O&P BUSINESS NEWS March 15, 2012
Selective Laser Sintering Creates Subtle Changes to Lower Limb
Selective laser sintering may potentially improve mobility on a variety
of terrains and increase comfort for the user.
Rick Neptune, PhD, professor, University of Texas at Austin Cockrell
School of Engineering and his team of graduate students are working in
collaboration with the Department of Veterans Affairs Center of Excellence for
Limb Loss Prevention and Prosthetic Engineering on a research project aimed at
understanding the influence of the
prosthetic foot the
stiffness profile on gait performance.
What is SLS?
A manufacturing technique developed at The University of Texas called
Selective laser sintering (SLS) makes it possible for researchers to quickly
create custom and lightweight prosthetic devices or sockets. Neptune and his
team create a lower limb device using a 3-D computer program and reproduce the
device using lasers that sinter a nylon powder. The nylon powder becomes hard
as it cools, allowing for adjustments in design to improve a subject’s
gait. With each layer, the powder is transformed to a solid but flexible custom
“SLS is convenient because it allows us to systematically change
the design characteristics of a prosthesis quickly while keeping with its
functionality,” Neptune said. “We are able to fine tune either
sockets or prosthetic foot-ankle sockets for an individual.”
||Following selective laser
sintering, Nepture and his team assemble the foot to test how its stiffness
affects the subjects' ability to walk.
||Image: Rick Neptune
A number of studies through the years have compared lower extremity foot
devices but the O&P industry has yet to discover any definitive results,
according to Glenn Klute, PhD, research health scientist, VA R&D Center for
Excellence for Limb Loss Prevention and Prosthetic Engineering.
Previous lower extremity foot comparison studies were conducted with
specific manufactured feet. Over time, those manufacturers improve their feet
or they stop making one model to make a new one.
“What’s unique about our research is that we are mechanically
testing the foot the patient is wearing when he or she walks in the door so we
know exactly what the mechanical properties are through the whole gait
cycle,” Klute said. “We then use SLS manufacturing to create feet
whose properties are varied in a systematic way.”
All told, Neptune will design five feet for the subjects — a
replica of the subject’s foot, one that is 25% less stiff, one that is 50%
less stiff, one that is 25% stiffer and another that is 50% stiffer. Neptune
and Klute measure across the stiffness profile throughout the subject’s
entire gait cycle.
“In this project, Dr. Neptune is designing feet and using SLS to
manufacture feet of very specific stiffness and mechanical properties,”
Klute said. “We are amplifying or subtracting the properties. The results
will not be manufacturer specific. Instead, we are varying the mechanical
properties of the feet so we can clearly understand the effects stiffness has
on the subjects’ ability to walk.”
Neptune, Klute and their research team put subjects through a variety of
tests and dynamic tasks such as walking at different speeds, up and down stairs
and along a curved path in Klute’s lab at the Seattle VA, which features
five force plates on the floor and 12 cameras that measure how they move. By
understanding the effects mechanical properties of prosthetic feet have on
amputee gait, researchers can help manufacturers refine how they build their
feet and better determine what kind of foot matches best with certain patient
“We’re not only making properties that mimic the stiffness
profile of saggital plane for straight walking, but we’re also varying
them in the coronal plane to understand more complex maneuvers like
turning,” Klute said. “That makes sense for active ambulators and for
those who walk in their homes. Cooking dinner is a great example. Every aspect
of cooking involves turning and maneuvering.”
For soldiers and civilians alike
The research team enjoys working with active personnel because they have
the motivation and fitness level to succeed. According to Neptune, military
subjects are the ideal candidates to test how new prosthetic components affect
overall physical fitness.
“One of the things that we are interested in is how do individuals
adapt to changes in design characteristics in O&P devices?” Neptune
said. “Using SLS, we can perform systematic studies to change the
stiffness characteristics of a prosthetic foot or ankle to determine how the
soldiers adapt from a compliant foot to a stiff foot or how they change their
Klute foresees a series of studies where their research teams determine
the effects specific design characteristics have on subjects’ ability to
walk and their overall mobility.
“We hope to improve the state of the art of the prosthetic foot by
better understanding how their properties affect how a subject walks in
straight lines and more complex maneuvers,” Klute said. — by
One of the wonderful characteristics of the intact foot and ankle is
that there is multidimensional motion, including saggital plane, coronal plane
and transverse plane. Not only is there multidimensional motion but the motion
is controlled by muscle actuators that can vary their stiffness characteristics
automatically based upon the functional goals of the movement. This can be seen
in the sagittal plane with walking on inclines, or walking at different walking
speeds; in the coronal plane, as athletes make side to side cuts, or in walking
on irregular terrain.
The study of stiffness and optimizing stiffness across these functional
tasks is obviously exceedingly complex. The research being undertaken by Klute
and Neptune is the first step in the process. By systematically being able to
modify stiffness while keeping the other mechanical characteristics constant
one can begin to both ascertain the biomechanical effects and the subjective
responses from the subjects, and then to link the two together. This is a
fundamental step in beginning to define an optimization strategy.
Although not mentioned in the article, the optimum stiffness is likely
to be task specific. The natural question that will arise then is “what
stiffness do we choose when prescribing a prosthetic foot?” The
development of prosthetic feet with built in controllers and control systems is
an emerging technology, so the task specific stiffness being derived by Klute
and Neptune could be used as the output for this type of a dynamic controller.
The SLS system used in their research is an excellent tool to rapidly
fabricate prosthetic feet with specific characteristics and the research will
make a significant contribution to our understanding of the effect of
prosthetic foot stiffness on mobility.
— Joseph Czerniecki, MD
Director, Department of Veterans Affairs National Amputation System of Care