Chapter 4 - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles
Prosthetic Management: Overview, Methods, and Materials
Michael J. Quigley, C.P.O.
PROSTHETIST'S ROLE AND PRACTICE
The prosthetist of today is a highly skilled individual who must meet significant educational and professional standards prior to obtaining board certification. Training in limb prosthetics has advanced from apprenticeship programs without formal academic standards in the 1940s to the present requirements for a baccalaureate degree, supervised internship, national certification examinations, and mandatory continuing education.
The prosthetists role in the rehabilitation team has become more significant as a result. In the period following World War II, prostheses were relatively simple, and prescriptions were therefore extremely specific, with the prosthetist given little latitude in exercising clinical judgement. Given the complexity of todays componentry, many of the details regarding prosthetic configuration are now based on clinical results observed by the prosthetist during the dynamic alignment procedure. The prosthetists function as a consultant to rehabilitation programs and hospitals is also now more clearly defined, and board certification in prosthetics is usually required if the rehabilitation programs are to receive national accreditation.
The vast majority of prosthetists are small businessmen or employees of small businesses. The field of orthotics is closely aligned with prosthetics, and many individuals achieve certification in both disciplines. Large rehabilitation hospitals may have their own prosthetic and orthotic department or may contract with local private prosthetists and orthotists to have on-site provision of services.
Prosthetists work within the same constraints as other health care workers, including the ubiquitous problems of rising malpractice insurance costs, federal and state cost containment programs, and increased difficulty in dealing with third-party payers. Most patient referrals come directly from orthopedic, physical medicine, and vascular physicians. Referrals also come from physical therapists, insurance carriers, and recommendations by former patients.
The field of prosthetics in the United States is represented by three national associations. The American Orthotic and Prosthetic Association (AOPA) was founded shortly after the turn of the century and deals with business issues. The American Board for Certification in Prosthetics and Orthotics, Inc. (ABC), is a certifying body that sets educational standards for prosthetists, holds examinations, maintains records, and accredits facilities that are accessible, clean, and well equipped. The third association is the American Academy of Orthotists and Prosthetists (AAOP), whose members must be certified by the ABC. The AAOP provides numerous national and regional meetings, maintains a mandatory continuing education program, and produces a number of publications including the Journal of Prosthetics and Orthotics, published jointly with the AOPA.
PATIENT EDUCATION AND ORIENTATION
When amputation of a limb is being considered, it is important to inform the patient as early as possible about future rehabilitation. It is not unusual for the recent amputee to become depressed, withdrawn, or angry; in fact, it is unusual when amputation seems to have no effect on the persons attitude. Patient education and counseling can come from several sources, including a formal clinic, an amputee support group, or individuals such as the prosthetist, nurse, or social worker. Whenever possible, both the prosthetist and a support group should be contacted immediately following amputation because each can provide the patient with valuable information.
Amputee support groups are not new in the United States, but they are now better organized and more widely available. Most support groups have two purposes: (1) to introduce the recent amputee to realistic role models who have gone through the rehabilitation process and are functioning normally in society and (2) to provide ongoing social and educational programs. Amputee support groups usually have special training sessions for their members to ensure that the initial visit with a new amputee will be a constructive one.
An early visit by the prosthetist can also be helpful. The prosthetist frequently has written information on prosthetic options and rehabilitation and can often demonstrate various types of prostheses to the patient or patient's family. The prosthetist can also answer many of the basic questions, such as "Is a prosthesis worn to bed every night?" or "Are special orthopedic shoes needed whenever the prosthesis is worn?" The prosthetist can also be certain that preprosthetic management with rigid dressings, elastic bandaging, or prosthetic shrinkers is employed to speed the maturation of the residual limb.
When the amputee is ready for prosthetic fitting, additional orientation information should be offered. An explanation of the different stages of the rehabilitation process is in order, including how long the preparatory prosthesis will be used and when the evaluation for a definitive prosthesis will occur. Many amputees are seen in a prosthetic clinic setting 1 or more months following their amputation. Unfortunately, in many cases they still have not been informed of the entire process and are confused by the number of health care professionals in attendance. Whenever a new patient is seen, it is best for the prosthetist to assume that no one has yet explained the process and to offer a concise overview of the prosthetic procedures about to begin. The amputee tends to develop confidence in the person willing to spend the time to provide a clear explanation of the rehabilitation process, and this can enhance the overall outcome.
TYPES OF PROSTHESES
There are five generic types of prostheses: postoperative, initial, preparatory, definitive, and special-purpose prostheses. Although progression through all five levels may be desirable, only selected amputees will receive the postoperative or initial prostheses, which are directly molded on the residual limb. Most amputees will have preparatory and definitive prostheses, but a much smaller number will receive special-purpose prostheses for showering or for swimming and other sports.
Postoperative prostheses are, by definition, provided within 24 hours of amputation. These are often referred to by various acronyms including immediate postsurgical fitting (IPSF) and immediate postoperative prosthesis (IPOP). Although technically feasible for virtually any amputation, postoperative fittings are currently most commonly prescribed for the younger, healthier individual undergoing amputation due to tumor, trauma, or infection. Its use in the elderly or dysvascu-lar individual is controversial but can be successful when meticulous technique and close supervision are available.
The initial prosthesis is sometimes used in lieu of a postsurgical fitting and is provided as soon as the sutures are removed. This is sometimes referred to as an early postsurgical fitting (EPSF). Due to the usual rapid atrophy of the residual limb, the EPSF is generally directly molded on the residual limb by using plaster of paris or fiberglass bandages. An alternative is to use a weight-bearing rigid dressing such as the technique reported by Wu. Such devices are used during the acute phase of healing, generally from 1 to 4 weeks after amputation, until the suture line is stable and the skin can tolerate the stresses of more intimate fitting. Postoperative and initial prostheses are most commonly used in rehabilitation units or in hospitals with very active amputee programs.
Preparatory prostheses are used during the first few months of the patient's rehabilitation to ease the transition into a definitive device. They are also used in marginal cases to assess ambulatory or rehabilitation potential and help clarify details of the prosthetic prescription. The preparatory prosthesis accelerates rehabilitation by allowing ambulation before the residual limb has completely matured. Preparatory prostheses may be applied within a few days following suture or staple removal, and limited gait training is started at that point.
Originally, the preparatory prosthesis was a very rudimentary design containing only primitive components. The modern preparatory limb, however, usually incorporates definitive-quality endoskeletal componentry but lacks the protective and cosmetic outer finishing to reduce the cost, (Fig 4-1). It allows the therapist and prosthetist to work together to optimize alignment as the amputee's gait pattern matures. Different types of knee mechanisms or other components can be tested to see whether individual patient function is improved.
Preparatory prostheses are generally used for a period of 3 to 6 months following the date of amputation, but that time can vary depending on the speed of maturation of the residual limb and on other factors such as weight gain, weight loss, or health problems. The new amputee may begin by wearing one thin prosthetic sock in the preparatory prosthesis; after 3 months, he may be wearing ten plies of prosthetic socks to compensate for atrophy. When the number of plies of prosthetic socks the patient must wear remains stable over several weeks, it is usually an indication that the definitive prosthesis can be prescribed.
The definitive prosthesis is not prescribed until the patients residual limb has stabilized to ensure that the fit of the new prosthesis will last as long as possible. The definitive prescription is based primarily upon the experience the patient had when using the preparatory prosthesis. The information learned during those months will demonstrate to the clinic team the patient's need for a lightweight design, special types of feet or suspension, or any special weight-bearing problems that may arise.
Unless a suction socket is used, the amputee wears prosthetic socks over the residual limb for the same reason that people wear socks when wearing shoes: the textile fibers provide cushioning and comfort, take up shear forces, and absorb perspiration. An additional advantage is the ability to accommodate minor volume fluctuations by wearing more or fewer layers (plies) of prosthetic socks. Once the amputee is wearing ten plies of prosthetic socks, the fit has degraded sufficiently that socket replacement should be considered.
A definitive prosthesis is not a permanent prosthesis since any mechanical device will wear out, particularly one that is used during every waking hour. The average life span for a definitive prosthesis is from 3 to 5 years. Most are replaced due to changes in the amputee's residual limb from atrophy, weight gain, or weight loss. Substantial changes in the amputee's life-style or activities may also dictate a change in the prosthetic prescription. Overall physical condition is also a factor since the more debilitated individual generally requires a very lightweight and stable prosthesis.
A certain number of patients will require special-use prostheses designed specifically for such activities as showering, swimming, or skiing. It is most economical if special-use devices are prescribed at the same time as a definitive replacement is necessary since both can be fabricated from the same positive model. Most require specialized alignment. For example, swimming prostheses are made waterproof and aligned so that the patient can walk without a shoe. In some cases the foot can be plantar-flexed and have a swim fin attached. Snow skiing prostheses require an increase in dorsiflex-ion at the ankle and may incorporate additional knee support or auxiliary suspension.
Special-use prostheses can be valuable to the amputee who wishes to expand his activities and participate in a full range of sports and recreational pursuits. There are sports-related amputee organizations in every major city in the country, with the greatest participation in golf and snow skiing. The value of amputee sports and recreation has been recognized by the Veterans Administration, which decided several years ago to reimburse patients for the cost of special-use prostheses.
GENERAL PRESCRIPTION GUIDELINES
There are many factors to be considered when a new prosthesis is prescribed, including weight bearing, suspension, activity level, general prosthesis structure, components, expense, and certain unique considerations. These will be discussed in order.
- Weight bearing.-For lower-limb prostheses, the weight-bearing characteristics of the socket are the first concern. If the patient has scarring, neuromas, or sensitive areas, specific provisions must be made in the design of the socket. Special impact-absorbing materials may be used, or modifications may be necessary to spread the load over a greater area. For example, in a trans tibial (below-knee) prosthesis, a thigh corset might be considered if weight bearing causes severe problems with the residual limb.
- Suspension.-There are many methods of suspension, ranging from very basic leather belts to sophisticated suction sockets. Each alternative must be evaluated individually; anticipated volume change in the residual limb is a key factor. It is important to review any previous experience with other suspensions to determine the optimum recommendation.
- Activity level.-A person using the prosthesis only indoors obviously presents different considerations from someone who anticipates being active in his job and in competitive sports. Activity level influences weight bearing, suspension, and structural strength of the prosthesis.
- Structure of the prosthesis.-There are two basic structural types: endoskeletal and exoskeletal. En-doskeletal prostheses consist of internal tubes and components covered with a soft foam outer cover. They are becoming increasingly popular because of the inter-changeability of componentry for trial or repair, their relatively light weight, and the good appearance they offer. Exoskeletal prostheses, on the other hand, consist of wood or polyurethane covered with a rigid plastic lamination. For very active patients, the exoskeletal prostheses are more durable since the foam coverings of the endoskeletal designs tear easily and need replacement at intervals.
- Prosthetic components.-Components need to be matched with the amputee's activity level, body weight, and functional goals. Obviously, the person with good strength and balance does not require a stance-control knee, while someone who intends to compete in the Boston marathon would require an artificial foot designed for a high activity level. Due to the large and expanding number of options now available in prosthetic componentry, close consultation with the prosthetist is imperative.
- Expense.-The expense of a prosthesis may vary greatly, primarily depending on the need for lightweight or sophisticated componentry. Lightweight prostheses are often made from titanium or carbon fiber, aerospace materials that are both expensive to obtain and difficult to manufacture, which may increase the cost of componentry 50% and more. Sophisticated componentry such as hydraulic knees will increase the cost of the prosthesis as well. Each feature of the prosthesis should be considered carefully to provide the most cost-effective solution that fully meets the needs of the individual amputee.
- Unique considerations.-Many patients present unique factors that need to be considered in the design of the prosthesis. For example, someone who lives near the ocean may need a prosthesis designed with maximum protection from salt corrosion and water damage; the finish carpenter needs more comfort from the prosthesis in the kneeling position than the average wearer does. Cultural background is also significant; Asian amputees require a foot that allows the shoes to be removed easily when entering a home since that is customary. Such personal factors must be added to the more generic factors discussed previously to ensure the proper match between prosthetic configuration and amputee goals.
It is important for the prosthetist to thoroughly evaluate the amputee before starting to design the prosthesis. The prosthetist's physical examination should be very detailed and record such factors as adherent scar tissue and neuromas, range of motion, edema, and muscular development. A careful personal history helps identify the likelihood of weight fluctuations as well as medical factors that may have a bearing on prosthetic fitting, such as previous fractures, any visual impairments, and the presence of concomitant disease including arthritis or diabetes.
Measurements are then taken of both the residual limb and sound limb. The length of the residual limb is measured, and circumferences are taken at intervals (Fig 4-2). Those who are being evaluated for powered upper-limb prostheses will have myoelectric control sites identified by electromyographic (EMG) testing. Once the measurements are completed, a negative impression of the residual limb is obtained.
The hollow plaster or fiberglass cast of the residual limb creates the negative impression. It is sealed and filled with liquid plaster, which hardens to form an accurate positive model of the patients limb. The impression is generally taken in a specified position and is usually hand-molded by the prosthetist to more clearly define key anatomic landmarks. A number of fixtures have been specifically developed for taking this impression, especially for the transfemoral (above-knee) amputee. The purpose of the casting fixture is to preshape the soft tissue so as to result in a negative impression that more closely resembles the finished prosthetic socket. Proper angulation for initial static alignment of the prosthesis is recorded by using plumb lines drawn onto the negative impression. Some specialized impressions are taken by using alginate, which is a gelatinous material commonly used for dental molds. In prosthetics, partial-hand and -foot amputations are often molded in alginate rather than plaster.
In summary, the impression procedure provides much more than simply a model of the patients limb; it also simulates the socket design and provides alignment information. When combined with an accurate physical examination and personal history, the impression forms the foundation for prosthetic design.
POSITIVE MODEL RECTIFICATION
The positive model of the residual limb is rectified or modified by the prosthetist to improve the pressure distribution. Judicious addition or subtraction of material relieves the bony prominences and tender areas while increasing pressure to more tolerant areas such as soft tissue and broad expanses of bone or tendon (Fig 4-3). For example, in the transtibial (below-knee) prosthesis, pressure is increased by removing material at the following areas: patella tendon, pretibial muscles, flare of the tibia, popliteal area, and the calf musculature. Conversely, pressure is relieved by adding material in the following areas: tibial crest, distal portion of the tibia, fibula head, hamstring tendons, and patella.
Positive model modification is a difficult and time-consuming procedure requiring much skill on the part of the prosthetist. With proper modification, the prosthetist can create a comfortable and stable socket with good suspension characteristics and can relieve any particular problem areas that the patient has experienced in the past.
COMPUTER-AIDED DESIGN/COMPUTER-AIDED MANUFACTURING
An alternative method of socket design and fabrication is beginning to enter clinical practice: use of the microcomputer to automate repetitive portions of the fabrication process. Current computer-aided design/ computer-aided manufacturing (CAD/CAM) systems consist of three major components:
- A digitizer that converts information from the negative impression of the patient's residual limb into numerical data that are read by the computer.
- A software system that provides a visual image of the patient's residual limb on a video screen.
- A carver that reads the modified computer image of the patient's residual limb and carves a rectified plaster model.
Presently, the most practical method of converting information about the residual limb into data that the computer can understand is by taking an impression of the patient's residual limb with plaster or fiberglass and then placing this impression in the digitizer machine, which holds it in a centered position. A probe, or digitizing arm, is then used to locate and identify different landmarks in the negative impression. This digitizing arm is attached directly to the computer and converts the location of each landmark into numerical data as it slowly spirals upward and records the dimensions of the entire negative impression (Fig 4-4). Other systems are presently under development that utilize light sources or ultrasound to obtain an image of the residual limb in the hope of obviating the need for a plaster or fiberglass impression in the future.
Once the digitized impression of the residual limb is in the computer, the prosthetist can look at an exact replica of the patients residual limb on the computer screen. The replica is generally shown as a wire frame drawing that gives a good three-dimensional perspective, but it can also be viewed as if it were a solid model. The prosthetist then modifies the image by sketching the desired changes on the screen by using either a mouse or keyboard control (Fig 4-5).
Once the prosthetist is satisfied with the socket design, a numerically controlled milling machine can be used to carve the rectified plaster model from a blank (Fig 4-6). The socket is then fabricated in the conventional manner or with a semiautomated machine (Fig 4-7). CAD/CAM systems in prosthetics were introduced in the mid-1980s and were not used on a clinical basis in the United States until the early 1990s. Although this first generation of CAD/CAM in prosthetics is very basic, it is a logical development that parallels related advances in other professions.
Future generations of CAD/CAM systems may change the face of prosthetics because the use of a computer enables those with fewer manual skills to make properly fitting sockets. Recently, several central fabrication companies have purchased the extremely expensive carving and socket manufacturing equipment so that a prosthetist need only purchase the digitizer, computer, and software. This should reduce the cost of this new technology since the highly expensive carving and manufacturing equipment can be amortized by a large number of prosthetists. Although prosthetists presently freely trade disks of information regarding the modification techniques that they have developed on the computer, it is likely that such information will be commercialized in the future. The extremely high cost of CAD/CAM systems will only be justified if it can be proved that they can provide the patient with a very well fitted prosthesis while saving time for the prosthetist and increasing his productivity and skills.
Regardless of the method chosen for rectification and manufacturing of the socket, test socket fitting is recommended to ensure that the socket fits the patient optimally before it is attached to an artificial limb. Test sockets are made over the modified positive model from a number of materials; a transparent plastic is the most common choice (Fig 4-8).
A separate appointment is usually necessary for test socket fitting. The patient is instructed in the use of prosthetic socks (when applicable), and these are put on the patient's limb before the socket is applied. When weight-bearing test sockets are fitted, the socket is placed on an adjustable stand and raised to the proper level so that the patient can bear equal weight on both limbs, or the test socket may be attached to the components of the prosthesis. Several methods may be used to evaluate the socket fit, but in most cases holes are drilled in the socket at strategic areas such as over bony prominences or areas critical to suspension. The tissue is then probed with a small, blunt rod to determine local skin pressures. Areas of excessive or inadequate pressure can also be noted by observing the amount of compression to the weave of the prosthetic sock or by the presence or absence of skin blanching if no sock is being worn. During the test socket fitting, the prosthe-tist will frequently split the socket to make volume adjustments or will heat-modify and trim away portions of the socket. Reference marks are made in those areas that cannot be fully relieved during the test socket fitting. When the test socket is filled with plaster to create a new positive model, the prosthetist can modify the mold in this area to ensure proper fit.
The following four factors are evaluated during the test socket fitting:
- Even distribution of weight-bearing pressure and biomechanical forces
- Freedom of motion at the next proximal joint
Since each patient has a unique gait pattern and activity level, dynamic alignment of the prosthesis must be done on an individual basis. The purpose of dynamic alignment is to provide maximum comfort, efficient function, and cosmesis by adjusting the relative position of the components while the patient is using the limb in a number of controlled situations.
During the alignment stage, the prosthesis must be durable and functional but must also be adjustable in all planes. For example, in an upper-limb fitting, the suspension and control harnesses, cable attachments, and forearm length are all adjustable, and the efficiency of various configurations is measured by the use of prehension gauges and force scales.
Some patients may require more than one visit to optimize the alignment of the prosthesis since the more complex alignments may require several hours of adjustments and new patients are frequently not able to stand and ambulate for more than a few minutes at a time. Complicated cases, of course, also require additional time. Lower-limb alignment generally takes place within parallel bars in a private walking room in the prosthetist's office. The following procedures summarize the basics of the alignment process:
- The function of the prosthesis is explained, and the amputee is instructed in how to don the prosthesis properly, including the use of prosthetic socks, if required.
- Contours are checked to ensure that the socket fits properly and comfortably.
- The length and angulation of the prosthesis is checked.
- The suspension is tested.
- The patient is instructed to stand in a relaxed attitude while wearing the prosthesis.
- Static alignment of the components is refined (Fig 4-9).
- The patient begins to use the prosthesis in a controlled manner by walking inside parallel bars or operating the terminal device. Dynamic function of components is checked during use and adjusted to provide maximum efficiency, comfort, and cosmesis (Fig 4-10).
- The prosthesis is checked with the patient sitting, and adjustments are made to increase comfort or function in this position as well.
Socket design and alignment complement each other and are the fundamental determinants of prosthetic function. No matter how sophisticated the components are, how well the prosthesis is finished, or how carefully it is fabricated, if it is malaligned or uncomfortable, the overall function will be drastically reduced.
Fitting and alignment of the prosthesis are not completed until both the prosthetist and amputee are convinced that the prosthesis is functioning as well as possible. More experienced individuals are usually able to provide accurate feedback concerning how the prosthesis fits and feels during walking. The prosthetist can then make adjustments in a rapid and accurate fashion, and the fitting proceeds smoothly (Fig 4-11). New amputees, however, cannot always provide accurate feedback; therefore, they are sometimes referred to a physical therapist for initial gait training prior to completion of dynamic alignment. The new amputee can then practice with the prosthesis, and further adjustments can be made as endurance and ability to use the prosthesis improve. Generally, 1 week in physical therapy with the prosthesis will afford adequate time for the prosthetist to make decisions concerning the final alignment. The therapist also helps the amputee master more advanced activities such as negotiating inclines, stairs, and irregular terrain. It is often useful for the amputee to return to the therapist following fitting with the definitive device to further refine his prosthetic skills (see Chapter 23).
Proper patient follow-up is of critical importance in prosthetics. New amputees in particular require follow-up at frequent intervals; they should be developing not only tolerance to pressures of the prosthesis against the skin but also general physical endurance. Patients will have many questions after wearing the prosthesis for a week or two, such as how to use the prosthesis while driving a car and during sports activities and dancing, choosing the proper shoes, and wearing the prosthesis to the beach. In addition, a number of minor problems can occur during the first few weeks of prosthetic wear from pressure areas in the socket, discomfort while sitting, or problems when wearing different shoes. These concerns can be easily corrected during a follow-up visit.
Patients should be seen, at the very minimum, every 4 to 6 months. The prosthesis contains many moving mechanical components that require cleaning, maintenance, or replacement at intervals. Some components, particularly joint mechanisms, must be cleaned and adjusted on a regular basis because they directly affect the function of the prosthesis.
Changes in the volume or shape of the patient's residual limb will frequently require socket adjustments, particularly during the first month of wearing a new prosthesis. In some cases, varying the thickness or ply of the prosthetic socks will improve the fit of the prosthesis, but in many cases more extensive modifications are required. Socket adjustments are made only after a careful analysis of the cause of discomfort is completed by the prosthetist. The prosthetist then has two choices: relieving the pressure area by removing material from the socket over the area of pressure or adding material elsewhere, thereby redistributing the forces. In some cases, minor alignment changes can be made to further reduce discomfort or pressures.
It is important for the prosthetist to keep a good record of all follow-up adjustments. Such information will help guide future decisions regarding socket or component modification and prosthetic design.
A wide variety of natural and man-made materials are used in prosthetics today. Whether natural or man-made, however, they must still conform to the special requirements of the profession: biocompatibility, strength, durability, light weight, and ease of fabrication. The most common materials used in prosthetics today are various plastics, but the more traditional materials such as wood, leather, metal, and cloth still have a role to play.
Wood is often used in lower-limb prostheses to provide shape and interior structural strength. The inherent properties of wood make it a very difficult material to replace: it is lightweight, strong, inexpensive, easy to work, and consistent in texture. Basswood (linden), willow, and poplar wood are most commonly used for prosthetic knees and shins because they are lightweight, strong, and free from knots and can be shaped easily by using standard woodworking tools. When a prosthesis is finished, it is hollowed out until the wood is only 6 mm (1/4 in.) thick to reduce weight.
Hardwoods are also used in lower-limb prosthetics. Solid-ankle, cushion-heel (SACH) feet have an interior hardwood keel that provides structural strength to the foot. This keel is bolted to the rest of the prosthesis and provides a strong anterior lever arm when the amputee stands and walks. Maple and hickory are commonly used for keels in prosthetic feet and to reinforce high stress areas of prosthetic knee units. Hardwoods are not used in areas where the prosthetist might make an adjustment since the inherent strength of these woods makes them very difficult to reshape.
Leather is another material still commonly used in prosthetics for suspension straps, waist belts, and socket linings. Leather is easy to work with, has a soft natural feel, and is biocompatible. Many years ago hides were available from horses, elk, deer, as well as cattle, but today cowhide is modified by the tannery to provide the same feel and working properties as the hides of other animals. Therefore, "horsehide" is actually cowhide that has been treated to provide the thin, soft flexible properties of the original horsehide. The properties of "horsehide" make it a very attractive material to use when the leather is to contact the skin; therefore, it is used to line waist belts, suspension straps, and inserts for patellar tendon-bearing (PTB) sockets. Since "horsehide" will stretch easily with wear, it must be reinforced with another material such as cowhide or synthetic fabric.
Other variations such as elk, calf, kip, and rawhide, are used for a variety of purposes in prostheses from such things as dorsiflexion stops in single-axis feet to laces for leather thigh corsets. Plastics such as Naugahyde and thermoplastic foam have replaced leather in some applications but will probably never completely replace this readily available biological material.
Cloth is used for prosthetic socks, waist belts, straps, and harnesses for upper-limb prostheses. Probably the greatest use of cloth is for prosthetic socks, which can be considered analogous to athletic socks since they keep the skin dry, cushion the limb, absorb shear forces, and take up volume to improve the fit. Prosthetic socks are commonly made of wool, cotton, or blends of these natural fibers often combined with nylon, Orion, acrylics, or other man-made materials.
Wool is the most common material used for prosthetic socks because of its characteristic elasticity, cushioning, and ability to absorb moisture without feeling damp. Wool also has good resistance to acids from perspiration. The blend of domestic and foreign wool fleece used in prosthetic socks provides greater resistance to shrinkage. Pure wool, however, must be washed carefully in a mild soap that will dissolve in lukewarm water. The sock should be rinsed in lukewarm water as well since a change in temperature will affect it adversely. Wool prosthetic socks should be dried carefully by first removing the excess water, wrapping them in a towel, and then drying them away from sunlight or any other direct heat. The recent development of machine-washable wool should reduce the need for hand washing in the future.
Cotton is also used for prosthetic socks but is more common in the form of a stockinette used to protect the limb during casting procedures. Cotton is also blended with wool in prosthetic socks, and some 100% cotton prosthetic socks are available. Cotton is a natural vegetable fiber that is soft, pliable, and absorbent, but falls short of wool in all of its properties. Cotton, however, is easier to care for and less expensive than wool, which makes it more practical for many uses.
Nylon is used for prosthetic sheaths, plastic laminations, bushings, suction valves, and nylon stockings to cover prostheses. The major advantages of this man-made fiber are its strength, elasticity, and low coefficient of friction. Nylon prosthetic sheaths are in common use for transtibial amputees. A thin sheath worn directly over the skin significantly reduces shear stresses and helps to pull moisture away from the skin into the outside prosthetic socks. A nylon stockinette provides inherent strength to nearly all prosthetic laminations (Fig 4-12). Three to eight layers of nylon are impregnated with polyester or acrylic resins during the lamination process to provide both structural strength and a pleasant appearance to the finished device. Nylon is a thermoplastic material, which means that it can be heated and remolded without adversely affecting its physical properties.
Acrylics are thermoplastics that have greater durability and strength than polyester resins do. Acrylic fibers are frequently used in the newer synthetic blends for prosthetic socks since this material is soft, durable, and machine washable. Acrylic resin is increasingly popular for laminations in prosthetics because its high strength permits a thinner, lighter-weight lamination and its thermoplastic properties allow easier adjustments of the prosthesis by reheating the plastic and remolding it locally. Acrylic resins tend to have a softer feel than polyester resins but are more difficult to use during fabrication. Clear acrylics have been used for years in the sign and building industry for skylights and enclosures for shopping centers; they do not yellow and have good weather resistance.
Polyester resin is a thermosetting plastic that is most commonly used for laminations in prosthetics. Thermosetting plastics cannot be heated and reformed after molding without destroying their physical properties. Polyester resins come in a liquid form that can be pigmented to match the patient's natural skin tone. A benzoyl peroxide catalyst is then added to this resin to initiate the setting process, and a promoting chemical is added to speed up the setting time.
Polypropylene is used for hip joints, pelvic bands, knee joints, and lightweight prostheses. Polypropylene is used in great quantities in industry for everything from fan shrouds in passenger cars to carpets and shipping containers. Polypropylene is an opaque white material that is relatively inexpensive, strong, durable, and easy to mold. This material can be welded by using hot air or nitrogen. Polypropylene sheets 1 to 9 mm (1/16 to 3/8 in.) thick are heated and vacuum-formed over the mold of a socket or complete limb.
Polyethylene is an opaque white thermoplastic that looks like polypropylene but feels waxier. The properties of polyethylene vary depending on the density of the material. Low-density polyethylene (LDPE) is very flexible and easy to heat and mold; it is used for triceps cuffs in transradial (below-elbow) prostheses and for tongues in plastic thigh corsets and hip disarticulation sockets. High-density polyethylene (HDPE) is more difficult to modify and is used to make bushings in joint mechanisms. Ultrahigh-molecular weight (UHMW) polyethylene is sometimes used in partial-hand or partial-foot prostheses due to its tear resistance.
Polyurethane foams are widely used in prosthetics for both soft cosmetic foam covers (Fig 4-13) and rigid structural sections. Polyurethanes, also called ure-thanes, are available in three broad groups: flexible foam, rigid foam, and elastomers.
Flexible urethane foams are generally purchased in prefabricated pieces from suppliers as covers for en-doskeletal prostheses. The foam is shaped by the pros-thetist from measurements and tracings of the patients limb. Flexible polyurethane foams are also widely used in the manufacture of prosthetic feet.
Rigid polyurethane foams compete with wood in providing structural stability to knee units and ankle blocks. Prosthetists routinely use this foam to provide both strength and shape to exoskeletal prostheses. A plastic lamination covers the foam to provide additional strength and cosmesis.
Silicones are used in prosthetics for distal end pads in sockets, to provide a flexible rubberlike end in air-cushion sockets, and for silicone gel insets. Silicones can be classified as fluids, elastomers, or resins, and all three are used in prosthetics. Silicone is synthesized from sand (a combination of silicon and oxygen) and undergoes a number of chemical reactions before liquid or solid silicone results.
The room-temperature-vulcanizing (RTV) silicones are used most widely in prosthetics. Silicones have relatively uniform properties over a wide temperature range, repel water, are chemically inert, resist weathering, and have a high degree of slip or lubricity. Silicone fluid is used for lubrication of moving parts, as the liquid inside hydraulic knee mechanisms, and as a parting agent. A two-component silicone elastomer is used for foaming end pads in sockets while the patient is weight-bearing to ensure total contact. Silicone gel-impregnated gauze is an excellent cushioning and force distribution material for weight-bearing prosthetic sockets. Although the gel adds weight and bulk to a prosthesis, it has been proved to work well for many problem cases, particularly those with burns or severe scarring.
Two basic types of high-strength fiber reinforcements are used in prosthetics today: glass and carbon. Fiberglass is commonly used to reinforce polyester resin laminations where mechanical attachments such as bolts and screws will fasten. It is also used to stiffen thin areas and to prevent breakage in vulnerable areas. Fiberglass is difficult to finish smoothly, so care must be taken to avoid exposed areas of this material. The added strength fiberglass provides is proportional to the amount used and also depends on the arrangement of the fibers relative to the stresses it must tolerate. A unidirectional arrangement of fibers found in continuous-strand roving allows the best reinforcement if placed directly in line with the stresses. Multidirectional fibers such as woven mat or fabrics provide equal strength in all directions but are less effective when only one stress must be tolerated.
Carbon fibers are more expensive than fiberglass but have superior strength and stiffness. They are also being used by component manufacturers to replace metal. Carbon fibers are generally set in epoxy and can provide a material with a stiffness twice that of steel at a fifth the weight. In addition to this high strength-to-weight ratio, carbon fiber composites have a fatigue resistance twice that of steel, aluminum, or fiberglass. Prefabricated carbon fiber prosthetic components such as pylon tubes, knee joints, and connectors can significantly reduce the weight of the prosthesis while increasing its strength.
Modern prosthetic care is far more complex and far more effective than was the case just a few decades ago. Lightweight endoskeletal devices allow clinical verification of the suitability of specific componentry or permit realignment as the new amputee's gait matures (Fig 4-14). Widespread availability of transparent test sockets allows more precise and more comfortable fittings, while sophisticated knee/ankle/foot mechanisms and myoelectrically controlled prehensors have increased the versatility of prosthetic devices. Rehabilitation begins prior to amputation and continues as the amputee progresses from the postoperative or initial device to the preparatory and definitive designs. Special-use prostheses for sports and recreational uses are now available. Although aerospace materials are increasingly common in prosthetic design, the traditional materials such as wood, cloth, and leather still have a role to play.
- Wu Y, Keagy RD, Krick HJ, et al: An innovative removable rigid dressing technique for below-the-knee amputation. J Bone Joint Surg Am 1979; 61:724-729.
- Wu Y, Krick HJ: Removable rigid dressing for below-knee amputees. Clin Prosthetics Orthotics 1978; 2:33-44.
Chapter 4 - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles