Selective Laser Sintering Creates Subtle Changes to Lower Limb Prostheses

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 made prosthesis.

“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.
  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

Unique research

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.”

Complex maneuvers

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 activities.

“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 biomechanics.”

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 Anthony Calabro


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
Interim National Director, Department of Veterans Affairs National Amputation System of Care

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