Mark D. Geil

Energy Loss and Stiffness Properties of Dynamic Elastic Response Prosthetic Feet

Dynamic elastic response prosthetic feet are designed to store and return energy during the gait cycle to assist the amputee with limb advancement. In so doing, the structural ability of the feet to store and return energy is significant, as is foot stiffness and the amount of input energy dissipated by the foot before it unloads. Manufacturers do not generally make subjective measures of stiffness and hysteresis available; instead, feet are classified according to manufacturer-specific stiffness categories. This investigation attempted to provide an independent measure of stiffness and hysteresis for eleven prosthetic feet. The feet were tested in constant strain rate and in cyclic tests on an Instron material-testing machine. Each foot demonstrated a stiffness value in one of only four categories, with multiple models of feet and even feet from multiple manufacturers showing essentially the same stiffness. Energy loss showed a broad range of feet in the middle of all values, with the greatest amount of loss occurring in the College Park feet and the least amount occurring in the Flex-Foot. Although structural property values do not fully explain a foot’s function in gait, data suggest the need for an independent classification scheme for stiffness and hysteresis among all manufacturers. Such a scheme would aid clinicians’ ability to appropriately prescribe and fit prosthetic feet.

Comparison of methods for the calculation of energy storage and return in a dynamic elastic response prosthesis

The standard method used to calculate the ankle joint power contains deficiencies when applied to dynamic elastic response prosthetic feet. The standard model, using rotational power and inverse dynamics, assumes a fixed joint center and cannot account for energy storage, dissipation, and return. This study compared the standard method with new analysis models. First, assumptions of inverse dynamics were avoided by directly measuring ankle forces and moments. Second, the ankle center of rotation was corrected by including translational power terms. Analysis with below-knee amputees revealed that the conventional method overestimates ankle forces and moments as well as prosthesis energy storage and return. Results for efficiency of energy return were varied. Large differences between models indicate the standard method may have serious inadequacies in the analysis of certain prosthetic feet. This research is the first application of the new models to prosthetic feet, and suggests the need for additional research in gait analysis with energy-storing prostheses.

Plantar foot pressure responses to changes during dynamic trans-tibial prosthetic alignment in a clinical setting

Alignment of a lower limb prosthesis refers to the spatial orientation of the prosthetic components and socket with respect to one another. During the process of dynamic alignment, a prosthetist repeatedly modifies this spatial orientation and observes the amputees resulting walking pattern, eventually arriving at an alignment that is judged to be optimal. Quantification of the effect of each alignment modification and correlation of the magnitude of modification with the changes in gait could improve understanding of the process and promote an evidential base for practice. This investigation quantified bilateral plantar foot pressures in six trans-tibial amputee subjects during the process of dynamic alignment at prosthetists’ clinics during regularly scheduled appointments. Outcomes of changes in prosthetic alignment during the clinical dynamic alignment process were determined to be quantifiable via plantar pedobarography. Changes in the angle between the pylon and the socket in the frontal plane produced predictable shifts in foot pressure between medial and lateral foot regions under the prosthesis, and typically shifted pressure to the lateral region of the contralateral foot, regardless of the direction of the modification. Temporal parameters revealed that subjects initially adopt a conservative locomotor pattern after an alignment change but within a few steps begin to refine their gait and approach more symmetrical single limb support times. Plantar pedobarography provides the clinician with potentially useful information to augment dynamic alignment and provides a tangible record of the results of the process.

An iterative method for viscoelastic modeling of prosthetic feet.

Prosthetic foot designs are growing in complexity, but a few material and structural properties, including stiffness and viscoelasticity, remain critical to foot function. Consistent identification of these critical properties would aid prosthesis prescription. This investigation evaluates a new technique to model prosthetic feet as a combination of springs and dampers, and therefore characterize a foots stiffness and viscoelasticity by means of spring and damper coefficients. A quasi-Newton iterative algorithm was developed to determine model coefficients for 9 prosthetic feet based on compressive creep, stress-relaxation, and constant strain rate tests. A broad range of current energy-storing feet including designs from Otto Bock, Seattle, Kingsley, and Ohio Willow Wood were very accurately modeled with the iterative technique. Feet without a solid ankle from Flex and College Park were the least accurately modeled. The Flex foot, tested without a cover, had a considerably lower damping coefficient. Damper coefficients were similar for most all other feet, suggesting similar material properties of the foam cover. Stiffness varied and generally agreed with published data. The ability of the model to produce two separate parallel spring stiffness constants might provide insight into foot structure. The model represents a means to objectively quantify material properties for a range of solid ankle dynamic elastic response prosthetic feet, but may be limited in its characterization of other foot varieties.

Consistency, precision, and accuracy of optical and electromagnetic shape-capturing systems for digital measurement of residual-limb anthropometrics of persons with transtibial amputation.

Computer-aided design (CAD) and computer-aided manufacturing systems have been adapted for specific use in prosthetics, providing practitioners with a means to digitally capture the shape of a patients limb, modify the socket model using software, and automatically manufacture either a positive model to be used in the fabrication of a socket or the socket itself. The digital shape captured is a three-dimensional (3-D) model from which standard anthropometric measures can be easily obtained. This study recorded six common anthropometric dimensions from CAD shape files of three foam positive models of the residual limbs of persons with transtibial amputations. Two systems were used to obtain 3-D models of the residual limb, a noncontact optical system and a contact-based electromagnetic field system, and both experienced practitioners and prosthetics students conducted measurements. Measurements were consistent; the mean range (difference of maximum and minimum) across all measurements was 0.96 cm. Both systems provided similar results, and both groups used the systems consistently. Students were slightly more consistent than practitioners but not to a clinically significant degree. Results also compared favorably with traditional measurement, with differences versus hand measurements about 5 mm. These results suggest the routine use of digital shape capture for collection of patient volume information.

Consistency and accuracy of measurement of lower-limb amputee anthropometrics

Lower-limb amputees often exhibit large fluctuation in residual-limb shape, necessitating careful observation and anthropometric measurement for prosthetists to ensure socket fit. Anthropometric measurement may become more important as an outcome measure indicating success in rehabilitation. This study investigated the accuracy and reliability of seven prosthetic anthropometric measurement devices as used by a group of eight prosthetic-orthotic practitioners and a group of five prosthetic-orthotic students to measure six common anthropometric dimensions on three foam positive models of transtibial amputee residual limbs. Two of the models were identical, enabling assessment of individual repeatability. Some clinically significant errors were noted in the results; however, the general variability in measurements was not clinically significant. Students were slightly more consistent than practitioners; students were more consistent with linear measurements, while practitioners were more consistent with circumferential measures. The results further demonstrated that the VAPC measurement device used in the study was both inaccurate and unreliable.

Custom-molded foot-orthosis intervention and multisegment medial foot kinematics during walking

CONTEXT Foot-orthosis (FO) intervention to prevent and treat numerous lower extremity injuries is widely accepted clinically. However, the results of quantitative gait analyses have been equivocal. The foot models used, participants receiving intervention, and orthoses used might contribute to the variability. OBJECTIVE To investigate the effect of a custom-molded FO intervention on multisegment medial foot kinematics during walking in participants with low-mobile foot posture. DESIGN Crossover study. SETTING University biomechanics and ergonomics laboratory. PATIENTS OR OTHER PARTICIPANTS Sixteen participants with low-mobile foot posture (7 men, 9 women) were assigned randomly to 1 of 2 FO groups. INTERVENTION(S) After a 2-week period to break in the FOs, individuals participated in a gait analysis that consisted of 5 successful walking trials (1.3 to 1.4 m/s) during no-FO and FO conditions. MAIN OUTCOME MEASURE(S) Three-dimensional displacements during 4 subphases of stance (loading response, midstance, terminal stance, preswing) were computed for each multisegment foot model articulation. RESULTS Repeated-measures analyses of variance (ANOVAs) revealed that rearfoot complex dorsiflexion displacement during midstance was greater in the FO than the no-FO condition (F(1,14) = 5.24, P = .04, partial η(2) = 0.27). Terminal stance repeated-measures ANOVA results revealed insert-by-insert condition interactions for the first metatarsophalangeal joint complex (F(1,14) = 7.87, P = .01, partial η(2) = 0.36). However, additional follow-up analysis did not reveal differences between the no-FO and FO conditions for the balanced traditional orthosis (F(1,14) = 4.32, P = .08, partial η(2) = 0.38) or full-contact orthosis (F(1,14) = 4.10, P = .08, partial η(2) = 0.37). CONCLUSIONS Greater rearfoot complex dorsiflexion during midstance associated with FO intervention may represent improved foot kinematics in people with low-mobile foot postures. Furthermore, FO intervention might partially correct dysfunctional kinematic patterns associated with low-mobile foot postures.

The Effects of Vibration on the Gait Pattern and Vibration Perception Threshold of Children With Idiopathic Toe Walking

The effectiveness of idiopathic toe walking treatments is not conclusive. The study investigated the use of vibration as a therapeutic/treatment method for children with idiopathic toe walking. Fifteen children with idiopathic toe walking and 15 typically developing children, aged 4 to 10 years, completed the study. The study included a barefoot gait examination and a vibration perception threshold test before and after standing on a whole body vibration machine for 60 seconds. Temporal-spatial parameters were recorded along with HR32, a calculation designed to distinguish on aspects of the toe-walking pattern. No significant gait pattern differences were found between children with idiopathic toe walking and typically developing children after one bout of vibration intervention. HR32 was found to be a means to identify the toe-walking pattern (P < .001). Hypersensitivity to vibration of children with idiopathic toe walking was not found in the current study (P = .921).

An Instrumented Wheel System for Measuring 3-D Pushrim Kinetics During Racing Wheelchair Propulsion

The purpose of this study was to design and validate an instrumented wheel system (IWS) that can measure 3-dimensional (3-D) pushrim forces during racing wheelchair propulsion. Linearity, precision, and percent error were determined for both static and dynamic conditions. For the static condition, the IWS demonstrated a high linearity (0.91 ≤ slope ≤ 1.41) with less than 2.72% error rate. Under dynamic loading, the IWS provided the well-matched measurement forces with the predicted values from the inverse dynamics method (0.96 ≤ slope ≤ 1.07) with less than 4.32% error rate. The results revealed that the IWS developed in the study can be used to measure 3-D pushrim reaction forces with acceptable accuracy. This was the first instrumented wheel device that can register 3-D pushrim forces during racing wheelchair propulsion. With the available kinetic information of the 3-D pushrim forces, the upper extremity joint reaction forces could be determined.

Variability among Practitioners in Dynamic Observational Alignment of a Transfemoral Prosthesis

The most innovative technology in prosthetic components cannot fulfill its objective without a proper patient interface and a proper alignment. Alignment, the orientation of components with respect to one another, is an area of prosthetic practice in which quantification and repeatability are not clearly defined. Researchers have sought an optimum standard and methods for automated alignment. Before automation is sought, variability among practitioners using current methods should be assessed. This investigation determined the outcomes of the alignment of five different prosthetic practitioners given the same subject and components using kinematic and kinetic gait analysis. Differences in static alignment were quantified through instrumented gait analysis; however, these differences were relatively small. Similar small differences were noted in gait velocity and ground reaction force, and bilateral joint angles during walking were very similar. This consistency among practitioners with varying levels of experience suggests that automated alignment is probably feasible but may not be necessary.