Perceptive Evaluation of Prosthetic Foot and Ankle Systems (2005)

The availability of advanced prosthetic components may be beneficial to the amputee, but the great variety of available options may also cause confusion in selecting the proper prosthetic prescription. Ideally, the selection of components in a prosthetic prescription would be derived from published scientific outcomes and supported by the prosthetist's successful clinical experience. However, clinical prescription is more often a result of anecdotal evidence and positive subjective feedback because objective amputee gait studies often fail to demonstrate significant differences among devices based on scientific measures. This void between anecdotal experience and scientific evidence is often acknowledged and even accepted in the literature, particularly in regards to prosthetic feet: “Most active patients subjectively prefer the dynamic response prosthetic foot to its predecessors although studies have yet to prove the dynamic response keel design to be significantly advantageous in improving ambulatory efficiency” (p. 69).1 Although scientific evidence does not significantly favor one prosthetic foot to another, there are alternative outcomes that can be used to assess prosthetic performance and amputee preference for prosthetic feet. If these perceptive measures are to be used to drive clinical decision making or prescriptions with respect to component selection, then these results must be presented in such a way as to convince clinicians that one type of prosthesis will offer an improved quality-of-life for their patients as compared with another. The purpose of this report is to summarize available outcomes measures used to assess prosthetic performance and preference and to review published outcomes that have been used to compare prosthetic feet. ASSESSMENT TOOLS Clinical assessments and patient input are essential components in the delivery of optimal health care, particularly in the evaluation of functional ability and health-related quality of life. The last decade has seen the development of a number of tools designed specifically to assess function, preference, health, and quality-of-life of amputees (Table 1). Many of these tools offer insight into an amputee’s preference for and performance with a prosthesis. These tools are commonly questionnaires or surveys and may be either administered by health care personnel or self-administered by the amputee. In addition, a number of physical measures have been used to similarly evaluate an amputee’s performance while using a prosthesis. Although many of these physical measures are not exclusive to persons with limb loss, they are often used in the clinical and research environments to assess an amputee’s function.Table 1: Established survey tools and physical measurement used to evaluate prosthetic performance and preferenceAlthough there is clearly a variety of tools and physical measures used to assess prosthetic performance, amputee preference, and functionality, there are little data to suggest they are sensitive enough to show advantages of one prosthetic foot over another. To date, only two of these outcomes, the PEQ and step activity, have been used in scientific research to evaluate the differences among prosthetic feet. Coleman et al.11 showed that in a comparison between the solid ankle cushioned heel (SACH) foot and the Flex-Foot, users walked farther, spent more time walking at a moderate activity level, walked with a longer stride, and scored an improvement on the Prosthesis Utility when wearing the Flex-Foot. All of these outcomes were reported to be significant. Another study by Hsu et al.12 compared the Flex-Foot, C-Walk, and SACH feet. The Flex-Foot was reported to have a significantly higher Frustration and Usefulness score when compared with the SACH foot. The C-Walk was reported to have a significantly higher Usefulness score. More often in scientific studies, unique measurement tools are developed to assess prosthetic function, preference, or performance. These measures fall into one of three categories: descriptive dialog, functional assessment questionnaires, and numerical rating scales. DESCRIPTIVE DIALOG Descriptive dialog is the subjective feedback obtained during a study. This often occurs in the form of a conversation, a directed question (e.g., What foot do you prefer?), or unsolicited feedback. Scientific studies are often complemented by this type of perceptive feedback when the amputee is allowed to keep one of the prosthetic feet they have worn in the study. The subject’s selection is often noted, and then used to support or contradict the results of the study. Additional details about prosthetic feet can be garnered through a descriptive dialog when the researchers question the amputee regarding perceived function and/or performance of a foot. This may allow a researcher to understand why one foot is preferred over another. Although informative, this type of perceptive evaluation is limited because it is difficult, if not impossible, to perform quantitative analyses of the results. Several studies have included a descriptive dialog, or subjective feedback, within the scope of the analysis.13,14 This method of determining preference for or functionality of a device has shown that high-end energy storage and return (ESAR) feet (such as the Flex-Foot) may offer improvements over conventional feet such as the SACH foot. Neilsen et al.13 reported that users perceived improvements in walking velocity and stability on uneven ground, but with the limitation of a decreased ability to walk downhill. Torburn et al.14 showed that five subjects preferred to keep an ESAR foot such as a Carbon Copy II or Seattle Foot over a conventional SACH foot. These particular feet were often preferred over more advanced feet like the Flex-Foot for esthetic reasons. These two studies are by no means a comprehensive recommendation for ESAR feet; however, they do agree with anecdotal evidence in the prescription of prosthetic feet. FUNCTIONAL ASSESSMENT QUESTIONNAIRES Beyond the subjective preference of the descriptive dialog, the functional assessment questionnaire is a unique, but standardized series of questions relating to prosthetic function, performance, or preference. These questionnaires may be similar in style to the aforementioned assessment tools, but they are often much simplified, consisting of only a small set of questions relating to the functionality and acceptance of the prosthetic device. Two studies in the literature have examined the comparison between prosthetic foot types using functional assessment questionnaires. Menard et al.15 polled 22 amputees with at least two months experience with the Flex-Foot. They were questioned as to their pain and ability to perform activities when compared to their previous prosthetic foot. Most amputees reported improved gait, activity level, pain, and skin problems. Similarly, Murray et al.16 designed a questionnaire of an unspecified number of questions that was completed by 31 amputees currently wearing a Seattle foot. The amputees were asked to compare their existing foot to their previous one (typically a SACH foot). Users reported improved ankle motion, joint stress, gait, balance, skin problems, and pain (Table 2).Table 2: Percent of subjects who report no change or an improvement in functional areas when comparing ESAR feet to a conventional SACH footNUMERICAL RATING SCALES The most detailed perceptive analyses comparing prosthetic foot types use a numerical rating scale to assess the preference for and performance of the feet. A numerical rating scale is a customized metric designed to assess the improvements with a prosthetic component change. The key advantage to this type of study is that it allows statistical analysis of the results. To date, only four comparative studies have used this level of analysis in the perceptive evaluation of prosthetic foot type. Alaranta et al.17 compared ranked gait improvement for users wearing Flex-Foot as compared with the SACH foot in 10 activity situations. A rating of disability for the Flex-Foot and SACH foot was given by 31 amputees. The results showed that the Flex-Foot provides a statistically significant improvement in walking or running in all conditions, save level indoor walking (Table 3).Table 3: Reported statistical significance between ratings of disability for the Flex-Foot as compared to the conventional foot (SACH) (Adapted from [17])A study by Macfarlane et al.18 compared the same two feet at three speeds and incline grades using a modified BORG rating of perceived exertion. Users ranked the feet at three speeds (2.0 mph, 2.5 mph, 3.0 mph) and three inclines (level, 8.5° incline, 8.5° decline) on a scale from 6 to 20 spanning from “very, very easy” to “very, very difficult.” The Flex-foot produced significantly lower ratings at all speeds and all grades of incline. Postema et al.19 subjectively compared two conventional feet (Multi-axial and Lager) and two ESAR feet (Dynamic and Quantum). The customized questionnaire posed 27 questions in the following categories: stability while standing, stability while walking, functional factors, and special activities. The mean score determined the final ranking, with a higher score representing better performance and preference. Results from 10 amputees showed that the Lager foot produced a significantly lower mean. The subject preferences for this study did not, however, match the ranked measures. A recent study by Underwood et al.20 compared the SAFE foot against the Flex-Foot using a numerical rating scale that assessed a patient’s perceived stability and mobility on a variety of terrains. Eleven amputees compared the feet in six performance categories (Table 4).Table 4: Reported ratings of perceived stability and mobility between a Flex-Foot and SAFE foot. Ranking is based on a 10-point scale with 10 being the best. (Adapted from [20])Although the Flex-Foot was favored over the SAFE foot, the authors found that only two activities, ability to walk quickly and stability on a compliant surface, were different by more than one point. No significant differences were noted. CONCLUSIONS In general, the results of the perceptive literature show preference for and increased performance from ESAR feet when compared with conventional feet, particularly in activities of daily living outside of level walking. Such data appear to confirm the clinical prescription of advanced foot components, although there is no evidence to suggest it is a driving factor in the clinical decision-making process. Ideally, the choice of a prosthetic prescription would be driven by documented evidence that supports the prosthetic components selected. In absence of strong scientific evidence in favor of one prosthetic foot over another, the clinician must then turn to outcomes measures other than the traditional biomechanical ones. This summary presents a number of survey tools, physical measures, and perceptive methods that may be used to gather this information. To date, little evidence shows that the survey tools and physical measures can be used to discriminate among prosthetic foot-ankle systems. However, these tools are not often used in comparative foot studies to evaluate these components. The reasons for this are likely the time required to administer and score the tests and the insensitivity to prosthetic foot type. The perceptive methods used to analyze prosthetic feet vary greatly in complexity and in power of the results. Descriptive dialog provides quick and easy feedback, but offers no way to standardize or statistically assess the results. Functional assessment questionnaires offer a way to standardize the results, but lack the ability to statistically analyze the results. Only numerical rating scales offer the power to assess the statistical significance when comparing prosthetic components. The key advantage to perceptive studies of prosthetic feet is the sample size of the population. Unlike traditional biomechanical studies, perceptive studies are able to recruit far more subjects in a given time and budget. Whereas published biomechanical studies are limited to between 1 and 17 subjects (mean 8.1), perceptive studies range from 5 to 31 subjects (mean 14.4).21 However, these tools also face limitations. Without the ability to perform a statistical analysis of the results (particularly for descriptive dialog and functional assessment questionnaires), the results lack power and conviction. In addition, because most perceptive analyses are based upon customized questions and scales, they are not validated tools. If the goal of scientific evaluation of prosthetic feet is to drive the clinical decision-making process; consistent trends and validated, reliable research must be presented to convince clinicians that one type of prosthetic foot is superior or equivalent (for less cost) to another. If perceptive analysis is to do this, it must be done with reliable, valid measures that offer a researcher the ability to statistically analyze the results. Anything less will continue to support the anecdotal evidence, and not drive it forward.