- Email: [email protected]
Degradation mechanisms of the burrs in rotablation
archives of civil and mechanical engineering 19 (2019) 1–9
Available online at www.sciencedirect.com
ScienceDirect journal homepage: http://www.elsevier.com/locate/acme
Original Research Article
Degradation mechanisms of the burrs in rotablation Zbigniew Gronostajski a,*, Marcin Kaszuba a, Wojciech Zimoch b, Krzysztof Reczuch b a b
Wroclaw University of Science and Technology, Poland Wroclaw Medical University, Poland
article info
abstract
Article history:
Rotablation is a percuteneous coronary procedure dedicated for treatment of highly calcified
Received 22 June 2019
or fibrotic coronary lesions. This procedure allows plaque modification using a diamond
Received in revised form
coated burr rotating at high speed. The literature lacks information on the principles for
2 August 2019
selection of the tools for such a process which would ensure the best efficiency (speed of
Accepted 3 August 2019
removing the calcified or fibrotic plaques). The starting point for this is the knowledge of the
Available online
wear mechanisms in the case of such tools. The present study examines 7 burrs after different operation times. The following mechanisms were considered: pulling out, spalling,
Keywords:
abrasion and diamond grains sticking. Based on the performed investigations, it was
Burrs
established that the basic wear mechanism is progressive sticking of the atherosclerotic
Rotablation
plaques onto the burrs. In the first place, the burr's front becomes stuck over, yet this should
Wear
still not have an effect on the speed of the atherosclerotic plaque removal also scarce sticking on the side surface of the burr is observed. During further operation, successive
Tools
plagues are stuck onto the ones stuck earlier, causing a reduction of the speed of their removal and the necessity of the use of a new burr in order to continue the rotablation. © 2019 Politechnika Wroclawska. Published by Elsevier B.V. All rights reserved.
1.
Introduction
The term ‘‘grinding’’ can be defined as a process of abrasion. The removal of the material takes place by means of a sharp abrasive material placed on the sides or on the surface of the grinding wheels. The abrasion wear is a result of material loss mainly through a separation of the material's particles from the surface [1]. The efficiency of the tools used for the material removal through abrasion treatment depends mainly on the type of the
abrasive material, the size of its grain and the binding material. At present, a wide range of abrasion materials is applied for the treatment of various components, from economical such as silicon oxides, to super-materials, such as boron nitride or expensive diamond grains [2]. In the course of time, research has shown that no abrasion material can fulfill all the implicational requirements. The mechanical and physical properties of the given abrasion material make it suitable for a certain application, while not for another. There are merely certain guidelines in the aspect of the known materials. The selection of abrasive material depends on the type of the
* Corresponding author. E-mail address: [email protected] (Z. Gronostajski). https://doi.org/10.1016/j.acme.2019.08.006 1644-9665/© 2019 Politechnika Wroclawska. Published by Elsevier B.V. All rights reserved.
2
archives of civil and mechanical engineering 19 (2019) 1–9
Table 1 – Knoop Hardness range for various materials [2]. Abrasive materials Common glass Flint, quartz Zirconium oxide Hardened steels Tungsten carbide Aluminium oxide
Knoop hardness
Abrasive materials
Knoop hardness
350–500 800–1100 1000 700–1300 1800–2400 2000–3000
Titanium nitride Titanium carbide Silicon carbide Boron carbide Cubic boron nitride Diamond
2000 1800–3200 2100–3000 2800 4000–5000 7000–8000
material which is to be ground [3]. Hard materials, with high strength, such as alloy steel, high-speed steel, should be treated with aluminium oxides [4]. Low strength ductile materials, such as: bronze, aluminium, copper and other non-metallic materials, are best ground by material made of silicon carbide [5]. At present, the best abrasion material characterizing also in higher hardness is diamond [6], which due to its costs, is often replaced with cubic boron nitride with similar hardness [7]. The hardness of a material is the main factor determining the selection of the grit sizes; hard and brittle materials require finer grit sizes, whereas soft and plastic ones are best treated with bigger grit sizes. Also, there are certain recommendations concerning the bonding material. For the optimal efficiency, a harder binder is recommended for soft and easily penetrating materials, whereas soft types are ideal for harder materials [2]. The speed of material removal through grinding is determined by the speed of tool wear. In the case of engineering materials, the dominating mechanisms are: pulling out, spalling, abrasion and sticking over of the grains [8]. All of them can occur to various extents, depending on the process conditions, the treated material and the tool. The two very important grinding parameters are the speed [9] and the lubrication technique [10]. In the case of abrasion treatment of engineering materials, there is a sufficient number of publications helpful in the selection of the optimal tools (ensuring the highest process efficiency), e.g. according to the hardness of the particular abrasive material (Table1) [2]. In the case of other materials, especially biological materials, the literature provides no information on the methods of selecting the tool for their removal. Such knowledge would be highly useful [3]. One of the processes similar to typical grinding is rotablation. Percutaneous coronary intervention (PCI) is a non-surgical procedure which allows to treat narrowing in coronary arteries (vessels which supply blood to the heart)- situation responsible for coronary artery disease and myocardial infarction. Procedure may, in general, be divided into two basic stages. The first one is lesion preparation which includes various methods of plaque modification such as: balloon inflation, cutting balloon inflation, orbital and rotational atherectomy (rotablation). The aim of this stage is to make the plaque and vessel wall susceptible to final balloon predilatation, which restores original diameter of the vessel. Full balloon expansion during predilatatation allows trouble free vascular prosthesis (stent) implantation which is the second and final stage of PCI. Correct stent implantation is essential for favorable long term
Fig. 1 – Schematic view of plaque modification during rotablation. The burr, rotating at high speed (a), creates crackles in homogenous calcium ring (b). This relatively small modification disturbs plaque structure which allows to break apart solid calcium ring (e) and therefore ensures full balloon expansion during predilatation (d) [12].
PCI results. Stent under expansion is one strongest predictors of PCI failure. Presence of highly calcified or fibrotic plaques pose a challenge for PCI operators as they may not allow complete balloon expansion and need additional facilitating procedures before stent implantation. Rotablation allows plaque modification by introducing into a coronary artery a diamond coated burr rotating at 140–160 000 revolutions per minute [11]. The aim of the procedure is not to reduce the volume of the plaque by grinding, but to modify its surface to allow further complete balloon dilatation and optimal stent implantation [12]. Schematic view of plaque modification during rotablation is presented in Fig. 1. The material which is removed from the arteries during rotablation is mostly calcified atherosclerotic plaque. These plaques are formed as a result of a slow process of cholesterol penetration from the blood and its accumulation on the inner side of the artery walls [13]. Additionally, the composition of the atherosclerotic plaque includes morphotic blood elements as well as fibrin. The accumulating material causes hardening and stiffness of the arteries. A typical atherosclerotic plaque is built of a centrally located (lipid) core and a surrounding layer made of collagen fibres. During everyday practice PCI operators sometimes come across challenging, rotablation resistant lesions. This plaques are very hard and require long grinding process during rotablation. Occasionally during the procedure burr degradation process is so advanced that it loses its grinding properties and needs to
archives of civil and mechanical engineering 19 (2019) 1–9
3
Fig. 2 – Rotablation burr.
be exchanged. In some procedures multiple burrs have to be used due to its degradation [14]. Despite 30 years of experience with rotablation the mechanism responsible for burr wear off was not yet described. Moreover optimal timing of burr exchange is unknown and completely subjective, based solely on operators personal experience. Describing burr durability and its optimal exchange moment in objective fashion may result in reducing the number of burrs used, therefore decreasing the risk of procedural complications [15]. Recognition of burr degradation mechanisms is a starting point for further research leading to procedure optimization and increasing its safety and efficacy. For this purpose we examined multiple (new ones and used) rotablation burrs. The main aim of manuscript is to determination of degrada-
tion mechanisms of the burrs in rotablation. Recognition of burr degradation mechanisms will be a starting point for further research leading to procedure optimization and increasing its safety and efficacy - speed of removing the calcified or fibrotic plaques. For this purpose, rotablation burrs (new ones and used) were examined.
2.
Burr construction
The starting point of the investigations was the determination of the construction of a typical burr used for rotablation, based on an examination performed by means of a scanning electron microscope. Fig. 2 shows a typical image of a rotablation burr.
Fig. 3 – Cross section of a rotablation burr.
4
archives of civil and mechanical engineering 19 (2019) 1–9
Fig. 4 – Chemical composition of (a) matrix and (b) abrasive material.
Rotablation burr is a part of the Rotablator and RotaPRO rotational atherectomy systems (Boston Scientific, Marlborough, MA, USA). It is olive shaped with the front half covered with grinding material. The length-to-width ratio is around 2. Burr sizes range from 1,25–2,5 mm diameter. Inside the burr there is a channel allowing guidewire passage. The burr itself
is embed on a spring which allows effortless bending and adapting to vessels curvature (Fig. 3). For the purpose of a more precise analysis of its construction, the chemical composition was analyzed (Fig. 4). The chemical composition analysis showed that the burr core consists mainly of copper and zinc, whose amount proves that
5
archives of civil and mechanical engineering 19 (2019) 1–9
Table 2 – Data for the analyzed burrs. No. 1 2 3 4 5 6 7
Fig. 5 – Burr division into 7 areas for the analysis.
it is monophase brass a (Fig. 4 a). Such a material characterizes in high plasticity, which is very important at the moment of its introduction into the artery. Hardness of matrix equals 165 HV0,1. Even on brand new burrs multiple empty slots were visualized, most probably after diamonds which dropped out during the manufacturing process. What's interesting, the number of empty slots differed between brand new burrs suggesting varying grinding potential of each burr.
3.
Wear mechanism description
At first, it was suspected that the reduction in the intensity of removing the atherosclerotic plaques results, similarly to grinding used in mechanical processes through: pulling out of grains, spalling of grains, abrasion of grains as well as sticking. To check it, 7 burrs were examined after different operation times, including two new ones, in order to reveal the basic wear mechanisms. The first stage consisted in elaborating the appropriate test methodology, which would be best suitable for the determination of the amount of pullouts, as the SEM examinations showed a high amount of such pull-outs. The tests were performed by means of a scanning microscope with high vacuum Tescan Vega 3 and with an SE and BSE detector, and with the use of image analysis software. Test subject The analysis was performed on 7 burrs. The denotation and information concerning the analyzed tools have been presented in Table 2.
Operation time [s]
Diameter[mm]
New New 62 171 180 180 540
1,25 1,25 1,5 1,5 1,5 1,25 1,5
The wear analysis of the burrs was carried out in 7 areas on their length (Fig. 5). Two methods were applied, which were to determine the degree of wear. One consisted in counting the number of pull-outs, whereas the other used image analysis to determine the dark phase share in the particular areas, which corresponds to the amount of the diamond phase. Fig. 6 shows the number of spallings for burr no. 1, in the particular areas. A similar analysis was performed for the remaining burrs and the collective results are shown in Fig. 7. As we can see, no relation between the amount of pull-outs and the operation time or the position on the burr's length was observed. In the other method, for the determination of burr wear, an image analysis program was used, by means of which the percentage share of the dark phase was determined for the analyzed areas, identified as diamond grit (Fig. 8). Similarly to the pull-outs, a diagram of the dark phase share for all the examined burrs and in different areas was generated (Fig. 9). The diamond phase share changes from 30 to over 60%; it is not possible to relate the grit phase share to the rate of burr wear, as, e.g. burr no. 7, which had worked the maximum of 540 seconds and had its grit phase share at the level of about 50%, according to diagram no. 7, obtained the smallest number of spallings. The grit phase share can prove only the quality of the prepared burr as well as its potential in the removal of atherosclerotic plaques. And so, it can be suspected that burr no. 3, with the highest diamond phase share, at the level of 60%, had worked the shortest time, as it was the fastest to remove the atherosclerotic plaques. These results are not unequivocal, as the mechanical properties of the atherosclerotic plaques of every patient have different properties. The investigations performed so far have shown that, during the operation, diamond grains are not pulled out, and so, the further part of the research focused on verifying whether spalling or wearing (blunting) of the grains could be observed. The tool working surface analysis performed in respect of the possible geometrical/visual differences did not reveal any special differences in the look of the tools. The mean heights of the protruding blades are similar (Fig. 10). The last examined burrs' wearing mechanism (reduction of the speed of the calcified atherosclerotic plaque removal) was their sticking over with the calcified atherosclerotic plaques. To that end, unwashed burrs right after the operation, after 200 and 540 seconds, were compared (Fig. 11). Considering the look of a new burr (Fig. 3), one can see progressive sticking of the
6
archives of civil and mechanical engineering 19 (2019) 1–9
Fig. 6 – Burr no. 1, analysis results for the selected areas.
Fig. 7 – Number of pull-outs for burrs after different operation times and in different areas on their length.
archives of civil and mechanical engineering 19 (2019) 1–9
7
Fig. 8 – Burr no. 6, analysis results for the selected areas.
Fig. 9 – Share of the diamond grit dark phase for different working times and in different areas on its length.
atherosclerotic plaques onto the burrs. In the first place, after 200 seconds, the front of the burr is stuck over, which, however, should still not reduce the atherosclerotic plaque removal. Rare stick-overs are observed on the working surface of the burr. Further operation causes very intensive sticking of
atherosclerotic plaques onto the burr. Probably, the areas previously stuck over by the plaques are glued together with new ones, thus reducing the removal speed and causing the necessity of the use of a new burr in order to continue the rotablation procedure.
8
archives of civil and mechanical engineering 19 (2019) 1–9
Fig. 10 – Look of the tool working surfaces after different operation times (a) new (b) after 250 s. and (c) 540 s.
Fig. 11 – A burr after rotablation right after the operation, after (a) 200 s and (b) 540 s.
archives of civil and mechanical engineering 19 (2019) 1–9
4.
Conclusion
- The literature provides no information on the selection of burrs for the process of rotablation ensuring the highest process efficiency. - The starting point for the creation of tools with higher durability (ensuring the highest efficiency in removing the atherosclerotic plaques) is the knowledge of the burr wearing mechanisms. - The study examines 7 burrs after different operation times, including two new ones, in order to reveal the basic wear mechanisms, such as: pull-outs, spalling, abrasion, diamond grain stick-overs. - Initially, it was thought that reducing the intensity of the atherosclerotic plaque removal results, similarly to the grinding wheels used on mechanical treatment, from: pulling out of the grains (the dominating form of wear for all grinding wheels). The investigations showed that this process does not occur. - It was demonstrated that the diamond phase share changes in a wide range of 30 to over 60% and it is impossible to relate the amount of the grit phase share to the rate of the burr wear. The impact of share diamond phase on burr ensuring the highest efficiency in removing the atherosclerotic plaques is not straightforward as it is also connected with plaque composition which varies widely between patients. - The dominating mechanism lowering the rate of atherosclerotic plaque removal is the burrs being stuck over by these plaques. Considering the dominating mechanism of wear and the price of burrs, the possibility of their multiple use, for example after washing and sterilization, should be discussed.
references
[1] Z. Gronostajski, M. Kaszuba, M. Hawryluk, M. Zwierzchowski, A review of the degradation mechanisms of the hot forging tools, Arch. Civ. Mech. Eng. 14 (4) (2014) 528–539.
9
[2] S. Bhowmik, R. Naik, Selection of abrasive materials for manufacturing grinding wheels, Mater. Today Proc. 5 (2018) 2860–2864. [3] ASM Handbook, Volume 16 – Machining, 1989. [4] F.R.L. Dotto, P.R. Aguiar, F.A. Alexandre, L. Simões, W.N. Lopes, D.M. D'Addona, E.C. Bianchi, Acoustic image-based damage identification of oxide aluminum grinding wheel during the dressing operation, Procedia CIRP 79 (2019) 298–302. [5] X. Xi, T. Yu, W. Ding, J. Xu, Grinding of Ti2AlNb intermetallics using silicon carbide and alumina abrasive wheels: tool surface topology effect on grinding force and ground surface quality, Precis. Eng. 53 (2018) 134–145. [6] P.W. Leech, X.S. Li, Comparison of abrasive wear in diamond composites and WC-based coatings, Wear 271 (2011) 1244– 1251. [7] M. Winter, S. Ibbotson, S. Kara, C. Herrmann, Life, cycle assessment of cubic boron nitride grinding wheels, J. Clean. Prod. 107 (2015) 707–721. [8] A. Kawalec, A. Bazan, M. Krok, Influence of grinding wheel velocity on the wearof electroplated cBN grinding wheel, Mechanik 8-9 (2017) 690–692. [9] S. Ghosh, S. Paul, A. Chattopadhyay, Experimental investigations on grindability of bearing steel under high efficiency deep grinding (HEDG), Int. J. Abras. Technol. 2 (2009) 154–172. [10] H. Esmaeili, H. Adibi, S.M. Rezaei, An efficient strategy for grinding carbon fiber-reinforced silicon carbide composite using minimum quantity lubricant, Ceram. Int. 45 (2019) 10852–10864. [11] M.-H. Chianga, W.-L. Leea, C.-R. Tsaoa, W.-C. Changa, C.-S. Sua, T.-J. Liua, K.-W. Lianga, C.-T. Ting, The use and clinical outcomes of rotablation in challenging cases in the drugeluting stent era, J. Chinese Med. Assoc. 76 (2013) 71–77. [12] K. Sakakura, Y. Taniguchi, M. Matsumoto, H. Wada, S. Momomura, H. Fujita, How should we perform rotational atherectomy to an angulated calcified lesion? Int. Heart J. 57 (2016) 376–379. [13] Myer Kutz, Standard Handbook of Biomedical Engineering & Design, 2003. [14] P.C. Ho, T.M. Weatherby, M. Dunlap, Burr erosion in rotational ablation of metallic coronary stent: an electron microscopic study, J. Interv. Cardiol. 23 (3) (2010) 233–239. [15] Y. Zheng, Y. Liu, J.J. Pitre, J.L. Bull, H.S. Gurm, A.J. Shih, Computational fluid dynamics modeling of the burr orbital motion in rotational atherectomy with particle image velocimetry validation, Ann. Biomed. Eng. 46 (4) (2018) 567– 578.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: http://www.elsevier.com/locate/acme
Original Research Article
Degradation mechanisms of the burrs in rotablation Zbigniew Gronostajski a,*, Marcin Kaszuba a, Wojciech Zimoch b, Krzysztof Reczuch b a b
Wroclaw University of Science and Technology, Poland Wroclaw Medical University, Poland
article info
abstract
Article history:
Rotablation is a percuteneous coronary procedure dedicated for treatment of highly calcified
Received 22 June 2019
or fibrotic coronary lesions. This procedure allows plaque modification using a diamond
Received in revised form
coated burr rotating at high speed. The literature lacks information on the principles for
2 August 2019
selection of the tools for such a process which would ensure the best efficiency (speed of
Accepted 3 August 2019
removing the calcified or fibrotic plaques). The starting point for this is the knowledge of the
Available online
wear mechanisms in the case of such tools. The present study examines 7 burrs after different operation times. The following mechanisms were considered: pulling out, spalling,
Keywords:
abrasion and diamond grains sticking. Based on the performed investigations, it was
Burrs
established that the basic wear mechanism is progressive sticking of the atherosclerotic
Rotablation
plaques onto the burrs. In the first place, the burr's front becomes stuck over, yet this should
Wear
still not have an effect on the speed of the atherosclerotic plaque removal also scarce sticking on the side surface of the burr is observed. During further operation, successive
Tools
plagues are stuck onto the ones stuck earlier, causing a reduction of the speed of their removal and the necessity of the use of a new burr in order to continue the rotablation. © 2019 Politechnika Wroclawska. Published by Elsevier B.V. All rights reserved.
1.
Introduction
The term ‘‘grinding’’ can be defined as a process of abrasion. The removal of the material takes place by means of a sharp abrasive material placed on the sides or on the surface of the grinding wheels. The abrasion wear is a result of material loss mainly through a separation of the material's particles from the surface [1]. The efficiency of the tools used for the material removal through abrasion treatment depends mainly on the type of the
abrasive material, the size of its grain and the binding material. At present, a wide range of abrasion materials is applied for the treatment of various components, from economical such as silicon oxides, to super-materials, such as boron nitride or expensive diamond grains [2]. In the course of time, research has shown that no abrasion material can fulfill all the implicational requirements. The mechanical and physical properties of the given abrasion material make it suitable for a certain application, while not for another. There are merely certain guidelines in the aspect of the known materials. The selection of abrasive material depends on the type of the
* Corresponding author. E-mail address: [email protected] (Z. Gronostajski). https://doi.org/10.1016/j.acme.2019.08.006 1644-9665/© 2019 Politechnika Wroclawska. Published by Elsevier B.V. All rights reserved.
2
archives of civil and mechanical engineering 19 (2019) 1–9
Table 1 – Knoop Hardness range for various materials [2]. Abrasive materials Common glass Flint, quartz Zirconium oxide Hardened steels Tungsten carbide Aluminium oxide
Knoop hardness
Abrasive materials
Knoop hardness
350–500 800–1100 1000 700–1300 1800–2400 2000–3000
Titanium nitride Titanium carbide Silicon carbide Boron carbide Cubic boron nitride Diamond
2000 1800–3200 2100–3000 2800 4000–5000 7000–8000
material which is to be ground [3]. Hard materials, with high strength, such as alloy steel, high-speed steel, should be treated with aluminium oxides [4]. Low strength ductile materials, such as: bronze, aluminium, copper and other non-metallic materials, are best ground by material made of silicon carbide [5]. At present, the best abrasion material characterizing also in higher hardness is diamond [6], which due to its costs, is often replaced with cubic boron nitride with similar hardness [7]. The hardness of a material is the main factor determining the selection of the grit sizes; hard and brittle materials require finer grit sizes, whereas soft and plastic ones are best treated with bigger grit sizes. Also, there are certain recommendations concerning the bonding material. For the optimal efficiency, a harder binder is recommended for soft and easily penetrating materials, whereas soft types are ideal for harder materials [2]. The speed of material removal through grinding is determined by the speed of tool wear. In the case of engineering materials, the dominating mechanisms are: pulling out, spalling, abrasion and sticking over of the grains [8]. All of them can occur to various extents, depending on the process conditions, the treated material and the tool. The two very important grinding parameters are the speed [9] and the lubrication technique [10]. In the case of abrasion treatment of engineering materials, there is a sufficient number of publications helpful in the selection of the optimal tools (ensuring the highest process efficiency), e.g. according to the hardness of the particular abrasive material (Table1) [2]. In the case of other materials, especially biological materials, the literature provides no information on the methods of selecting the tool for their removal. Such knowledge would be highly useful [3]. One of the processes similar to typical grinding is rotablation. Percutaneous coronary intervention (PCI) is a non-surgical procedure which allows to treat narrowing in coronary arteries (vessels which supply blood to the heart)- situation responsible for coronary artery disease and myocardial infarction. Procedure may, in general, be divided into two basic stages. The first one is lesion preparation which includes various methods of plaque modification such as: balloon inflation, cutting balloon inflation, orbital and rotational atherectomy (rotablation). The aim of this stage is to make the plaque and vessel wall susceptible to final balloon predilatation, which restores original diameter of the vessel. Full balloon expansion during predilatatation allows trouble free vascular prosthesis (stent) implantation which is the second and final stage of PCI. Correct stent implantation is essential for favorable long term
Fig. 1 – Schematic view of plaque modification during rotablation. The burr, rotating at high speed (a), creates crackles in homogenous calcium ring (b). This relatively small modification disturbs plaque structure which allows to break apart solid calcium ring (e) and therefore ensures full balloon expansion during predilatation (d) [12].
PCI results. Stent under expansion is one strongest predictors of PCI failure. Presence of highly calcified or fibrotic plaques pose a challenge for PCI operators as they may not allow complete balloon expansion and need additional facilitating procedures before stent implantation. Rotablation allows plaque modification by introducing into a coronary artery a diamond coated burr rotating at 140–160 000 revolutions per minute [11]. The aim of the procedure is not to reduce the volume of the plaque by grinding, but to modify its surface to allow further complete balloon dilatation and optimal stent implantation [12]. Schematic view of plaque modification during rotablation is presented in Fig. 1. The material which is removed from the arteries during rotablation is mostly calcified atherosclerotic plaque. These plaques are formed as a result of a slow process of cholesterol penetration from the blood and its accumulation on the inner side of the artery walls [13]. Additionally, the composition of the atherosclerotic plaque includes morphotic blood elements as well as fibrin. The accumulating material causes hardening and stiffness of the arteries. A typical atherosclerotic plaque is built of a centrally located (lipid) core and a surrounding layer made of collagen fibres. During everyday practice PCI operators sometimes come across challenging, rotablation resistant lesions. This plaques are very hard and require long grinding process during rotablation. Occasionally during the procedure burr degradation process is so advanced that it loses its grinding properties and needs to
archives of civil and mechanical engineering 19 (2019) 1–9
3
Fig. 2 – Rotablation burr.
be exchanged. In some procedures multiple burrs have to be used due to its degradation [14]. Despite 30 years of experience with rotablation the mechanism responsible for burr wear off was not yet described. Moreover optimal timing of burr exchange is unknown and completely subjective, based solely on operators personal experience. Describing burr durability and its optimal exchange moment in objective fashion may result in reducing the number of burrs used, therefore decreasing the risk of procedural complications [15]. Recognition of burr degradation mechanisms is a starting point for further research leading to procedure optimization and increasing its safety and efficacy. For this purpose we examined multiple (new ones and used) rotablation burrs. The main aim of manuscript is to determination of degrada-
tion mechanisms of the burrs in rotablation. Recognition of burr degradation mechanisms will be a starting point for further research leading to procedure optimization and increasing its safety and efficacy - speed of removing the calcified or fibrotic plaques. For this purpose, rotablation burrs (new ones and used) were examined.
2.
Burr construction
The starting point of the investigations was the determination of the construction of a typical burr used for rotablation, based on an examination performed by means of a scanning electron microscope. Fig. 2 shows a typical image of a rotablation burr.
Fig. 3 – Cross section of a rotablation burr.
4
archives of civil and mechanical engineering 19 (2019) 1–9
Fig. 4 – Chemical composition of (a) matrix and (b) abrasive material.
Rotablation burr is a part of the Rotablator and RotaPRO rotational atherectomy systems (Boston Scientific, Marlborough, MA, USA). It is olive shaped with the front half covered with grinding material. The length-to-width ratio is around 2. Burr sizes range from 1,25–2,5 mm diameter. Inside the burr there is a channel allowing guidewire passage. The burr itself
is embed on a spring which allows effortless bending and adapting to vessels curvature (Fig. 3). For the purpose of a more precise analysis of its construction, the chemical composition was analyzed (Fig. 4). The chemical composition analysis showed that the burr core consists mainly of copper and zinc, whose amount proves that
5
archives of civil and mechanical engineering 19 (2019) 1–9
Table 2 – Data for the analyzed burrs. No. 1 2 3 4 5 6 7
Fig. 5 – Burr division into 7 areas for the analysis.
it is monophase brass a (Fig. 4 a). Such a material characterizes in high plasticity, which is very important at the moment of its introduction into the artery. Hardness of matrix equals 165 HV0,1. Even on brand new burrs multiple empty slots were visualized, most probably after diamonds which dropped out during the manufacturing process. What's interesting, the number of empty slots differed between brand new burrs suggesting varying grinding potential of each burr.
3.
Wear mechanism description
At first, it was suspected that the reduction in the intensity of removing the atherosclerotic plaques results, similarly to grinding used in mechanical processes through: pulling out of grains, spalling of grains, abrasion of grains as well as sticking. To check it, 7 burrs were examined after different operation times, including two new ones, in order to reveal the basic wear mechanisms. The first stage consisted in elaborating the appropriate test methodology, which would be best suitable for the determination of the amount of pullouts, as the SEM examinations showed a high amount of such pull-outs. The tests were performed by means of a scanning microscope with high vacuum Tescan Vega 3 and with an SE and BSE detector, and with the use of image analysis software. Test subject The analysis was performed on 7 burrs. The denotation and information concerning the analyzed tools have been presented in Table 2.
Operation time [s]
Diameter[mm]
New New 62 171 180 180 540
1,25 1,25 1,5 1,5 1,5 1,25 1,5
The wear analysis of the burrs was carried out in 7 areas on their length (Fig. 5). Two methods were applied, which were to determine the degree of wear. One consisted in counting the number of pull-outs, whereas the other used image analysis to determine the dark phase share in the particular areas, which corresponds to the amount of the diamond phase. Fig. 6 shows the number of spallings for burr no. 1, in the particular areas. A similar analysis was performed for the remaining burrs and the collective results are shown in Fig. 7. As we can see, no relation between the amount of pull-outs and the operation time or the position on the burr's length was observed. In the other method, for the determination of burr wear, an image analysis program was used, by means of which the percentage share of the dark phase was determined for the analyzed areas, identified as diamond grit (Fig. 8). Similarly to the pull-outs, a diagram of the dark phase share for all the examined burrs and in different areas was generated (Fig. 9). The diamond phase share changes from 30 to over 60%; it is not possible to relate the grit phase share to the rate of burr wear, as, e.g. burr no. 7, which had worked the maximum of 540 seconds and had its grit phase share at the level of about 50%, according to diagram no. 7, obtained the smallest number of spallings. The grit phase share can prove only the quality of the prepared burr as well as its potential in the removal of atherosclerotic plaques. And so, it can be suspected that burr no. 3, with the highest diamond phase share, at the level of 60%, had worked the shortest time, as it was the fastest to remove the atherosclerotic plaques. These results are not unequivocal, as the mechanical properties of the atherosclerotic plaques of every patient have different properties. The investigations performed so far have shown that, during the operation, diamond grains are not pulled out, and so, the further part of the research focused on verifying whether spalling or wearing (blunting) of the grains could be observed. The tool working surface analysis performed in respect of the possible geometrical/visual differences did not reveal any special differences in the look of the tools. The mean heights of the protruding blades are similar (Fig. 10). The last examined burrs' wearing mechanism (reduction of the speed of the calcified atherosclerotic plaque removal) was their sticking over with the calcified atherosclerotic plaques. To that end, unwashed burrs right after the operation, after 200 and 540 seconds, were compared (Fig. 11). Considering the look of a new burr (Fig. 3), one can see progressive sticking of the
6
archives of civil and mechanical engineering 19 (2019) 1–9
Fig. 6 – Burr no. 1, analysis results for the selected areas.
Fig. 7 – Number of pull-outs for burrs after different operation times and in different areas on their length.
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Fig. 8 – Burr no. 6, analysis results for the selected areas.
Fig. 9 – Share of the diamond grit dark phase for different working times and in different areas on its length.
atherosclerotic plaques onto the burrs. In the first place, after 200 seconds, the front of the burr is stuck over, which, however, should still not reduce the atherosclerotic plaque removal. Rare stick-overs are observed on the working surface of the burr. Further operation causes very intensive sticking of
atherosclerotic plaques onto the burr. Probably, the areas previously stuck over by the plaques are glued together with new ones, thus reducing the removal speed and causing the necessity of the use of a new burr in order to continue the rotablation procedure.
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Fig. 10 – Look of the tool working surfaces after different operation times (a) new (b) after 250 s. and (c) 540 s.
Fig. 11 – A burr after rotablation right after the operation, after (a) 200 s and (b) 540 s.
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4.
Conclusion
- The literature provides no information on the selection of burrs for the process of rotablation ensuring the highest process efficiency. - The starting point for the creation of tools with higher durability (ensuring the highest efficiency in removing the atherosclerotic plaques) is the knowledge of the burr wearing mechanisms. - The study examines 7 burrs after different operation times, including two new ones, in order to reveal the basic wear mechanisms, such as: pull-outs, spalling, abrasion, diamond grain stick-overs. - Initially, it was thought that reducing the intensity of the atherosclerotic plaque removal results, similarly to the grinding wheels used on mechanical treatment, from: pulling out of the grains (the dominating form of wear for all grinding wheels). The investigations showed that this process does not occur. - It was demonstrated that the diamond phase share changes in a wide range of 30 to over 60% and it is impossible to relate the amount of the grit phase share to the rate of the burr wear. The impact of share diamond phase on burr ensuring the highest efficiency in removing the atherosclerotic plaques is not straightforward as it is also connected with plaque composition which varies widely between patients. - The dominating mechanism lowering the rate of atherosclerotic plaque removal is the burrs being stuck over by these plaques. Considering the dominating mechanism of wear and the price of burrs, the possibility of their multiple use, for example after washing and sterilization, should be discussed.
references
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