Prosthetic Liners for Lower Limb Amputees: A Review of the Literature

Abstract

Prosthetic liners exist to improve amputee safety and comfort by adding a cushioning layer between the residual limb and the prosthetic socket. Many choices in liner technology are available, and clinicians often rely on personal intuition and experience to choose which liners are appropriate for which patients. The purpose of this study was to examine the literature to find what scientific evidence exists to inform prescription practices. ‘Prosthetic liner’ was used as a search term in the Web of Science and PubMed research databases. Fourteen scientific articles met the eligibility criteria and are discussed in this review. The results of this review suggest that there is little scientific evidence to inform prosthetic liner prescription practices. Liner material properties have been well-studied, but their influence on in vivo performance is not well understood. Understanding liner effect on function would be an area of great usefulness.

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

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 Daly ⁹ 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 others ⁸ 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 Slater ⁵ performed a study with 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 others ¹⁴ 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 others ¹⁵ 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 suggestions have been made based on the experimental data. For example, Sanders and others ¹⁵ 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 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 including 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 others ²⁵ 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 others ²⁶ 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 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 others ²⁷ 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 others ²⁸ 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 others ²⁸ 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 others ³⁴ sent surveys to 72 physicians and prosthetists experienced in prescribing the system. The respondents felt that the ICEROSS neither provided advantages or disadvantages to 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. Furthermore, 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 others ³⁵ 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 others ³⁶ 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 others ³⁷ 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 Geertzen ³⁸ 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 Geertzen ³⁸ 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 others ³⁴ 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.

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 others ³⁶ 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.