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Overview of Outcome Measures for the Assessment of Prosthetic Foot and Ankle Components

Brian J. Hafner, PhD

The ability to properly prescribe one prosthetic component over another is an essential talent for any clinician. Basic questions such as "is foot A a better choice than foot B?" or "does knee C function better than knee D?" are at the root of the clinical decision-making process. The wealth of available components makes this a daunting, if not impossible, task. Often, this skill is the result of years of clinical experience. Unfortunately, the scientific literature does not offer the clinician a wealth of evidence to support or refute his decision. But what does the literature show? This question, when applied to the clinical prescription of prosthetic feet, was recently posed to a group of experts who met at the American Academy of Orthotists and Prosthetists State of the Science Conference on Prosthetic Foot and Ankle Mechanisms. The goal of that conference was to review the literature comparing prosthetic foot and ankle mechanisms and to make recommendations for future research. The purpose of this article is to describe outcome measures commonly used to assess prosthetic intervention, to review the application of these outcomes to the comparative study of prosthetic feet, and to summarize outcomes-based recommendations from the Prosthetic Foot/Ankle Mechanisms State-of-the-Science Conference (SSC) held April 14 through 16, 2005.

OUTCOME MEASURES

Outcome measures in lower limb prosthetics may be subdivided into several groups, including biomechanical outcomes, functional outcomes, and unique assessment tools. Each type has been used to assess the performance of and/or the preference for a prosthetic intervention. These outcomes and their ability to discern differences among prosthetic feet are reviewed here.

BIOMECHANICAL OUTCOMES

Biomechanical outcome measures are used to quantify function and performance by way of established, validated biomechanical assessment tools. These measures are typically acquired during quantitative gait analysis and fall into several broad categories including temporal, spatial, kinetic, kinematic, energy expenditure, and muscular activity. Examples of key parameters from each category are included in Table 1 .

STRIDE ANALYSIS

Stride analysis incorporates both temporal and spatial measures. Biomechanical studies of prosthetic components commonly include stride analysis as a key component of the evaluation. The most frequently used measures are velocity, stride length, and cadence, but detailed analyses may also incorporate a number of coronal plane parameters including base-of-support width and foot angle with respect to the direction of motion (Figure 1 .¹) Such variables are important when assessing balance, coordination, and effectiveness of an intervention, such as the use of a new prosthetic device.

FORCE ANALYSIS

Kinetic or force analysis is also commonly used to measure prosthetic component intervention. Kinetic biomechanical measures are most typically derived from measures of the ground reaction force (GRF). This is the resultant force that opposes weight of the body striking and moving across the ground during gait. The GRF is equal, and opposite in direction, to the force being supported by the stance limb. This resultant GRF is commonly resolved into three independent components: the vertical ground reaction force (VGRF), the anterior-posterior (AP) force, and the medial-lateral (ML) force (Figure 2 ).

The VGRF is commonly identified by the characteristic "M" shape that occurs in walking gait. In able-bodied walking gait, the two large peaks (often referred to as the weight acceptance and propulsive peaks) oscillate about full body weight (BW), reaching a maximum of about 110% BW and a minimum between peaks of approximately 80% BW.² The small spike in the VGRF that occurs early in stance phase is called the "impact peak" or "impact spike." It is thought to represent the change in moment of body segments as the body strikes the ground in loading response. It is not often present in the VGRF profile of amputee gait.

The ML reaction force or medial-lateral shear force is a result of transferring body weight from one limb to the other across the line of progression. The ML force rarely exceeds 10% BW and is the smallest of the forces resolved from the GRF.² The AP reaction or fore-aft shear force is a result of the anterior braking force and posterior propulsive force in late stance. The maximal AP force is typically less than 25% of body weight.²

MOTION ANALYSIS

Kinematic or motion analysis is the measurement of a body's motion during gait. This pattern is inherently complex, as it involves the motion of myriad body parts moving with respect to one another in three-dimensional space at various speeds. Because of this complexity, kinematic models are often used to simplify the body into basic segments that represent the primary motions of the body. Lower limb segments used to analyze prosthetic foot and ankle mechanisms commonly include the foot, leg (or shank), and thigh. The upper body is often considered a single segment called the "trunk" or "head, arms, and torso" (HAT) (Figure 3 ).

In complex analyses, the pelvis, arms (hand, forearm, and arm) and head may be separated from the HAT, but this significantly increases the complexity (i.e., the degrees of freedom of the model). To simplify evaluation, motion is usually analyzed with respect to one plane (e.g., sagittal, coronal or transverse) at a time. Motion analysis of the sagittal plane typically details the respective motion of the foot, leg, thigh, and trunk segments. Such motion parameters include ankle plantarflexion/dorsiflexion, knee flexion/ extension, and hip flexion/extension. Detailed sagittal analysis may also include pelvic tilt. Coronal and transverse plane analyses similarly involve motions between these same body segments but instead includes the motion parameters of foot and leg internal/external rotation, foot inversion/eversion, thigh abduction/adduction, pelvic drop (i.e., pelvic obliquity), trunk lean, and trunk rotation. The measures most commonly measured when assessing prosthetic foot and ankle components include hip flexion, knee flexion, foot plantar/ dorsiflexion, and foot inversion/eversion.

Joint moments (i.e., muscle moments) may be obtained through a combination of both kinetic and kinematic measures. Joint moments may be referenced as either a demand moment or a response moment.² A demand moment is that moment created by the external forces (GRF and body segment inertias), whereas a response moment is created by the muscular activity required to resist such forces. They are equal in magnitude but opposite in direction. These conventions are best explained through the following example. In loading response, the GRF vector passes through the ankle joint, posterior to the knee joint, and anterior to the hip joint (Figure 4 ).

If referenced as demand moments, this would be reported as no moment at the ankle joint, a positive moment at the knee joint, and a negative moment at the hip joint. These knee and hip demand moments may also be referenced as flexion moments.³ Because of this demand, the hip and knee extensors contract to prevent collapse of the stance limb. If the joint moments are referenced as reaction moments, this would be reported as no ankle moment, a negative knee muscle moment, and a positive hip muscle moment. Similarly, these hip and knee reaction muscle moments may be referenced as extension muscle moments. Researchers may report results by either convention, so results should be interpreted with caution, especially when comparing across studies. It is also important to understand that muscle moments are the net muscular reaction at any joint and may not necessarily reflect the true muscular activity. Behaviors such as co-contractions are difficult to detect in the muscle moment analysis but may be easily detected in a muscular activity analysis.

MUSCULAR ACTIVITY

Muscular activity measures quantify the action and timing of muscles or muscle groups during the gait cycle. These include the state of activity (i.e., active or passive), the magnitude of the contraction, and the duration of the contraction. The use of muscular activity analysis in pathological gait analysis is common, but rare when comparing prosthetic components.

ENERGY EXPENDITURE

The fundamental goal of locomotion is the efficient progression of the body through space. Energy expenditure analysis attempts to measure the metabolic cost of this effort to move the body during locomotion. Energy expenditure may be measured by a number of direct and indirect methods. In simplest form, relative energy expenditure may be measured as a timed distance (e.g., 6-minute walk). More advanced methods quantify the oxygen consumed during an activity as a means to create a permanent record that allows direct comparisons between interventions. In such cases, exhaled gases [i.e., oxygen (O₂) and carbon dioxide (CO₂)] are collected with laboratory equipment and analyzed to determine the amount of O₂ consumed. These values are then commonly normalized to body weight and reported as a function of time or distance.

BIOMECHANICAL OUTCOME RESULTS

A review of the scientific literature reveals that biomechanical outcomes in prosthetic feet have been used to compare a number of different prosthetic feet, including the SACH foot (various manufacturers), the Seattle foot (Seattle Systems, Poulsbo, WA), the Flex-Foot (Φssur, Aliso Viejo, CA), and others. Commonly, prosthetic feet are grouped into two categories: energy storage and return (ESAR) feet and conventional feet (CF). Most comparisons in the literature involve comparison between the CF and one or more ESAR feet.

Few statistically significant results were consistently reported across multiple studies in the literature when comparing the ESAR foot to a conventional device like the SACH foot. Of all the biomechanical outcomes listed, the only ones to consistently (or even predominantly) report a significant difference when comparing an ESAR foot with a CF include peak ankle moment (Flex Foot > SACH foot)^4–6 and maximum dorsiflexion (Flex Foot > SACH foot).^4,7,8 No other significant changes were reported across multiple studies. However, a number of trends were detected that suggest that ESAR feet may offer increased self-selected walking velocity, increased stride length, decreased sound side weight acceptance force, increased affected side propulsive force, and increased total ankle range of motion when compared to the CF.⁹ In comparison to the commercial and clinical success of these devices, this lack of supporting evidence is unfortunate, but may be understandable. Scientific evaluation of prosthetic components is often hindered by limitations of clinical research. These include small sample sizes, mixed populations (i.e., traumatic and vascular amputees), test environments (often limited to gait labs), inadequate accommodation time, insufficient training, and outcomes that may be insensitive to prosthetic componentry.

FUNCTIONAL OUTCOMES

Functional outcome measures are used to evaluate functional ability and quality of life (QoL). When applied to the amputee population, these measures may be used in an effort to assess surgical outcome, the rehabilitation process, or prosthetic intervention. Several types of functional outcome measures may be used to evaluate an amputee's preference for and performance with a prosthesis. These tools may be loosely grouped into two categories: survey tools and physical assessments. Survey tools are designed to assess performance, function, preference, health, and/or QoL through questionnaire-based feedback. Physical measures are intended to assess function and performance through an administered assessment or evaluation of functional tasks. Examples of measures used to assess amputee performance are included here.

SURVEY TOOLS

Survey tools are a questionnaire-based subgroup of functional outcome measures. Often, these tools are specifically designed and validated for use with the amputee population. Survey tools are administered and/or self-administered and must be scored by the clinician or researcher.

The Prosthesis Evaluation Questionnaire (PEQ) is a validated, self-administered questionnaire consisting of 82 questions.¹⁰ The PEQ is divided into nine scales computed from 42 of the questions. These scales include ambulation, appearance, frustration, perceived response, residual limb health, social burden, sounds, utility, and well-being. The 40 remaining items pertain to other evaluation areas and are not grouped into scales. Individual questions of the PEQ are answered with respect to the amputee's recollection of the previous 4 weeks. Answers are recorded on a visual analog scale that records the amputee's response between two extremes (Figure 5 ).

The Orthotic and Prosthetic User's Survey (OPUS) is a self-administered questionnaire consisting of 91 questions.¹¹ The questions are grouped into four categories, including lower limb functional measure, health-related QoL, satisfaction with device, and satisfaction with services. Questions are recorded on a Likert scale rating of the amputee's perception (Figure 6 ).

The Prosthetic Profile of the Amputee (PPA)¹² is a validated, administered or self-administered questionnaire used to evaluate the factors contributing to the use of a lower extremity prosthesis. The PPA contains the Locomotor Capabilities Index (LCI), a 14 question sub-scale designed to measure the functional status of the lower limb amputee. The LCI uses a Likert scale to self-assess the amputee's ability to accomplish a functional task (Figure 7 ).

The Orthotics and Prosthetics National Office Outcomes Tool (OPOT)¹³ is a self-administered questionnaire based on the Short-Form 36 (SF-36),¹⁴ designed to assess health, satisfaction with the prosthesis, and ambulation. The OPOT uses a Likert scale to record the amputee's self-assessment of QoL and function (Figure 8 ).

PHYSICAL MEASURES

Physical measures are tools designed to assess and/or predict physical function and mobility. Such measures are used to evaluate various pathologies, including amputee gait. These measures are most commonly administered by a clinician or researcher.

The Timed Up-and-Go (TUG) is a validated instrument that rates and times an individual as he or she performs a set of functional tasks.¹⁵ These include rising from a chair, walking 3 meters, turning around, returning to the chair, and sitting down. Although commonly used to assess elderly gait, the TUG is used rarely to assess amputee gait.

Timed Walk Tests (TWTs) are a group of validated tools that include time-based test such as the 2-, 6-, and 12-minute walk tests^16,17 and distance-based tests like the 10-meter walk test.¹⁸ Each test measures mobility by recording the distance traveled by an individual as he or she ambulates at selfselected speed over the specified period of time or the time required to travel the specified distance. These measures have been used to assess a variety of pathological conditions, including lower limb amputation.¹⁹

The Amputee Mobility Predictor (AMP) is a validated assessment tool designed to predict the potential of an amputee to ambulate.²⁰ It consists of 21 functional tasks rated by a clinician. It may be used to assess walking potential without a prosthesis (AMPnoPRO) or with a prosthesis (AMPPRO). It is specifically designed to assess amputee subjects or patients.

Step activity is a functional measure of daily activity.²¹ It is most commonly acquired using a small, unobtrusive recording device (e.g., CYMA StepWatch™, Mountlake Terrace, WA) attached to the ankle. Steps are recorded over a period of up to 2 months and may then be downloaded to a computer for advanced analysis. Outcomes such as time spent at low, medium, and high activity levels and overall step counts can be assessed by using the interface software.

FUNCTIONAL OUTCOME RESULTS

Despite the availability of functional outcomes tools to assess amputee performance and function, few have been used to compare prosthetic feet. To date, only two (PEQ and step activity) have been reported to compare the differences among or between prosthetic feet. Coleman et al.²² showed that amputees wearing the Flex Foot (Ossur) walked significantly farther, spent significantly more time walking at a moderate activity level, walked with a significantly longer stride, and scored a significantly higher score on the Prosthesis Utility scale of the PEQ than when wearing the SACH foot. A study by Hsu et al.²³ compared the Flex-Foot, C-Walk (Otto Bock, Minneapolis, MN), and SACH feet. Users scored the Flex-Foot significantly higher on the Frustration and Usefulness scales of the PEQ than they did the SACH foot. Additionally, the C-Walk scored significantly higher on the Usefulness scale than did the SACH foot.

UNIQUE ASSESSMENT TOOLS

Despite the availability of measures to assess prosthetic performance, amputee preference, and functionality, there is little evidence to suggest they are sensitive to the use of different types of prosthetic feet. Because of this, researchers often turn to unique measures of preference and performance to evaluate the influence of foot type. Several types of unique assessment tools have been used in the literature to show differences among feet. These unique tools have been categorized into three groups, termed descriptive dialog, functional assessment questionnaires, and numerical rating scales.⁹

DESCRIPTIVE DIALOG

Descriptive dialog is described as the basic, subjective feedback obtained during an evaluation. This type of feedback is often used to assess patient preference for a particular device or to infer why that device may be preferred. It does not include any form or standardized reporting, and is the weakest level of evidence of the outcomes discussed here.

FUNCTIONAL ASSESSMENT QUESTIONNAIRES

The functional assessment questionnaire tools are unique, standardized questions posed to patients or subjects relating to prosthetic function, performance, or preference. These questionnaires are similar to survey tools, but are not validated, and typically consist of a small number of questions specifically targeting a particular function of, or preference for, the prosthetic device.

NUMERICAL RATING SCALES

Like functional assessment questionnaires, numerical rating scales are often customized by the researchers to assess the performance of, or preference for, a prosthetic device. These rating scales use ordinal metrics to rank the users' perception of function. This allows the collected data to be scored and statistically analyzed.

UNIQUE ASSESSMENT RESULTS

Researchers who have reported results of a descriptive dialog when comparing prosthetic feet have indicated that ESAR feet are generally preferred over conventional or SACH feet.^4,24 Improvements perceived and discussed included increased velocity and stability on uneven ground. Although these results are extremely limited by the nature of the data, they do appear to corroborate anecdotal evidence that suggests energy storing feet offer improved function in these domains.

Functional assessment questionnaires have been used to compare prosthetic feet in two studies reported in the literature. ^25,26 In both cases, the researchers asked the subjects to compare an ESAR foot (either the Seattle or Flex Foot) to a SACH foot. Subjects noted if they had an improvement with the ESAR foot, an improvement with the SACH foot or had no change in improvement between the feet in several functional categories. The majority of users noted no change or an improvement with the ESAR foot in nearly every functional category. Functions where the majority of users reported improvements with the ESAR foot included gait, recreational activity level, ankle motion, awareness of keel action, balance, and skin problems. Once again, this information appears to support the clinical experience and anecdotal evidence.

To date, several studies have used numerical rating scales to analyze the influence of prosthetic feet. Alaranta et al.²⁷ found that the Flex Foot scored significant improvements in functional ability when compared with a SACH foot. These improvements were noted in a variety of activities that included stairs, uneven ground, and rapid speeds. Only in level walking did users not report a difference between feet. Mac- Farlane et al.²⁸ used a modified BORG scale to assess users' perception of the Flex Foot and the SACH foot. Subjects reported significant reductions in exertion when using the Seattle foot at all three speeds (2.0, 2.5, and 3.0 mph) and all three grades (level, 8.5 incline, 8.5 decline) tested. Postema et al.²⁹ compared several ESAR and conventional feet using a numerical scoring system that focused on standing stability, walking stability, function, and special activities. The only significant result was that one of the conventional feet scored a significantly lower mean using the designed metric. Last, Underwood et al.³⁰ compared the Flex Foot and the SAFE foot (Campbell-Childs, White City, OR), using a rating scale of perceived stability and mobility. Although no statistical analysis was performed, the Flex foot scored higher than the SAFE foot in all categories, most noticeably when walking quickly or standing on a compliant surface.

STATE OF THE SCIENCE REVIEW

The literature and evidence regarding the clinical use and prescription of prosthetic foot/ankle mechanisms was reviewed by clinical and scientific experts in April 2005.³¹ It was determined that prescription of prosthetic feet was more a function of clinical experience and subjective preference than based upon the limited scientific evidence in support of energy storage and return prosthetic devices. The reviewers and experts agreed that there were promising examples of applicable and relevant research, but that there was simply insufficient evidence to use these results as drivers in the clinical decision-making process.

A number of reasons for the inconsistency between clinical experience and the scientific evidence were brought forth. These included small sample sizes, mixed populations, lack of accommodation and training, and the lack of a consistent categorization system for the devices. It was also believed that many outcomes lacked "ecological validity." Examination of the literature revealed that the functional outcomes and unique assessments most often found differences between foot types when the feet were being used in an environment other than level, indoor walking. Yet, the majority of the biomechanical outcomes were collected in just that environment.

The assembly recommended a number of future research directions. Of those related to outcomes, a clear need for the development of sensitive and function-specific outcomes designed to differentiate prosthetic components was noted. Similarly, it was recommended that research should target the functional needs of the patients to develop assessment tools that would assist clinicians in properly prescribing prosthetic components for an individual. In both cases, it was noted that these tools should be sensitive to prosthetic intervention, applicable to the activities of daily life, reliable for use by clinicians, and validated for use in the amputee population.

CONCLUSIONS

A number of outcome measures have been used in the analysis of prosthetic foot/ankle mechanisms. These include biomechanical measures, functional outcome measures, and unique assessment tools. Ideally, these outcomes should be used as evidence to recommend and support a specific prosthetic prescription. However, a review of the literature and evaluation of the evidence by experts at a recent State-of-the-Science conference revealed that clinical decision-making was driven primarily by experience and preference rather than by support of the scientific evidence. Although selected differences among feet were detected, the evidence in support of energy storage and return prosthetic feet was underwhelming when compared with the clinical acceptance and patient preference for such devices. This may be because many of the outcomes chosen in the scientific evaluations are not sensitive enough to discriminate among prosthetic feet. Moreover, many of the outcomes have been used in environments inconsistent with the perceived performance of, and preference for, ESAR feet.

Due to the inherent limitations with prosthetics research, it is suggested that new outcome tools be developed to assess prosthetic feet in functional areas of daily activity. These tools have the potential to complement existing measures and provide specificity to componentry where existing measures are better suited to health and quality of life. In particular, these tools should be able to assess function and performance in domains beyond flat, level walking. Additionally, these tools should be sensitive to changes in prosthetic devices and be validated for use by targeted amputee populations.

Although the suggestions here are based on a review of the literature regarding prosthetic foot and ankle mechanisms, it is possible, if not likely, that reviews of evidence in support of other lower limb prosthetic components such as liners, sockets, and knees would reveal similar findings. Because existing outcomes tools are largely insensitive to changes in prosthetic components, it will be necessary to develop new measures of function, preference, and performance to evaluate and prescribe one component with respect to another. With such tools available, researchers and clinicians will be able to better discern the differences among components, and amputees will be able to receive the prosthetic care most applicable to the activities of their daily lives.

Correspondence to: Brian J. Hafner, PhD, 675 South Lane Street, Suite 100, Seattle, WA 98104; e-mail: .

BRIAN J. HAFNER, PhD, is affiliated with Prosthetics Research Study, Seattle, Washington.

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