Chapter 24C - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles
Special Considerations: Emerging Trends in Lower-Limb Prosthetics: Research and Development
Charles H. Pritham, C.P.O.
In the period beginning shortly after World War II, a revolution occurred in the provision of prosthetic care. While it is not appropriate to review this transformation in depth, a few major points will provide perspective about present circumstances and speculation regarding future developments.
- The new techniques and devices that appeared during that time were not necessarily invented but rather synthesized from the ofttimes conflicting body of contemporary practice and theory as reiterated in the light of fundamental studies of human gait. Radcliffe and others showed that applying engineering concepts of rational analysis and development to clinical problems would bear fruit many times over.
- As stated, many of the concepts now identified with the new practice of prosthetics were readily available to all. What reshaped the practice of prosthetics was the dissemination of this now rationally ordered and internally logical body of knowledge. Instructional courses were held, and from these, long-term courses of study in prosthetics were developed, thereby creating a new profession in the place of a craft. Prepared in clinical practice and inspired with the example of past efforts, this new cadre of better-educated prosthetists embarked on research and development efforts of their own.
- The standardization of prosthetic practice along with the education of a new generation gave impetus to the development of a fledgling industry in the manufacture of prosthetic components. Previously, a limb fitter would often custom-fabricate every element of a prosthesis and use a minimum of purchased items. It took many years to learn the requisite skills, it was labor-intensive, and the results varied widely. The new generation of prosthetists was unequipped and unwilling to adopt this method of practice. Their willingness to accept standardized manufactured components freed them from bench work, gave them time to pursue new avenues of development, and created a market. With the creation of a market and the revenue that it generated, it became attractive for prosthetic suppliers to engage in research and development efforts of their own. Today, competition among the suppliers ensures a continuous process of innovation to allow not only growth but economic survival.
- All of the above happened because the federal government provided leadership, coordination, and funding. The result was an integrated process of innovation, development, dissemination, and feedback. Many disparate groups with widely varying interests were brought together in a common purpose through the intercession of this outside agent with its independent base. Today, the federal government has fundamentally abandoned its leadership, and the feedback loop has been disrupted. Financial support of the schools of prosthetics and orthotics is gradually being withdrawn. While the loss of central coordination and leadership is regretted, it is a tribute to the excellence of the work done in the post-World War II period that developmental efforts by clinicians, manufacturers, and others have continued. Present and future efforts, as shaped by these four factors, promise to produce continuing results in a number of different areas.
As prosthetists grow ever more willing to accept standardized components, manufacturers strive to outdo each other in offering improved devices. This trend has been marked by a shift from exoskeletal wood components, which demand a considerable amount of labor, to endoskeletal components that can be bolted together with a minimum of labor.
This development was sparked by the introduction of the Otto Bock modular endoskeletal system some 20 years ago. It was designed with an interchangeable series of components in such a fashion that prostheses to accommodate virtually every level of amputation could be fabricated. The original system has been continuously refined and updated with an increasing array of options available. As a result, the design parameters adopted by Otto Bock have become the worldwide de-facto standards for development by other companies. Components that fit into the Otto Bock system and complement it are now being designed by competitors.
Paradoxically, standardization has led to increased compatibility of components produced by different manufacturers and has increased the ability to readily mix and match components. Most such structural systems make provision for changing the alignment of the prosthesis without performing major structural changes. These two factors enable the prosthetist to readily vary the prosthesis throughout its life to meet the changing needs of the patient.
Newer, more sophisticated materials such as titanium and carbon composite are being used to design systems that are both more robust and lighter in weight. Smaller-size units for children are also being fabricated. It seems likely that future research efforts will be focused on working within the context of the endoskeletal rather than the exoskeletal structure.
New and improved methods of providing cosmetic finishing for the structural components are needed. Cosmetic fairings are laborious to shape and fragile and, in transfemoral applications, can impede the proper function of knee control units. Spray-on or paint-on prosthetic skins offer, at best, a partial solution to these problems while creating new ones of their own.
Component parts within the endoskeletal system have also received attention. In regard to knee control units, the trend has not necessarily been to develop new units, but rather to adapt exoskeletal designs to the endoskeletal context. Today, an endoskeletal analogue for every exoskeletal knee can be found, including those controlled by hydraulic and pneumatic units. In the process, composite structures of carbon fiber and epoxy have been used to offset the often formidable weight of the control unit. Otto Bock has now introduced a new composite knee incorporating a Mauch S-N-S control unit.
Physically active amputees have pressed for prostheses suited to a range of activities more demanding than just walking. The availability of newer materials at reasonable cost, among other factors, has led manufacturers to respond to the need with new feet. One general category, termed dynamic elastic response or energy-storing feet, is designed to absorb energy from the early portion of stance phase and to release it at the end of stance phase to assist with forward propulsion. Although such feet, epitomized by the Seattle Foot (Fig 24C-1.), Carbon Copy II Foot, and Flex-Foot, were originally envisaged as being most suitable for younger amputees interested in athletic activities, they have found favor with geriatric amputees as well. The other group of feet, including the stationary-ankle, flexible-endoskeleton (SAFE) foot, Quantum, and Multiplex, are designed for improved compliance and range of motion at the ankle to adapt more readily to irregular terrain. Decreased weight has also come to be a design parameter to be carefully considered.
The development of these feet has been marked not just by a concern with performance but with appearance as well. Feet are now routinely designed with such sculpted details as toes and veins. This attention to cosmetic effect will probably continue and be heightened in the future.
Many of these developments and others in the area of socket design were only made possible by the availability of new materials at reasonable cost. Materials such as titanium and carbon fiber/epoxy composites found their first applications in the aerospace industry where weight was at a premium and cost was of little object. As these materials proved their worth and as confidence grew in the ability of engineers to work with them, they found ever wider application and allowed a reduction in cost. With reasonable costs ensured and with a body of engineering data to work with, it became possible for prosthetic manufacturers to design and produce new components with the new materials.
Significant costs in product design have been incurred by manufacturers developing such components, and it has only been attractive to the manufacturers to take the risk because of the existence of a market for commercially available components. The existence of a standardized context within which to design components and a body of data describing human gait and the performance of prostheses has smoothed the way.
Market size has grown as a result of the globalization of the prosthetic market. At the end of World War II, it was possible to discern distinctly different national styles of prosthetic fitting and construction. With the growth in standardization, these differences have become less clear. It is now possible to find manufacturers from around the world who are developing ever more sophisticated products from expensive materials, all within a common context. This has, in turn, spurred the pressure to develop an internationally agreed-upon standard for physical strength and performance.These interacting trends will doubtlessly accelerate in the future.
Another group of materials primarily used in socket construction has considerably more mundane origins. Thermoplastic polymers such as polyethylene, polypropylene, ionomer, and polycarbonate find their most common applications in such everyday products as packaging, illuminated signs, toys, consumer appliances, and cars. These materials and some of the techniques used to shape them have been borrowed to produce flexible sockets, transparent check sockets, and prostheses themselves. Most of these latter applications have been made by clinical prosthetists working singly or in small groups. They have been abetted in these efforts by their growing sophistication about design principles and the biomechanical basis for prosthetic fitting, as fostered by their improved educational status.
SOCKET DESIGN/FITTING METHODS
These various factors have interacted with growing expectations among amputees and a concern with meeting the needs for extra-ambulatory activities to create an entirely new style of interaction between prosthetists and amputees, including new fitting methods.
Regardless of amputation level, these new methods emphasize the use of multiple transparent check socket fittings along with such aids as alginate and radiographic examination of the fit. They also rely on the use of structural components that can be readily adjusted in alignment and that can be set up with different feet and knee control units. When these tools are used, various options and variations can be explored during the trial fitting stage. The net result is a much greater level of patient involvement in the fitting process and presumably a higher level of satisfaction.
Transparent check sockets have also given rise to innovative socket designs, most notably for transfemoral (above-knee) amputees. While these various designs have been given a variety of names such as normal shape-normal alignment (NSNA), contoured-adducted trochanteric, controlled alignment method (CAT-CAM), and Sabolich contoured-adducted trochanteric, controlled alignment method (SCAT-CAM), they all can be classified as ischial containment Sockets.
The technology used in fabricating transparent check sockets is also being employed to fabricate socket elements of other more durable thermoplastic materials such as copolymer polypropylene. A variety of concerns motivate this switch in technique, including weight, function, economics, and fabrication safety.
Thermoplastic techniques are particularly suitable for use with endoskeletal structural components, thus leading to increasing reliance on such devices. This, in turn, has stimulated manufacturers to develop new endoskeletal components such as the Endolite system developed in England and the Carbon Copy III (Fig 24C-2.) system developed in the United States.
Of particular importance among the efforts of clinical prosthetists are those of the Icelandic prosthetist Os-surr Kristinsson. He has been responsible for such innovations as the flexible socket concept and the use of silicone elastomer components to provide suction suspension for levels at which it was not previously practical. In the United States, these developments, or similar ones derived from his work, are variously known as the Scandinavian flexible socket (SFS) or Icelandic-Swedish-New York (ISNY) in the case of flexible sockets and Icelandic Roll-On Suction Socket (ICEROSS) or silicone suction socket (3-S) in the case of suction suspension sockets. His work epitomizes a trend in the field: the growing interest in exploiting the characteristics of elastomeric polymers,primarily silicone or urethane, in socket fabrication in order to make more functional and comfortable devices.
It seems likely that the future will see continued efforts to exploit newly available materials in creative ways to meet the expressed needs of amputees for prostheses that do more than just walk. This demands more sophistication about materials and performance upon the part of prosthetists and other clinicians and creates more opportunities for manufacturers. While many of these efforts have been and will be directed to the needs of younger, more active patients, older, more sedentary patients have benefited and will continue to do so from the emphasis on light weight and comfort.
The foregoing sections have reviewed the developments that have had the most immediate impact on current practice. One other emerging trend, while still in its early stages, promises even more far-sweeping impact on the field. This is the application of the computer-aided design-computer-aided manufacturing (CAD-CAM) concept to prosthetics.
Automated production of prostheses has been sought for some 20 years. In the most basic manner, it can be described as consisting of three elements (Fig 24C-3.). The first is acquisition of dimensional information from the involved body segment. The second is manipulation of this information to generate the specifications for a socket model to be produced by the third element, an automated carver. These last two elements correspond exactly to the CAD-CAM process. Their availability to prosthetics and orthotics only occurred with the widespread availability of personal computers, CAD-CAM software to be run on such personal computers, and the possibility of directing numerically controlled carving machines with them.
The units needed to automatically record information from all segments of a patient's body are not yet available and do not take into account tissue density. Commercial units depend on information generated by the prosthetist and entered indirectly into the system. As a result, human error is not eliminated, and it is still necessary to use check socket procedures in order to ensure a socket fit that equals the best that is currently available by conventional means. This means that it is not yet possible to generate true productivity gains. These limitations, coupled with the fact that currently available CAD-CAM systems are only capable of dealing in a limited fashion with just transtibial and trans-femoral sockets, have limited their commercial appeal. Nonetheless, a number of installations have been made in the past few years. The feedback from prosthetists using the systems should prove to be invaluable in the efforts of their designers to improve them and develop new applications.
As noted, the outcome of the CAD-CAM system is a socket model. This model is generally utilized in producing a thermoplastic vacuum-formed socket, by either automated or nonautomated means, to be attached to endoskeletal components to produce a prosthesis. In effect, the trend that began with the prosthetist ceasing to craft elements such as feet, knees, and shins will culminate with relinquishment of any direct hands-on role in producing the socket. The impact that this will have on the nature of prosthetic practice remains to be seen.
As in so many areas of medicine, the introduction of new technology to prosthetics has troublesome and contradictory implications for cost containment. Productivity in the field has increased since the end of World War II as a result of the introduction of more efficient means of fitting and the standardization of components. Likewise, our ability to fit patients comfortably has increased, and we have been able to provide them with new measures of comfort and function as a result of the application of new materials. These advances, however, have not been without their price.
It is costly to adequately educate and train a prosthetist. Many of the new materials are expensive to purchase and to fabricate into finished components. The capital investments needed to work such new materials are high, and the costs of research and product development must be reckoned with. If CAD-CAM comes to be an integral part of clinical prosthetic practice, the capital expense of the equipment will have to be borne as well. The prosthetist of the future will doubtlessly be able to produce more and better prostheses, but the capital expenses at all levels of the delivery system necessary to support this advance will have to be recognized and met. If they are not, the incentives to develop even newer techniques will cease to exist. Coping with the implications of this quandary and answering the questions it raises are likely to play a prominent role in future prosthetic research.
The federal government has withdrawn from its active role in shaping prosthetic research and development and has left it to manufacturers and prosthetists to take the lead. The former will seek to gain competitive advantage by exploiting the possibilities of newly available materials and methods to produce components that are stronger, lighter, and more functional. The latter will explore the possibilities of new materials for the same advantages in constructing prostheses and in producing sockets with novel shapes and characteristics. By common consent, both will work in the context of the endoskeletal structural system. De facto acceptance of Otto Bock's design parameters and the globalization of the market will aid and abet this trend. The need for marketplace incentives and well-educated prosthetists will become even more important.
The growing self-assurance and assertiveness of younger, more active amputees will probably also shape developments. This and other factors are likely to lead to more functional feet and knees, prostheses that can be more readily adapted to the varying needs of amputees, and improved methods of providing cosme-sis. However, it must be recognized that as the population ages, the needs of these amputees will change. There is also likely to be a concurrent increase in the number of geriatric amputees. These factors will shift priorities and developmental efforts.
Ever more CAD-CAM production systems will be purchased for use in clinical practice. This will accelerate the rate of development of such systems, but the ultimate realization of their fullest potential must await the introduction of devices that will automatically acquire information about the shape and physical characteristics of the involved body segment. In all likelihood, this development will first occur in some field peripheral to prosthetics and will only become available in the field of prosthetics when the price is reduced to manageable levels.
All these factors may interact in unexpected ways with the desire of society to curtail the growth of medical expenses. Research efforts could well be shifted from developing newer means of providing prosthetic care to justifying and defending the expense of present methods. Cost containment efforts might extinguish the marketplace incentives spurring research and development and could formalize the present trend of two distinctly different levels of prosthetic care. One would provide a tolerably comfortable, minimally functional lightweight prosthesis, most likely produced by CAD-CAM methods and available to the beneficiaries of funding systems stressing a fixed fee for service structure and to amputees in the Third World. The other level of care would be provided to patients for whom price is of little concern, with more elaborate attention being paid to such factors as fit and function.
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Chapter 24C - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles