J. Biomedical Science and Engineering, 2010, 3, 1108-1116 JBiSE
doi:10.4236/jbise.2010.311144 Published Online November 2010 (http://www.SciRP.org/journal/jbise/).
Published Online November 2010 in SciRes. http://www.scirp.org/journal/jbise
Experimental and numerical investigation of orbital
atherectomy: absence of cavitation
Reza Ramazani-Rend1, Srikar Chelikani1, Ephraim M. Sparrow1, John P. Abraham2
1Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN, USA;
2School of Engineering, University of St. Thomas, St. Paul, MN, USA.
Email: jpabraham@stthomas.edu
Received 30 September 2010; revised 18 October 2010; accepted 20 October 2010.
A definitive, quantitative investigation has been per-
formed to determine whether orbital atherectomy
gives rise to cavitation. The investigation encom-
passed a synergistic interaction between in vitro ex-
perimentation and numerical simulation. The ex-
perimentation was performed in two independent
fluid environments: 1) a transparent tube having a
diameter similar to that of the superficial femoral
artery and 2) a large, fluid-filled, open-topped con-
tainer. All of the experimental and simulation work
was based on the geometric model of the Diamond-
back 360 atherectomy device (Cardiovascular Sys-
tems, Inc., St. Paul, MN). Rotational speeds ranged
from 80,000 to 214,000 rpm. The presence or absence
of cavitation in the experiments was assessed by
means of high-speed photography. The photographic
images clearly display the fact that there was no
cavitation. Flow visualization revealed the presence
of fluid flows driven by pressure gradients created by
the geometry of the rotating crown. The numerical
simulations encompassed the fluid environments and
the operating conditions of the experiments. The key
result of the numerical simulation is that the mini-
mum fluid pressure due to the rotational motion was
approximately 50 times greater than the saturation
vapor pressure of the fluid. Since the onset of cavita-
tion requires that the fluid pressure falls below the
saturation vapor pressure, the computational out-
come strongly supports the experimental findings.
Keywords: Orbital Atherectomy; Cavitation; Arterial
Disease; Numerical Simulation
There are currently four types of in vivo devices for the
debulking of plaque. Among these, two are based on
abrasive removal of the plaque while the others utilize a
shaving technique. All four of the techniques make use
of rotation to create relative motion between the abra-
sive-debulking surface and the plaque proper. The pres-
ence of rotation gives rise to radial pressure variations
across the cross section of the artery being debulked.
These pressure variations could lead to the creation of
cavitation bubbles if the local pressure were to fall be-
low the saturation vapor pressure of the fluid.
The issue of cavitation is one that requires careful
consideration in that collapsing cavitation bubbles are
known to do damage to adjacent bounding materials [1-8].
In particular, if cavitation bubbles were to collapse in a
blood vessel, substantial tissue damage might occur.
There is an extensive literature on cavitation, its crea-
tion, and its ramifications. Numerous means exist by
which cavitation bubbles may be created in addition to
fluid rotation. Indeed, any flowing liquid which sustains
a substantial pressure drop may cavitate. The subject is
so extensive and the literature so numerous that multiple
reviews have appeared over the years. For example, in
chronological order, reviews and state-of-the-art assess-
ments have been published by [9-12]. A number of pa-
pers have been concerned with the interaction of cavita-
tion and the human body. In one major category, cavita-
tion is caused by imposed ultrasound [13-16]. In another
category, separation of flow passing through artificial
heart valves has been identified as a cause of cavitation
Of special relevance are papers which deal with cavi-
tation in blood flows. In [20], an analytical study was
performed to compare the life cycle of a single bubble in
blood and in water, from which it was concluded that
bubble collapse in blood is more violent than that in wa-
ter. For a wide range of initial bubble radii, collapse
times were calculated, and it was found that the collapse
time was approximately 100 µs times the initial bubble
radius in mm. Other studies that encompassed in vivo
and in vitro experiments are reported in [21-24]. The
first of these, which included both clinical studies and
R. Ramazani-Rend et al. / J. Biomedical Science and Engineering 3 (2010) 1108-1116
Copyright © 2010 SciRes. JBiSE
benchtop tests, purports to have identified the presence
of cavitation bubbles caused by orbital atherectomy. On
the other hand, the second article, a totally clinical study,
made little mention of cavitation. The last two articles
dealt with bubbles created by ultrasound and were fo-
cused on damage due to bubble collapse within simu-
lated blood vessels.
Attention will now be turned to a discussion of what
appears to be the only report of the creation of cavitation
bubbles by rotational atherectomy [22]. There are sev-
eral reasons why the observations reported in this refer-
ence may be viewed with some uncertainty. First, the
time of bubble collapse reported there is on the order of
minutes. Since the involved bubbles had radii on the
order of 100 µm, the collapse time should have been in
the range of 10 µs according to the aforementioned
findings of [21]. Figure 1 of [22] shows that the radial
distribution of the bubbles is the same both adjacent to
the rotating crown and adjacent to the shaft to which the
crown is affixed. However, the rotational velocities at
the surface of the crown and at the surface of the bare
shaft appear to differ by an order of magnitude, thereby
giving rise to very different pressures adjacent to the
bare shaft and the crown. In this light, it is difficult to
justify comparable distributions of bubbles adjacent to
the crown and the bare shaft. Photographs of the bubble
field presented in Figures 6 and 7 of [22] show the pres-
ence of bubbles in regions distal to the crown that do not
contain a rotating device. The mechanism for the crea-
tion of the latter-named bubbles is, therefore, unclear.
2.1. The Orbital Atherectomy Device
The specific orbital atherectomy device that has moti-
vated this investigation is displayed in Figure 1. The
special feature of this device is that the crown which
functions as a sanding surface is positioned eccentrically
Figure 1. Photograph of the rotating atherectomy device that
motivated the present investigation (Diamondback 360, Car-
diovascular Systems, Inc., St. Paul, MN).
on the shaft. This off-center positioning creates a secon-
dary motion in addition to the main rotational motion of
the shaft. The secondary motion is a precession. It has
the virtue of following the contour of the surface of the
plaque even as the plaque is removed and the lumen is
enlarged. The primary application of the device is for the
treatment of peripheral artery disease (PAD). This device
has been operated in a large number of clinical settings
but no reports have been received which suggest the
presence of cavitation. On the other hand, the findings
reported by [22] are disquieting and justify a careful
evaluation of the cavitation issue.
2.2. The in Vitro Experimental Models
The experiments were performed in two in vitro envi-
ronments. The first is a large open-topped glass con-
tainer having a diameter of 80 mm (3.1 in.) and a height
of approximately 80 mm (3.1 in.). A schematic diagram
of this environment with the rotating atherectomy device
in place is exhibited in Figure 2. As pictured in the fig-
ure, in this model the crown is a symmetric widening of
the shaft. The shaft diameter is 1.1 mm (0.043 in.). Both
symmetric and asymmetric crowns of various dimen-
sions were used in the experiments. Rotational speeds of
the shaft were varied between 80,000 and 214,000 rpm.
The second experimental environment is a horizontal
circular glass tube having a diameter of 6 mm (0.24 in.)
and a length of approximately 25 cm (10 in.). This setup
is pictured schematically in Figure 3. A throughflow
was superimposed on the rotational motion of the ath-
erectomy device. The use of a throughflow was moti-
vated by the in vivo situation wherein blood and a lubri-
cant co-flowed through the artery being debulked.
Figure 2. Diagram of the experimental setup with the atherec-
tomy device situated in the vertical orientation in a large water
R. Ramazani-Rend et al. / J. Biomedical Science and Engineering 3 (2010) 1108-1116
Copyright © 2010 SciRes. JBiSE
Figure 3. Diagram of the experimental setup for the atherec-
tomy device situated in a tube with superimposed throughflow.
A number of different shaft and crown arrangements
were used in this setup. Both symmetric and asymmetric
crowns of different dimensions were employed in con-
junction with both rigid and flexible shafts. Once again,
the rotational speeds were varied between 80,000 and
214,000 rpm.
The results of these experiments were obtained opti-
cally, both by human observation and by high-speed
photography. For the latter, information could be col-
lected at the rate of 10,000 frames/sec. To enable flow
visualization, air bubbles were injected in some of the
experiments. In addition, in certain experiments, sand
was dispersed throughout the water bath in order to pro-
mote the nucleation of bubbles. The sand was selected to
have a distribution of particle sizes that closely mirrored
that of plaque that is debulked by the atherectomy device.
Figure 4 presents the particle-size distributions of both
the sand and the debulked plaque.
2.3. The Numerical-Simulation Models
The simulation models were chosen to reflect the ex-
perimental work. For the vertical orientation of the ath-
erectomy device, the simulation model was a true rendi-
tion of the experimental setup shown in Figure 2 with an
axisymmetric crown. For the horizontal tube situation, a
larger variety of cases was modeled, including two dif-
ferent tube diameters and several sizes of axisymmetric
crowns. The model for the horizontal-tube situation also
took throughflow into account. The use of an axisym-
metric model enabled the problem to be modeled as
The numerical work was performed for both the fluid
properties of blood and of water. For the former, results
Figure 4. Particle size distributions for the
dispersed sand and debulked plaque.
were obtained for both Newtonian and non-Newtonian
viscosities. For the latter, the power-law model
was used. Rotational speeds were varied
from 80,000 to 214,000 rpm.
2.3.1. Governing Equations for the Numerical
The high rotational speeds encountered with the use of
orbital atherectomy create a turbulent flow. Consequent-
ly, the governing equations must reflect this reality. Two
sets of equations are necessary for the description of the
flow: 1) the RANS form of the Navier-Stokes equations
and the equation of continuity and 2) the renormalized
group (RNG) theory k-ε turbulence model proposed by
Yakhot et al. (1994). In the model, steady-state condi-
tions and an incompressible fluid were assumed.
Mass conservation and RANS equations
i turb
iii i
xxx x
 
 
 
 
 (2)
RNG k-ε turbulence model
uk k
 
 
ii i
xx xk
 
 
 
 (4)
2.3.2. Numerical Details
The issue of solution accuracy was explored by numeri-
cal experimentation with regard to numbers of elements
and deployment of the elements. To this end, solutions
based on 150,000 and 230,000 elements were performed.
Since the result of greatest significance for this work is
the minimum rotation-induced pressure, this quantity
was used as the metric for comparison of the solutions
for different numbers of elements. For the aforemen-
tioned cases, the minimum pressure differed by less than
5%. The deployment of the elements was governed by the
requirement that the nodes nearest the bounding surfaces
satisfied the requirement that y+ < 5,
where y is the distance of the nearest node to the wall
R. Ramazani-Rend et al. / J. Biomedical Science and Engineering 3 (2010) 1108-1116
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3.1. Experimental Results
The experimental results consist of photographic evi-
dence that demonstrates the absence of cavitation. The
photographs shown here are selected frames from mo-
tion pictures recorded at 10,000 frames/second. The ob-
servations that were extracted from the photographs
corroborate human visual observations.
The first set of photographic results pertains to the ro-
tating atherectomy device deployed vertically in the
large water environment. The operational variables were
214,000 rpm and a concentric crown with a diameter of
2.5 mm, which is the largest of those used in the inves-
tigation and also is larger than those employed in prac-
tice. The choice of the largest crown and the highest ro-
tational speed was made to promote the onset of cavita-
tion. Figures 5(a)-(c) represent a succession of photo-
graphic recordings at two-second intervals. Careful in-
spection of the entire field of flow does not reveal the
presence of bubbles.
The next experimental results pertain to an eccentri-
cally mounted crown operating at 185,000 rpm. The in-
dividual photographs shown in Figures 6(a)-(c) were
selected to exhibit the eccentricity, and they also display
(a) (b) (c)
Figure 5. A sequence of photographs extracted at 2-second
intervals from high-speed photography for operation with a
concentric 2.5 mm crown rotating at 214,000 rpm.
(a) (b) (c)
Figure 6. A sequence of photographs extracted at 2-second
intervals from high-speed photography for operation with an
eccentric crown rotating at 185,000 rpm.
the absence of cavitation. It might be noted in the lower
part of the figure that there are small imperfections in the
glass through which the photographs were taken. These
imperfections are not cavitation bubbles.
To further explore the possibility of cavitation and to
display the pattern of fluid flow, a tracer medium, air
bubbles, was introduced into the fluid environment, and
the results are displayed in Figures 7(a)-(e). The left-
most of this grouping, photograph (a), illustrates the
physical situation prior to the injection of the air bubbles.
In photograph (b), the first appearance of air bubbles,
introduced through the sheath, is evident. With the pas-
sage of time, photographs (c)-(e), the bubbles are seen to
migrate longitudinally toward the rotating crown. This
direction of motion can be attributed to the fact that the
lowest pressure in the flow occurs at the location of the
crown. The pressure gradient that is therefore created
drives the bubble motion. It is noteworthy that the bub-
ble migration is arrested at the widest part of the crown.
At that location, the bubbles are seen to be flung radially
outward. In other experiments, not shown here, bubbles
were introduced below the crown, and an upward migra-
tion, once again driven by an axial pressure gradient,
was observed.
R. Ramazani-Rend et al. / J. Biomedical Science and Engineering 3 (2010) 1108-1116
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(a) (b) (c) (d) (e)
Figure 7. A sequence of photographs extracted at 2-second intervals from high-speed photography for operation with a
concentric 2.5 mm crown rotating at 214,000 rpm and with introduced air bubbles serving as a tracer.
3.2. Simulation Results
The first result to be presented from the numerical simu-
lations relates to the radial pressure variation created by
the rotating atherectomy device positioned in a tube.
That information is conveyed in Figure 8 where the
minimum pressure is plotted as a function of the rota-
tional speed. In addition, there is a reference line in the
figure which corresponds to the saturation vapor pres-
sure of water at 20oC. The working fluid for this simula-
tion is water, and the crown was the largest available
with an outer diameter of 2.5 mm. The diameter of the
simulated artery was 5 mm, and there was a throughflow
at the rate of 80 ml/min. The figure clearly shows that
the fluid pressures are much higher, by a factor of 50,
than the saturation vapor pressure. Since the onset of
cavitation requires that the fluid pressure be lower than
the saturation vapor pressure, cavitation is thereby pre-
cluded. This outcome offers strong support for the visual
photograph evidence. The information conveyed in Fig-
ure 8 is representative of a large number of simulations
for a 2.5 mm diameter crown and host tube diameters
which ranged from 3-6 mm. Furthermore, both blood
and water were simulated, and complementary simula-
tions were also performed in the large environment.
Properties for the water simulations were taken at 25oC
while blood properties were evaluated at 37oC. In every
case, the minimum pressures that were encountered were
at least as large as that shown in Figure 8. No combina-
tion of crown diameter, tube diameter, or fluid type led
to pressures within an order of magnitude of those re-
quired for cavitation.
To complement the information conveyed in Figure 8,
a color contour diagram showing the pressure distribu-
tion in the vicinity of the rotating crown is presented in
Figure 9. The figure consists of a main body which cor-
Figure 8. Variation of the minimum pressure caused by rota-
tion of the atherectomy device in a tube.
R. Ramazani-Rend et al. / J. Biomedical Science and Engineering 3 (2010) 1108-1116
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Figure 9. Variation of the pressure in the vicinity of the rotating atherectomy device positioned within a tube.
responds to an overall portrayal of the pressure field and
an inset which is focused on the immediate neighbor-
hood of the crown. Both parts of the figure show that the
major pressure variations are confined to the neighbor-
hood of the crown. Clearly, the lowest pressures are at-
tained at the very surface of the crown and, with in-
creasing distance from the surface, the pressure increases.
The inset is focused on the pressure distribution which is
most relevant to the issue of cavitation. The pressures
shown there are well above the saturation vapor pres-
To illustrate the pattern of fluid flow caused by the
rotating motion of the atherectomy device, a simulation
was performed with the device immersed in the large
environment. The results of the numerical work are pre-
sented in the vector diagram of Figure 10(a). The vector
directions coincide with the local flow directions. These
vectors show the presence of oppositely directed wall
jets moving along the surface of the crown which collide
at the apex of the crown. That collision leads to the crea-
tion of a radial jet emanating from the neighborhood of
the crown. The wall jets themselves result from longitu-
dinal pressure gradients. The pressure at the apex of the
crown is a minimum, whereas both the upstream and
downstream pressures are larger. A result similar to that
shown in Figure 10(a) was obtained from numerical
simulations in a tube. The flow patterns shown in the
figure are representative of flow patterns for all rotation
rates which were investigated.
A counterpart of Figure 10(a) is the display of
streamlines presented in Figure 10(b). The streamlines
are a reinforcement of the flow pattern that was describ-
ed in the discussion of the (a) part of the figure.
3.3. Verification of the Numerical Model
As a verification experiment for the numerical simula-
tions, fluid velocities were tracked by means of an in-
jected tracer medium. The tracer medium was injected
air bubbles, and the motion of the bubbles was observed
by means of a high-speed camera. The bubbles in ques-
tion are displayed in Figure 7. Particular attention was
focused on the flow field in the neighborhood of the
crown that was visible in Figures 10(a) and (b). The
investigated domain extended upstream of the crown
which is vertically above the crown as it is exhibited in
these figures. Velocities were deduced by carefully re-
cording the position of the moving bubble front observed
in consecutive frames of the high-speed recording. In
view of the size of the observed bubbles, it was possible
only to determine a local-average velocity over a finite
radial expanse. To confirm the accuracy of the visual
observations, five observers were individually involved
with the obtainment of the results. Each observer inde-
pendently measured the velocities, and those measure-
ments were averaged.
The experimental results obtained for the axial veloc-
ity are presented in Figure 11 where they are compared
with the radial distribution of the axial velocity taken
from the numerical simulation and represented by the
continuous curve. The horizontal lines represent the av-
erage velocities over a radial span whose dimension is
approximately equal to the size of the bubbles being
R. Ramazani-Rend et al. / J. Biomedical Science and Engineering 3 (2010) 1108-1116
Copyright © 2010 SciRes. JBiSE
traced. The solid horizontal line corresponds to the re-
sults of the numerical simulation while the dashed line
was taken from the visual observations. The spread in
the latter due to the several individual visual observa-
tions is too small to be seen in the figure.
Figure 10. (a) Vectors showing the direction of the flow adja-
cent to the crown of a rotating atherectomy device situated in a
water bath; (b) Streamlines of the flow adjacent to the crown of
a rotating atherectomy device situated in a water bath.
Comparison of the two horizontal lines indicates ex-
cellent agreement between the local-averaged velocities.
This level of agreement lends strong support to the va-
lidity of the numerical simulations.
Results were also obtained for the radial velocity in
the same region as that for which the axial velocity re-
sults have already been presented. Once again, the ex-
perimental results correspond to local-average velocities.
These results are presented in Figure 12. In that figure,
the continuous curve corresponds to the radial distribu-
tion of the radial velocity taken from the numerical
simulations. In addition, there is a pair of horizontal
lines which represent local-average radial velocities. The
solid horizontal line depicts the numerical results while
the dashed line corresponds to the observed bubble mo-
tion. The observations were made by visually tracking
bubbles which moved radially outward from the apex of
the crown. Bubbles were tracked to a radial distance of
2.5 mm from the crown. From the figure, it is seen that
Figure 11. Comparison of observed and calculated axial veloc-
ity results upstream of the rotating crown. The horizontal lines
represent local-averaged velocities, respectively from the nu-
merical simulations and from the visual observations.
Figure 12. Comparison of observed and calculated radial ve-
locity results at the apex of the rotating crown. The horizontal
lines represent local-averaged velocities, respectively from the
numerical simulations and from the visual observations.
R. Ramazani-Rend et al. / J. Biomedical Science and Engineering 3 (2010) 1108-1116
Copyright © 2010 SciRes. JBiSE
the experimental results reinforce those extracted from
the simulations. This observation, taken together with
what has been already found in Figure 11, strongly af-
firms the validity of the numerical model and its imple-
A synergistic approach involving in vitro laboratory ex-
perimentation and numerical simulation was employed
to determine whether an orbital atherectomy device can
cause cavitation. The importance of this issue relates to
the fact that bursting cavitation bubbles create powerful
jets which can erode surfaces on which they impinge. In
particular, if cavitation bubbles were to collapse in a
blood vessel, substantial tissue damage and hemolysis
might occur.
The only report of cavitation bubbles due to rotating
atherectomy appears to be that of [22]. While their find-
ings can be questioned on phenomenological grounds
(see Introduction), the importance of the cavitation issue
demands a quantitative investigation such as that under-
taken here.
The specific rotating atherectomy device employed by
Zolz, the Rotablator, reflects the limited availability of
such devices at the time of his study, the early 1990s.
The present investigation made use of a more contem-
porary device, the Diamondback 360. The main differ-
ence between these devices is that the crown of the
Diamonback 360 is positioned eccentrically on its shaft
whereas the crown of the Rotablator is positioned axi-
The experimental part of the present investigation was
performed in two different fluid environments: 1) a
transparent horizontal tube whose diameter was chosen
to model that of the superficial femoral artery and 2) a
large open-topped transparent container. Both eccentric
and axisymmetric crowns of various dimensions were
employed, as were rigid and flexible shafts. Rotational
speeds ranged between 80,000 and 214,000. High-speed
photography (10,000 frames per second) was used to
record the flow field. Water was the working fluid. Un-
der no conditions were cavitation bubbles in evidence.
Supplementary flow visualization experiments revealed
axial flows driven by pressure gradients created by the
difference in diameter between the shaft and the crown.
The numerical simulations reflected the operating
conditions of the experiments, but were limited to axi-
symmetric crowns and rigid shafts. The key finding of
the simulations is that the lowest rotation-induced pres-
sure in the fluid is about 50 times greater than the satura-
tion vapor pressure. Since the onset of cavitation re-
quires that the fluid pressure be below the vapor pressure,
the numerical results support the absence of cavitation.
The axial and radial flows observed in the flow visuali-
zation experiments were corroborated by the numerical
R. Ramazani-Rend, S. Chelikani, and J. P. Abraham gratefully ac-
knowledge support from Cardiovascular Systems, Inc. for this re-
Support of H. Birali Runesha and the Supercomputing Institute for
Digital Simulation & Advanced Computation at the University of
Minnesota is gratefully acknowledged.
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tion of a bubble confined inside a micro pseudoelastic
blood vessel and the corresponding vessel wall stresses.
International Journal of Solids and Structures, 44, 7197-
C turbulence model constants
k turbulent kinetic energy
p pressure
P production term
u velocity
x coordinate
y+ dimensional normal distance from wall
turbulence dissipation
dynamic viscosity
v kinematic viscosity
shear strain rate
Prandtl-number-like diffusion parameters
shear stress
i, j tensor indices
turb turbulent