodiment, the handles may be omitted from the tissue testing device  40 . Other forms of a suitable tissue testing device may include two linear pieces joined at the center by a pivot, similar to a pair of scissors. If a test strip portion  30  is attached to two of the adjacent ends of the scissors-like device and load spring is attached to the side of the scissors-like device at or near the ends of the device, the load spring would exert the known force on the test strip portion  30 . If the test strip portion  30  does not break when subjected to the known load, the tissue is considered suitable for use in preparing a heart valve.  
     [0035] Various modifications and alterations of this invention will be apparent to one skilled in the art without departing from the scope and spirit of this invention. It should be understood that the invention is not limited to the embodiments disclosed therein, and that the claims should be interpreted as broadly as the prior art allows.