What’s the thickness of the prosthetic liners you use? If you don’t know the answer, don’t feel bad—most amputees don’t. But the information is highly relevant, according to a new study published in Nature Scientific Reports. Using advanced imaging and biomechanical simulation technology, a research team found that just two millimeters of extra padding can significantly relieve pressure in lower-limb prosthetic devices.
Unlike sockets, liners haven’t been extensively studied—which is a little odd, since liners interface more directly with the residual limb than sockets do. Most research to date has focused on the pros and cons of various liner materials, but there hasn’t been extensive work on variables such as thickness.
The paper appearing in Nature was based on a computational modeling exercise that analyzed the mechanical properties of two common liner materials (gel and silicone) across three thicknesses (2 mm, 4 mm, 6 mm). The authors studied four main factors: shear stress (the key factor in surface-level skin irritation), pressure distribution (which affects subsurface tissues and nerves), vertical displacement (corresponding to how much the limb moves within the socket); and maximum strain.
They began by constructing an anatomically precise 3D model of a transfemoral residual limb, using CT scan and MRI imaging to capture accurate bone, muscle, and nerve architecture. Next, they modeled in a liner and socket with a realistic set of prosthetic fit characteristics. Finally, they used a finite element modeling (FEM) program to assess performance under varying conditions. Their key findings:
- Thinner liners lead to higher pressures. For gel liners, pressure readings at 6mm were nearly 18 percent lower than at 2mm. The effect in silicone liners was even more dramatic, with pressure dropping more than 50 percent between 2mm and 6mm.
- Gel liners produce higher maximum strain readings than silicone liners. This effect is especially pronounced in thinner gel liners, which displayed concentrated areas of high strain. Silicone liners had a much lower peak strain and more even distribution. Overall,
- Gel liners produce higher shear stress than silicone. Mirroring the pattern in maximum strain, the gel liners had very high shear stress readings at 2mm thickness, with only modest improvement at 4mm and 6mm. Silicone liners are less stress-prone at all thicknesses, and stress distribution becomes more even as thickness increases.
- Thicker liners allow for more displacement. For both silicone and gel liners, the residual limb experiences more “slippage” within the socket as liner thickness increases. This effect was especially pronounced for silicone liners.
The bottom line is that liner selection inevitably involves tradeoffs. Thicker liners reduce pressure, shear stress, and strain, all of which tend to minimize pressure sores, skin irritation, nerve pain, and other discomforts. But they also allow for greater displacement, which could hinder healthy gait habits, undermine stability, increase fall risk, and reduce confidence. Similarly, silicone liners do a better job of reducing pressure, strain, and stress, but gel liners provide better stability. Unfortunately, there’s no combination of material and thickness that carries the day on every account.
Keep in mind that these conclusions are based on a purely theoretical exercise. They haven’t been validated in a clinical setting using human subjects. The findings are only as valid as the model they’re based upon, and no model is perfect; one or more relevant variables inevitably gets left out. But if nothing else, this study provides a framework for making some informed decisions about your liner. If you’re experiencing sores, skin problems, or other discomfort—or if your socket fit feels a little loosey-goosey—it might be discussing the characteristics of your liner with your prosthetist.
To read the full paper, log on at Nature Scientific Reports.