The senior design project involves the design and construction of an

Transcription

1 Executive Summary: The senior design project involves the design and construction of an endovascular tissue removal device that can cut and remove calcified valve leaflet tissue from the aortic root with as little invasiveness as possible. It must be able to fit into a 24F catheter. This device will specifically help the elderly population, who are more prone to develop aortic stenosis. The device involves the novel technique of first removing diseased and calcified tissue percutaneously from the body. The process in use today involves the valve leaflet tissue being only pushed to the side of the aortic valve by the prosthesis, causing undue pressure and problems in both the aortic valve wall and the prosthesis. The proposed device will be threaded through the femoral artery through vascular pathways to the aortic root. It will then actively cut out the calcified tissue using power from an external motor. The debris will then be vacuum-sucked out, and collected. The device must then travel through the same pathway through the body in reverse in order to be removed so that the prosthesis can then be inserted into the heart. The product will increase the prosthesis performance and longevity and decrease the risk to the patient, making this endovascular tissue removal device unique and valuable. The research group is aiming to obtain a patent for the device as soon as possible. Background: The device is designed to perform an endovascular procedure which will cut out or macerate calcified or diseased tissue in order to prepare the site for a valve replacement prosthesis. This surgery is intended to be much less evasive and more effective than current techniques to treat these issues. By entering via an artery, the device would be threaded all the way to the aorta inside of a catheter. Using a technique like this would improve the recovery times of patients, especially those who are elderly

2 or have weakened immune systems. However, working in such small space contraints will force the tool to be as small and simplistic as possible while still generating enough force to cut through hardened leaflets. Many previous devices and procedures such as stents and angioplasty simply leave the valve leaflets intact and pressed against the aortic wall, causing distortion of the prosthesis, paravalvular leakage, or regurgitation. Endovascular procedures reduce the risk of severe complications of the surgeries that these procedures would be replacing, especially in patients with advanced age or comorbidities. This product would be greatly useful due to the fact that aortic stenosis is the most common valvular heart disease in the western world. Purpose of the Project: The purpose of this project is to create an endovascular tissue removal device that operates with the most minimal invasive procedure possible in the aortic root to remove valve leaflet tissue, both healthy and severely calcified, to prepare for the introduction of a prosthesis. Due to the tight path that the tool must travel before reaching the aorta, the device must be collapsed inside the catheter, with the ability to expand when at the destination. Because of the high lack of ability to do much manipulation of the tool once inside the aorta, the blades must be constructed out of a material that has high enough strength to still cut the leaflets, while having the ability to expand and recollapse before and after the procedure. This cutting blade would be attached to a thin wire that would be driven by a motor external to the body. While not the direct focus of our portion of the project, the floating debris would then be required to be suctioned into an attached catheter via a funneling filter which would still allow blood to flow past. This filter would need to be attached to the cutting blade in a manner that doesn t impede the ability of the driveshaft to rotate. The device will be run using a computer program specifically designed to work with the device. The program will allow the user to control the blade rpm, as well as control the electrical impulse needed to expand the device once it reaches the aorta.

3 Previous Work Done by Others: Over the last three to four years, a few groups have looked at ways to improve the efficiency of the surgical techniques to remove damaged valve leaflets. While looking at previous works, there are a few designs that attempt to accomplish the same desired result as this project. Most of the research that was found was very recent, ranging from the earliest of 2005 until present. A few teams had some success using various mediums for clearing the calcified leaflets; however, none quite could combine speed and accuracy of cutting in the in vitro environment with the necessary task of debris removal. Products A team from Germany s University of Kiel had experimented with using high pressured water streams to resect the valves. While this technique was successful in cutting the leaflets, the procedure damaged the aortic wall in multiple cases, as well as superficial lesions that were revealed with microscopic analysis. This same team came back with another design in early Instead of using high pressured water, this time lasers were used. Using an Er:YAG laser with a purpose specific fiber coupling device and fiber tip was used to cut the leaflets. It was seen that while this technique was quicker than the previous design (~15 mm/min) it was still very slow in their minds, as well as creating pieces of debris up to 1 mm in size. More recently (March 2009) Florian Hauke et al. created a collapsable cutting blade using Nitinol metal. This memory metal allows the device to go into the artery collapsed, and then expands once situated. Meanwhile, the blade itself was machined from a nickel-titanium alloy. The blade was used in tandem with a slightly larger diameter holder which was placed on the other side of the leaflet. This provides the blade with some leverage to cut the tissue. With the blade having a saw-tooth like design, the method for cutting that was chosen that of a rotating punching technique. This saw the leaflet being sandwiched between the holder and the blade, with the blade approaching the holding ring with a combination of translation and rotation, until the leaflet is chipped away. The team determined that valve resection could be performed in less than 30 seconds with this technique. However, this experiment lacked actual in vitro testing, or any sort of suction filter for debris removal.

4 Patent Search Results: Searching on the U.S. Trademark and Patent Office s website yielded no results for searches including calcified valve leaflet removal and over 1,600 results for the less specific valve leaflet. After sifting through most of them, however, it was found that nothing was in any way related to our project, most of them being primarily related to the ballooning technique that pushes the calcified tissue further toward the aortic walls. All of the research that was discovered on the topic seemed to stem from Europe, and specifically Germany. Most of the research was done mainly by groups that were working in research at universities. Dr. Sun has expressed interest in acquiring a patent for the system upon completion of the project next year. Objective: The endovascular valve resection tool in discussion will include a foldable cutting blade system capable of being manipulated into a standard 24F catheter in order to be delivered to the heart in a minimally invasive procedure that will replace traditional openheart methods and accommodate for 100% endovascular valve replacement surgeries. The device will consist of circular cutting blades which will have a matching cutting plate which will be placed on either side of the aortic valve leaflet which will be cut. The two pieces will be pressed together, and the circular cutting blades will rotate to cut away the tissues of each leaflet individually leaving approximately 1mm of the leaflet tissue for adequate insertion of the replacement valve. Since this is a circular cutting tool, a rotating, flexible, mechanical shaft must fit into the catheter and run the length of catheter that would take to reach from the insertion point (generally in the femoral artery) to the aortic heart valve where the resection would take place. Not only does this shaft need to be sturdy enough to rotate the blades with sufficient force, but it also must be able to be snaked all through the body, including one hundred and eighty degree turns while still being able to rotate and transmit its mechanical properties to the surgical site. The device will be controlled by outside human controls. These controls would consist of a controller box which has a speed (RPM) knob to regulate the speed of the rotating cutting blades within the body for better precision. This rate would be displayed

5 on a small LCD screen on the controller box. The controller box would also include a power transformer and various power and instrument switches to operate and control the tool such as a overall power switch, a switch to toggle the tool motor on and off, as well as displays for these states such as a power LED and message on the LED screen stating the motor is in use. Other controls would include a sort of mechanical tool operated by the surgeon which would expand the outer catheter tip, exposing the blade and end plate. Another control would extend/expand and retract/fold the two cutting implements from the inside of the catheter s pieces and allow for the two to be pressed against one another in order to apply the necessary pressure and to hold the leaflet being cut. The whole device would need to be portable enough to be a table-top model and be module, meaning that the catheter portion of the device would be able to be replaced, without touching or removing the controller unit. This means that some sort of quickconnect mechanism will be implemented in order to facilitate this aspect of the device. This would allow for decreased cost of ownership, and allow for the device to have increased longevity as well as be able to incorporate future designs, as well as be able to eventually have several different sizes to accommodate all anatomical situations and anthropometries. Once completed, the device will go through several tests for functionality, such as tests done on leaflet tissues acquired from porcine sources as an inexpensive analog for the human anatomy as a proof of concept. Calculations of the forces and moments required in order to perform their operation will be recorded in documentation that will accompany the product. Maximum revolutions per minute of the cutting blades, as well as approximate minimum RPM to achieve the goal will also be included in documentation that would be included. Inspection of all removed tissues and the surrounding environment would be examined for any damage the tool would cause and methods for minimizing external damage would be analyzed and suggested to the user. Methods: The device can be broken up into four different parts; the cutting device, the electronic motor controller and readout, the manual mechanical controller, and the catheter/shaft portion. Each portion of this is vital and feeds off of the other. A very

6 basic block diagram can be viewed as fig. 1 on the next page. This diagram illustrates the basic inputs and outputs that will be in sequence in order to control and operate the valve resection tool. Each portion will be furthered discussed in the following documentation. The cutting device is made up of a circular cutting blade attached to a rotating shaft. This blade will be made out of shape-memory material, most likely Nitinol or Nickel-Titanium alloy that allows for very high elasticity. The need for this high elasticity is based on the fact that these blades will need to be bent down into a size that will fit within a 24F catheter. They need to then be able to expand to their cutting size within the body quickly and become rigid enough to perform the cutting process that needs to be accomplished within the aortic arch. These blades will have to be assembled in parts, utilizing rivets or some other fixation mode. As a result, the actual cutting blade would be separate from the supports that connect it to the shaft and later attached. This is due to the fact that the blades need to be sharpened before they are assembled and bent down into the assembly that will be collapsed into the catheter. Since we are working with very small stock in this project, a method of sharpening these blades will be adapted from Florian Hauke et al. (2009). Before shaping into the rounded blades, the straight metal stock will be systematically sharpened on a grinder for this method. Using a blocking mechanism, this will ensure that the sheet will be sharpened at a uniform angle and intensity on both sides. This method provided an even and uniformly sharpened blade without burrs to avoid tearing of the tissue and provide a clean cut at the resection site. In conjunction with this cutting blade is a back plate opposite of the cutting blade. This back plate acts as a cutting block for the rotating blades and allows for fixation of the leaflet tissues. This solves the issue of controlling or grabbing the leaflet tissues during the heart beating process, as this device would be used in in vivo conditions with a regular heartbeat as opposed to traditional methods at valve resection and replacement where the heart is taken out of the circulatory loop and a machine instead pumps blood through the patient s body. These cutting blades would be able to leave approximately 1 mm of the material on the outer wall of the aortic valve fixation point to allow for a good seating of the artificial valve that would come immediately after the resection of the defective or diseased valve.

7 This whole cutting apparatus would need to be enclosed in a capsule which would have the exact same diameter as the 24F catheter (~8 mm O.D.). This capsule will have to be mechanically opened by the user, which will expose the folded cutting blades. This capsule must be as small in length as possible as it must be rigid and would prove difficult to navigate through the circulatory system without being so. In all, both the capsule and the blade-back plate assembly need to be able to be actuated separately or independently in order to facilitate the functions outlined before. The soft shaft will prove very difficult to model as it needs to both be very flexible, bending 180 degrees in some places and at the same time able to translate the motor s circular motion in order to rotate the cutting blades. All the while, it must be small enough to snake its way through the human circulatory system from the thigh all the way to the aortic arch. This shaft can be found in the form of a helical coil which will accommodate the speeds of the blades as well as the forces. The helical coils can be bent in all shapes and still deliver their load on the end in a circular motion. This shaft would be affixed to the cutting blades and be within the catheter. It would have a connector in order to interface with the controller box. This would be in the form of a quick disconnect, so that it may be interchanged for different catheter set ups. As a result, the device can accommodate different sizes or shapes of the individual s anthropometry. Smaller diameter catheters for a pediatric patient rather than an adult patient would be necessary. Within this shaft, there need to be wired controls in order to expand or retract the blades and the capsule which can be within the hollow helical coil shaft. However, as much space within the helical shaft should be conserved as possible in order to accommodate future advancements to this product, such as suction in order to remove any and all tissues resected, as well as a grinder in order to isolate the material resected though these are not in the scope of this project. The shaft will connect to a brushless motor which would be mounted an aluminum box which will house all electronics needed to be used for the system. The reason for choosing a brushless motor is for cleanliness as it will accumulate less dust than a brushed motor, as well as lower maintenance and longer life to ensure the device has a long lifespan and few repairs would be necessary. Also, a brushless motor runs

8 cooler and produces less heat, and since we are mounting the motor in an enclosed space, heat production becomes an issue. The actual mechanism to attach and detach the cutting device and catheter from the controller station needs to be further researched but the goal is to make the catheter attachments to be module and easily interchangeable to accommodate various anthropometric dimensions of a patient such as discussed before. The interface of this controller would include, first and foremost, a controller knob or series of buttons to regulate the speed of the rotating cutting blades once they have been inserted into the aortic arch of the heart. This controller would have to be able to regulate the speed of the blades within certain tolerances. For example, the device would need to have a resolution of at least 100 RPM (or whatever is determined after experimentation). The speed of the motor would have to be both measured or calculated based on applied voltage and be outputted as a numeric value to the LCD screen on the front of the controller box. The box will also have switches with corresponding LED indicators for system power as well as one that regulated the motor s power independently to act as an emergency kill switch. In case something failed during surgery, this would avoid any damage to the patient s body. Various electronics would be required within the controller box, the first of which being a power supply. Since most brushless motors run on DC power, there would have to be an AC to DC converter and power supply, which will be determined once a motor is chosen to accommodate the requirements needed by the device. A variety of circuitry and controllers would also have to be included in the internal design of the controller box. Circuitry to regulate the speed along with the power to the motor would be one requirement. In order to regulate and display the motor s speed, a microcontroller chip would need to be implemented into the design. A speed controller would also be employed to better control the exact speed of the motor as it is adjusted to the technician s desired rate. In order to display the motor s speed on the LCD screen, programming of the memory in C-code would be required. This would then be converted to assembly to do the necessary analog to digital conversions of the voltage applied to the motor. Next, it can be connected to a digital readout of revolutions per minute. Along with the standard circuitry, an emergency cutoff switch would also be included in order to remedy any failures in the device immediately.

9 Additionally, a breaking system will also be looked into in order to stop the motor immediately to avoid any damage to the patient in the event of some mechanical failure. There are other mechanical considerations to take into account. Some parts of the design need to be able to be adjusted, including the blades, back plate, as well as the opening of the end cap of the catheter. This all needs to happen in order to release the enclosed blades once inserted into the body at the appropriate area. The blade and back plate need to be able to be moved back and forth from one another, in order to position and hold the leaflet in place during the cutting process. This can be done either by hand, using a hand controller, or via the controller box. Implementing a hand controller would allow the user to mechanically manipulate the blade, while the controller box would utilize servo motors or linear actuators. The method for controlling the linear position of the blade and the capsule will be determined later once further brainstorming and research has been completed on the subject. Though an electric servo controlled version would be idea, the budgeting of purchasing the motor and necessary electronics may be beyond the scope of the project which is what is necessitating a deeper look into the actual problem at hand and both solutions feasibility as the appropriate resolution to this question. Once a prototype has been completed, the device will go though several tests of its integrity and function. Forces will be measured using force plates to determine the applied force on the leaflet during resection as well as the measurements of torque and calculated moments in order to provide a complete specifications sheet to the consumer and to have proof that no harm comes to the patient. Also, all of the fail-safes put into the product will be tested. The breaking system (if it is indeed implemented) as well as the emergency cut off switches will be tested and be scrutinized to ensure that if the product were to fail it will be controlled and fail safely. Budget: The budget at this point in the project timeline, the budget is not very solid as we have not yet decided upon a design. Some items would be included in virtually all of the modalities that we have discussed and these items are listed below. Many of these items have been ball-parked due to the fact that a much more solidified budget should be

10 available once a design has been finalized and accepted by the client and all parts necessary have been accounted for and vendors have been contacted. Component Estimated Price Suggested Source brushless instrument grade motor $ Transmotec aluminum sheet metal (controller box) $ F catheter (deliver device) $ flexishaft (stainless steel, 2m) $ LCD screen (controller box readout RPM, etc) $60.00 Jameco Electronics Microcontroller $10.00 Power adapter AC/DC (specstbd) $60.00 Various wiring/ electronic components (sw, led, etc) $60.00 Nitinol Sheet Stock (Blade and back plate) $ Alfa Aesar Capsule (plastic) $20.00 Speed Controller (Kit) $50.00 Pig Aorta (For final testing and proof of concept) $20.00 Total $ Conclusion: In conclusion, the overall purpose of the project is to design and construct an endovascular heart tissue removal device that can fit into a 24F catheter in order to remove calcified valve leaflet tissue from the aortic root with as little invasiveness as possible. This device will specifically help the elderly population, who are more prone to develop aortic stenosis. The device involves the novel technique of first removing diseased and calcified tissue percutaneously from the body, before an aortic valve prosthesis is inserted also percutaneously into the body. In the past, open heart surgery was performed to insert the prosthesis, which carries a high risk of morbidy and mortality, especially for the weak and elderly. In the more recent past, percutaneous procedures were developed. However, the valve leaflet tissue was only pushed to the side of the aortic valve by the prosthesis, causing undue pressure, paravalvular leakage and blood regurgitation, as well as distortion of the prosthesis. The proposed device will increase in prosthesis performance and longevity and decrease the risk to the patient,

11 making this endovascular tissue removal device unique and valuable. The research group is aiming to obtain a patent for the device as soon as possible. If a patent is received, the device can then be marketed. A limiting monetary factor for constructing the device is the material in which to build the soft shaft. A wire material that is able to stretch and able to bend with the artery through its whole length is needed to reach the aortic valve. Once the monetary problem of the device is resolved, the product can be sold to hospitals all over in order to save lives.