Gel Liners May Decrease Sensory Feedback in Transtibial Amputees

  • O&P Business News, June 2012

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.

  Erin Boutwell, MS
  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.

Perspective

  • This article presents a discussion about the effect of liner thickness on amputee performance. A reference is made to a comparative study conducted at Northwestern University. Subjectively, the subjects with a more bony residuum preferred the 9-mm liners, while the response from the subjects with a padded residuum was mixed.

    Data obtained in the trials included peak pressures in specified zones, ground reaction forces (GRF) and joint angles. After averaging measurements in the bony and padded groups, a statistically significant increase was found in vertical GRF loading peak (2.3% of body weight) and in stance-phase knee flexion (1.8°) when the subjects wore the 9-mm liners.

    The authors suggest that the thick liners decrease impact forces during stance phase, but might mask the sensory feedback needed for the wearer’s perception of stability and security in gait.

    An increase in the stance-phase knee flexion with thicker liners would be an indication of a better restoration of the ballistic knee-ankle-hip synergy. However, the Journal of Rehabilitation and Development paper doesn’t present data on ankle and hip articulation. Therefore, it remains unclear whether the stance-phase knee angle owed its increase to the greater angle between the thigh and the prosthesis, or between the thigh and the residuum, which could more easily move relative to the socket in a thicker gel.

    The authors did not offer practical recommendations regarding prescription preferences for the liner thickness, indicating that further studies are required. If such recommendations are developed, they should not ignore the moments of resistance to bending in prosthetics joints. That dependence is obvious, as the articulation in prosthetic joints is actuated by the residuum, and in turn, the prosthesis’s resistance is the source of pressure on the residuum.

    • Mark Pitkin,, PhD, ScD
    • Research professor of physical medicine and rehabilitation, Tufts University School of Medicine, Boston.

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