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

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.Table 1: Biomechanical outcome measuresSTRIDE 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.Figure 1.: Coronal and sagittal stride characteristics (From C.L. Vaughan, B.L. Davis and J.C. O'Conners, 1992, Dynamics of Human Gait, page 12, figure 2.8. Adapted with permission from Human Kinetics Champaign, IL.).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).Figure 2.: Components of the ground reaction force (Adapted with permission from Perry J, Gait Analysis: Normal and Pathological Function, Thorofare, NJ: Slack Incorporated; 1992.).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).Figure 3.: Example of a kinematic model.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).Figure 4.: Joint moments and muscle actions during loading response. A, Referenced as demand moments. B, Referenced as response moments.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 (Össur, 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).Figure 5.: The PEQ uses a visual analog scale to record answers.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).Figure 6.: The OPUS uses a Likert scale to assess perception.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).Figure 7.: The PPA-LCI uses a Likert scale to rate the ease of performing activities.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).Figure 8.: The OPOT uses a Likert scale to rate QoL and physical function with the prosthesis.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 tests16,17 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 self-selected speed over the specified of time or the time required to the specified distance. These measures have been used to assess a of pathological including lower limb The Amputee is a validated assessment designed to predict the of an amputee to It of functional by a clinician. It may be used to assess walking a or with a It is specifically designed to assess amputee or activity is a functional measure of It is most commonly acquired a recording device (e.g., to the are recorded over a of to and may then be to a for advanced analysis. Outcomes such as time at and activity and be by the FUNCTIONAL OUTCOME RESULTS the of functional outcomes tools to assess amputee performance and function, have been used to compare prosthetic feet. To only two and have been reported to compare the differences among or between prosthetic feet. that the Foot significantly significantly more time walking at a activity with a significantly and scored a significantly on the Prosthesis scale of the PEQ than when the SACH foot. A study by compared the and SACH feet. scored the Flex-Foot significantly on the and scales of the PEQ than the SACH foot. the scored significantly on the scale than the SACH foot. TOOLS the of measures to assess prosthetic performance, amputee preference, and is evidence to suggest are to the use of different types of prosthetic feet. Because of often to unique measures of preference and performance to evaluate the of foot Several types of unique assessment tools have been used in the literature to differences among feet. These unique tools have been into three groups, functional assessment and rating is as the obtained during an evaluation. This type of is often used to assess preference for a device or to that device may be It does not include any or and is the of evidence of the outcomes here. FUNCTIONAL The functional assessment questionnaire tools are questions posed to or to prosthetic function, performance, or These are to survey but are not validated, and typically of a small number of questions specifically a function or preference the prosthetic functional assessment rating scales are often by the to assess the performance or preference a prosthetic These rating scales use to the perception of This allows the collected to be scored and statistically RESULTS Researchers who have reported results of a when comparing prosthetic feet have that ESAR feet are over conventional or SACH perceived and included increased and on Although these results are limited by the of the to evidence that energy feet offer function in these Functional assessment have been used to compare prosthetic feet in two studies reported in the In both cases, the the to compare an ESAR foot the Seattle or to a SACH foot. if an with the ESAR foot, an with the SACH foot or no change in between the feet in several functional The of no change or an with the ESAR foot in functional the of reported with the ESAR foot included gait, activity ankle of balance, and this to support the clinical and To several studies have used rating scales to analyze the of prosthetic feet. that the Foot scored significant in functional ability when compared with a SACH foot. These were in a of that included and speeds. in walking not report a difference between feet. used a scale to assess perception of the Foot and the SACH foot. reported significant in when the Seattle foot at all three and and all three compared several ESAR and conventional feet a that on walking function, and The only significant result was that one of the conventional feet scored a significantly lower the designed compared the Foot and the foot a rating scale of perceived and mobility. 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A number of for the between clinical and the scientific evidence were These included small sample sizes, mixed lack of accommodation and training, and the lack of a for the It was also that outcomes of the literature that the functional outcomes and unique most often differences between foot types when the feet were being used in an other than the of the biomechanical outcomes were collected in that The a number of future Of to outcomes, a for the of and outcomes designed to prosthetic components was Similarly, it was that should the functional of the to assessment tools that would in properly prosthetic components for an In both cases, it was that these tools should be to prosthetic intervention, to the of for use by and validated for use in the amputee population. 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