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A Rotating Cutting Tool to Remove Hard Cemented Deposits in Heart Blood Vessels without Damaging Soft Vessel Walls
A Rotating Cutting Tool to Remove Hard Cemented Deposits in Heart Blood Vessels without Damaging Soft Vessel Walls M. Nakao (2)', K. Tsuchiya', W. Maeda', D. lijima2
' Department of Engineering Synthesis, School of Engineering, The University of Tokyo, Japan 2
Nan0 Corporation, Tokyo, Japan
Abstract A rotating cutting tool was developed to remove hard cemented deposits in the heart blood vessels without damaging the soft vessel walls. The new tool has a "grater-like'' configuration which is made of anodized aluminum with 20pm high micro-blades on a 2mm diameter tip, and it rotates at 200,000 rpm underwater. An evaluation test demonstrated the feasibility of the new tool by cutting the hard shell, and not the soft white, of a hardboiled egg. The water pressure forms a hydrodynamic film around the tool tip to press down the soft tissue, protecting it from any unwanted cuts. Keywords: cutting, ceramic, grinding
1 INTRODUCTION A medical procedure, percutaneous coronary intervention (PCI) treats plugged or narrowed coronary arteries from the inside by inserting a catheter from the thigh or arm through a blood vessel until it reaches the heart. The catheter for this procedure has special tips like, balloon, stent, DCA (directional coronary atherectomy), PTCRA (percutaneous transluminal coronary rotational atherectomy), and so on. The first two expands the narrowed coronary artery, and the later two removes deposits like atheroma or plaque. Of the later two atherectomy procedures, DCA scratches off the soft atheroma with a low speed linear tool, whereas PTCRA grinds off the hard atheroma with a high speed rotating tool. The de facto standard of PTCRA is "Rotablator (Boston Scientific Corporation)" which has a grinding wheel with diameter about 2mm with diamond grains of diameters in 10s of micro-meters. This tool rotates at 200,000rpm to grind off the calcified atheroma as shown in Figure 1. Invented in 1988 by Auth et al. [I], it was clinically tested in the same year by Bertrand et al. [2]. The tool is the only one to remove calcified atheroma, and for example, it has been used in about 5% of all the around 100,000 PCI surgeries in Japan in 2004. This PTCRA, however, has its own problems; the diamond grains grinding tool (we call "a diamond tool" in this paper) can penetrate the vessel wall or the grains can fall off the tool and plug the vessel, causing death. The authors developed an alternative design solution that is free of such accidents. The solution is a rotating grater-like cutting tool which has a dense set of micro-blades arranged like a grater. The ultimate evaluation test for the new tool should be performed to a coronary artery of a living human. On the leg bones which are known not to cause fatal accidents, the grinding was verified with bones of dead human specimen [3]. The possibility of aforementioned fatal accidents places ethical constraints on us from even performing animal experiments to extracting atheroma samples. For our study, we set an evaluation criterion of cutting the hard shell, which resembles atheroma, of a hardboiled egg without damaging the soft egg white which resembles vessel wall. Elasto-Hydrodynamic Lubrication (EHL) was applied to a
mechanism for preventing damage to the vessel wall. EHL produces elastic deformation to the soft vessel wall with the hydrodynamic pressure in the blood induced by the tool rotation. Studies [4] [5] have shown the effect of having machining fluid between the tool and the work-piece, however, in terms of how it helps cutting the work-piece and not of how it prevents cutting.
2 DESIGN OF ROTATING GRATER-LIKE CUTTING TOOL The following functional requirements apply to the PTCRA procedure: Remove hard cemented deposits, i.e. calcified atheroma. FR2: Do not damage the soft vessel walls. FR3: Do not let tool tips wear or fall off. FR4: Make chips of 10pm or less, or capture them. FR5: Finish cutting the calcified atheroma within 30 seconds or less. FR6: Be visible with X-ray. FR7: Stop the cutting upon excessive cutting force. FR8: Do not produce any side-effects induced by temperature rise or cavitations from the rotation. F R I and FR 2 require a tradeoff. Generally speaking, a tool FRI:
Figure 1: Schematic of PTCRA.
that effectively cuts the hard ceramic will damage the soft tissue as well. These two FR's are called "differential cutting" requirements which the current Rotablator roughly satisfies. There have been reports, however, of accidents that penetrated the vessel walls. Such accidents so far have made about 1% of all surgeries and some fatal [6]. FR3 and FR4 are necessary for preventing wear particles or cutting chips from clogging smaller blood vessels downstream. The tool in this case enters the coronary artery from upstream and moves downstream as it makes the cuts (Figure 1); thus, failing to capture the large particles or chips can clog narrow blood vessels downstream. Particles of size 10pm or less, however, are the same as the red cell and are assumed to get buried in the blood vessel wall without stopping the blood flow. A paper [ I ] reported an animal experiment that Rotablator produced particles, of which over 90% measured smaller than the red cell. Another paper [6], however, reported a half population of particles which are larger than the red cell, approximately 20% of clinical surgeries resulting in temporary slowdown of the blood flow, and about 1% resulting in myocardial infarction. The procedure temporarily stops the blood flow until the tool completes the cutting, thus, we have the 30 seconds requirement of FR5. On an average, the narrow section of a blood vessel is about 5mm long and the procedure widens the diameter from 0.8mm to 1.6mm. The required removal rate is therefore, 0.25mm3/sec. In our experiment, we cut a flat work-piece surface with a round tool on the one-fifth cutting area of the actual surgeries; the removal rate requirement thus interprets to 0.05mm3/sec. A metal tool meets the requirement of observing the cutting process with an X-ray image (FR6). The operator pushes the tool by hand, and can immediately stop the procedure upon feeling a large reaction force of I N or more (FR7). Many surgeons in Japan believe the no side-effect requirement FR8 is met, however, some phenomena persist. In fact, cavitations of diameter O.lmm have been observed as well as 7.7degrees Celsius temperature rise after rotating a tool for 40seconds at 150,000rpm [6]. The authors developed a design solution, a rotating grater-like cutting tool (we call "a grater tool" in this paper) that satisfies the set of FRs. The cutting points of the grater are convex bulges produced by pressing a single point punch against the side of a metal material. As Figure 2 shows, the punch opened dents where it was pressed against, while the material was pushed out towards the front side to produce 5pm or higher bulges. These bulges were placed at about a 0.2mm pitch served as tool tips on a 3mm long cylindrical or bullet shaped material with a 2mm diameter. We used a lathe for small diameter parts (Nano Corporation, MTS3) for producing this tool. For material we selected brass (Zn 35%), tool steel (C 1.O%), and aluminum (A1 99.5%), and after producing the bulges, plated the brass tool with a 10pm thick nickel film, quenched the tool steel under vacuum, and anodized the aluminum tool with a 5pm thick oxidization film.
3
EXPERIMENTAL METHODS
Table 1 on next page lists the 14 kinds of the tools we used. We prepared three different heights of micro-blades of the grater tools; 5, 20, and 30pm (#I to #7). For the 20pm high tools, we prepared two types of micro-blade orientations, one where the bulge leads to the dent (bulge first, #3), and the other with the dent leads to the bulge (dent first, #2). The 5pm high micro-blade tools had two different pitches; one with the blades placed every 0.2mm (#4) and the other
Figure 2: Fabrication of the grater tool.
Figure 3: Shapes of the cut surfaces.
Figure 4: Measured cutting force. type with a O.lmm pitch (#5). The reference diamond tools had diamond grains adhered to a cylindrical or bullet shaped bars of diameter 2mm with nickel plating. We had 5 types with varied micro-blade heights of 7, 10, 15, 20, and 30pm ( # I 0 to #14). The 10pm high tool (#13) was one that had actually been used in surgeries. For our study, we cut the shell of hardboiled eggs to check if F R I and FR2 are met, that is, if the tool can cut the hard shell without damaging the soft egg white. Observing the cut surfaces (Figure 6 mentioned later) revealed that the human calcified atheroma and the eggshell were removed with the same mechanism of cutting, not one of melting or ploughing. We also tested Young's ratio of vessel walls to about 300kPa by measuring the outer diameter of a live pig blood vessel with laser, and also conducted a compression test of a thin piece of boiled egg white to find its Young's ratio to be 20 to 50kPa. These tests confirmed similar magnitudes of Young's ratio for vessel walls and egg white. A high speed dental turbine with a maximum speed of 200,000rpm turned the tool which was entirely submerged underwater with the work-piece egg. Observing the profile with a laser microscope allowed us to calculate the removal rate from the volume removed. Figure 3 shows the shapes cut with bullet type tools, one with the 10pm high diamond tool (#13) and the other with the 20pm high, bulge first grater tool (#3). Continuous cutting of 30 seconds produced these shapes. The shells were left with the bullet shape
Table 1: Used tools in the experiments.
Table 2: Experimental result (screening columns meet the FRs).
Figure 5: Surfaces of used tools. marks from the cutting, whereas the egg whites remained flat without being cut. The shells were cut by the grater (diamond) tool through within the first 5 (10) seconds, and the volume removed was 0.37 (0.87)mm3 at a rate of 0.074 (0.087)mm3/sec, which satisfied 0.05mm3/sec or more of FR5. We also measured the cutting force in the thrust direction using an electronic balance scale (AND Co., Ltd., EK300i). Figure 4 shows the cutting force during the cuts in Figure 3. The cutting force was about I N , producing enough strong reaction force to the operator's hand.
4
EXPERIMENTAL RESULTS AND DISCUSSIONS
Table 2 shows the experimental results. First for the eggshell, its removed rate is faster with higher micro-blades for both the grater tools ( # I , #3 and #5) and the diamond tools ( # I 0 to #14). We ran reference tests with only dents (#8) and one with just a cylindrical bar (#9); however they failed to remove the shell. These reference tests prove that only the water flow from rotation cannot cut the shell. The table also reveals that with the same 20pm high blades (#3 and # l l ) , the grater tools produce about a 60% smaller material removal rates compared to the diamond tools. Next, for the egg white, cutting with the grater tools didn't
take any visible scratches on the egg white, with aluminium and steel materials, when the micro-blades oriented to bulge first were 20pm high or shorter (#3, #4 and %). They meet both F R I and FR2. The grater tool with densely arranged 5pm high blades (#5), contrary to our expectation, slightly scratched the egg white. The diamond tools satisfied FR2 when the micro-blades were 10pm high or shorter ( # I 3 and #14), leaving no scratches on the egg white. Even using the satisfied tool (#13), the egg white was scratched when the process took place without water or when the rotor speed was dropped to 6,000rpm or less. This suggests that the water with hydrodynamic pressure from the tool rotation affects the mechanism of not cutting the soft tissue. Through the results, the micro-blade height is important in meeting FR2. For the diamond tool, arranging the micro-blades to a constant height with the same size grains is difficult as shown in Figure 5(c). In this aspect, the grater tools are easier to produce with a constant blade height. Our method of fabrication, for example, made the 20pm high micro-blades within +/- 2pm as shown in Figure 2(b). We then checked the damages on the micro-blades after the cuts. After 3 minutes of cutting, only the anodized aluminum types were free of wear (Figure 5(a)). This type satisfies FR3. The nickel plated brass and quenched steel types showed gradual wear on the bulges (Figure 5(b)).
Figure 7: Simulated pressures on grater tools. Table 3: Calculated gap heights (HminR) by Herrebrugh equations [7]. The diamond tools on the other hand showed some grains had fallen off (Figure 5(d)), for example, with a frequency of 1 for every 50. We also observed the cut surfaces with a scanning electron microscope. Figure 6(d) [6] shows the cut surface of calcified atheroma of a human's blood vessel with the typical diamond grains tool; Figure 6(a)(b)(c) show the almost similar surfaces with series of grooves cut by the rotating micro-blades of the grater tools (#3 and #6) and the diamond tool (#13). The fine chips, however, were washed away in the water flow; FR4 couldn't be evaluated. Now we analyze the mechanism of not damaging the soft vessel wall tissues with Elasto-Hydrodynamic Lubrication (EHL). We applied known theories [7] for the line contact between a cylinder and plane and the point contact between a sphere and plane. Table 3 shows the results. In case of line contact, when the work-piece is hard like an eggshell, the gap is 0.065pm (Table 3(a)), whereas when it is soft like the egg white, the gap is 10pm (Table 3(b)). These results quantitatively explain why the egg white is not cut when the micro-blades are 5 to 20pm high. Bernoulli's theorem predicts cavitations because the water flow induced by 135,000rpm rotation dynamically drops the pressure to -latm, causing the blood to boil. These bubbles shall narrow the gap to 0.93pm when filled with air (Table 3(c)), and may cause penetrating the vessel walls during surgeries. Figure 7 shows the simulation results for the micro-blade orientation of the grater tools. The egg white was deformed by the pressure distribution which had been calculated on condition that the egg white is flat. The bulge first has 50% higher positive pressure and 30% lower negative pressure than the dent first, preventing the contact of the white and the tool. The dents and its negative pressure may satisfy FR4; when the grater tool cuts plaster solidified in the tube, the chips, but with only a total 0.01mm3, were collected in the grater dents (Figure 8). As we showed above, the grater tool for PTCRA can satisfy F R I to FR8. The tool seems more promising compared to the diamond tool for FR2 (no damage to the vessel walls), FR3 (no wear on the tool tip), and FR4 (capturing the chips). We need to further optimize this grater tool design.
5 CONCLUSION A new PTCRA was designed and prototyped for removing calcified atheroma from coronary arteries. Our design has grater-like micro-blades instead of diamond grains. The prototypes were evaluated by cutting the shell of a hardboiled egg in place for the calcified atheroma and the
Figure 8: Grater tool just after cutting plaster. egg white in place for the blood vessel walls. The evaluation tests demonstrated that the grater tool, when the blades are designed to 20pm in height, cuts the shell at 0.07mm3/sec without damaging the egg white at all. The hydrodynamic pressure in the water between the tool and the egg white pressed back the elastic egg white and prevented its contact with the tool.
REFERENCES Ahn, S.S., Auth, D., Marcus, D.R., et al, 1988, Removal of Focal Atheromatous Lesions by Angioscopically Guided High-speed Rotary Atherectomy, Preliminary Experimental Observation, J Vasc Surg 7:292. Bertrand, M.E., Fourrier, J.L., Dietz, U., et al, 1990, European Experience with Percutaneous Transluminal Coronary Rotational Ablation, Immediate Results (abstr.), Circulation, 82/l I I: 310. Mitsuishi, M., Warisawa, S., Sugita, N., 2004, Determination of the Machining Characteristics of a Biomaterial Using a Machine Tool Designed for Total Knee Arthroplasty, Annals of the CIRP, 53/1:107-112. Komanduri, R., Lucca, D.A., Tani, Y., 1997, Technological Advances in Fine Abrasive Processes, Annals of the CIRP, 46/2:545-596. Evans, C.J., Paul, E., Dornfeld, D., Lucca, D.A., Byrne, G., Tricard, M., Klocke, F., Dambon, O., Mullany, B.A., 2003, Material Removal Mechanisms in Lapping and Polishing, Annals of the CIRP, 52/2:611-633. Topol, E.J., Serruys, P.W. (Edition), 1994, Current Review of lntervantional Cardiology (Chapter 10: Percutaneous Transluminal Coronary Rotational Angioplasty), Current Medicine, Philadelphia, USA, 10.1-10.26. Herreburgh, K., 1968, J. Lub. Tech., Trans. ASME, 90:262.
' Department of Engineering Synthesis, School of Engineering, The University of Tokyo, Japan 2
Nan0 Corporation, Tokyo, Japan
Abstract A rotating cutting tool was developed to remove hard cemented deposits in the heart blood vessels without damaging the soft vessel walls. The new tool has a "grater-like'' configuration which is made of anodized aluminum with 20pm high micro-blades on a 2mm diameter tip, and it rotates at 200,000 rpm underwater. An evaluation test demonstrated the feasibility of the new tool by cutting the hard shell, and not the soft white, of a hardboiled egg. The water pressure forms a hydrodynamic film around the tool tip to press down the soft tissue, protecting it from any unwanted cuts. Keywords: cutting, ceramic, grinding
1 INTRODUCTION A medical procedure, percutaneous coronary intervention (PCI) treats plugged or narrowed coronary arteries from the inside by inserting a catheter from the thigh or arm through a blood vessel until it reaches the heart. The catheter for this procedure has special tips like, balloon, stent, DCA (directional coronary atherectomy), PTCRA (percutaneous transluminal coronary rotational atherectomy), and so on. The first two expands the narrowed coronary artery, and the later two removes deposits like atheroma or plaque. Of the later two atherectomy procedures, DCA scratches off the soft atheroma with a low speed linear tool, whereas PTCRA grinds off the hard atheroma with a high speed rotating tool. The de facto standard of PTCRA is "Rotablator (Boston Scientific Corporation)" which has a grinding wheel with diameter about 2mm with diamond grains of diameters in 10s of micro-meters. This tool rotates at 200,000rpm to grind off the calcified atheroma as shown in Figure 1. Invented in 1988 by Auth et al. [I], it was clinically tested in the same year by Bertrand et al. [2]. The tool is the only one to remove calcified atheroma, and for example, it has been used in about 5% of all the around 100,000 PCI surgeries in Japan in 2004. This PTCRA, however, has its own problems; the diamond grains grinding tool (we call "a diamond tool" in this paper) can penetrate the vessel wall or the grains can fall off the tool and plug the vessel, causing death. The authors developed an alternative design solution that is free of such accidents. The solution is a rotating grater-like cutting tool which has a dense set of micro-blades arranged like a grater. The ultimate evaluation test for the new tool should be performed to a coronary artery of a living human. On the leg bones which are known not to cause fatal accidents, the grinding was verified with bones of dead human specimen [3]. The possibility of aforementioned fatal accidents places ethical constraints on us from even performing animal experiments to extracting atheroma samples. For our study, we set an evaluation criterion of cutting the hard shell, which resembles atheroma, of a hardboiled egg without damaging the soft egg white which resembles vessel wall. Elasto-Hydrodynamic Lubrication (EHL) was applied to a
mechanism for preventing damage to the vessel wall. EHL produces elastic deformation to the soft vessel wall with the hydrodynamic pressure in the blood induced by the tool rotation. Studies [4] [5] have shown the effect of having machining fluid between the tool and the work-piece, however, in terms of how it helps cutting the work-piece and not of how it prevents cutting.
2 DESIGN OF ROTATING GRATER-LIKE CUTTING TOOL The following functional requirements apply to the PTCRA procedure: Remove hard cemented deposits, i.e. calcified atheroma. FR2: Do not damage the soft vessel walls. FR3: Do not let tool tips wear or fall off. FR4: Make chips of 10pm or less, or capture them. FR5: Finish cutting the calcified atheroma within 30 seconds or less. FR6: Be visible with X-ray. FR7: Stop the cutting upon excessive cutting force. FR8: Do not produce any side-effects induced by temperature rise or cavitations from the rotation. F R I and FR 2 require a tradeoff. Generally speaking, a tool FRI:
Figure 1: Schematic of PTCRA.
that effectively cuts the hard ceramic will damage the soft tissue as well. These two FR's are called "differential cutting" requirements which the current Rotablator roughly satisfies. There have been reports, however, of accidents that penetrated the vessel walls. Such accidents so far have made about 1% of all surgeries and some fatal [6]. FR3 and FR4 are necessary for preventing wear particles or cutting chips from clogging smaller blood vessels downstream. The tool in this case enters the coronary artery from upstream and moves downstream as it makes the cuts (Figure 1); thus, failing to capture the large particles or chips can clog narrow blood vessels downstream. Particles of size 10pm or less, however, are the same as the red cell and are assumed to get buried in the blood vessel wall without stopping the blood flow. A paper [ I ] reported an animal experiment that Rotablator produced particles, of which over 90% measured smaller than the red cell. Another paper [6], however, reported a half population of particles which are larger than the red cell, approximately 20% of clinical surgeries resulting in temporary slowdown of the blood flow, and about 1% resulting in myocardial infarction. The procedure temporarily stops the blood flow until the tool completes the cutting, thus, we have the 30 seconds requirement of FR5. On an average, the narrow section of a blood vessel is about 5mm long and the procedure widens the diameter from 0.8mm to 1.6mm. The required removal rate is therefore, 0.25mm3/sec. In our experiment, we cut a flat work-piece surface with a round tool on the one-fifth cutting area of the actual surgeries; the removal rate requirement thus interprets to 0.05mm3/sec. A metal tool meets the requirement of observing the cutting process with an X-ray image (FR6). The operator pushes the tool by hand, and can immediately stop the procedure upon feeling a large reaction force of I N or more (FR7). Many surgeons in Japan believe the no side-effect requirement FR8 is met, however, some phenomena persist. In fact, cavitations of diameter O.lmm have been observed as well as 7.7degrees Celsius temperature rise after rotating a tool for 40seconds at 150,000rpm [6]. The authors developed a design solution, a rotating grater-like cutting tool (we call "a grater tool" in this paper) that satisfies the set of FRs. The cutting points of the grater are convex bulges produced by pressing a single point punch against the side of a metal material. As Figure 2 shows, the punch opened dents where it was pressed against, while the material was pushed out towards the front side to produce 5pm or higher bulges. These bulges were placed at about a 0.2mm pitch served as tool tips on a 3mm long cylindrical or bullet shaped material with a 2mm diameter. We used a lathe for small diameter parts (Nano Corporation, MTS3) for producing this tool. For material we selected brass (Zn 35%), tool steel (C 1.O%), and aluminum (A1 99.5%), and after producing the bulges, plated the brass tool with a 10pm thick nickel film, quenched the tool steel under vacuum, and anodized the aluminum tool with a 5pm thick oxidization film.
3
EXPERIMENTAL METHODS
Table 1 on next page lists the 14 kinds of the tools we used. We prepared three different heights of micro-blades of the grater tools; 5, 20, and 30pm (#I to #7). For the 20pm high tools, we prepared two types of micro-blade orientations, one where the bulge leads to the dent (bulge first, #3), and the other with the dent leads to the bulge (dent first, #2). The 5pm high micro-blade tools had two different pitches; one with the blades placed every 0.2mm (#4) and the other
Figure 2: Fabrication of the grater tool.
Figure 3: Shapes of the cut surfaces.
Figure 4: Measured cutting force. type with a O.lmm pitch (#5). The reference diamond tools had diamond grains adhered to a cylindrical or bullet shaped bars of diameter 2mm with nickel plating. We had 5 types with varied micro-blade heights of 7, 10, 15, 20, and 30pm ( # I 0 to #14). The 10pm high tool (#13) was one that had actually been used in surgeries. For our study, we cut the shell of hardboiled eggs to check if F R I and FR2 are met, that is, if the tool can cut the hard shell without damaging the soft egg white. Observing the cut surfaces (Figure 6 mentioned later) revealed that the human calcified atheroma and the eggshell were removed with the same mechanism of cutting, not one of melting or ploughing. We also tested Young's ratio of vessel walls to about 300kPa by measuring the outer diameter of a live pig blood vessel with laser, and also conducted a compression test of a thin piece of boiled egg white to find its Young's ratio to be 20 to 50kPa. These tests confirmed similar magnitudes of Young's ratio for vessel walls and egg white. A high speed dental turbine with a maximum speed of 200,000rpm turned the tool which was entirely submerged underwater with the work-piece egg. Observing the profile with a laser microscope allowed us to calculate the removal rate from the volume removed. Figure 3 shows the shapes cut with bullet type tools, one with the 10pm high diamond tool (#13) and the other with the 20pm high, bulge first grater tool (#3). Continuous cutting of 30 seconds produced these shapes. The shells were left with the bullet shape
Table 1: Used tools in the experiments.
Table 2: Experimental result (screening columns meet the FRs).
Figure 5: Surfaces of used tools. marks from the cutting, whereas the egg whites remained flat without being cut. The shells were cut by the grater (diamond) tool through within the first 5 (10) seconds, and the volume removed was 0.37 (0.87)mm3 at a rate of 0.074 (0.087)mm3/sec, which satisfied 0.05mm3/sec or more of FR5. We also measured the cutting force in the thrust direction using an electronic balance scale (AND Co., Ltd., EK300i). Figure 4 shows the cutting force during the cuts in Figure 3. The cutting force was about I N , producing enough strong reaction force to the operator's hand.
4
EXPERIMENTAL RESULTS AND DISCUSSIONS
Table 2 shows the experimental results. First for the eggshell, its removed rate is faster with higher micro-blades for both the grater tools ( # I , #3 and #5) and the diamond tools ( # I 0 to #14). We ran reference tests with only dents (#8) and one with just a cylindrical bar (#9); however they failed to remove the shell. These reference tests prove that only the water flow from rotation cannot cut the shell. The table also reveals that with the same 20pm high blades (#3 and # l l ) , the grater tools produce about a 60% smaller material removal rates compared to the diamond tools. Next, for the egg white, cutting with the grater tools didn't
take any visible scratches on the egg white, with aluminium and steel materials, when the micro-blades oriented to bulge first were 20pm high or shorter (#3, #4 and %). They meet both F R I and FR2. The grater tool with densely arranged 5pm high blades (#5), contrary to our expectation, slightly scratched the egg white. The diamond tools satisfied FR2 when the micro-blades were 10pm high or shorter ( # I 3 and #14), leaving no scratches on the egg white. Even using the satisfied tool (#13), the egg white was scratched when the process took place without water or when the rotor speed was dropped to 6,000rpm or less. This suggests that the water with hydrodynamic pressure from the tool rotation affects the mechanism of not cutting the soft tissue. Through the results, the micro-blade height is important in meeting FR2. For the diamond tool, arranging the micro-blades to a constant height with the same size grains is difficult as shown in Figure 5(c). In this aspect, the grater tools are easier to produce with a constant blade height. Our method of fabrication, for example, made the 20pm high micro-blades within +/- 2pm as shown in Figure 2(b). We then checked the damages on the micro-blades after the cuts. After 3 minutes of cutting, only the anodized aluminum types were free of wear (Figure 5(a)). This type satisfies FR3. The nickel plated brass and quenched steel types showed gradual wear on the bulges (Figure 5(b)).
Figure 7: Simulated pressures on grater tools. Table 3: Calculated gap heights (HminR) by Herrebrugh equations [7]. The diamond tools on the other hand showed some grains had fallen off (Figure 5(d)), for example, with a frequency of 1 for every 50. We also observed the cut surfaces with a scanning electron microscope. Figure 6(d) [6] shows the cut surface of calcified atheroma of a human's blood vessel with the typical diamond grains tool; Figure 6(a)(b)(c) show the almost similar surfaces with series of grooves cut by the rotating micro-blades of the grater tools (#3 and #6) and the diamond tool (#13). The fine chips, however, were washed away in the water flow; FR4 couldn't be evaluated. Now we analyze the mechanism of not damaging the soft vessel wall tissues with Elasto-Hydrodynamic Lubrication (EHL). We applied known theories [7] for the line contact between a cylinder and plane and the point contact between a sphere and plane. Table 3 shows the results. In case of line contact, when the work-piece is hard like an eggshell, the gap is 0.065pm (Table 3(a)), whereas when it is soft like the egg white, the gap is 10pm (Table 3(b)). These results quantitatively explain why the egg white is not cut when the micro-blades are 5 to 20pm high. Bernoulli's theorem predicts cavitations because the water flow induced by 135,000rpm rotation dynamically drops the pressure to -latm, causing the blood to boil. These bubbles shall narrow the gap to 0.93pm when filled with air (Table 3(c)), and may cause penetrating the vessel walls during surgeries. Figure 7 shows the simulation results for the micro-blade orientation of the grater tools. The egg white was deformed by the pressure distribution which had been calculated on condition that the egg white is flat. The bulge first has 50% higher positive pressure and 30% lower negative pressure than the dent first, preventing the contact of the white and the tool. The dents and its negative pressure may satisfy FR4; when the grater tool cuts plaster solidified in the tube, the chips, but with only a total 0.01mm3, were collected in the grater dents (Figure 8). As we showed above, the grater tool for PTCRA can satisfy F R I to FR8. The tool seems more promising compared to the diamond tool for FR2 (no damage to the vessel walls), FR3 (no wear on the tool tip), and FR4 (capturing the chips). We need to further optimize this grater tool design.
5 CONCLUSION A new PTCRA was designed and prototyped for removing calcified atheroma from coronary arteries. Our design has grater-like micro-blades instead of diamond grains. The prototypes were evaluated by cutting the shell of a hardboiled egg in place for the calcified atheroma and the
Figure 8: Grater tool just after cutting plaster. egg white in place for the blood vessel walls. The evaluation tests demonstrated that the grater tool, when the blades are designed to 20pm in height, cuts the shell at 0.07mm3/sec without damaging the egg white at all. The hydrodynamic pressure in the water between the tool and the egg white pressed back the elastic egg white and prevented its contact with the tool.
REFERENCES Ahn, S.S., Auth, D., Marcus, D.R., et al, 1988, Removal of Focal Atheromatous Lesions by Angioscopically Guided High-speed Rotary Atherectomy, Preliminary Experimental Observation, J Vasc Surg 7:292. Bertrand, M.E., Fourrier, J.L., Dietz, U., et al, 1990, European Experience with Percutaneous Transluminal Coronary Rotational Ablation, Immediate Results (abstr.), Circulation, 82/l I I: 310. Mitsuishi, M., Warisawa, S., Sugita, N., 2004, Determination of the Machining Characteristics of a Biomaterial Using a Machine Tool Designed for Total Knee Arthroplasty, Annals of the CIRP, 53/1:107-112. Komanduri, R., Lucca, D.A., Tani, Y., 1997, Technological Advances in Fine Abrasive Processes, Annals of the CIRP, 46/2:545-596. Evans, C.J., Paul, E., Dornfeld, D., Lucca, D.A., Byrne, G., Tricard, M., Klocke, F., Dambon, O., Mullany, B.A., 2003, Material Removal Mechanisms in Lapping and Polishing, Annals of the CIRP, 52/2:611-633. Topol, E.J., Serruys, P.W. (Edition), 1994, Current Review of lntervantional Cardiology (Chapter 10: Percutaneous Transluminal Coronary Rotational Angioplasty), Current Medicine, Philadelphia, USA, 10.1-10.26. Herreburgh, K., 1968, J. Lub. Tech., Trans. ASME, 90:262.