Research and Clinical Selection of Foot-Ankle Systems

Joseph M. Czerniecki, MD

It is important to realize that the "recommendation of ankle-foot systems for specific amputees" in today's climate of evidence-based medicine would require that there is scientific evidence (a double-blind, placebo-controlled trial, using well-established and validated outcome measures) that shows enhanced function when a given component is provided compared with other suitable components. There are no studies that have been done that would meet this standard. This is supported by a systematic review of the literature between 1966 and 2001 by van der Linde et al.¹ Of the reviewed studies, only one achieved a methodological level of A, and only 15 achieved a methodological rating of B. The majority of these studies addressed outcome variables of metabolic oxygen consumption, stride kinematics, ground reaction forces, or more sophisticated joint moment and joint power output characteristics. These studies generally showed few significant differences between feet. Three B-level studies did show an increase in self-selected walking speed with different energy-storing feet compared with the solid ankle cushioned heel (SACH) foot. Additional studies showed that the increase in walking speed was accomplished by a greater stride length and reduced cadence. In addition to kinematic measures, three of nine B-level studies showed a reduction in metabolic oxygen consumption with the Flex or the Re-Flex VSP compared with the SACH foot.¹

Hafner et al.² suggest that one of the important factors that have limited the achievement of statistical significance in these biomechanical and metabolic studies is the small sample sizes. This review compiled all the results from multiple studies and showed that there may be some important trends that can be inferred even though each individual study was not able to show significance. The graphic display of the cumulative data from multiple studies is very illustrative of this feature.

Rietman et al.³ reviewed the literature between the years 1990 and 2000 related to "instrumented gait analysis" after amputation. This review concluded that gait analysis provides insight at the level of impairment but does not give insights at the level of disability or functional impairment. The authors conclude that gait analysis will continue to be of value in assessing the fundamental biomechanics of new prosthetic components, but further assessment of subjective perception and function in day to day functional tasks will be important.

A fundamental question needs to be asked at this point. Even if all of these studies showed "improvement" in an isolated outcome variable such as self selected, stride length, ground reaction forces, or metabolic oxygen consumption, would that have been adequate evidence to "recommend their use in specific amputees"? From a theoretical perspective the answer would be no. What these studies would have demonstrated in a defined population, tested under specific laboratory conditions, was that there was a beneficial effect. This does not mean that there would have been any perceived or quantifiable beneficial effect in real-world use. The two relevant issues here are 1) Is the statistical difference of adequate magnitude to be clinically relevant?, and 2) Are the significant findings in the laboratory relevant to real world mobility? Hofstad et al.⁴ use the term "ecological validity" from Mulder.⁵ An example of the first issue is that a drug may cause a statistically significant 10% reduction in cholesterol but have no effect on the development of clinical atherosclerosis. An example of the second issue is that the metabolic costs may be reduced while walking in linear constant speed ambulation on a linoleum floor, but these conditions are rarely seen in day-to-day ambulation, and that walking in real-world conditions with stopping, starting, inclines, and stairs may in fact result in an increased metabolic oxygen consumption and an adverse effect on functional mobility. These questions should not be interpreted as a rejection for the need for fundamental biomechanical research. This research will be necessary to further understand the response of the amputee to changes in design and to assist in the development of novel prosthetic designs.

Along with laboratory measurement, an additional approach used to quantify the effect of prosthetic feet is through subjective analysis. That is, amputees through a variety of mechanisms are asked to provide their subjective perceptions or define, in a quantifiable way, their subjective impressions in various domains. Hafner et al.² have reviewed the literature in this regard. Unfortunately, few studies have met adequate quality to be interpreted as to their relevance. In the systematic literature review of van der Linde et al.,¹ only one level-A study and two level-B studies were acceptable. One showed no particular benefit of energy-storing feet over conventional feet and the other two showed some benefit of the Flex-Foot over the SACH foot; however, these studies were not conducted in a double-blind fashion. Blinding of subjects and investigators is clearly an essential part of studies that include a subjective evaluation.^1,2,4

CONCLUSION

There is limited useful research to aid the clinician in the recommendation of specific foot-ankle mechanisms for specific patients.

IMPORTANT FUTURE DIRECTIONS

BETTER DEFINE PATIENT NEEDS
The majority of the research on prosthetic feet has classified the subject population with a fairly "coarse mesh." That is, the populations have been defined on the basis of age, amputation level and etiology of amputation. Part of the reason that the research on subjective evaluation of prosthetic components has been inconsistent is that the needs and priorities of subjects in these populations may be different. If we are going to get to the point where we can use scientific evidence to help in the prescription of the prosthetic feet, we must be able to more clearly define the needs, requirements, and priorities of individual patients so that we can better fit the prosthetic component to the patient.

There are a number of outcome tools available that measure the level of mobility of patients; for example, the Medicare K levels, the Stanwood,⁶ the Locomotor Capability Index,⁷ or the Special Interest Group in Amputee Medicine (SIGAM)⁸ mobility grades. These are typically general mobility scales but do not allow one to measure, for instance, the intensity of mobility and the potential need for impact absorption or torsional activities that the patient might experience in their day-to-day functional tasks. As an example, Medicare K levels divide the functional mobility status of the amputee into five levels, K0–K4, but should not be used to prescribe a prosthetic foot. This is illustrated by the K2 level description: "...has the ability for ambulation with the ability to traverse low level environmental barriers such as curbs stairs or uneven surfaces. Typical of the limited community ambulator." Would you prescribe the same prosthetic foot to someone who lived in an apartment in an urban environment who walked exclusively on level surfaces, compared with someone who lived in a doublewide mobile home in a rural environment with extensive irregular terrain, who also enjoyed fishing as an avocational interest? The clinical history, in contrast, attempts to identify special patients' needs and priorities when arriving at a prosthetic prescription. Unfortunately, as it is used in the clinical context, it cannot be quantified and used for scientific investigation.

The approach used by Postema et al.9 may provide the foundation for a needed future direction: the development of a tool that quantifies a patient's mobility in key domains and also quantifies the importance of function within each of the domains for an individual patient. Postema et al.⁹ developed two questionnaires. The first questionnaire was an evaluation of the prosthetic foot measure that included 27 questions grouped into four separate domains. The amputee rates how well the prosthetic foot performed on a 0–10 scale for each question. The second questionnaire included 12 factors that relate to prosthetic foot function. The subjects then were presented with 66 pairs of these 12 factors and were asked to rank the importance of the factor of each presented pair. These questionnaires allow one to define a patient's needs and priorities in a more comprehensive way and then measure the effect of differences in prosthetic foot on their function. For example, if one were doing a research investigation on impact absorbing pylons in transtibial amputees, one might find no beneficial effect with conventional subjective or objective biomechanical measures. But if one used the approach Postema et al.9 described, one might find that an impact absorbing pylon benefited those who walked over irregular terrain or on inclines and who prioritized comfort as an important criterion. This would then suggest that a clinician could administer a questionnaire at the time of clinic visits to assess the needs and priorities of a given patient and then provide a prosthetic foot that had been shown to be beneficial in that population.

BETTER-VALIDATED TOOLS FOR EVALUATION AND MEASUREMENT OF FUNCTION THAT HAVE ADEQUATE SENSITIVITY TO DETECT CHANGE IN FUNCTION OVER TIME AS WELL AS PSYCHOPHYSIOLOGIC MEASURES
Currently, the tools that are available to the investigator to measure the effect of prosthetic components on key functional parameters are limited. We need measurement tools that have adequate sensitivity and specificity, adequate ability to detect change in function over time, and with adequate floor and ceiling characteristics for the population under study. For example, some key domains that could be measured are perception of exertion, fatigue, mobility, and stability.

DEVELOPMENT OF TOOLS THAT CAN BE USED IN BIOMECHANICAL OR METABOLIC MEASUREMENT DURING REAL WORLD FUNCTIONAL TASKS
The majority of the scientific biomechanical research has been done in a laboratory and oftentimes walking on a treadmill. This is inadequate to comprehensively quantify the effects of prosthetic feet on function. Equipment is available to accurately and reliably measure VO2 in the field. Techniques to measure impact transients, residual limb torque, forces and activity are needed. This should help better quantify the influence of prosthetic feet on function in a more "ecologically valid" way.

ADDITIONAL CONCERNS

PROSTHETIC FOOT EVOLUTION AND RATE OF PRODUCTION
The current practice of prosthetic foot development and introduction to the marketplace will pose significant challenges to the development and utilization of patient-specific criteria for prosthetic foot prescription. New prosthetic feet are being introduced regularly. These prosthetic feet are introduced without any published quantification of even their most basic functional characteristics. Any research done on evaluation and quantification of performance will always lag behind the marketplace. Although a long way from being a perfect solution, a key question that needs to be answered is, Is it possible to develop some simple instrumented quasi-static load deflection evaluations that all manufacturers will be required to perform on their prosthetic feet before bringing them to the marketplace? For example, a heel strike simulation, a forefoot keel dorsiflexion and release, and a medial and a lateral foot load to simulate the effect of walking on irregular terrain. These would allow some objective measurement that a clinician could understand so that new feet could be viewed in the context of feet available in the marketplace. So, if a new foot was developed and its loading characteristics were quantified, one might be able to say that it is "better than a Seattle Light Foot in absorbing energy at the heel, but stores and returns less energy in the forefoot keel than a Renegade, and is about as stiff in the medial lateral plane as a Luxon Max." Ultimately new clinical studies would need to be done but this could be used in an interim way to help a clinician predict the utility of a foot, for a patient with a given constellation of functional needs.

TIME DELAYS IN FUNDING RESEARCH, CONDUCTING AND PUBLISHING RESEARCH
Currently, the average time delay required to design and write a research proposal may span 2 or 3 months. Subsequently, there are additional delays from submission of the research grant through review, and, if successful, waiting for funding to arrive. After receipt of funding, depending on the study design, it will then take 2 or 3 additional years to complete the study. There will then be an additional 12- to 18-month delay before it is in print. Thus from inception to completion, 5 years may pass. This is incompatible with the rate of prosthetic foot development and testing. The use of modeling of different gait activities and the effect of foot design may ultimately allow the prediction of the effect of a novel foot without needing to conduct a 3-year clinical trial. The development of these models will, of course, take time, and so will their validation.

Correspondence to: Joseph M. Czerniecki, MD, Director RCS, VAPSHCS, 1660 S Columbian Way, Seattle, WA 98108; e-mail: .

JOSEPH M. CZERNIECKI, MD, is affiliated with the Department of Rehabilitation, University of Washington, Seattle, and VA Rehabilitation Research Center of Excellence, Limb Loss Prevention and Prosthetic Engineering, VAPSHCS, Seattle, Washington.

References:

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Hofstad C, Linde H, Limbeek J, Postema K. Prescription of prosthetic ankle-foot mechanisms after lower limb amputation. Cochrane Database Syst Rev 2004;(1):CD003978.
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Hanspal RS, Fisher K. Assessment of cognitive and psychomotor function and rehabilitation of elder people with prostheses. BMJ 1991;302:940.
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Ryall NH, Eyres SB, Neumann VC, et al. The SIGAM mobility grades: a new population-specific measure for lower limb amputees. Disabil Rehabil 2003;25:833–844.
Postema K, Hermens HJ, de Vries J, et al. Energy storage and release of prosthetic feet Part 2: subjective ratings of 2 energy storing and 2 conventional feet, user choice of foot and deciding factor. Prosthet Orthot Int 1997;21:28–34.