1. Design of an End-effector to be Used in Conjunction with a
Dexterous Manipulator in Minimally-Invasive Surgery
Charles Dou, Dr. Kang Li
Abstract
Ultra high molecular weight polyethylene (UHMWPE) is
commonly used in hip implants due to its low friction,
biocompatibility, excellent wear resistance, and good
mechanical performance. However, over time, tiny UHMWPE
particles wear from the surface of the implant and enter the
surrounding tissue, causing an inflammatory response that
leads to deterioration of nearby bone and subsequently,
loosening of the implant [1]. Removing these micro to nano-
sized UHMWPE particles may reduce the rate of bone
degradation surrounding the joint implant. To do this, we
propose a minimally invasive procedure using a hollow stem
bur (end-effector) that grinds away hard lesions caused by
the immune response to UHMWPE and removes debris
through its hollow shaft. This 4.3-mm diameter bur is driven by
an embedded micro-motor and is housed within a long
flexible shaft (dexterous manipulator) whose orientation can
be controlled by the surgeon through the tension of the two
cables that run the length of the shaft. The instrument will
minimize bodily impact during surgery and extend the period
of time before another total hip arthroplasty (THA) is
necessary.
Design Theory
The criteria for the end-effector (EE) design are that it needs
to retract within the dexterous manipulator (DM), which has
an outer diameter of 5.99mm and an inner diameter of 4mm
while still being durable enough to withstand the shear
stresses associated with grinding against the lesion surface.
Additionally, it needs to be capable of extracting material
(UHMWPE particles and lesion tissue) at a reasonable rate. The
head of the EE is patterned so that any material that is ground
away will be channeled towards its center as the micro-motor
rotates. The design of the EE is similar to that of certain dental
burs, except it has a center shaft that is hollow. Its hollow shaft
allows for the extraction of polyethylene particles and other
debris when the EE is attached to a vacuum outside of the
body. The minimum thickness of the hollow shaft is 0.5mm,
which should be sufficient for withstanding the amount of
shear that the EE will likely encounter without deforming given
that the EE is made of a very hard material such as tungsten
carbide or titanium.
Design Process
First, all parts were designed in Solidworks 2011 including the
end-effector (EE), and the mold pieces. The dexterous
manipulator (DM), which was designed by Mike Kutzer and
Ryan Murphy’s group, was also modeled in Solidworks 2011.
An assembly was then made containing the end-effector and
dexterous manipulator.
Next, the mold that will be used to cast the first prototype of
the end-effector was produced in V-Flash® FTI resin, a UV
curable epoxy resin, by a V-Flash® 3D printer. After thorough
sanding, the two mold segments were pressed together to
form the final mold.
After creating the mold, the aim was to create a cast a low
melting point metal to see if the surface geometry would be
suitable for grinding.
Results/Conclusions
Although the molds were printed at 400% of the true scale,
the surface of the drilling end of the end-effector mold was
poorly preserved in the printed epoxy resin, with the sharp
edges of the drilling surface being extremely blunted or
almost indistinguishable in each of the three trials that were
done. This is most likely due to the accuracy limitations of the
printer that was used.
Due to these limitations I believe it would be wise to try
different method for creating the end-effector (EE) besides
casting using a mold that may produce parts with a higher
surface integrity at the millimeter scale. Direct Metal Laser
Sintering is one such option that we are currently exploring.
Design Validation
Ultimately, the goal of the project is to measure the material
removal rate of the end-effector in order to determine
whether the removal of the UHMWPE induced lesion could be
accomplished by a surgeon within a reasonable period of
time. This is dependent on two factors: force applied, and
speed of the micromotor. In order to determine what force
should be used, it is necessary to find the minimum force to
remove lesion at a reasonable rate without removing
significant amounts of bone. After finding the speeds of
known micromotors, a regular motor could be set up to rotate
at the same speed by adjusting the voltage applied. A test
setup could then be made to simulate the force applied by
the doctor on the EE, and a motor can be set to turn the EE at
micromotor speeds. Samples of materials with similar
mechanical properties to that of the UHWMPE containing
lesions could then be tested and the decrease in weight of
the sample material over time would simulate the material
removal rate. With knowledge of the total amount of lesion
material, the time for the minimally invasive surgery could be
estimated to determine whether or not the end-effector
could clean the cavity behind the acetabular shell
effectively.
Acknowledgements
I would like to thank Professor Li for advising me on this
project, Mike Kutzer and Ryan Murphy from JHU APL for
designing the dexterous manipulator, Professor Danforth for
assisting me in printing the molds, and the Aresty Research
Center for giving me this unique research opportunity.
References
[1]I. Catelas, M. A. Wimmer, and S. Utzschneider.
"Polyethylene and Metal Wear Particles: Characteristics
and Biological Effects." Seminars in Immunopathology, vol.
33, pp. 257-271, Jan. 2011.
[2]M. D.M. Kutzer, R. J. Murphy, and M. Armand. “Cable
length estimation for a compliant surgical manipulator,”
Robotics and Automation (ICRA), 2012 IEEE International
Conference on, pp. 701-708, May 2012.
[3]M. D.M Kutzer, S. M. Segreti, C. Y. Brown, R. H. Taylor, S. C.
Mears, and M. Armand. “Design of a new cable-driven
manipulator with a large open lumen: preliminary
applications in the minimally-invasive removal of
osteolysis,” Robotics and Automation (ICRA), 2011 IEEE
International Conference on, pp. 2913-2920, May 2011.
Figure 1: Example of the DM entering an osteolytic lesion in a surro-
gate pelvis through a screw hole in the metallic component of an
acetabular cup [2].
Figure 3: Conceptual depiction of revision surgery highlighting the
acetabular shell with removed polyethylene liner, and the dexter-
ous manipulator accessing an osteolytic lesion through a screw
hole [3].
Figure 2: Dextrous manipulator with an end-effector mounted to end (scale in mm)