Research Indicates Complex Relationship Between Interface Pressure and Discomfort

The discomfort level for upper extremity prosthesis wearers varies based
on the position of the arm, according to preliminary findings by Orthocare
Innovations and Johns Hopkins University’s Applied Physics Laboratory.

Additionally, discomfort increases significantly at 5 psi of skin
pressure, as reported at the
13th ISPO World Congress in Leipzig, Germany.

  Wayne K. Daly
  Wayne K. Daly

As part of DARPA’s Revolutionizing Prosthetics 2009 Program,
subcontracted by Johns Hopkins University, Wayne K. Daly, CPO, LPO, FAAOP, and
others at Orthocare Innovations set out to develop a high-function upper limb
prosthesis. One of the first components of the task, however, was to determine
the amount of force that would be appropriate for the wearer. When they
researched skin tolerance and comfort for pressure, they found the available
information significantly lacking, he said.

Daly, research prosthetist at Orthocare, discovered several studies
concerning lower limb interface issues, including those by Edward Neumann, PhD,
at the University of Nevada, Las Vegas, and Joan Sanders, PhD, at the
University of Washington in Seattle. This information was useful, but —
because the positioning and loads are completely different for an arm versus a
leg — not entirely relevant to the task.

Existing articles on the upper limb population were somewhat
inconsistent, he said.

The team proposed a research project, which was accepted and funded by
DARPA as part of the larger project, to study the forces and comfort level
within upper limb prostheses. Team members outfitted nine upper extremity
amputees — three transradial, three transhumeral and three shoulder
amputees — with TekScan sensors attached to their own prostheses.

“We started out knowing that the position and loading was going to
influence the comfort. We determined that we’d have to position the arm in
space,” Daly said.

Researchers counted the possible degrees of flexion in the arm and ended
up with 160 different possible combinations of loading, which they then
reproduced for 0-, 1- and 2-kg loadings for each; this was too many data points
for the subject, and for the researchers, to complete, Daly told O&P
Business News
. Instead, they used a process called experimental design, and
dropped the number to 74 discrete points for each patient. Within the original
number of load combinations, the missing points overlapped. Each patient had a
different set of points, but the nine subjects together covered all of the
necessary points.

“We found that … the discomfort level increased significantly
at 5 psi of skin pressure,” Daly said. “There was some variability in
the comfort tolerance — different folks have much higher tolerance to
pressure than others — but that seemed to be the most consistent breaking

The team currently is analyzing the data, but one point is certain:
along with new high-function prostheses must also come new designs of sockets
and additional different interface materials to distribute force more evenly
than existing designs, without increasing the discomfort level.

“With a high-function prosthesis you can’t just put it on a
standard design socket and expect it to function comfortably and
effectively,” he said.

Both Randall Alley, BSc, CP, FAAOP, CFT, chief executive officer of
biodesigns inc. and Practitioner Advisory Council member for O&P
Business News
, and Jay Martin, CP, LP, director of Advanced Systems
Group at Orthocare, already are working on this matter.

Additionally, Daly said the researchers found a significant error
problem when working with the TekScan sensors, which do not apply smoothly on
the curved surface inside the socket. This led to an unnecessarily high
measurement error rate. — by Stephanie Z. Pavlou


I appreciate the relevance of this research project. I commend the
research group for this significant undertaking.

We really are just beginning to understand the many differences and
complexities of socket pressure on upper limbs. Unlike lower limb, the
prosthesis is in suspension nearly 100% of the time, which creates some unique
physical dynamics. Then consider all the different pulling and pushing and
lifting motions we do with an arm. The fitting scenario is further complicated
by the fact that most upper limb deficiency is secondary to trauma, setting the
stage for compromised, pressure sensitive tissues.

This study takes the necessary initial steps to helping prosthetists
understand how to create more comfortable socket interfaces for patients.

— Chris Lake, L/CPO, FAAOP
Chief clinical
director, Lake Prosthetics and Research, and chair, Academy Upper Limb
Prosthetics Society

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