Extensive research has been conducted to examine how different
prosthetic components affect movement patterns, pressure distribution and
ground reaction force during walking in below knee amputees. However, little
research has been done to investigate how gel liners, specifically the
thickness of the liner, affect pressure within the socket.
“Our lab has been interested for a while in how different shock
absorbing prosthetic components can influence overall gait,” Erin
Boutwell, MS, a PhD candidate in the department of biomedical engineering
at Northwestern University, told O&P Business News. “The
interface between the residual limb and the prosthetic socket is a key area in
which shock absorption can be influenced, and a prosthetic gel liner is one
component that can be used for this purpose.”
Study design
To investigate this relationship, Boutwell and her colleagues recruited
11 unilateral transtibial amputees with a mean age of 55.9 years for the study,
which was funded by the Department of Veterans Affairs and recently published
in the Journal of Rehabilitation Research and Development. The
researchers had two experimental prostheses fabricated for each participant
based on a digitization of the subject’s residual limb. Each prosthesis
consisted of an Otto Bock 1D35 prosthetic foot, a rigid aluminum pylon and a
TSB socket fabricated to fit either an Alpha 3-mm or 9-mm gel liner. The
participants were given 2 weeks to orient themselves with their new prosthesis.
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| Erin Boutwell |
Once accommodated, the participants were asked to walk at a freely
selected speed over a 10-meter walkway wearing the 3-mm liner and the 9-mm
liner. Pressures were recorded in three anatomical locations on the residual
limb: patellar tendon, distal anterior tibia and fibular head. Reflective
markers were also placed on the participants to collect motion data. Trials
were repeated until five clean force plate strikes were obtained for each foot
for each liner condition.
At the conclusion of the trials, the participants completed a survey
relating to the comfort level and functionality of each liner and were asked to
indicate which liner they preferred.
Padded vs. bony
As the researchers collected the data, they noticed that the
participants were affected differently based on their type of residual limb.
Using this information, they subjectively separated the participants into three
groups: bony, average or padded, based on the amount of soft tissue on the
residual limb.
“We hadn’t previously realized how much the amount of soft
tissue on the residual limb might influence the variables we were
studying,” Boutwell said. “[The classification of groups] was
something we determined during the course of data collection.”
Based on the survey results, the bony limb group overwhelmingly
preferred the thicker liner, while the padded limb group had mixed preferences.
“Subjects that we classified as padded appeared to already have
sufficient tissue at the limb-socket interface to help with shock absorption,
and therefore they weren’t focused as much on the high peak
pressures,” Boutwell said. “They had more complaints regarding other
types of concerns, such as a perceived increase in temperature or difficulty
bending the knee.”
Surprising results
In addition to the reported comfort of the 9-mm liner, the researchers
determined that there was a significant reduction in peak pressures at the
fibular head while wearing the 9-mm liner, which was what they had expected.
However, they also found that the ground reaction force increased while wearing
the 9-mm liner even though there was no increase in walking speed.
“This was surprising because nothing else appeared to change
between the two trials other than the thickness of the liner,” Boutwell
said. “So our hypothesis coming out of this was that perhaps the greater
thickness of the liner was adversely affecting the sensory information that was
transmitted to the residual limb.”
Boutwell suspects that participants were stepping more forcefully with
their prosthesis to increase sensory feedback, which could affect stability and
safety while walking.
“The greater thickness of the gel liner could allow the subjects to
contact the ground harder while reducing peak forces transmitted through the
prosthesis, but the thickness could also be problematic in terms of the user
knowing when the prosthesis is in contact with the ground.” Boutwell said.
“More research has to be done to figure out exactly what sensory
mechanisms lower limb amputees are using in order to have confidence in their
prosthesis, so we can design a prosthesis that provides both shock absorption
and appropriate sensory feedback.”
Boutwell said that future studies will need to be conducted to fully
understand the effect of compliant prosthetic components such as gel liners and
the role they play in the relationship between shock absorption and sensory
feedback.
“We’re interested in determining how those two concepts relate
to each other and what the tradeoff is between prosthetic shock absorption and
sensory feedback,” she said. “We’re looking at how different
prosthetic components can play a role in both of these areas.” —
by Megan Gilbride
For more information:
Boutwell E, Stine E, Hansen A, Tucker K, Gard S. Effect of prosthetic
gel liner thickness of gait biomechanics and pressure distribution within the
transtibial socket. J Rehabil Res Dev. 2012; 49(2): 227-240.
