Clinical Prescription and Use of Prosthetic Foot and Ankle Mechanisms: A Review of the Literature (2005)

A State-of-the-Science Conference (SSC) is designed to provide a systematic review of literature and ranking of evidence on a conference topic. The purpose is to evaluate the available scientific information on an aspect of orthotic and prosthetic (O&P) care and develop statements that advance understanding of the issues in question that will be useful to health professionals and the public. It may also serve as a mechanism to document clinical belief systems in O&P care, based on what is understood through sound research or from expert opinion. The end goals of an SSC are to publish a document that identifies and ranks the available evidence and defines the current status of patient care, to develop consensus on controversial issues where possible, and to identify research priorities. The focus of this SSC and this literature review is the body of scientific evidence that supports the clinical prescription and use of prosthetic foot and ankle mechanisms. LITERATURE REVIEW The literature review was conducted by searching a number of scientific and publisher databases for relevant articles. Articles were limited to those in the English language or those that had been translated into English. The following sources were searched: MEDLINE (1966–2005), Current Contents (1996–2003), Cochrane Library, and Elsevier Science Direct (1997–2005). In particular, the following journals were also targeted for review of relevant articles: Journal of Biomechanics, Clinical Biomechanics, Gait & Posture, Archives of Physical Medicine & Rehabilitation, Journal of Prosthetics & Orthotics, Prosthetics & Orthotics International, and Journal of Rehabilitation Research & Development. Keywords (and relevant combinations) included such words and phrases as “prosthesis,” “prosthetic foot,” “below-knee,” and “transtibial.” References from each of the selected articles were also searched for key articles not identified in the database search. PROSTHETIC FEET: TERMINOLOGY Prosthetic foot-ankle units are often differentiated by their physical design, mechanical behavior, and functionality. Four primary categories of feet have evolved and are used commonly in the clinical environment: conventional, single-axis, multi-axis, elastic keel, and energy storage and return. The term “conventional foot” (CF) is often attributed to basic solid-ankle designs, although it can be argued that such components are no longer the norm. The term “single-axis” (SA) refers to prosthetic feet with a single hinge in the direction of motion that provides plantar flexion and dorsiflexion. “Multi-axis” (MA) feet are those with a multidirectional hinge that flexes not only in the direction of motion, but also in the coronal plane, providing inversion-eversion characteristics to the prosthetic foot. The term “energy storage and return” (ESAR) is one chosen by the author to most accurately represent the behavior and mechanical function of modern prosthetic feet, capable of storing energy generated during the early phases of gait and then returned later in stance to propel the swing phase.1 Other terms that have been used to describe similar feet include “energy storing,” “energy storing prosthetic feet,” and “dynamic elastic response.”2–4 This category of feet may also be subdivided into the those feet with a flexible keel internal to the foot (e.g., Seattle Foot) and those with a flexible keel that extends into the ankle or shank of the prosthesis (e.g., Flex Foot). It is also important to recognize that some types of feet possess characteristics of multiple categories. For example, a College Park TruStep could be said to be both an MA foot as well as an ESAR foot. Prosthetic foot categories and examples of corresponding, commercially available prosthetic feet are listed in Table 1.Table 1: Common classifications of prosthetic feet and examples of manufacturer-specific feetRANKING THE LITERATURE One challenge that faces the SSC panel is a comprehensive ranking of the relevant scientific literature. To our benefit on this topic, researchers from the Rehabilitation Center Sint Maartenskliniek (Nijmegen, the Netherlands) recently completed an exhaustive review of the literature and ranking of evidence pertaining to transtibial (and transfemoral) prosthetic components. The results of the literature review were documented as a Cochrane review and report in 2004.5 The ranking analysis from the Cochrane report was subsequently presented as a literature review in the Journal of Rehabilitation Research and Development.6 This article summarized the methodology and findings of the Cochrane review. As there are no randomized control trials reported in the body of literature investigating prosthetic feet, the reviewers developed a unique scale applicable to the body of scientific evidence reported. For the purpose of the SSC, this scale was adopted as a suitable method for ranking the scientific evidence. Each reviewed manuscript7–32 was ranked based on 13 scores in three categories: selection of patients, intervention and assessment, and statistical validity (Table 2).Table 2: Literature ranking criteriaThe criteria were used to categorize each study into one of three levels: A, B, and C. A-level studies were those found to score in at least 11 of the categories, including six or more points from the A (Selection of Patients) and B (Intervention and Assessment) level criteria. A-level studies were also required to score in categories B7 (Blinding) and B8 (Timing of the Measurement). B-level studies were those found to score in at least six (and less than 10) of the categories. B-level studies were also required to score in category B8 (Timing of the Measurement). C-level studies were those found to score at least six points in the A (Selection of Patients) and B (Intervention and Assessment) level criteria, but did not score in categories B7 (Blinding) and B8 (Timing of the Measurement). Descriptions of each study, year, population, feet evaluated, and measured outcomes are listed in Table 3.Table 3: Ranking of the scientific literatureTable 3: ContinuedThe scientific ranking shows limited evidence to support the prescription of prosthetic foot-ankle systems. Evidence does show that ESAR prosthetic feet do offer some advantages over traditional, SACH feet, including increased walking velocity10,23,25 and improved gait symmetry.20 Single-axis feet may allow an amputee to reach foot-flat sooner, a characteristic often desired by inactive users for stability. However, these feet may lack late-stance stability22 that is offered by SACH and Flex feet.17 Given that more than 20 years of scientific research has examined the effect of prosthetic feet on amputee gait, this limited evidence to support prescription or use of prosthetic feet appears to fall short of expectations. OTHER RELEVANT LITERATURE Along with the scientific literature presented in the ranking, a small body of review literature exists that has attempted to examine the evidence regarding prosthetic foot-ankle mechanisms from a variety of viewpoints. Several authors have undertaken the task of evaluating the state of knowledge regarding these prosthetic components from the clinical anecdotal perspective, the scientific subjective perspective, and the biomechanical objective perspective. CLINICAL REPORTS Clinical reports are manuscripts based upon the clinical assessment of prosthetic devices. These are most often based upon anecdotal evidence and experience. The last 20 years have seen a number of publications we may refer to as “clinical review articles.”2,3,33–36 These are manuscripts that address and summarize the state of prosthetics technology in terms of anecdotal evidence, clinical experience, and general consensus. They do not contain significant scientific evidence or outcomes, but rather attempt to address the state of clinical prescription and foot-ankle recommendations. Several of these studies review the mechanical and functional characteristics of a spectrum of prosthetic feet, from the traditional SACH foot to modern ESAR feet.2,3,36 The authors then present clinical recommendations for these feet based upon such influences as device cost, patient weight, patient activity, other prosthetic components (i.e., prosthetic knees), and other key aspects (Table 4).Table 4: Factors for prescription of prosthetic feetIt is important to consider that many of these feet have been updated by the manufacturer to preserve the advantages and eliminate many of the limitations originally reported. Additionally, a wide variety of feet from many manufacturers have been released to the market in the past few years. No evidence yet exists to support the recommendation of these feet save for clinical experience. PERCEPTIVE ANALYSES Perceptive analyses are studies that evaluate prosthetic devices through the use of patient assessment. Typically this information is sourced from feedback, questionnaires, or product ratings. One recent review of the evidence correlating patient perception to biomechanical analysis of ESAR prosthetic feet found three types of perceptive analyses are used in the body of scientific literature: descriptive dialog, functional assessment questionnaires, and numerical rating scales.37 Several studies have included a descriptive dialog, or subjective feedback, within the scope of the analysis.26,31 This method of determining preference for or functionality of a device has shown that high-end ESAR feet (such as the Flex-Foot) are perceived to offer increased walking velocity and stability on uneven ground, but with the limitation of a decreased ability to walk downhill.31 Similarly, subjects often prefer to keep an ESAR foot such as a Carbon Copy II or Seattle Foot over a conventional SACH foot.26 These feet were often preferred over more advanced feet like the Flex-Foot for esthetic reasons. The next level of perceptive analysis beyond simple preference is the functional assessment questionnaire. These are custom-designed questionnaires designed to offer insight into amputee preference for and the performance of particular prosthetic devices. Results from these types of analyses showed that subjects felt many functional aspects such as gait, activity level, pain, skin problems, ankle motion, joint stress, balance, and endurance to be improved when they wear an ESAR prosthetic foot.4,38 The final level of perceptive analysis is the numerical rating scale, defined metrics 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. One study ranked gait improvement for users wearing Flex-Foot as compared to the SACH foot in 10 activity situations.39 The results showed that the Flex-Foot provides a statistically significant improvement in walking or running in all conditions, save level walking. Another study compared the same two feet at three speeds and incline grades using a modified BORG rating of perceived exertion.19 The Flex-Foot produced significantly lower ratings at all speeds and grades. A third study comparing two conventional feet and two ESAR feet showed that one CF produced a significantly lower mean of the ranked performance factors.8 The subject preferences for this study did not, however, match the ranked measures. In general, the results of the perceptive literature show strong preference for, and increased performance from, ESAR feet when compared to conventional feet, particularly in activities of daily living outside of level walking. Such data appear to confirm the clinical prescription of advanced foot components, though there is no evidence to suggest it is a driving factor in the clinical decision-making process. BIOMECHANICAL STUDIES Biomechanical studies are those that evaluate or compare prostheses using scientific or mechanical test equipment. Common measures for biomechanical analyses include stride and temporal characteristics, kinetics (force data), kinematics (motion analysis), muscle activity, and energy expenditure. Despite a rather large quantity of scientific articles directed toward prosthetic foot-ankle mechanisms, there have been few reviews of the biomechanical or perceptive literature to date. A recent review in the ISPO journal Prosthetics & Orthotics International examined the literature for the effect of prosthetic components (including feet and knees) on gait of amputees.40 The scientific results from 12 studies focused on two primary prosthetic foot characteristics: range of motion (ROM; kinematics of the ankle joint) and energy storing (mechanical properties of the prosthetic foot, including energy absorption, storage, and return). The review noted that ESAR prosthetic feet possessed improved ROM when compared with conventional prosthetic feet (SACH); however, it was noted that SA feet possess even greater ROM. The authors suggest that active patients would benefit from feet with larger ROM, although those with limited mobility and a need for balance would be best served from a conventional foot. The capabilities of these feet to store and return energy were also reviewed through the literature. Several studies showed that ESAR feet produced increased energy-release at push off when compared to conventional feet7,15,41; however, it was noted in one study that the timing of the energy release may not coincide with push-off of the foot.30 The amount of energy storage at the heel in loading response was not noted to be significantly different between foot types, although the authors suggest that amputees may indeed consider a loss in energy at the heel “comfortable.” One noted problem regarding the reported studies was a low number of subjects. The largest studies of transtibial subjects had only 10 subjects. Another review, published in Clinical Biomechanics, assessed the perceptive and biomechanical literature pertaining to ESAR prosthetic feet.37 This article examined the body of scientific evidence that has been used in comparative, biomechanical evaluations of prosthetic feet. The accumulated evidence suggests that the use of ESAR feet does offer improvements in a number of gait parameters when compared with conventional SACH feet. These include improved velocity, stride length, midstance support time, sound side weight acceptance force (vertical), affected side propulsive force, ankle power generation, ankle power absorption, peak plantar flexion moment, ankle ROM, and late-stance dorsiflexion. Reduced shock transmission (at low velocities) was also evident. Despite these apparent advantages, few of the reported variables were consistently statistically significant when comparing foot types. The authors noted that the small sample sizes and mixed populations (e.g., traumatic and vascular amputees) used in most of the studies were likely reasons that many of these variables were not significant. CONCLUSIONS This article reviews the scientific and clinical literature pertaining to prosthetic foot and ankle mechanisms. The ranking of evidence conducted by Hofstad et al.5 and the literature review by Hafner et al.1 support the conclusion that limitations in the research studies conducted to date preclude the direct application of scientific evidence to clinical decision making. It seems clear that clinical experience and subjective feedback from amputees show clear preference for and improved performance with particular prosthetic devices. However, the scientific literature to date seems more suited to support such experience rather than to affect clinical conclusions. A key element that hinders the application of scientific evidence to prescription appears to be the small sample sizes available for research studies. Without larger samples or a standardization of prosthetic feet between studies, it seems clear that statistically significant outcomes will be difficult to obtain. Additionally, specific selection of study populations may serve to limit the variability of outcome measures and have a better chance to provide statistically significant results. One additional limitation with analyzing the literature is extensive availability of prosthetic feet and the significant time required to obtain funding for, perform, and publish a scientific study. The net result of this time delay is that published data are often applicable to outdated components. One proposed solution is the standardization of prosthetic foot characteristics and/or mechanical behavior. Hafner et al.1 proposed standardization for prosthetic foot performance and function, based on energy storage and efficiency of the heel and keel regions of the foot. Such a system could be one method for better using scientific research as a basis for clinical prescription. Lastly, researchers must consider revising the test environments under which these prosthetic components are evaluated. The perceptive analyses showed the greatest difference between feet in activities not commonly studied in scientific research. Propelling research to match real-world environments such as stairs, hills, and uneven terrain may serve to better drive clinical prescription of prosthetic feet. If such conditions are those that offer amputees the most benefit when using a particular prosthetic foot, then these should be the conditions under which they are evaluated. There is currently little compelling scientific evidence to guide the clinical prescription of prosthetic foot-ankle systems. There is, however a great deal of clinical consensus regarding the advantages of certain components, particularly energy storage and return prosthetic feet. Problems such as small sample sizes, mixed populations, outdated components, and limited test environments plague the application of scientific results to clinical prescription of components. Understanding and resolving these issues is a key step toward driving clinical decision making through scientific evidence. Once these issues are addressed, it seems reasonable and logical that scientific research would then play a key role in the clinical prescription of prosthetic feet.