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http://poi.sagepub.com/Prosthetics and Orthotics International
http://poi.sagepub.com/content/34/2/146The online version of this article can be found at:
DOI: 10.3109/03093641003645528
2010 34: 146Prosthet Orthot IntGlenn K. Klute, Brian C. Glaister and Jocelyn S. Berge
Prosthetic Liners for Lower Limb Amputees: A Review of the Literature
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What is This?
- Jun 1, 2010Version of Record >>
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Prosthetic liners for lower limb amputees: A reviewof the literature
GLENN K. KLUTE1,2,3, BRIAN C. GLAISTER1,2, & JOCELYN S. BERGE1
1Department of Veterans Affairs, RR&D Center of Excellence, Seattle, WA, and Departments of2Mechanical and 3Electrical Engineering, University of Washington, Seattle, WA, USA
AbstractProsthetic liners exist to improve amputee safety and comfort by adding a cushioning layer betweenthe residual limb and the prosthetic socket. Many choices in liner technology are available, andclinicians often rely on personal intuition and experience to choose which liners are appropriate forwhich patients. The purpose of this study was to examine the literature to find what scientific evidenceexists to inform prescription practices. ‘Prosthetic liner’ was used as a search term in the Web ofScience and PubMed research databases. Fourteen scientific articles met the eligibility criteria and arediscussed in this review. The results of this review suggest that there is little scientific evidence toinform prosthetic liner prescription practices. Liner material properties have been well-studied, but theirinfluence on in vivo performance is not well understood. Understanding liner effect on function wouldbe an area of great usefulness.
Keywords: Prosthetics, prosthetic interface mechanics, testing of prosthetic components, prostheticdesign, prosthetic liners, lower limb prosthesis, lower limb amputee
Introduction
Protecting residual limb soft tissues for lower limb amputees is a difficult challenge. Unlike
the plantar tissues of the intact foot, residual limb soft tissues are not accustomed to bearing
loads.1 Consequently, loads imparted on the residual limb by the prosthetic socket can
frequently cause ulceration and other skin conditions.1–3 This is problematic as treatment
can often require the temporary disuse of a prosthesis which greatly impedes an amputee’s
ability to perform activities of daily living.
To help cushion the transfer of loads between the prosthetic socket and the residual limb,
soft prosthetic liners have been developed.4–7 Liners also provide additional functionality by
contributing to the suspension of the prosthesis. Historically, liners were made from open
and closed cell foams formed around the residual limb.8,9 Foam liners are still used in
practice, but modern liners are typically made from silicone or other elastomers and are
rolled onto the residual limb. These roll on liners are believed to offer better suspension,
durability and cushioning than foam.7,10–13
Correspondence: Dr Glenn Klute, Department of Veterans Affairs, RR&D Center of Excellence, 1660 S Columbian Way MS151,
Seattle, WA 98108 USA. E-mail: [email protected]
Prosthetics and Orthotics InternationalJune 2010; 34(2): 146–153
ISSN 0309-3646 print/ISSN 1746-1553 online � 2010 ISPO
DOI: 10.3109/03093641003645528
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Many liners exist on the market, but clinical prescription practices are still primarily
informed by experience. That is, a clinician will choose a particular liner for a specific patient
based on intuition, product literature, colleague recommendations, and/or prior experience.
If the liner appears to be successful with that patient, the clinician will then often prescribe
that liner to other similar patients. However, there is a growing desire in the prosthetics field
to include scientific evidence when making prescription decisions. The purpose of this paper
was to review the scientific evidence that exists regarding prosthetic liners to assist with
prescription practices and to identify potential areas of future research.
Methods
‘Prosthetic liner’ was entered in as a search term into the Web of Science and PubMed
databases searching for articles written in English. Fifteen articles were found from the
search to be related to lower limb amputees, and thirteen additional articles were found in
the citations of the articles from the search results. Upon inspection, fourteen articles were
found to present experimental evidence (as opposed to clinical observation or opinion) and
are reviewed in detail in this report.
Mechanical testing of prosthetic liners
Mechanical properties: Scientific literature
The main purpose of prosthetic liners is to cushion the transfer of loads from the prosthetic
socket to the residual limb. Naturally, cushioning performance depends on the mechanical
properties of the liner materials. Consequently, a large portion of prosthetic liner research
has focused on liner mechanical properties.
To understand how vacuum forming affects the mechanical properties of Pelite, a closed-
cell foam used as a prosthetic liner, Sanders and Daly9 tested material samples under
compression and shear loading for both unformed and vacuum-formed conditions. The
study found that vacuum-forming weakens the cell structure of the foam, particularly on the
surfaces. The authors suggested that by controlling the amount of cell degradation at
different areas of the liner, the mechanical properties of the foam could be controlled to
provide unique performance to individual patients.
To understand how mechanical properties differed between liners, Sanders and others8
investigated the compressive stiffness and coefficient of friction for eight types of liner
materials commonly used at the time of the study. Spenco, Poron, silicone, soft Pelite,
medium Pelite, firm Plastazote, regular Plastazote and Nickelplast were used in the study.
Based on load-displacement data from the compressive stiffness tests, Spenco, Poron, and
silicone were recommended for situations where it is desirable for the liner to maintain
thickness and volume since these materials had the least non-recovered strain. Soft Pelite,
medium Pelite, regular Plastazote, and firm Plastazote all had the highest non-recoverable
strains, but they stiffened with increasing displacements similar to biological tissues
suggesting that these materials may be advantageous in cases when matching liner
properties to those of biological tissues is important. Nickelplast behaved linearly but had a
very high stiffness suggesting that it would not give much of a cushioning effect to the
residual limb. Materials with smooth surfaces or coverings, such as Spenco or Poron, had
the lowest coefficients of friction which could be problematic in maintaining suspension.
To investigate the mechanical properties of liners under cyclic compressive loading, cyclic
shear abrasive loading, and frictional loading, Emrich and Slater5 performed a study with
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Bock-Lite, Pedilin, silicone and polyurethane. Bock-Lite and silicone had the greatest cycles
to failure under compressive loading while the Pedilin and polyurethane samples lasted
orders of magnitude less. While the investigators were unable to test silicone and
polyurethane under abrasive loading due to tearing of the samples, the Bock-Lite was
shown to survive 15 times as many cycles as the Pedilin under shear abrasion.
Polyurethane and silicone had the highest coefficients of friction while Pedilin and Bock-
Lite had coefficients that were much lower.
To investigate the compressive behavior of several liner materials under geometric
constraints and different loading rates, Covey and others14 performed a study using two
silicone liners (ICEROSS and Alps Easy Liner), a urethane liner, and a thermoplastic
elastomer liner. The authors believed that urethane was the optimal liner material since it
was found to be the stiffest of the four yet provide the best impact protection and the lowest
residual displacement. This suggests it will transfer loads without a time lag yet protect from
impact and not compress from loading. However, it should be noted that the manufacturer of
the urethane samples (TEC Systems) provided funding for the study.
To further understand the mechanical behavior of liners, Sanders and others15 tested
15 commercially-available liners under compression, friction, tension, and shear. The
liners included samples made of silicone elastomer, silicone gel, and urethane. Silicone
gels, which bleed fluid upon compression, were the softest during the compression tests
and were most similar to biological tissues suggesting that they would be most
appropriate for cushioning bony prominences. Silicone elastomers, which are highly
cross-linked, were the stiffest in compression suggesting that these would be
advantageous for residual limbs with excessive soft tissue since the liners would not
add any more deformation on top of the tissue. The urethane sample had a similar
compressive stiffness to some of the silicone elastomer samples. None of the samples
had coefficients of friction low enough to induce slipping in a clinical setting. Urethane had
the highest coefficient of friction suggesting that it would adhere well to weak skin sites
and protect them from breakdown. Shear stiffness trends were similar to the compressive
tests suggesting that silicone elastomers and urethanes would be most appropriate for
residual limbs with excessive residual limb soft tissues to prevent the limb from sliding into
the socket, while silicone gels would be appropriate for limbs with bony prominences to
provide some cushioning effect. The silicone gels and urethane were soft in tension as
were three of the silicone elastomer samples though most of the elastomers had higher
tensile stiffness. This suggests that the elastomers with higher tensile stiffnesses would
provide better suspension.
Mechanical properties: Discussion
Prosthetic liner material properties have been well studied under a number of loading
conditions (e.g., tension, compression, shear and friction) and many prescription sugges-
tions have been made based on the experimental data. For example, Sanders and others15
suggested that stiff liners would be best for patients with excessive soft tissue while soft
liners would be best for cushioning bony prominences. However, the leap from the benchtop
data to patient performance has yet to be made. Testing the hypotheses suggested from
the benchtop tests in controlled experiments with human subjects could make a tremendous
effect on the field in terms of providing clinical guidelines for liner prescription.
Specifically, the following hypotheses should be tested: (1) soft liners provide better
cushioning than stiff liners, (2) stiff liners provide a faster response to movement, (3) silicone
gels provide superior cushioning over bony prominances, (4) silicone elastomers and
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urethanes provide superior soft tissue stabilization, (5) urethanes provide superior weak
skin site breakdown protection, and (6) silicone elastomers provide better suspension.
In addition to human subjects testing, a few opportunities for new research remain in
the area of benchtop testing of prosthetic liners. For instance, the vibration transmissibility
properties of prosthetic liners have yet to be investigated. Intact individuals attenuate
shock loads at heel strike through a combination of soft tissue compression and joint
movement,16 but lower limb amputees must also rely on prosthetic components includ-
ing liners to attenuate shock loads. Understanding how prosthetic liners transmit
transient loads could lead to better prescription practices and the development of new
technologies.
Lastly, the material properties of prosthetic liners are likely to be affected by use and wear.
It would be useful to understand how the properties degrade with use as old, worn liners
may be more apt to contribute to discomfort or soft tissue injury. This information could lead
to new liner technologies as well as prescription guidelines for replacing old liners.
Heat and moisture transfer properties
Heat and moisture transfer: Scientific literature
While much attention has been paid to mechanical properties of liners, heat and moisture
transfer properties have also received attention. Excessive heat and moisture retention
within the socket are common complaints of lower limb amputees.17–21 Research has
shown that the prosthetic socket/liner interface acts as a heat capacitor and retains elevated
residual limb temperatures even after activity is ceased.22 Combined with the pooling of
perspiration against the skin, the environment between the liner and skin is perfect for
forming a host of residual limb skin problems including contact dermatitis, hyperhydrosis,
and bacterial infections.3,23,24 As such, researchers have investigated the heat and moisture
transfer properties of prosthetic liners.
To understand how prosthetic sockets and liners may influence the retention of
perspiration, Hachisuka and others25 investigated the moisture permeability properties of
liner and socket materials. The study included a silicone liner, an ICEROSS liner, Degaplast
plastic, Pelite, and a sample of poplar from a wooden socket. The poplar sample was about
four times as permeable as everything else, and the liners and Degaplast samples were
more than 80 times less permeable than tests without a sample suggesting that liner
materials are highly impermeable to moisture transfer.
To understand how liner and socket materials may influence residual limb skin
temperatures, Klute and others26 investigated the thermal conductivity of prosthetic socket
and liner materials. Twenty-three liners were tested as were samples of carbon fiber
laminate and thermoplastic socket materials. Across the board, thermal conductivity
coefficients were small ranging from 0.085–0.266 W/m-K for liners and 0.148–0.0150 W/m-
K for socket materials. This suggests that liners and sockets are highly resistive to heat
conduction and could be a major contributor to elevated skin temperatures.
Heat and moisture transfer: Discussion
There exists a tremendous opportunity to make a positive improvement of amputee comfort
by moving heat and sweat away from the surface of the residual limb. This could be
achieved via pumps or other mechanical means, but augmentations to liner designs could
possibly be successful as well. Future research should focus on improving heat transfer
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coefficients in liners as well as finding a way to remove perspiration while also maintaining
suspension.
Human subjects’ experiments
Human subjects’ experiments: Scientific literature
In addition to material properties, researchers have investigated the performance of liners
with human subjects. To investigate the effect of liner materials on interface pressures,
Sonck and others27 measured pressures at four sites on the sockets of 26 amputees under
three liner conditions: (1) no liner, (2) a soft insert called Kem-Blo, and (3) a silicone liner.
The results of the study showed reduced pressures at all the socket sites with the silicone
liner compared to the other conditions suggesting that silicone has an ability to distribute
pressure evenly to the residual limb.
Lee and others28 investigated residual limb pain threshold and tolerance using an indenter
device lined with Pelite liner material and polypropylene socket material. At all locations on
the residual limb, subjects could withstand greater force with Pelite than with polypropylene
suggesting that Pelite distributes load over a greater area of soft tissue. Additionally, the
study found that thin, stiff layers of tissue tolerated pain better than thick, soft layers of
tissue.
Human subjects’ experiments: Discussion
Given the important role that liners play in amputee comfort, it is surprising that only two
studies were found in the literature investigating liner performance in human subjects.
Nonetheless, these studies provide valuable information in that they show that liners
distribute pressures over the residual limb which is their intended purpose. The study by Lee
and others28 also found that pain tolerance varies according to soft tissue type. This
information suggests that liners that have different geometries or material properties in
different areas could perhaps provide additional functionality. Additionally, several future
human subjects’ experiments have already been suggested in the review of the material
properties literature.
Outcomes research
Outcomes research: The scientific literature
In addition to controlled trials with human subjects, many investigators have performed
outcomes research related to prosthetic liners. The measurement and reporting of
outcomes can help in understanding the mobility, function, and quality of life of lower limb
amputees. Importantly, the results from outcome studies can be used in a number of
different ways that benefit lower limb amputees. Identifying problems experienced by lower
limb amputees is one application that can aid in specifying areas of patient care that need
improvement.17,19 Describing prescription practice is another valuable application that can
assist in creating or evaluating clinical guidelines.29,30 They can also assist in quantifying
factors associated with costs related to rehabilitation or prosthesis fabrication.31–33
In order to gather clinical opinion about the ICEROSS liner, McCurdie and others34 sent
surveys to 72 physicians and prosthetists experienced in prescribing the system. The
respondents felt that the ICEROSS neither provided advantages or disadvantages to
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patients with long residual limbs, conical residual limbs, peripheral vascular disease,
residual limb pain, poor alignment stability, or poor cosmesis. Additionally, the system would
be neither advantageous nor disadvantageous for elderly or traumatic patients. Further-
more, the respondents felt the ICEROSS would be advantageous to patients with sensitive
skin or skin grafts, those prone to pistoning, and those unable to use a cuff suspension.
Lastly, the respondents were divided on some subjects with some respondents advocating
the use of the ICEROSS for patients with skin breakdown, neuropathies, sensitive skin,
residual limb volume instability, and bony prominences while other respondents
discouraged the use of the ICEROSS for the same patients. The study results further
highlighted the diversity of clinical opinions and the need for scientific evidence to inform
prescription practices.
To perform a qualitative comparison between silicone and Pelite liners, Boonstra and
others35 used a survey instrument with transtibial amputees wearing both systems.
Unfortunately, only eight subjects were included in the study, and a definitive preference for
one liner over the other was not demonstrated. Some subjects preferred the silicone liner
because it distributed pressure differently and had a closer connection between the liner and
the socket. Others rejected the silicone liner for the same reasons.
To understand outcomes differences between mineral oil gel liners with locking pin
suspensions and Pelite liners and neoprene suspension sleeves, Coleman and others36
performed a randomized crossover trial with 13 lower limb amputee subjects. The study
found that 77% of the subjects preferred the Pelite system, took 83% more steps, and wore
the system 6 h per day longer than the gel liner. Subjects however, when interviewed,
expressed advantages and disadvantages of both systems and both systems performed
similarly in terms of pain and comfort.
To investigate the relationship between hand function and residual limb skin problems,
Baars and others37 performed a chart review with 60 amputees using silicone liners. The
study found that 70% of subjects with impaired hand function suffered residual limb skin
problems compared to 32% of patients with normal hand function suggesting that hand
function should be considered when making a decision to prescribe silicone liners.
Lastly, to determine what evidence exists to support the prescription practices of silicone
liners for transtibial amputees, Baars and Geertzen38 performed a review of the literature.
The study concluded that little evidence existed to support the prescription of silicone liners,
but that the conclusion was more a reflection of the quality of research that existed not the
liners themselves. The authors recommended future work to include definitive clinical
metrics and homogeneous subject groups.
Outcomes research: Discussion
Prosthetic liner outcomes research represents another opportunity to positively impact
amputee care because, as Baars and Geertzen38 noted, few studies have used objective
clinical measures or homogenous patient groups which makes drawing conclusions
difficult. However, with that said, the existing literature helps identify future areas of
research.
McCurdie and others34 highlighted the problems that arise when prosthetic components
are prescribed based on clinician experience and intuition; as the clinician group surveyed
was divided as to if the ICEROSS system is useful for patients with skin breakdown,
neuropathies, sensitive skin, residual limb volume instability, or bony prominances.
Targeted outcomes research with those patient groups could greatly aid prescription
practices.
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Cost is becoming increasingly more important in prescription decisions as well. New
technologies come on to the market constantly promising improved function but usually at a
higher cost. Coleman and others36 performed the only study to report costs in their
comparison of Alpha liners ($945 reimbursement rate) and Pelite liners ($280). Patients
overwhelmingly preferred the Pelite and took 82% more steps with that system. More cost/
benefit studies could help clinicians prescribe devices that provide maximum function for
minimum cost which could result in a potentially large savings for the healthcare system.
Conclusion
The results of this paper suggest that there is little scientific evidence to inform the
prescription practices of prosthetic liners. Research has shown that liners can help distribute
loading and reduce pain, but research discerning differences between liners to aid with
clinical choices is lacking. While the material properties of prosthetic liners have been well-
studied, how those properties affect function in vivo is not well understood. Additionally, new
technology is constantly being introduced that promises improved function but often at a
higher cost. Research focused at quantifying a cost:benefit ratio could be a great aid for
prescription practices aimed at cost savings. Plenty of opportunities remain to make
research impacts regarding prosthetic liners.
Acknowledgements
This work was supported by Department of Veterans Affairs Grant A4843C and by U.S.
Department of Education Grant entitled ‘‘Advancing Orthotics & Prosthetic Care through
Research Standards of Practice and Outreach’’ (H235J060001) CFDA# 84.235J to the
American Academy of Orthotists and Prosthetists.
Declaration of interest: The authors report no conflicts of interest. The authors alone are
responsible for the content and writing of the paper.
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