Clinical Classes of Gel Liners
Flexible gel liners are currently regarded as the predominant suspension choice for a majority of lower limb prostheses, because they serve multiple purposes such as suspension, shear relief, and cushioning. These needs become especially important at the boundary layer between the interface and the skin, where the sensate limb meets the solid surface of the prosthesis. Historically wool socks, polyethylene foam, urethane foam rubber, silicone, and a variety of other materials have been used to increase load and shear comfort at the skin boundary layer. Gel liners, introduced in the mid 1990's, have been largely successful in accomplishing these various goals and are evolving into various clinical classes based on Material Type, Softness, Construction Matrix, Surface Matching, and Suspension.
Clinical Need for Comfort
In a often referenced study by Board, Street, and Caspers on vacuum assisted suspension, a total of 528 skin problems were documented from 337 lower extremity limbs. The primary conditions included ulcers, irritations, cysts, calluses, and verrucous hyperplasia, accounting for 79.5% of the skin disorders.3 A 2001 study by Ehde, Czerniecki, and Smith surveyed 255 amputees and 74% had residual limb pain with a mean intensity of 5.4 on a 10 point scale.6 60% of patients described the pain as being "moderately to severely bothersome".6 These repetitive high-magnitude loads are directly related to the shear at the boundary later at the instant of heel strike. Klute in a 2001 paper on shock absorbing pylons relates this rapid deceleration as a high frequency "shock wave" detected in spectral analysis above a frequency of 50Hz. The overall stiffness of the prosthesis is typically arranged in series with the foot providing the dynamic spring, shock absorber acting to dampen impulse load, and the interface material against the skin. The mechanical vibrational system is said to be in series since they are placed in line from most distal to proximal. As Gard notes, a series vibrational system is only as stiff as the softest member which is not the shock absorber but the soft gel interface.8 This translates to a large amount of shear and load still being placed at the boundary layer with the reduces surface area of the residual limb. In a 2008 review by McKenzie, Bosker, and Walden, a survey was done of approximately 1,200 transtibial casts. The average limb was 14 cm long, 31 cm in circumference at MTP and 21 cm in circumference 25 mm proximal to the distal end.1 This translates into roughly 335 square centimeters of surface area. If total surface bearing was applied this would be about 174 mm-Hg of pressure just in simple loading.1 Pressures within the interface may be higher than those found at the distal end of the interface. Biel measured these pressure values in 2002 from 31mm-Hg to 89mm-Hg.1 Arjon Buis, Ph.D. at the University of Strathclyde, Scotland has also mapped these pressure points when comparing standard PTB style sockets with hydostatic methods during walking.9 The peak loads of hydrostatic method is concentrated at the fibular head and distal tibial whereas peak load on the PTB are visually more broadly identified (Fig. 1 ).9 According to the Rogers and Wilson tissue tolerance curve (Fig. 2 ), skin can tolerate high loads of 100 mm-Hg, but only for short periods of time.10 Pressures as low as 30mm-Hg can produce tissue breakdown in access of 10 hours. As mentioned by Carlson the presence of shear forces greatly limit the amount of force the skin can tolerate. A reduction of the coefficient of friction by 30% results in triple the amount of load cycles that the skin can tolerate.11 Anecdotally this shear relief may be one reason fleshy patients experience relief with gel liners even though they have ample subcutaneous padding.
Evolution of Gel Liners
Alleviating friction and load magnitude is nothing new. Wool socks were the first type of cushioning material and provided a modicum of cushioning and shear relief. Kemblo a foam rubber material was paneled on the exterior of a leather liner to provide increased cushioning, but did not conform to shape. Michigan Gel was a PVC gel made in a cookie jar in the late 1960's, was the first gel type material to create cushioning and flow, but had to be sandwiched in a leather liner as well because it had little structural properties.14 In 1986 Ossur Kristinsson with Bo Klasson developed the ICEROSS system which originally utilized a flexible urethane interface, then later a thin silicone liner.14 Carlton Fillauer, inspired by Kristinsson's method, developed the 3-S custom liner system in 1987 using a custom laminated silicone, but it was intended for suspension only with no cushioning.14 Carl Caspers introduced the use of a thicker custom urethane liner that could flow to increase cushion and shear properties with vacuum attachment in 1992.14 Silicone sheaths and fitting socks with gel sandwiched in between also provided added cushion and shear in the early 90's. Although many prosthetic manufacturers knew of the properties of thermoplastic elastomer it was not viable as a prosthetic interface material because it stretched and tore so easily. With the application of an elastic fabric to the elastomer in the 1994, gel liners became a possibility.14 Since then a dozens of liners have evolved using different reinforcements and materials evolved to the variety of designs we have presently. While the diversity of designs brings with it many alternatives, it also can be difficult to choose the best one or compare the range of designs. Liners have differentiated themselves into different clinical design classes based on a variety of factors including: Material Type, Softness, Construction Matrix, Suspension Type, and Degree of Surface Matching. Using these as comparative subsets may be beneficial when making a recommendation.
Liners have developed with three main types of elastic materials: Urethane, Silicone and Thermoplastic Elastomer (TPE) each with different advantages and disadvantages due to different manufacturers formulations.
Urethane elastomer is essentially a man-made form of rubber with a high degree toughness and abrasion resistance. It is usually a simple two-part mixture that does not require injection molding and can be made inexpensively, although air bubbles frequently result in these low-cost slush molds. It has excellent flow characteristics and resists packing out even under a high number of loading cycles so it can be made thinner. It is a thermoset with significant cross-linking so it cannot be reformed nor will it deform. Thermosets will burn at high temperatures and thermoplastics melt due to the presence of cross-linking. However it has a higher density and it is heavier than water with a specific gravity of 1.05-1.25. Urethanes typically discolor and can degrade when exposed to sunlight. Urethanes for prosthetic liners vary in durometer from the mid-30s to the low-50s on the Shore 00 scale.11
Silicone rubber is also a thermoset, as are most elastomers, and has even greater elasticity than Urethane although it may not flow as easily. Silicones are more expensive due to their longer cure times and material costs. A two-part material, silicone is extremely sensitive to reactant proportions including water or latex gloves. Once reacted is very elastic and impervious to water or UV damage. Silicones have a less dense specific gravity at .95-1.2, but need to made thicker because of poor notch sensitivity and tear resistance. Silicones come two main types: Tin catalyst (Condensation cure) and Platinum catalyst (Addition cure). Tin cure included many of the early off-the-shelf liners. Tin cure silicone are basic two part silicones that are less expensive, easier to mix, and have less odor. Tin cure silicones usually have a shorter life span, lose elasticity, and become toughed after 2-4 years. There is slight shrinkage during the manufacturing process. Platinum cure silicones are used in most custom or thick gel-type liners. The curing is internalized, fully cures after 10 minutes, and is only \affected by temperature with no shrinkage. Also Platinum cure produces less volatiles or by-products, which can irritate the skin surface. Platinum cured silicones preserve their elasticity, but require attention to proportions to avoid contamination such as latex gloves. Silicone liners have excellent memory and usually have a durometer ranging from the low-30s to mid-40s, but can go to the high-40s to low-50s on the Shore 00 scale.1 (Image )
Thermoplastic elastomer or (TPE) combines the advantages thermoforming like a thermoplastic with the cushion and resiliency of urethane. TPE exhibits elements of an elastomer with thermoplastic binding in a SBS block structure. SBS or styrene-butadiene-styrene creates a structure with butadiene "blocks" in a microscopic cross-linked nanostructure linked by polystyrene clusters linked in a covalent thermoplastic structure. The polystyrene acts as microscopic crosslinks that can be loosened and broken with heat, but reform during cooling. This allows the material to be stretched twice its original length repeatedly and return to the original shape. TPE can be formed in a semi liquid state with high-pressure injection molds and the cross-linking will reform. Although TPE's require very little compounding, they are relatively expensive due to the molding process when compared to urethanes. Being a thermoplastic, TPE can be reformed to a shape using moderate temperature but has the tendency of thinning more than urethane with repeated loading. TPE can be made extremely soft and can be made softer with the addition of skin friendly softening oils such as mineral oil soften the material further. TPE can be extremely soft from the mid 20s to low 30's on the shore 00 scale.1 The original use of TPE for prosthetic liners is said to have come from a prosthetic wearer who melted fishing lure worms into a socket shape after being impressed with its softness.
The type of material utilized depends on a variety of factors desired including perceived softness, durability, expense, and thermoformability. The best materials absorb load and shear with little thinning. During normal gait silicones and urethanes show very similar properties with TPE displacing slightly more. Convery utilized relatively high force loads of 550N to achieve a greater separation material performance with urethane exhibiting the greatest load and shear absorption properties with minimal thinning. However these load forces would primarily be found in high activity athletic activities which would be 8 times higher than normal ambulation.1,2
Different Softness Measures
With the range and formulation of material design, liners can vary greatly even within the same family of material. Softness is measured in durometer with different scales originally developed by Albert Shore in the 1920's. An insrument called a durometer measures a material's resistance to permanent indention using a specific presser "foot" and load (Durometer is used with polymers, elastomers, and rubbers whereas Brinell, Rockwell, and Vickers scales are used for metal toughness). There are several scales most notably for prosthetic liners shore A, 0, and 00. While liner materials can be measured in the Shore A scale, shore 0 and 00 scales are more sensitive to the softest of materials. Human skin is 20 Shore A which would be similar to a very tough "dummy" liner used for fabrication. Chewing gum is softer so it would be a 15 Shore 00. TPE liners would have a 4 Shore A, but the test is really not as sensitive as it should be so the 00 scale is used to have a more accurate 28 Shore 00 value. Softness is can be typically correlated to durability, but not necessarily to thermoformability. Materials that are softer typically have less shear resistance and poor notch sensitivity meaning they tear more easily. TPE for example has a very low durometer, and was not a viable product until it could be reinforced with fabric. Silicone and urethane can be very soft, but because they are thermosets, they will not thermoform even when subjected to heat and pressure.
The construction matrix has historically been the design challenge for elastomeric liner construction. In fact the liner gels have been in existence for some time, but the construction matrix is what makes liners practical. It is the construction matrix that is so valuable and the primary subject of most of the patent protection with liner designs. The Michigan gel material, advanced for its time in 1969, required a traditional material, leather, to provide the construction matrix to hold it together. Ossur Kristinsson's original liner was a supple urethane, but it ripped and tore easily so he needed to develop a silicone liner with an internal matrix. Carlton Fillauer, saw promise Kristinsson's flexible liner system and also improved on the urethane design in a different direction with a custom laminated silicone and nylon stockinette.12 The addition of elastic external fabric to TPE made offthe- shelf gel liners possible. In a sense all of these designs can be considered a composite or hybrid of materials with flexible interface material proximally and a more structural element distally to bind the interface and provide support for the distal attachment. The matrix was also necessary to limit the periodic distal distraction or "milking sensation" that occurred when the elastic only material was used. The matrix of can be constructed of any fiber nylon, fabric, or polymer mesh and can be external or internal but needs to eliminate longitudinal stretch. This reinforcement does have a rounding effect to the limb shape can be placed at the distal 1/3 where there is less physiologic shape. Internal matrices are difficult to manage during the molding process to prevent wrinkles. They must be made in two layers or fixtured in the mold to prevent buckling or wrinkling. An internal fabric matrix has the issue of silicone delaminating and providing a porous surface for bacterial growth. External matrices can be glued to the liner or heat melded to prevent delaminating. These can consist of fabrics with differing amounts of elasticity or composition. Usually stiffer urethane or plastic distal ends are utilized to attach the pin ends. This process of attaching the distal end takes considerable design effort and cost due to the localized load. Some liner designs attempted to eliminate this attachment option to produce a less expensive device, but this excluded too many possible wearers.
There was a disadvantage to the addition of fabric matrices; they changed the shape of the liner to a more rounded generic shape. The more stiff the matrix type the greater the affect to the liner and socket shape. The residual limb with its triangular shape would be become more rounded. This was also required by the material, especially TPE, which could not tolerate high localized loading. To insure long term where, Total Surface Bearing had to be employed to load the entire limb equally even over prominences. This is different than the "Hydrostatic" methods first described by Robin Redhead that advocated elongation to encourage overall circumferential tension. The original proponents of elastic interfaces, notably Bo Klasson and others, spoke of the need for surface matching.13 This is why the elasticity of the liner was so important to conform to the overall surfaces. With thicker gel liners, especially urethane, material flow became the key to create relief for bony prominences. Without surface matching or material flow, rounded generic shapes actually increase pressure on prominent areas rather than equailize them. Since the bony areas are of much greater density, the ring tension applies greater surface load. Klasson said this was inversely proportional to the radius of the interface or up to five times more pressure.13 Fortunately the softness of the material counteracts this affect. Comfort can be achieved with surface matching even with relatively firm surfaces such as arch supports. For example when standing on thick TPE the metatarsal heads and heel feel most of the load instead of the arch, because there is insufficient flow around the bony prominences. It is interesting to consider that musculoskeletal contouring is considered novel in the transfemoral brim shape, but outdated for the transtibial interface. In reality most practitioners produce a hybrid design with incorporates more general surface bearing with relief for the fibular head and distal tibia.
Suspension type greatly influences liner construction, classification, and choice. Distal pin attachment greatly affects liner construction and volumetric control. This is a result from the pin distracting the liner during stance, which decreases distal circumference and volume. With minimal or no matrix the patient can feel the "milking" sensation of increased distal end suction and shear. Among athletes this can be accentuated during high activity motion, but not as readily apparent to one-speed geriatric ambulators. Distal attachment also increases the localized load on the liner itself, which can result distal material break down and the pin attachment pulling free. Pin attachment is not restricted to a distal pin and can utilize proximally placed pins. The largest difficulty comes in the donning of the device where the patient has circumferential tension applied to the proximal liner. The user must have sufficient balance of forces with in the interface to maintain limb position, but so much that they can easily push the pin into the catch distally. If the patient has a fleshy limb or redundant tissue they may not have sufficient substructure to push the limb distal enough to achieve suspension especially in circumferential tension. In these cases a lanyard or pin guide is required to position and help distract the liner to achieve locking. (Image )
Vacuum systems eliminate the need for a distal pin and rely suction fit of the liner to the interface. They can be classified as passive or active in nature. Passive suction can be used in conjunction with pin attachment and involves the used of a passive check valve and proximal seal. The suction is possible to accomplish with cushion gel liners that advocate total surface bearing techniques of a 5-10% reduction in circumference. Active suction uses an electric pump or the secondary action of a distal shock absorber to actively extract air. Board, Street, and Caspers examined volume control comparing passive and active vacuum systems at the University of St. Cloud in 2001 and found that after 30 minutes of walking there was on average - 6.5% loss of volume ranging from -1.7 % to - 11.3% using a passive check valve. Active vacuum assisted devices provided an average gain of volume from 3.7% ranging from -1.6% to 8.0%.3 With either system the fit of the prosthesis and the amount of tension applied is crucial. If there is a change of volume or shape the suspension may be compromised. The material must have sufficient flow over the characteristic limb shape and the interface is made more cylindrically to make vaccum possible. Too many contours would create channels where are could re-enter the system during motion. Another disadvantage is the increased limb bulk of the liner interface and proximal seal. This has the affect of increasing proximal limb bulk over 20mm medial and lateral and circumferences over 120mm (using a 9mm liner, 3mm plastic interface, 5mm laminated frame, 6mm suspension liner). This large bulk can also limit motion and increase overall maintenance costs of liner and suspension sleeves. The benefit of vacuum to wound healing and skin condition was analyzed by the University of Texas Health Sciences center measuring transcutaneous skin condition (TcpO2) and skin condition continues to be debated. A patient was observed to a change from 40mm-Hg to 50mm-Hg.1 (Image )
Another form of liner adapted for vacuum suspension uses a flexible silicone baffle that seals itself against the socket wall not proximally where motion occurs, but distally where motion is minimal. This simple suspension that grew from hypobaric socks and membranes, involves relatively view parts and benefits from less proximal limb bulk. The main disadvantage is that the membrane must be accommodated in the socket and spacer socks must also be split. (Image )
Using the main criteria of Material Type, Softness, Construction Matrix, Suspension Type, and Degree of Surface Matching can be utilized to advocate the best liner choice. One of the first determinations is the main type of suspension distal pin or vacuum. This often leads to the construction matrix based on durability needs of the distal attachment. Characteristic limb shape is addressed with greater surface matching and less stiff construction matrix. On the durability scale Urethane may be the most durable for loading and shear with Silicone and TPE behind. TPE is typically the most soft with Silicone and Urethane only slightly harder. Urethane and Silicone can be custom made so surface matching is greatest among these custom devices. Off the shelf TPE may be contoured under slight heat and pressure, but the external matrix prevents dramatic shape contours. Finally the liner should match the suspension choice. Pin locking provides distal locking without proximal bulk, but can be difficult to find the insertion and may place excessive shear on the distal limb. Passive vacuum avoids distal distraction and shear, but depends on the stabilized volume of the prosthesis. Active vacuum increases the ability to expel air and may achieve the quasi-hydrostatic support Robin Redhead suggest years ago, but involves another system the must be maintained.
- Bosker, G., Walden, G., The Interfaces between the Transtibial Residual Limb and the Socket Design: The Lower-Limb Prosthetics Society, The Academy Today, February 2008, Vol. 4, No. 1.
- Covey SJ, Mounio J, Street GM. Flow constraint and loading rate effects on prosthetic liner material and human tissue mechanical response. JPO, 2000 Vol. 12 Num. 1, pp 15-32).
- Board WJ, Street GM, and Caspers C. A Comparison of trans-tibial amputee suction and vacuum socket conditions. Prosthet Orthot Int, 2001; 25(3): 202-9.
- Dudek NL, Marks MB, Marshall SC. Skin problems in an amputee clinic. Am J Phys Med Rehabil, 2006 May; 85(5): 424-9.
- Beil TL, Street GM, and Covey SJ. Interface pressures during ambulation using suction and vacuum-assisted prosthetics Sockets. Journal of Rehabilitation Research and Development, 2002; 39(6): 693-700.
- Ehde D, Czerniecki JM , Smith DG, Campbell KM, Edwards WT, Jensen MP, Robinson LR. Chronic phantom sensations and pain following lower limb amputation. Arch Phys Med Rehabil 2000;81(8):1039-44.
- Klute, G., Mechanical properties of prosthetic limbs: adapting to the patient, Journal of Rehabilitation Research and Development, Vol. 38, No. 3, May/June 2001.
- Gard, S., The effect of a shock-absorbing pylon on the gait of persons with unilateral transtibial amputation, Journal of Rehabilitation Research and Development. Vol. 40 No. 2, March/April 2003, pp. 109-124.
- Convery P, Buis AWP. Socket/stump interface pressure dynamic pressure distributions recorded during the prosthetic stance phase of gait of a trans-tibial amputee wearing a Hydro-Cast socket. Prosthet Orthot Int 1999;23:107–112.
- Kahle, J., Conventional and Hydrostatic Transtibial Interface Comparison, JPO, 1999:11(4):85-91.
- Carlson, JM., Functional Limitations from Pain Caused by Repetitive Loading on the Skin: A Review and Discussion for Practitioners, with New Data for Limiting Friction Loads, JPO, 2006:18(4):93-103.
- Fillauer, C., Pritham, C., Fillauer, K., Evolution and Development of the Silicone Suction Socket (3S) for Below-Knee Prostheses, JPO, 1989:1(2):93-103.
- Klasson B. Appreciation of Prosthetic Socket Fitting From Basic Engineering Principles . Technical report WE 172 KLA. Glasgow: National Centre for Training and Education in Prosthetics and Orthotics, University of Strathclyde, 1995.
- Karolewski, T., "A History of Gel Liner Suspension, Lecture" Transtibial Prosthetic Section, Northwestern University Prosthetic-Orthotic Center, Northwestern University, Chicago, Illinois, 2008.
Gerald Stark, CPO/L, FAAOP; The Fillauer Companies, Inc., Chattanooga, Tennessee, USA