Lower-Limb Prosthetics Society:
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Figure 1: Showing percentage and level of amputations. (Delisa JA, Gans BM, Walsh NE, Physical Medicine & Rehabilitation, Principles and Practice, 4th Ed, Philadelphia, PA: Lippincott Williams & Williams 2005.) |
The Residual Limb
The soft tissue of the lower-extremity residual limb transfers the entire load between the skeletal components and the prosthesis. One orthopedic surgeon described it as "an artificial joint developed by a mechanical device to the residual limb to help the amputee ambulate." It is the design and make-up of this artificial joint or socket that has become one of the most significant challenges for practitioners. For example, how does a practitioner design and obtain a comfortable and functional socket when there is daily fluid loss under positive pressures3,4,5 or when the volume changes over time due to maturation?6 Researchers are attempting to understand such fluctuations and possibly assist in designing a more compliant socket for the amputee.7
Other factors involved in obtaining a comfortable socket fit are how the dynamic forces change inside the prosthetic socket during normal movements of gait. From the initial ground contact through the terminal swing, forces and pressures on or against the residual limb dynamically change, which could then lead to breakdown.8
Dudek et al.9 have shown that out of 337 lower-extremity residual limbs, a total of 528 skin problems were documented. The five most common skin problems were ulcers, irritations, inclusion cysts, calluses, and verrucous hyperplasia. These common skin problems represent approximately 80 percent of all documented skin disorders.9 This information should clearly exemplify the need for a better understanding of what makes a comfortable socket and what is needed to keep the comfortable fit. We also understand that the liner functions as the primary interface between the residual limb and socket. In this role it must complement the socket fit to ensure optimal pressure distribution while also attempting to eliminate harmful shear forces and providing a favorable moisture, temperature, and chemical environment that prevents skin breakdown and maintains a comfortable fit during volume loss in the residual limb.
The typical method of dealing with volume loss and the accompanying poor socket fit is to add additional socks. There are also socket designs that add bladders that can be inflated over the day to compensate for volume loss. However, Sanders and Cassisi10 found that the increase in pressure from these compensatory methods may cause further fluid loss. There is also evidence that continued pressure can occlude blood flow and lead to breakdown.11,12,13
Specific Weight Bearing and Total Surface Bearing Sockets
The specific weight-bearing sockets [i.e., patella tendon bearing (PTB), patella tendon bearing supracondylar (PTBSC), or the patella tendon bearing supracondylar, supra-patella (PTBSCSP) (see figure 2)], more commonly use a liner made from closed-cell polyethylene foam such as Pelite® for improved comfort. Using a PTB design socket with a Pelite liner is often an advantage in the preparatory prosthesis because of the relative ease with which the liner can be modified to accommodate changes in residual limb volume.14,15,16
Roll-on silicone, urethane, or elastomeric liners are other options that can be used with a PTB socket but are generally recommended for use with total surface bearing (TSB) socket designs. These types of liners are thought to enhance comfort and reduce shear, making them the initial choice for residual limbs with scarring or skin grafts that compromise skin integrity.14,16 These different liners can have a cushioned distal end or be equipped with a locking pin system. It has been suggested that the locking pin system works because the combination of high coefficient of friction with the skin and low compressive stiffness helps minimize displacement relative to residual-limb skin during walking,17 but compression may also assist in suspension with this design. However, these liners result in more sweating and are generally tolerated less in warm climates than other liners. Contraindications to the use of these liners are residual limbs with open wounds, poor hygiene, or a history of contact dermatitis. It has become apparent that more practitioners are using these different liners due to the theory that they distribute pressure more equally across the entire surface of the residual limb.
Elastomeric Liners
In general there are three main elastomeric liners/materials used in the prosthetic industry for the manufacture of prosthetic liners: thermoplastic elastomer (TPE), silicone, and urethane. Each in its own way has its uses. Comparing these materials is difficult since each liner manufacturer uses its own formula, altering the material properties as tested by the original material manufacturer. With the lack of research concerning the liner materials and some confusion about their properties, we first must identify properties of the materials used in the liners.
Thermoplastic Elastomer
Thermoplastic elastomer (TPE) is composed mainly of a one-part material, which is liquefied either by heat or the addition of solvent. Any liner that can be heated and then formed to a patient mold is a TPE. In addition to the one-part base material, other materials such as mineral oil can be added. TPE liners generally follow the same characteristics as Pelite liners, (i.e., they can be heat molded and tend to thin out over areas of high pressure but have poor memory). TPE liners generally have a lower hardness, ranging from the mid-20s to the low- 30s on the Shore 00 scale. This durometer makes the liner easy to reflect but also requires a thicker material in order to cushion the forces in a prosthetic socket. The specific material description by one manufacturer18 is that the liner is composed of styrene isoprene/butadiene block copolymer or styrene ethylene/butadiene-styrene block copolymer with the addition of purified mineral oil. Another manufacturer also lists this material as the base material.18
Silicone
Silicone is composed of a two-part mixture that is mixed together in varying ratios depending on the type. Silicone products can be separated into two types: condensation, or tin cured; and addition, or platinum cured. Platinum-cured silicones are generally used for medical purposes because of the low risk of skin reaction. Tin-cured silicones are also used in liner manufacturing, but may have a higher skin reaction. Since silicone liners are produced using a catalyst and a resin, once set, there is no modifying the shape. 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. These liners can require less material thickness to absorb the forces in a prosthetic socket.
Urethane
Urethane, similar to silicone but with different flow characteristics, is also a two-part mixture, and once set produces a liner that has good memory. Urethane liners can vary in durometer from the mid-30s to the low-50s on the Shore 00 scale. As with silicone liners, a urethane liner would require less material thickness to absorb the forces in a prosthetic socket.
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Figure 2: PTB style sockets (specific weight bearing). Courtesy of the University of Texas Health Science Center, Dept. of Rehabilitation Medicine. |
Selection of Elastomeric Liners
When evaluating a liner, three component issues need to be addressed: 1) will the liner supply enough comfort for the patient; 2) will it maintain its shape and thickness; and finally, 3) can it support the limb and tolerate the pressures created during the gait cycle?
A review was made of 1,200 transtibial casts.19 It was found that the average residual limb was 5.5 inches long, 12 inches in circumference at the mid-patella tendon, and 9 inches in circumference 1 inch up from the distal end. Using a standard formula, we can calculate that the average transtibial residuum has 52 square inches of surface area. If we were to believe that a total surface weight-bearing socket controlled the forces equally over the entire surface below the mid-patella tendon, then an amputee weighing 175 pounds would produce a force of approximately 3.36 pounds per square inch during static conditions.
According to the study by Beil reported in 2002,5 pressures inside the socket are not even. Higher pressures were measured in the distal portion of the socket. These pressure values range from 6 pounds per square inch to a maximum value of 17.2 pounds per square inch during initial contact during the stance phase of normal ambulation.
If we examine the information supplied in a study by Covey,20 silicone, urethane, and TPE were all subjected to a force of 550 newtons or 124 pounds of force over a 1-inch diameter fixture or 0.785 square inch. This would equal out to 158 pounds per square inch, which is much larger than the forces found above during normal ambulation. The force used for testing in Covey's study is more similar to high-activity sports, which show forces eight times that of normal ambulation.
Using the normal gait forces, silicone and urethane show a similar reduction in thickness, and TPE shows slightly more material displacement. This can be explained by the difference in durometers of the materials. The most appropriate material for use in any socket would be the one that absorbs forces with minimal loss of thickness. Only when the forces are taken to an extreme as in Covey's study do they show a variance of loading rate.
Using the average transtibial residuum that was calculated above, one could then calculate that a decrease of 1mm of liner wall thickness is equal to an increase of 3 percent volume of the socket. Use of the vacuum-assisted, total-surface, weight-bearing socket changes the positive and negative pressures exerted on the residual limb during ambulation. Pressure impulse and peak positive pressures are reduced during the stance phase, while the magnitude of the impulse, average, and peak negative pressures are increased during the swing phase. To eliminate the possibility of failure with a vacuum-assisted socket system (VASS), a material with little deformation during ambulation and excellent memory are essential for a proper fit.
Volume, VASS, and Limb Health
Previous studies on the vacuum-assisted socket focused on residual-limb volume measurements and interface pressures. It was shown that a 3.7 percent gain in residual-limb volume after a 30-minute walk in a total-surface-bearing suction socket with a vacuum (-78kPa) applied.3 In contrast, the same socket with only a check valve showed an average of 6.5 percent residual-limb volume loss after a 30-minute walk. In addition, pistoning of the residual limb in the socket was reduced in the vacuum-assisted version. Further, Beil et al. showed significantly lower positive pressures under the vacuum-assisted condition, resulting in a shift in fluid balance, while Goswami et al. reported that under the same condition, the residual limb could accommodate undersized, neutral, and oversized sockets.4,5 This would indicate that the volume of the residual limb can be controlled but does not address the issue of limb health. There is little research concerning circulation in the residual limb in relation to the prosthetic socket. There have been cases introduced that imply that the VASS does indeed improve limb health, but no research has been completed to substantiate this claim.
An amputee at the University of Texas Health Science Center was evaluated by obtaining transcutaneous oxygen tension (TcpO2) measurements before placing him in the VASS socket, after one month wearing the VASS prosthesis, and then after two years wearing the VASS prosthesis. The beginning reading (before fitted with the VASS system) was below 40mmHG, indicating that if a sore did develop on the residual, it possibly would not heal. After one month, the readings were above 50mmHG, possibly indicating a healthier residual. After two years, the reading was 50mmHG, possibly indicating a maximum condition was reached in the first month of wearing the VASS. This leads the authors to ask whether using the VASS improved the oxygen supply to the transcutaneous tissue. This clinical evaluation only indicates that further studies are needed to evaluate the VASS and all other sockets to find which would promote better limb health.
Conclusion
There are numerous factors that affect a comfortable fit for a prosthesis. Fit, alignment, componentry, suspension system, and materials are just a few. We should remember, however, that the amputee is the key factor in making the prosthesis as functional as its design permits. If the prosthesis is not comfortable, for whatever reason, this will have a negative impact on the amputee's quality of life, preferred lifestyle, and overall well-being.
Further research is required to understand what promotes better limb health and outcomes. There are numerous socket designs, but which ones best promote a healthier limb while providing a functional and comfortable interface is not so clear. For amputees with diabetes, this matter is especially important, given their compromised circulation and propensity for poor wound healing.
References
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Centers for Disease Control and Prevention website (September 26, 2007) Data and Trends. www.cdc.gov/diabetes/statistics/incidence/fig2.htm
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Bennett L. et al. Shear vs pressure as causative factors in skin blood flow occlusion. Arch Phys Med Rehabil, 1979; 60(7): 309-14.
Colin D and Saumet JL. Influence of External Pressure on Transcutaneous Oxygen Tension and Laser Doppler Flowmetry on Sacral Skin. Clin Physiol, 1996; 16(1): 61-72.
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Kahle JT. Conventional and hydrostatic interface comparison. JPO, 1999; 11:85-90.
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Sanders JE, Nicholson BS, Zacharia SG, Cassisi DV, Darchin A, Fergason JR. Testing of elastomeric liners used in limb prosthetics: Classification of 15 products by mechanical performance, JRRD, 2004; 41(2); 175-86.
U.S. patent office (October 7, 2007) Patent Numbers 5,830,237 & 6,117,119, www.uspto.gov
Data supplied by Evolution Liners Inc. Orlando, FL.
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).
