Patent Publication Number: US-2022233210-A1

Title: Cutting Grasper for Valve Leaflet Laceration

Description:
BACKGROUND 
     Millions of people each year are diagnosed with heart valve disease, the most commonly diseased valves being the aortic and mitral valves. If left untreated, valve disease can lead to life-altering symptoms, congestive heart failure, and death. Often medical management is insufficient, and many patients require surgical intervention such as valve replacement or repair. Some patients are candidates for minimally invasive procedures such as transcatheter mitral valve replacement (TMVR). 
     TMVR can be complicated by left ventricular outflow tract (LVOT) obstruction. This can occur when a transcatheter mitral valve pushes the anterior leaflet into the left ventricular outflow tract significantly limiting the outflow through the LVOT and aortic valve, which can be fatal. Concerning anatomy for LVOT obstruction is one of the primary reasons that a patient may be denied a transcatheter mitral valve replacement. 
     To reduce the risk of LVOT obstruction, medical practitioners have developed techniques that involve lacerating the anterior leaflet of the mitral valve, so that when a transcatheter replacement valve is inserted, the leaflet does not obstruct the LVOT. One conventional technique for lacerating the anterior leaflet of the mitral valve concomitantly with mitral valve replacement is the LAMPOON procedure. This procedure requires the use of two transfemoral guide catheters, which are advanced retrograde through the aortic valve and positioned in the LVOT and the left atrium, respectively. An electrified guide wire is advanced through the catheter in the LVOT and is used to perforate the base of the anterior leaflet. It is then snared through the other catheter in the left atrium. Once the guidewire has been successfully snared, tension is applied, and the guide wire is electrified once again to lacerate the anterior leaflet. The wires are then retracted through the LVOT. 
     This procedure is technically difficult and requires significant operator experience. Accordingly, the LAMPOON procedure is not commonly performed. Additionally, the LAMPOON procedure carries an inherent risk such as inadvertently injuring other anatomical structures. Due to the complexities and risks of such conventional techniques, LVOT obstruction by the anterior leaflet remains a significant barrier to TMVR. Often, there are no alternative treatments, and thus patients are left untreated. 
     SUMMARY 
     To minimize LVOT obstruction during TMVR, a percutaneous cutting grasper for valve leaflet laceration is deployed. In one or more implementations, a device includes a flexible shaft and a cutting grasper. The cutting grasper extends from a distal end of the flexible shaft, which is configured to advance the cutting grasper through a vascular system of a patient and into a chamber of the patient&#39;s heart. The device can be used to target a valve leaflet, e.g., an anterior leaflet of the patient&#39;s mitral valve. The cutting grasper includes a base member having a proximal end that is attached to the distal end of the flexible shaft. The cutting grasper also includes an actuating member. A proximal end of the actuating member is rotatably attached to the distal end of the base member forming a hinge of the cutting grasper. The actuating member is configured to rotate relative to the base member about the hinge between open and closed positions of the cutting grasper. In operation, the cutting grasper is controlled to grasp a target leaflet (e.g., the anterior leaflet of the mitral valve) in the open position, close into the closed position with the target leaflet disposed between the base and actuating members, and then divide the target leaflet while disposed between the base and actuating members. 
     This Summary introduces a selection of concepts in a simplified form that are further described below in the Detailed Description. As such, this Summary is not intended to identify essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description is described with reference to the accompanying figures. 
         FIG. 1  depicts an example of the cutting grasper in use in an example environment. 
         FIG. 2  depicts examples of the cutting grasper in a closed configuration and in an open configuration. 
         FIG. 3  depicts an example of an alternative implementation in which the cutting grasper includes an independent anchor component. 
         FIG. 4  depicts an example in which the cutting grasper advances from a left atrium to a mitral valve. 
         FIG. 5  depicts an example in which the cutting grasper enters a left ventricle and opens into an open position. 
         FIG. 6  depicts an example in which the cutting grasper retracts so that an anterior leaflet of a mitral valve is disposed between a base member and an actuating member of the cutting grasper. 
         FIG. 7  depicts an example in which the cutting grasper closes into a closed position with the anterior leaflet disposed between the base and actuating members. 
         FIG. 8  depicts an example in which the cutting grasper cuts the anterior leaflet and retracts into the left atrium. 
         FIG. 9  depicts example posterior views of the mitral valve including the mitral valve before and after laceration by the cutting grasper. 
         FIG. 10  depicts example implementations in which the cutting grasper is configured to rotate about a base hinge. 
         FIG. 11  depicts an example implementation in which the cutting grasper is configured to circumvolve about a rotational component. 
     
    
    
     DETAILED DESCRIPTION 
     Overview 
     Transcatheter mitral valve replacement (TMVR) is a minimally invasive procedure, which may be used for heart valve replacement or repair. However, TMVR can be complicated by left ventricular outflow tract (LVOT) obstruction. This can occur when a transcatheter mitral valve pushes the anterior leaflet into the left ventricular outflow tract significantly limiting the outflow through the LVOT and aortic valve, which can be fatal. Accordingly, medical practitioners have developed techniques that involve intentionally lacerating the anterior leaflet of the mitral valve, so that when a replacement valve is inserted, the leaflet does not obstruct the LVOT. 
     One conventional approach for laceration of the anterior leaflet of the mitral valve, to prevent LVOT obstruction, is the LAMPOON procedure. This procedure is technically difficult and requires significant operator experience. Additionally, the LAMPOON procedure carries an inherent risk such as inadvertently injuring other anatomical structures. Due to the complexities and risks of such conventional techniques, LVOT obstruction by the anterior leaflet remains a significant barrier to TMVR. Often, there are no alternative treatments, and thus patients are left untreated. 
     To overcome these problems, a percutaneous cutting grasper for valve leaflet laceration may be deployed. The percutaneous cutting grasper includes a base member having base proximal and distal ends. This cutting grasper also includes an actuating member, which is attached to the distal end of the base member. The junction of the base member and the actuating member forms a hinge of the cutting grasper, and the actuating member is configured to rotate relative to the base member about the hinge. In this way, the cutting grasper may be disposed in and also between open and closed positions, as described in more detail below. 
     In accordance with the described techniques, the cutting grasper may be attached to a distal end of a flexible shaft that is configured to advance the cutting grasper through a vascular system of a patient, e.g., a person or some other animal. In some scenarios, for example, the flexible shaft can advance the cutting grasper from a femoral vein accessed through an incision in the groin, through the inferior vena cava, to the right atrium of a patient&#39;s heart. Following a transeptal puncture, the flexible shaft can further advance the cutting grasper to the left atrium. From there, the flexible shaft can further advance the cutting grasper through the mitral valve and into the left ventricle. This singular catheterized deployment of the cutting grasper in an antegrade direction overcomes limitations of conventional techniques that involve complicated deployment of medical devices, such as the deployment of two separate catheters as part of performing the LAMPOON procedure. 
     In operation, once the cutting grasper is advanced by the flexible shaft to a desired position, such as disposed in the left ventricle just inferior to the mitral valve, the actuating member rotates into an open position. In one or more implementations, the cutting grasper can be configured to open into the open position automatically responsive to entry into the chamber. Alternatively or in addition, the cutting grasper can be controlled to open into the open position by a user of the device, e.g., a surgeon handling one or more controls communicably coupled to a proximal end of the flexible shaft. While disposed in the open position, the cutting grasper can be retracted towards a target leaflet, e.g., the anterior leaflet of the mitral valve, so that the actuating member is positioned on a ventricular side of the leaflet and the base member is positioned on an atrial side of the leaflet. With the target leaflet disposed between the base and actuating members, one or more grasper closing components may close the cutting grasper into a closed position and secure the target leaflet between the base member and the actuating member. 
     By way of example, the grasper closing components can include, or otherwise be implemented using, rare earth magnets to draw the members together and securely grasp the leaflet. Additionally or alternatively, the grasper closing components may comprise a system of wires and pulleys. In one or more implementations, the grasper closing components may be configured with a release mechanism which is operable to release the actuating member from the base member. In this way, the cutting grasper can be repositioned, enabling an alignment of the laceration to be adjusted based on the anatomy of the patient. In other words, the cutting grasper is not prevented from being positioned to accurately grasp a target portion of the anterior leaflet of the mitral valve, regardless of an anatomy of the patient. 
     With the target leaflet grasped between the base and actuating members, the cutting grasper is operable to divide the target leaflet. The target leaflet can be divided by cutting it using one or more lacerating components of the cutting grasper, including, for example, an electrified wire, one or more magnets, and/or one or more blades, to name just a few. Such lacerating components may be located on either or both the base member and the actuating member. In one or more implementations, the lacerating components may include a mechanism to remain idle until the target leaflet is securely grasped by the cutting grasper. In this way, the cutting grasper can mitigate the likelihood of inadvertently damaging healthy tissue. Once the target leaflet is cut, the cutting grasper can be retracted, e.g., by retracting the flexible shaft back through the patient. Additionally, once the target leaflet is cut, the valve is suitably prepared for replacement with minimized risk of LVOT obstruction. 
     In the following discussion, an example device is described that is configured to employ the techniques described herein. Any reference or depiction of anatomical features in this Application are not necessarily drawn to scale and are not necessarily anatomically accurate, but rather are drawn to illustrate the described device and techniques using the device. 
     Example Device 
       FIG. 1  depicts an example  100  of a heart  102  at different stages  104 - 114  and in which cutting grasper  116  is operated to divide an anterior leaflet  118  of the heart  102 &#39;s mitral valve. The heart  102  includes left ventricular outflow tract (LVOT  120 ), relative to which subsequent transcatheter mitral valve replacement surgery may be performed. 
     In the illustrated example  100 , the cutting grasper  116  is attached to a flexible shaft  122  having proximal and distal ends. The cutting grasper  116  extends from the shaft distal end so that the flexible shaft  122  can advance the cutting grasper  116  through a vascular system of a patient. For instance, as depicted in first stage  104  the flexible shaft  122  is configured to advance the cutting grasper  116  through inferior vena cava  124  and into right atrium  126 . The flexible shaft  122  further advances the cutting grasper  116  through a transeptal puncture  128  into left atrium  130 . 
     As shown in second stage  106 , the cutting grasper  116  is then advanced through mitral valve  132 . In general, the mitral valve  132  includes two leaflets, one of which is the anterior leaflet  118 . While in left ventricle  134 , an actuating member  136  of the cutting grasper  116  may be actuated to rotate relative to a base member  138  of the cutting grasper  116  into an open position as shown in third stage  108 , e.g., once the cutting grasper  116  has advanced past the mitral valve  132  and into the left ventricle  134 . In operation, the cutting grasper  116  may be retracted while in the open position so that the actuating member  136  is positioned on the ventricular side of the anterior leaflet  118  and so that the base member  138  is positioned on the atrial side, as shown in fourth stage  110 . 
     Based on the anterior leaflet  118  being disposed between the actuating member  136  and the base member  138 , the cutting grasper  116  may be said to “grasp” the anterior leaflet  118 . The actuating member  136  and the base member  138  are further configured to close against the anterior leaflet  118  as depicted in fifth stage  112 . In particular, the actuating member  136  and the base member  138  may be closed against the anterior leaflet  118  by one or more grasper closing components. In accordance with the described techniques, the cutting grasper  116  is operable to divide the anterior leaflet  118  while the anterior leaflet  118  is disposed between the actuating member  136  and the base member  138 , such as by using one or more lacerating components. Sixth stage  114  shows the cutting grasper  116  retracting into the left atrium  130 , e.g., after the anterior leaflet  118  has been successfully divided such that the anterior leaflet  118  is bisected. 
       FIG. 2  depicts examples  200  of the cutting grasper  116  in both a closed position  202  and an open position  204 . The cutting grasper  116  extends from the distal end of the flexible shaft  122 . The flexible shaft  122  may be configured to have a variety of sizes or thicknesses, depending on the desired application or the particular anatomy of the patient without departing from the spirit or scope of the described techniques. Here, the cutting grasper  116  is depicted in more detail with the base member  138 , which has proximal and distal ends. In this example, the proximal end of the base member  138  is attached to the distal end of the flexible shaft  122 . The actuating member  136 , which has proximal and distal ends, is also depicted in the illustrated example  200 . In this example  200 , the proximal end of the actuating member  136  is rotatably attached at the distal end of the base member  138  forming a hinge  206  of the cutting grasper  116 . 
     The actuating member  136  is configured to rotate relative to the base member  138  about the hinge  206  between the closed position  202  and the open position  204 . In this way, the cutting grasper  116  may be disposed in and also between (e.g., at angles between) the open position  204  and the closed position  202 . In one more implementations, the cutting grasper  116  is advanced through the vasculature of a patient by the flexible shaft  122  while in the closed position  202 . The actuating member  136  may be configured to rotate, causing the cutting grasper  116  to be disposed in the open position  204 , automatically, such as in response to entry into a desired heart chamber. Alternatively or additionally, the actuating member  136  can be controlled to cause the cutting grasper  116  to be disposed in the open position  204  by a user of the device, e.g., a surgeon. As noted above, the cutting grasper  116  and or the flexible shaft  122  may be coupled to one or more controls that enable a surgeon to control the flexible shaft  122  and the cutting grasper  116 . Similarly, the actuating member  136  can be configured to rotate so that the cutting grasper  116  is disposed in the closed position  202  automatically, such as responsive to a targeted leaflet becoming disposed between the base member  138  and the actuating member  136 . Alternatively or additionally, the cutting grasper  116  may be controlled to close into the closed position  202  by a user of the device, e.g., a surgeon. 
     The cutting grasper  116  may also include one or more grasper closing components that are configured to close the cutting grasper  116  into the closed position  202 . In one or more implementations, the grasper closing components may be configured as a system of wires and pulleys  208  as depicted in the illustrated example  200 . Additionally or alternatively, the grasper closing components can include, or otherwise be implemented using rare earth magnets to draw the members together. Grasper closing components may include a variety of mechanisms and/or or configured in a variety of ways to close the cutting grasper  116  into the closed position in the spirit and scope of the described techniques. 
     In one or more implementations, the grasper closing components may be configured with a release mechanism, which is operable to release the actuating member  136  from the base member  138 , e.g., returning the cutting grasper to the open position  204 . In this way, the cutting grasper  116  can be repositioned into a different position before cutting target tissue. 
     In accordance with the described techniques, the cutting grasper  116  includes one or more lacerating components to divide targeted tissue. Examples of such lacerating components include, but are not limited to, one or more electrified wires and one or more blades, to name just a few. The lacerating components can be disposed on or within either or both of the base member  138  and/or the actuating member  136 . Additionally or alternatively, the lacerating components may be configured so that they remain idle until a target leaflet is disposed between the base member  138  and the actuating member  136 . In this way, the cutting grasper  116  can mitigate the risk of accidentally damaging healthy tissue. 
       FIG. 3  depicts an example  300  of an alternative implementation of the cutting grasper  116 . In addition to the base member  138  and the actuating member  136 , the cutting grasper  116  includes an anchor component  302  in the illustrated example  300 . The anchor component  302  is depicted as being attached to the hinge  206  of the cutting grasper  116 . Alternatively or additionally, the anchor component  302  may be attached to the distal end of the flexible shaft  122 . When the target tissue is disposed between the base member  138  and the actuating member  136 , the anchor component  302  may be configured to rotate about the hinge  206  and bind to the actuating member, as a mechanical clasp. Operation of the anchor component  302  in this way can help secure the target tissue between the base member  138  and the actuating member  136 . Due to securing the target tissue, the anchor component  302  can ensure the target tissue is disposed within the cutting grasper  116  so that the tissue is divided at an intended location. In one or more implementations, the anchor component  302  may also be configured with a release mechanism, which is operable to release the anchor component  302  from the actuating member  136 . In this way, the cutting grasper  116  can be repositioned into a preferred orientation before cutting the target tissue. It is to be appreciated that an anchor component may not be used in one or more implementations, but that the cutting grasper  116  is nevertheless configured to suitably secure the target tissue between the actuating member  136  and the base member  138 , e.g., based on the actuating member  136  and the base member  138  closing with suitable force on the target tissue while in the closed position  202 . In implementations without the anchor component  302 , the cutting grasper  116  may therefore also enable the targeted tissue to be divided at an intended location. 
       FIG. 4  depicts an example  400  in which the cutting grasper  116  is advanced from the left atrium  130  to the mitral valve  132 . This advancement is depicted in the illustrated example  400  in detail via first stage  402  and second stage  404 . 
     The first stage  402  depicts the cutting grasper  116  in the closed position  202  while disposed in the left atrium  130 . In this example, the flexible shaft  122  is depicted advancing the cutting grasper  116  from the left atrium  130  to the mitral valve  132 , as shown in the second stage  404 . In one or more implementations, the cutting grasper  116  may be configured to remain in the closed position  202  while being advanced by the flexible shaft  122 . This enables the device to move efficiently through vasculature of a patient. 
       FIG. 5  depicts an example  500  in which the cutting grasper  116  is advanced from a position within the mitral valve  132  and into the left ventricle  134 . This advancement is depicted in the illustrated example  500  in detail via first stage  502  and second stage  504 . 
     As noted above, the actuating member  136  is configured to rotate about the hinge  206  into the open position  204 , such as based on the cutting grasper  116  entering the left ventricle  134 . In one or more implementations, the actuating member  136  may “spring” into the open position  204  responsive entry into the left ventricle  134 . This may occur when the cutting grasper  116  is biased in the open position  204  and when the cutting grasper  116  requires force to overcome such biasing to be disposed in the closed position  202 . Additionally or alternatively, the cutting grasper  116  can be controlled to open by a user of the device, i.e., a surgeon. 
       FIG. 6  depicts an example  600  in which the cutting grasper  116  is retracted while disposed in the open position  204  from an advanced position in the left ventricle  134  to make contact with the anterior leaflet  118  of the mitral valve  132 . This retraction is depicted in detail in the illustrated example  600  via a first stage  602  and a second stage  604 . At the second stage  604 , the actuating member  136  is depicted positioned on the ventricular side of the anterior leaflet  118  and the base member  138  is depicted positioned on the atrial side of the leaflet. In one or more implementations, the angle between the base member  138  and the actuating member  136  can be adjusted to better contact the anterior leaflet  118 , for instance, as controlled by a user of the device, i.e., a surgeon. 
       FIG. 7  depicts an example  700  in which the cutting grasper  116  grasps the anterior leaflet  118  and closes into the closed position  202 . This grasping is depicted in detail in the illustrated example  700  via a first stage  702  and a second stage  704 . The cutting grasper  116  may use one or more grasper closing components to close into the closed position  202 . By way of example, grasper closing components may include or otherwise be configured using one or more rare earth magnets and/or a system of wires and pulleys. In one or more implementations, such grasper closing components may be configured with a release mechanism, which is operable to release the actuating member  136  from the base member  138 , e.g., returning the cutting grasper  116  to or substantially to the open position  204 . In this way, the cutting grasper  116  can be repositioned into different positions so that an alignment of a laceration made using the cutting grasper  116  can be adjusted in real-time (e.g., while the cutting grasper  116  is deployed within the patient) based on the anatomy of the patient. 
       FIG. 8  depicts an example  800  in which the cutting grasper  116  lacerates the anterior leaflet  118  and then, while in the closed position  202 , retracts through the mitral valve  132  into the left atrium  130 . This laceration and retraction are depicted in more detail in the illustrated example  800  via a first stage  802  and a second stage  804 . 
     To divide the anterior leaflet  118 , the cutting grasper  116  may use one or more lacerating components. By way of example, such lacerating components may include an electrified wire, one or more magnets, and/or one or more blades, to name just a few. It is to be appreciated that the lacerating components may be configured to lacerate bodily tissue using different materials and or mechanisms without departing from the spirit or scope of the described techniques. 
     Further, such lacerating components may be disposed on or otherwise included as part of either or both the base member  138  and the actuating member  136 . In one or more implementations, the lacerating components may be configured to remain idle until the target tissue (e.g., the anterior leaflet  118 ) is securely grasped by the cutting grasper  116 . In this way, the cutting grasper  116  can mitigate the likelihood of inadvertently damaging healthy tissue. With the anterior leaflet  118  successfully bisected using the cutting grasper  116 , as depicted in the second stage  804 , the heart  102  is considered surgically “ready” for mitral valve replacement such that a risk of LVOT obstruction is reduced or eliminated. 
       FIG. 9  illustrates example posterior views at  900  of the mitral valve  132 , i.e., as if looking down at the mitral valve  132  from the left atrium  130 . The mitral valve  132  is shown before being divided  902  as well as after being divided  904 . An optimal cut site  906  is also depicted, located on the anterior leaflet  118 . The cutting grasper  116  is configured so that the actuating member  136  and the base member  138  may be disposed above and below the optimal cut site  906 . Grasper closing components may then be actuated to grasp the target leaflet along the optimal cut site  906 . The grasper closing components may also be configured with a release mechanism which is operable to release the actuating member  136  from the base member  138 . In this way, the cutting grasper  116  can be repositioned into a different orientation, including along the optimal cut site  906  if the target tissue is not grasped along the optimal cut site  906 . This helps overcome limitations of conventional techniques. For instance, the alignment of a laceration made using the LAMPOON procedure may be limited to a single plane which may not be optimal depending on specific anatomy of an individual patient. 
     With the cutting grasper  116  oriented along the optimal cut site  906 , lacerating components of the cutting grasper  116  may then cut the anterior leaflet  118 . Due to its ability to release tissue and be repositioned, use of the cutting grasper  116  mitigates the likelihood of inadvertently damaging healthy tissue and ensures that a cut is made only once the lacerating components are aligned with the optimal cut site  906 . The resulting cut is shown at  904 , with the anterior leaflet  118  bisected along the optimal cut site  906 . Lacerating the anterior leaflet  118  along the optimal cut site  906  mitigates the risk of LVOT obstruction upon subsequent mitral valve replacement. 
       FIG. 10  illustrates an examples  1000  in which the cutting grasper is configured to rotate at a base hinge. 
     Here, the cutting grasper  116  is depicted in four different orientations  1002 - 1008  and further includes a base hinge  1010 . In one or more implementations, the junction of the base member  138  and the flexible shaft  122  forms the base hinge  1010  of the cutting grasper  116 . In such implementations, the base member  138  is configured to rotate relative to the flexible shaft  122  about the base hinge  1010  in a first direction and/or a second direction. In the first orientation  1002 , for instance, the cutting grasper  116  is in a closed position  202 , i.e., the base member  138  and the actuating member  136  are disposed together. In the first orientation  1002 , the base member  138  is also illustrated rotated about the base hinge  1010  in a first direction. As depicted in the second orientation  1004 , the cutting grasper  116  is also deployed in the closed position  202 , but the base member  138  is depicted rotated about the base hinge  1010  in a second direction that is opposite the above-mentioned first direction. 
     In the third orientation  1006 , the cutting grasper  116  is depicted in the open position  204  and the base member  138  is rotated about the base hinge  1010  in the first direction. By way of contrast, in the fourth orientation  1008  the cutting grasper  116  is depicted in the open position  204  and the base member is depicted rotated about the base hinge  1010  in the second direction. In one or more implementations, the cutting grasper  116  can be configured to rotate about the base hinge  1010  automatically. Alternately or in addition, the cutting grasper  116  can be controlled to rotate about the base hinge  1010  by a user of the device, e.g., a surgeon. In this way, the cutting grasper  116  enables a surgeon (or a robot) to have more control generally and finer control over the position and orientation of the device, which overcomes limitations of conventional techniques that involve complicated deployment of medical devices, such as the deployment of two separate catheters as part of performing the LAMPOON technique. 
       FIG. 11  depicts an example  1100  of an implementation in which the cutting grasper  116  is configured to circumvolve about a rotational component. 
     This example  1100  includes a side view  1102  of the cutting grasper  116  as well as a second view  1104 , which looks down on a superior end of the cutting grasper  116 . In this example  1100 , the cutting grasper  116  is configured to circumvolve about a rotational component  1106 . In one or more implementations, the cutting grasper  116  is configured with the rotational component  1106 . By way of example and not limitation, the rotational component  1106  is disposed between the distal end of the flexible shaft  122  and the proximal end of the base member  138 . In this way, the cutting grasper  116  is configured to circumvolve, clockwise or counterclockwise, substantially about a longitudinal axis that runs substantially through the flexible shaft  122 . In one or more implementations, the rotational component can be controlled to cause the cutting grasper  116  to rotate so that it circumvolves substantially about the longitudinal axis. Additionally, the rotational component can be controlled by a user of the device, e.g., a surgeon, to enable the cutting grasper  116  to circumvolve the axis in this way. Advantageously, a user of the device has enhanced control over the position and orientation of the cutting grasper  116 , which allows the user to more effectively direct the cutting grasper  116  through the vasculature and position the cutting grasper  116  along an optimal cut site  906 . 
     CONCLUSION 
     Although the systems and techniques have been described in language specific to structural features and/or methodological acts, it is to be understood that the systems and techniques defined in the appended claims are not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as example forms of implementing the claimed subject matter.