American Academy of Orthotists & Prosthetists
Professionals Advancing Care Through Knowledge
Summary | Acknowledgments
HOW HAVE FEET BEEN STUDIED?
Question 1: What scientific methods have been used to determine the functional performance of prosthetic feet and ankle systems in current use?
Human subject testing
Perceptive analyses
Patient and clinician assessment
Focus on preference or performance
Levels of sophistication
Descriptive dialog
Functional assessment questionnaire
Numerical rating scale
Biomechanical studies
Temporal/Spatial
Kinetic
Kinematic
Mechanical energy
Metabolic energy
Muscular activity
Mechanical testing
Materials properties
Mechanical properties of the foot/ankle system
Load versus deflection properties
Hysteresis
Natural frequencies with physiologic loading
Fatigue
Ultimate strength testing
Roll-over shape
Damping ratio estimates
Mechanical energy
Emerging technologies
Plantar pressure measurement
Multisegment foot models for measurement of kinematics/kinetics
Finite element models for evaluation of kinematics/kinetics
Induced acceleration & forward dynamic simulation
Neural networks and fuzzy intelligence models
Total daily energy expenditure
The outline reflects the articles included in Section I, ensuing discussion and the group's summarized conclusions.
DOES THE SCIENCE RELATE TO CLINICAL USE?
Question 2: What is the correlation between the published scientific measurements and the clinical methods used to recommend foot/ankle systems for specific amputees?
In short, limited correlation exists. This is because:
Clinical decision making has not been well studied
Currently available scientific evidence has limited clinical usefulness
Scientific studies inform but do not greatly influence clinical practice
Scientific information lags behind clinical practice
Relationships between laboratory data and real world functions have not been clearly demonstrated
Although the group agreed that a correlation between the state of the science and the clinical use of feet was not well documented, the articles in Section II lend valuable insight into clinical decision making regarding prosthetic foot-ankle mechanisms.
DOES FOOT CHOICE INFLUENCE OUTCOME?
Question 3: What is the correlation between prosthetic foot-ankle systems and prosthetic outcomes such as performance, patient acceptance, durability, etc.?
The current outcome tools and or methods to measure the functionality of prosthetic foot and ankle components are not yet fully developed to an acceptable level of specificity or sensitivity for the heterogeneous amputee population.
This is because:Influence on patient acceptance is not well established.
Influence of patient outcomes has not been clearly established.
Current outcomes reports are inconclusive, but trends suggest that prosthetic foot prescription can affect functional outcomes.
The influence of prosthetic gait training on prosthetic function has not been thoroughly studied.
Subjects must be able to exhibit minimum capacity to utilize the component and be adequately trained as part of inclusion criteria.
The correlation to outcomes is also limited because there is no widely agreed upon evidence-based prosthetic foot classification system.
The correlation to outcomes is limited because there is no widely agreed upon performance-based mechanical test to compare individual prosthetic feet.
FUTURE RESEARCH AND RECOMMENDATIONS
Question Four: In light of the literature review and the panel's discussion, what are the primary future research priorities?
The long-term goal is to develop an accepted classification and comparison system for prosthetic ankle-foot devices that includes perceptive, clinical, and mechanical measurements. This is critical to facilitate communication among clinicians, manufacturers, and researchers and to ensure that results from investigations can be compared.
Four key areas or classifications for future research have emerged:
1. Comparison Tools: Better, standardized means for defining the mechanical and clinical characteristics of feet are needed. Researchers and manufacturers would benefit by knowing in advance, what kinds of mechanical tests are valid and important?
2. Standardized Nomenclature and Classification: How feet are classified needs to be clarified, broadly used, and based on science.
3. Literature ranking system: Prosthetic research should be evaluated and ranked according to specific merits and standards.
4. Qualitative Tools and Assessments that drive the clinical decision making process are needed.
To elevate the quality and comparability of research on prosthetic feet and ankles, it is important that researchers answer key clinical questions that will enhance both scientific research and clinical practice. Some examples include:
What defines "clinical significance"?
What is the optimal timing for energy storage and return in prosthetic feet to benefit the patient?
What is the optimal adaptation time for new prosthetic feet? (Time after amputation, etc.)
Have patients in studies demonstrated minimum functional criteria in the use of their prosthesis, and have they had proper gait training? (The group observes that many studies inadequately address the patient's degree of physical therapy and conditioning or whether a component was appropriate for them prior to being entered in a study. Recommendation: Based on their history and physical, clearly describe the degree and type of therapy they received and how close to "optimum" their gait is.
What is the impact of gait training on performance with prosthetic feet? Are all patients in a study trained and experienced more or less equally?
Is symmetry in amputee gait the optimal goal of prosthetic design?
How are clinical decisions currently made? More formal scientific review of this is needed.
What are some of the confounding variables? (For example, patient fatigue, and variability in materials)
It is important to develop standardized qualitative tools and assessments.
It is suggested that some future research or consensus efforts should focus on developing or agreeing upon a uniform assessment tool. This would be a set of easily implemented, activity-level questions that practitioners could ask patients, as well as appropriate functional tests that could be conducted. These would help narrow down foot/ankle recommendations for patients in a more uniform way, reducing bias and enabling multi-center studies to occur. (This process could of course apply to the entire prosthesis.) A follow-up assessment would also be needed to determine if the predicted outcomes were achieved. The work of Gailey and others is a good example of the direction this research should go.
t is important to develop "real world" testing and measurements that are ecologically valid. This is vital, for example, for high activity amputees, particularly injured war fighters who may be returning to active duty, or moving toward an active civilian life. Testing equipment, measurements and protocols should be introduced and be appropriate for various common age groups and etiologies that may present to a prosthetic practice.
It is important to define key needs and functional metrics for feet and ankle systems between pediatric, adult, geriatric, and high-demand populations.
It is important to apply newly emerging technologies to the development of prosthetic foot and ankle mechanisms to address identified functional needs. New feet should be introduced, but not without a review of what is needed, a comparison to other feet, and clear definitions for how the foot should be classified.
The list above reflects the broadest degree of agreement and primary focus of discussion among conference participants. Other areas of future research or development that were felt to be important, but perhaps less "global" are mentioned briefly below.
1. There is a need for the professions dealing with persons with amputations to agree upon a preferred nomenclature system for describing gait. (i.e., Perry, etc.?)
2. Questions arose about how clinicians could identify and support that their patients would benefit from different feet for different activities (for example, a day-to-day limb and a sports leg). Are there activities that clearly indicate the need for additional feet (i.e., track, vs. alpine climbing, vs. household ambulation)?
3. Do we need a revised and standardized nomenclature to define current and future feet (to replace the SACH, Single Axis, Flexible Keel, Dynamic Response scheme)? Are there reasonably concrete divisions—where are the lines for categories? (In many cases, patient functional differences with some feet in the same category or even across categories are not yet clarified. Yet, is there benefit in keeping a broader classification for purposes of understanding the intended design of the feet?)
4. Methods of accounting for confounding variables such as variability in materials should be developed (e.g., carbon fiber or urethane may differ from foot to foot).
5. Rollover shape (discussed in "Scientific Methods to Determine Functional Performance of Prosthetic Ankle/Foot Systems" in this Supplement) reflects the progression of the center of mass. Rollover shape alignment is the concept that, although different strategies and components may be used, prosthetists align prosthetic feet toward an ideal shape. With the rollover shape concept it is the deformation or articulation that gives the foot its unique curve. This is a worthwhile approach to evaluate as a means of comparing feet, evaluating alignment. However, not all feet are amenable to aligning to rollover shape. They may differ enough in design and mechanical test that choices about including them in a study would have to be made.
6. What constitutes energy storage and release in prosthetic foot/ankle systems needs to be identified and documented so that future studies may use this standard, and feet released to the market could be appropriately classified and compared.
7. More attention should be paid to the role of the prosthetic heel lever and its shock (energy) absorption properties as well as dynamic response capabilities. This might help researchers and clinicians understand how various heel con- figurations affect energy return.
8. A qualitative review of what physicians, prosthetists, therapists and biomechanists consider as being important elements of foot and ankle designs should be undertaken. This should also shed light on the clinical decision making process, how feet are selected, recommended and evaluated from a clinical perspective.
9. Durability of prostheses is not widely studied. Some developing world prospective reviews have looked at prosthesis durability, where 3 years of service is considered a target goal for improvement. Although this is more of a development issue, an information clearinghouse website that was widely used to classify prosthetic components, including feet and ankles, would enable clinicians to make quick searches and comparisons, and might encourage manufacturers to post results of how their component matches up to established metrics.
10. There is a need to describe "domains of function" and the hierarchical importance of these to the patient and the prosthetic design, training, and components he or she will receive. These "domains" would include issues such as degree of comfort and patient's status, i.e., what is indicated for their situation, and issues such as standing stability, dynamic stability, fatigue, management of stairs, irregular terrain, speed, agility, and frequency of participation in activities such as swimming, hiking, backpacking, standing in one place, and others. Should this be undertaken, researchers might subselect certain patient characteristics that help narrow down the prosthetic needs and choices.
11. Much study has used treadmills or walking indoors on flat surfaces, ramps, and so forth, but these activities do not truly mimic conditions in the "real world." Better tools and methods are needed and should be used in a uniform way to evaluate feet/ankles. Real-world studies would include mechanical load stresses, number of turns, linear gait, inclines, uneven ground, various stairs and steps, turning, stopping, running, carrying heavy objects, and even battlefield simulations (as appropriate for certain amputees who plan to return to active duty).
12. The potential utility of advanced technology foot/ankle components for elderly patients is an important future area of research. A mechanism for evaluating the appropriateness and benefits of providing energy return feet (i.e., Flex Foot and other such ESAR components) for elderly patients should be investigated. Outcome studies might clarify how or if a higher functional class foot can benefit elderly patients in various ways (i.e., survival of opposite foot) and return them to relatively normal function. In addition, how these feet contribute to high-level functions such as running or walking on uneven ground should be evaluated.
13. There is a need to evaluate differences and similarities between "low-profile" J-shaped feet and higher profile feet (i.e., Flex Walk vs. Modular 3).
14. Energy storage and return should not be the primary focus or the measure of function. Force dissipation is valuable (i.e., inversion/eversion, etc.) as a means of enhancing function, protecting the residual limb and reducing forces that can injure the limb. Thus, there is a need to evaluate multi-axial feet in multi-use modes. For example, one could investigate how a very flexible mobile ankle/foot impacts the patient's ability to function on uneven terrain, versus standing in place while shooting a bow at a target (an activity that requires one to stand still while concentrating on some other activity).
15. The majority of what is known about the biomechanics of the foot is related to its function in linear walking on level surfaces. Ultimately, the function of the foot is multiplanar. In addition to sagittal plane motion and forces, researchers must also consider motion and forces about the long axis of the foot as well as torsional forces about the long axis of the tibia. The foot has the ability to exhibit multiplanar adaptive stiffness that suits the functional demands of the individual. Although developing a paradigm in which we characterize stiffness and energy storage and return in the sagittal plane is important, ultimately we need to also develop a framework of considering prosthetic feet in terms of the multiplanar stiffness and force/impact absorption.
16. No current "best measure" to evaluate patient outcomes with a prosthesis and its various designs, interfaces, and components has been established. Perhaps the Berg scale might be a reasonable measure, combined with a self report and 6-minute test as a standardized means for evaluating prosthetic foot/ankle systems. Areas that should be considered in evaluating outcomes include (in addition to energy efficiency, walking speed, before-and-after measures, and other standard approaches): quality of life and cultural appropriateness, fatigue, exertion, durability, cosmesis, and others not covered in most studies.
17. The perception of cosmesis and body image are influenced not only by what someone walks with, but also how they walk. This area merits further investigation into acceptance and usage of prosthetic components.
18. The role and benefits of separate shock/torque absorbing "add-ons" should be determined. In addition, how they change the function of the rest of the device and change the patient's gait are important areas to investigate.
19. Adaptive control prosthetic ankles that are powered and responsive are likely to arrive in the market in the next few years. Researchers should be prepared to evaluate and compare such systems, and clinicians should consider looking into the technology in preparation for future changes.
20. Additional recommended future research and development suggestions include:
Applications of emerging composite technology, including, for example, carbon nanotube materials
Evaluate how materials and the geometry of the device influence gait, function, etc.
Investigate applications of active variable stiffness components
Rollover geometry principles and practical clinical applications
Possibilities of developing adjustable active ankles (variable settings)
CONCLUSIONS
A final general observation that conference participants made was that there were good, promising examples of foot and ankle studies, but not nearly enough of the high "A" or "B" caliber. The participants debated about how studies regarding prostheses should be ranked. It is frankly very tough, timeconsuming, and expensive to test enough prosthetic users, particularly using "A" level protocols. Also, the minimal link between solid research and clinical usefulness of the results was anticipated, but disappointing. The participants agreed that prosthetic/orthotic publications and authors of future research on foot/ankle mechanisms should be encouraged to keep aspiring to the standards of major peer-reviewed medical journals. Readers of current publications can also encourage advancement by taking a more active role in critiquing publications, that is, communicating to the editors or authors their questions or concerns regarding the means and methods used in particular studies.
Participants in this SSC hope that this review and the accompanying perspective papers and summaries will provide helpful information and will stimulate advancements in the state of the science of prosthetic foot and ankle mechanisms.
ACKNOWLEDGMENTS
The SSC participants thank the American Academy of Orthotists and Prosthetists and the AAOP Quantum Leap initiative, the U.S. Department of Education, the Journal of Prosthetics and Orthotics, Prosthetic Research Study (PRS), and Texas Scottish Rite Hospital for Children, for supporting this activity.
