Pediatric amputees often begin ambulating in our clinic using an energy-storing type of prosthetic foot, often the Seattle Lightfoot® (Seattle Limb Systems, Poulsbo, WA) because of its common usage, relative cost, acceptance, and durability. With improving ambulation and participation in recreational activities, choices of multiaxial dynamic prosthetic feet are available. To allow more high-performance activities for athletically inclined patients, we have presented two such feet in our prosthetic clinic, the College Park TruStep® (College Park Industries, Fraser, MI) and Otto Bock Luxon Max® (Otto Bock Health Care, Minneapolis, MN). Both the College Park TruStep® and Otto Bock Luxon Max® are multiaxial feet, allowing inversion and eversion motions.
Patients approaching middle school age often ask for prosthetic feet that allow more dynamic participation in activities involving running and jumping and are provided multiaxial dynamic feet in an effort to increase performance. After being fit with a multiaxial dynamic prosthetic foot, patients have reported subjective improvements in functional activities and increased athletic performance. Can these improvements be measured objectively?
Previous studies to compare prosthetic feet have found varied results on several measures and tests of function. Several studies have not found statistically significant differences in gait parameters, such as velocity, cadence, and prosthetic side step length, of amputees while using different prosthetic feet. 1–7 However, amputees have been reported to select a faster walking speed using an energy-storing foot compared with a solid-ankle, cushion heel (SACH) foot. 8 Symmetry of uninvolved limb compared with prosthetic limb has also been reported to change with different prosthetic feet in measures of gait parameters. 9
Energy cost (oxygen consumption normalized by velocity) for walking and running has been found not to be significantly different for amputees using different prosthetic feet. 2,3,5–7,9 Other studies have found improved energy costs for energy-storing prosthetic feet compared with the SACH foot, generally at higher velocities, 1,10 and for traumatic amputees as compared with vascular amputees. 8
Studies using computerized gait analysis have reported ankle and foot motion differences between different prosthetic feet. Thomas et al. 5 reported the Genesis II foot (Mica Corporation, Kelso, WA) to have greater peak dorsiflexion in stance and power generation at push-off during walking than the Seattle foot. Ankle range, midstance duration, and dorsiflexion moment in walking were found to be greater for the Flex-Foot (Ossur, Aliso Viejo, CA) compared with the Seattle foot and SACH foot. 2 However, Colborne et al. 1 and Schneider et al. 4 did not find significant differences in ankle motion measures between the SACH foot and Seattle or Flex feet. Schneider et al. 4 did find significant moment and power differences in gait with the Flex foot compared with the SACH foot. A study of stair ambulation with amputees using five different prosthetic feet found no differences in stance and swing phase durations or stride characteristics while the amputees were ascending or descending stairs. 11
Many studies of prosthetic feet have used adult amputee subjects, although Colborne et al., 1 Schneider et al., 4 and Thomas et al. 5 specifically addressed prosthetic feet for children and adolescents. For children and adolescents, different designs of prosthetic feet have also resulted in mixed results in gait parameters, oxygen cost during walking, and ankle/ foot kinematics and kinetics.
It remains unclear whether different prosthetic feet allow children and adolescents to improve performance in functional activities. Thomas et al. 5 had adolescents perform five agility tests (vertical jump, long jump, sprint, shuttle run, and figure-eight run). No significant differences were found between the two prosthetic feet in measures of performance on these agility tests. Seven of ten subjects selected the Genesis foot over the Seattle foot at the end of the study.
In reviewing previous studies comparing prosthetic feet, it is not clear if the subjects were blinded to the type of feet they were wearing. Several measures to compare prosthetic feet can be influenced by effort or subtle differences attributable to knowledge of the prosthetic foot type. Foot selection or choice can also be influenced by knowledge of foot characteristics not related to performance.
Different designs of energy-storing or multiaxial dynamic prosthetic feet still have not been shown to allow child and adolescent amputees to perform better in functional activities. Can any changes in activities be objectively measured under conditions in which the subjects are not biased by knowledge of prosthetic foot characteristics? The purpose of this study was to determine if one of three common energy-storing or multiaxial dynamic prosthetic feet used by athletically inclined unilateral below-knee pediatric amputees allow significantly better performance, in objective measures of high-performance functional activities.
MATERIALS AND METHODS
Sixteen children and adolescents participated in the study. The local Institutional Review Board approved the study, and informed consent was obtained from all subjects before participation. All subjects were active with unilateral below-knee amputations (11 Syme, 5 transtibial) secondary to fibular hemimelia (10 subjects), trauma (3 subjects), amniotic band syndrome (2 subjects), or congenital tibial pseudoarthrosis (1 subject). All were physically active, athletically inclined, and had previously asked for a prosthetic foot that would allow higher performance activities. The mean age of the subjects was 11.8 years (range, 7.2–20 years), average height was 149 cm (range, 125–174 cm), mass was 44 kg (range, 24–67 kg). All subjects were prosthesis users for many years.
MATERIALS AND APPARATUS
Each subject tested all three prosthetic feet: one baseline energy-storing foot (Seattle Lightfoot®), and two multiaxial dynamic prosthetic feet (the College Park TruStep® and Otto Bock Luxon Max®). Three of the 16 subjects were fit with a College Park TruPer® foot, instead of a College Park TruStep® foot, secondary to foot size limitations. The two feet are collectively referred to as the College Park TruStep® foot throughout this study. The Seattle Lightfoot® consists of a Delrin keel (Seattle Limb Systems) that is designed to deflect with loading in stance and then return energy in terminal stance/push off. It is described as providing a combination of fatigue resistance, durability, spring-like resilience, and shock absorption. Because of its keel construction, it does not allow inversion or eversion of the prosthetic ankle. The College Park TruStep® foot employs bumpers and bushings that can be adjusted and a forefoot split longitudinally to allow dorsiflexion and plantarflexion, inversion and eversion, and transverse plane rotation movement. The deflection of the bumpers and carbon fiber footplate provides energy-returning properties. The split toe is designed to allow improved ground compliance. The Otto Bock Luxon Max® consists of two parallel plates with the bottom plate split transversely to allow independent prosthetic forefoot and hindfoot motion. Deflection of the plates allows dorsiflexion and plantarflexion and energy return. The split of the bottom plate allows inversion and eversion. It is described as providing greater resilience, reduced energy loss, and increased range of motion.
Each subject performed six tests: cutting drill, 37-meter (40-yard) sprint, vertical jump, long jump, oxygen cost on a treadmill, and walking speed three-dimensional computerized gait analysis. For the cutting drill, subjects were asked to run around a set of eight cones as fast as possible. The cones were spaced seven feet apart and staggered at a distance of four feet (Figure 1). An electronic timing device was started when the subjects tripped a beam at the start line and ended when they tripped a beam at the finish. Each day the subjects completed two warm-up tests and then three tests with 3 minutes rest between each test. The best of the three times was used for analysis. The cutting drill was designed to measure the function of a prosthetic foot stressed in inversion and eversion. If multiaxial prosthetic foot designs allow inversion and eversion, can subjects use this functionally?
The 37-meter sprint time was tested indoors in a carpeted skywalk with the length marked off. An electronic timing device was started when the subjects tripped a beam at the start line and ended when they tripped a beam at the finish. The test was completed twice, with the better time used in the analysis.
For the vertical jump, subjects were instructed to begin standing straight, take off from the floor with both feet leaving the ground at the same time, and jump as high as possible. The positions of three reflective markers placed on the pelvis (left and right anterior superior iliac spine and sacrum) were measured with a Vicon motion system (Oxford Metrics, Oxford, United Kingdom). The average vertical height of the markers at the peak of the jump minus the starting vertical height of the markers determined the vertical jump distance. The pelvis was used as the reference to measure height of the jump to reduce possible inconsistencies in upper body extension.
For the standing long jump, subjects were instructed to begin standing with their feet flat on the floor, take off from the floor with both feet leaving the ground at the same time, and jump as far as possible. The positions of two reflective markers placed on the foot (heel and toe) were measured with a Vicon motion system. The horizontal position of the markers on the foot closest to the starting position at the end of the jump minus the starting position of the foot markers determined the long jump distance. A start line was not used, but instead the position of the feet at the start was employed. After warm-ups, each vertical and long jump was completed three times, and the best performance was used in the analysis.
The oxygen cost at maximum tolerated pace was completed with subjects using a treadmill. Subjects were familiarized with the operation of the treadmill, including speed selection and safety features. Subjects selected a starting speed that was the maximum pace they could safely maintain for the duration of the test. Once comfortable with treadmill operation, subjects were fit with oxygen consumption equipment. Oxygen consumption was measured using a Sensor-Medics VMax29 (SensorMedics Corporation, Yorba Linda, CA) metabolic cart in breath-by-breath mode. Headgear was placed on the subjects to support the mouthpiece and sensor assembly. When the subjects were comfortable, the nose clip was placed to ensure breathing through the sensor only. The subjects sat for 5 minutes to establish baseline oxygen consumption; then they started walking or running at the maximum speed they could maintain on the treadmill. The subjects warmed up for 2 minutes. During this time, the subjects could adjust the treadmill speed to arrive at a self-selected speed that could be maintained for the duration of the test. Participants were then instructed to maintain the selected speed for 5 minutes. At the end of the treadmill period, subjects recovered for 5 minutes. The two dependent variables that were measured in this test were self-selected speed that was maintained for 5 minutes, and oxygen cost with units of milliliters oxygen per kg per meter (mL O2/kg/ meter). Oxygen cost is the oxygen consumption (mL O2/kg/minute) divided by speed (m/minute). This test was done only once per prosthetic foot.
Although walking is not a high-performance activity, gait analysis was performed because walking is such a common activity. Three-dimensional walking gait analysis was performed using a six- or eight-camera Vicon motion measurement system and two AMTI force plates (Advanced Mechanical Technology, Inc., Watertown, MA). Kinematic (joint motions) and kinetic data (joint moments and powers) were generated using the modeling and reduction techniques of Vicon Clinical Manager, using a set of 13 reflective markers placed on the lower limbs of patients in standard locations used by the modeling program. Subjects were asked to walk at a self-selected speed across the laboratory. Data were collected until there were three trials in which the subject had the prosthetic foot fully on one of the force plates and three with the uninvolved side striking fully on one of the force plates. The following dependent variables were averaged for the three trials for analysis: walking velocity, stride length, prosthetic side step length, cadence, maximum dorsiflexion in stance, maximum plantarflexion in stance, dorsiflexion/plantarflexion range, peak ankle power absorption, and peak ankle power generation.
The subjects completed a survey rating characteristics of each foot. At the end of the study, subjects reported the foot they liked best while still blinded to the identity of each prosthetic foot. The subjects were then fitted (by a certified prosthetist) with the foot they had selected. During final fitting, the identities of the feet were revealed to the subjects.
Testing of the three different prosthetic feet was completed within a 1-week period. On the first day, the subjects were fitted with one of the three prosthetic feet by a certified prosthetist. The second day subjects performed the six tests and were not told any results of testing. At the end of testing, subjects filled out the questionnaire rating that foot. When testing for the first foot was completed, the second foot was fit. Testing on the second foot took place on the third day, again without knowledge of the results or foot identify. When testing for the second foot was completed, the third foot was fit. Testing on the third foot took place on the fourth day. At the end of all testing and again without knowledge of the results or foot identity, subjects were asked their prosthetic foot preferences. At the end of the last day, a certified prosthetist fit the preferred foot on the prosthetic leg for each subject.
The order of testing of prosthetic feet was done so that every possible order was completed twice by 12 subjects. For the remaining four subjects, order of testing was random. At the end of the study, the Seattle foot was tested five times on the first test day, five times on the second, and six times on the third. The College Park foot was tested six times on the first test day, five times on the second, and five times on the third. The Otto Bock foot was tested five times on the first test day, six times on the second, and five times on the third.
A one-factor ANOVA was used to assess the results of this study. The main factor, prosthetic foot type, had three levels consisting of the three different prosthetic feet tested in this study. A follow-up analysis used a two-factor ANOVA to analyze the results. The first factor remained foot type with three levels (the three different types of prosthetic feet), and the second factor was subject age with two levels (older than 10 years and younger than 10 years).
No significant differences were found comparing the three prosthetic feet for cutting drill time, sprint time, vertical jump height, long jump distance, self-selected treadmill velocity, or oxygen cost during treadmill testing (Table 1). During the treadmill test, 11 of 16 subjects were not able to maintain a jogging pace because of stability concerns, inexperience, or safety issues. The average speed for all tests was about 91 m/minute, which would be considered a fast walk. Although five subjects did maintain a jogging rate throughout the treadmill tests, the oxygen cost during treadmill testing should be interpreted as energy usage during the maximum tolerated pace.
Three-dimensional gait analysis showed no significant differences among the three feet for velocity, cadence, stride length, or prosthetic side step length. The College Park TruStep® had significantly greater maximum plantarflexion in stance, greater maximum dorsiflexion in stance, greater plantarflexion/dorsiflexion range, greater power absorption, and greater peak power generation in late stance (Table 2). There was no difference in any of these measures between the Seattle Lightfoot® and Otto Bock Luxon Max® feet.
Subjective ratings of the prosthetic feet on the average of seven questions were not significantly different. At the end of the study, six subjects preferred the College Park TruStep®, five preferred the Seattle Lightfoot®, and five preferred the Otto Bock Luxon Max®. Fifteen of the 16 subjects were currently using one of the three prosthetic feet tested in this study (three Seattle; five College Park, seven Otto Bock; one Flex-Foot). Ten of the 16 subjects selected a different prosthetic foot than the one used before the start of the study. Once the identities of the feet were revealed to the subjects, two changed their selection, one because it was the same foot they had used before the study and the other for unknown reasons. No relationships were found in prosthetic foot preference and subjects’ characteristics, such as age, weight, height, or prosthetic foot worn before study (Table 3).
An analysis was conducted to determine if any differences existed between testing days. Vertical jump was found to be different among first, second, and third testing days. Subjects jumped significantly higher on the first test day (mean 23.5 cm, p = 0.04) than on the second and third test days (mean 22.2 and 21.8 cm, respectively). None of the other dependent variables was significantly different among days. Eight subjects selected the foot tested on the first day; six selected the second day’s foot, and two the third day’s foot.
Seven subjects were older than 10 years and nine younger than 10 years. Analysis of the dependent variables did not reveal any additional differences in performance between prosthetic feet when age is considered. The older group had significantly faster cutting drill times, faster sprint times, higher vertical jumps, longer standing long jumps, and faster self-selected treadmill speeds (p < 0.01). However, there were still no significant differences among the three feet in either age group. There was no significant difference in oxygen cost between the age groups (p = 0.26). Older and younger subjects had the same self-selected velocity during gait analysis (p = 0.33), but older subjects had significantly greater stride and step length with a significantly lower cadence (p < 0.05). Prosthetic foot plantarflexion/dorsiflexion motions and power characteristics did not vary by age group (p > 0.20). The same relative differences in gait remained between prosthetic feet, regardless of the age. The group of subjects might have also been divided by weight, instead of age. Generally, an age or weight breakdown resulted in the same grouping of subjects and would not have altered the results.
The purpose of this study was to determine if child and adolescent unilateral below-knee amputees performed better on objective measures of high-performance functional activities using different prosthetic feet: one energy-storing foot represented by the Seattle Lightfoot®, and two multiaxial dynamic prosthetic feet represented by the College Park TruStep® and Otto Bock Luxon Max®. There was no significant difference in performance comparing the three prosthetic feet during a cutting drill; sprint; vertical jump; standing long jump; self-selected speed on a treadmill; oxygen cost during the treadmill activity; or self-selected speed, cadence, stride length, and prosthetic side step length during walking analysis. When subjects used the College Park TruStep®, they had significantly increased ankle and foot motions and ankle power generation during gait analysis. There was no difference between the Seattle Lightfoot® and Otto Bock Luxon Max® on any measures. The three prosthetic feet were selected equally as the preferred foot at the end of the study.
Other studies found similar results of no difference in self-selected velocity, comparing prosthetic feet in children and adolescents. 1,4,5 Thomas et al. 5 also found a similar lack of difference in energy cost between prosthetic feet. They also reported differences in walking kinematic and kinetic variables between a multiaxial prosthetic foot (Genesis II) and the Seattle foot similar to results found for the College Park TruStep® tested in the current study. Thomas et al. 5 found a similar lack of difference in agility test performance between prosthetic feet.
This study was limited in the adaptation period the subjects were given to become accustomed to each prosthetic foot before testing. However, even with an extended acclimation period of 1 month, Thomas et al. 5 found results similar to those of our study. With a 2-month acclimation period for a new prosthetic foot, Schneider et al. 4 found significant differences only in kinetic measures of symmetry. Although the subjects in our study were given 1 day to acclimatize, nearly all subjects expressed confident clear selections of prosthetic foot preferences. The tests to assess objective functional performance were carefully selected and expected to elicit differences among prosthetic feet. Conceivably, additional tests could be performed, such as stair climbing, but we were interested in those activities encountered in athletics. The subjects were self-screened and were active children and adolescents who wanted more high-performance athletic function. Still, most of the subjects could not maintain a jogging pace for more than 5 minutes on a treadmill, despite orientation. The challenge to balance on a treadmill in these subjects with a prosthesis might have consumed their energy and attention and was less a test of their pace potential. Still, at the pace selected, a prosthetic foot that might allow improved ground compliance should have resulted in improved energy cost.
Some subjects did not initially perform the jumps by taking off the ground with both feet simultaneously and did not plant and cut around the cones in the cutting drill and instead ran a curved path around them. If amputees do not perform tasks emphasizing the prosthetic side, it is difficult for the foot design to have an impact. It is also recognized that objective performance during functional tasks is not the only factor in prosthetic foot prescription. Other factors considered in foot selection include maintenance records, cost of the feet, and familiarity of the different prosthetic feet by the family and patient, orthopedist, physical therapist, and prosthetist.
The three prosthetic feet (Seattle Lightfoot®, College Park TruStep®, and Otto Bock Luxon Max®) tested by children and adolescents in this study were comparable according to objective assessment during high-performance functional activities. The College Park foot showed significant motion differences in walking. The subject preference for prosthetic feet was evenly distributed and occurred without subjects knowing which foot they were wearing. Perhaps amputees should be given an opportunity to try different prosthetic feet. Subjective preference may be the determining factor in pediatric foot selection.
The authors thank Seattle Systems, College Park Industries, and Otto Bock for contributing the feet tested during the study and for donating the feet selected by the subjects.
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