Since the introduction of prosthetic feet with integrated hydraulic ankle units, many patients have tried and adopted these feet as part of their ongoing prosthetic care. The first foot to introduce this concept, the Echelon foot, is a dynamic carbon-fiber foot comprising independent toe and heel springs with a double cylinder hydraulic self-aligning ankle. Since then, design variations have been added with additional features such as an integrated shock module and the Avalon foot, a lighter version with a single dual-function hydraulic cylinder and a three-quarter-length keel specifically designed for lower-activity K2 patients.1 Many benefits have been recorded over and above the ability of the foot to adjust to uneven slopes and surfaces. The work by Kristal2 shows that the foot can help to descend and ascend stairs, and Johnson et al.3 demonstrated that the foot reduces the likelihood of falling by aiding toe clearance in swing phase. This new category of prosthetic components also provides biomechanical advantages such as smoother center of pressure transition and reduced interface pressures.4,5 Perhaps most importantly from a user's perspective, Sedki and Moore6 reported that patients using this foot have reported high satisfaction levels in many facets of prosthetic use. One of the possible reasons felt for this increase in reported levels of satisfaction was that using a prosthetic foot with the hydraulic ankle unit offers a more symmetrical gait pattern than prosthetic feet does that work along more conventional means.
Marinakis7 stated in 2004 that “For most rehabilitation professionals, the achievement of symmetry of the lower limbs during walking has been an unquestioned goal of gait reeducation.”
This goal has been evaluated many times, with many aspects of gait and symmetry of persons with amputation having been investigated. Some of these aspects include the effect that asymmetry has on the center of pressure, studied by Schmid et al.8; ground reaction forces, studied by Engsberg et al.9; Marinakis'7 work on the effect that differing prosthetic foot choice has on symmetrical function in the more proximal joints; and the research done by Kaufman et al.10 on the improvements arising from the more symmetrical gait offered by microprocessor knee units. The debilitating effects of this asymmetry on persons with amputation have also been explored. It has been documented by Ephraim et al.11 that this asymmetry leads to an increased burden on the remaining limb and is linked to degenerative change and musculoskeletal problems. Lloyd et al.12 looked at muscular asymmetry in persons with amputation and the increased risk of osteoarthritis. The elevated risk of osteoporosis in later life was explored by Kulkarni et al.13
The purpose of this study is to assess in our patient group the stance phase14 of the gait cycle (the time the foot spends in contact with the ground), specifically the asymmetry between the time the prosthetic foot and the time the nonamputated foot spend in contact and then the difference made with the inclusion of a foot containing a hydraulic ankle unit.
Looking at the Smith and Martin15 study, where it is recorded that the average amputee takes 5586 steps a day, a reduction in asymmetry may translate into a significant decrease in the amount of extra time spent on the nonamputated side over the amputated side over the course a year. Interestingly, it has already been shown that the actual amount of steps taken does not differ when using a prosthetic foot with a hydraulic ankle unit.16
Twenty-two patients were chosen to be fitted with either the Echelon foot or the Avalon foot. The selection was not random because of ethical and financial issues based on prosthetic provision; instead, the decision to alter the patient's prescription was based on clinical need and taken by the multidisciplinary team working within the prosthetics department.
All the patients included in this case series supplied informed consent to be used, and as they were not randomly selected, the Luton and Dunstable Hospital National Health Service Foundation Trust Ethics Committee deemed this as a service evaluation and no ethical approval was required.
The main determinant as to whether the patient was supplied with an Echelon or an Avalon foot was his/her activity level. For this, the manufacturer's guidelines were used, these being based on the commonly used K-code activity levels,17,18 a scale ranging from 0 to 4, with levels based on an individual's potential functional ability. These were derived by the American Orthotic & Prosthetic Association from the grading scale used by the US national social insurance program Medicare,19 administered by the US Centers for Medicare & Medicaid Services, and are as follows:
- K0. Ability to transfer safely with or without assistance
- K1. Ability or potential to use a prosthesis for transfers or ambulation on level surfaces at fixed cadence
- K2. ability or potential for ambulation with the ability to traverse low-level environmental barriers such as curbs, stairs, or uneven surfaces
- K3. Community ambulator who has the ability to traverse most environmental barriers and may have vocational, therapeutic, or exercise activity that demands prosthetic utilization beyond simple locomotion and has ability or potential for ambulation with variable cadence
- K4. Ability or potential for prosthetic ambulation that exceeds basic ambulation skills, exhibiting high impact, stress, or energy levels typical of the prosthetic demands of the child, active adult, or athlete.
In addition, the multidisciplinary team discussed and prepared a list of prescription criteria for both feet. The criteria for the Echelon foot were as follows:
- K3 activity level
- Special Interest Group in Amputation Medicine (SIGAM) mobility grade of D/b (able to walk ≥50 m on level ground in good weather with one stick/crutch) or higher20
- Patient body weight under the manufacturers' recommended 125 kg weight limit21
- A need to manage slopes or uneven surfaces on a regular basis
- No other component changes required
- No current socket problems or fitting issues
The criteria for the Avalon foot were as follows:
- An activity level of K2 (as per manufacturers' guidelines)1
- A minimum mobility grade of SIGAM Da (able to walk ≥50 m on level ground in good weather with two sticks/crutches)20
- Individuals with unilateral amputation with musculoskeletal problems in the contralateral limb (e.g., arthrodesis)
- No current prosthetic socket fitting problems
- No other component changes required
- Body weight under 150 kg (as per manufacturers' guidelines)1
The patient demographic was 22 male patients and 2 female patients, comprising 16 individuals with unilateral transtibial amputation, 3 persons with bilateral transtibial amputations, and 5 individuals with unilateral transfemoral amputation. Twelve of the patients were assessed at activity level K2; all of these patients were using the same foot prescription, the Endolite Multiflex. The 10 patients with K3 activity level were using more varied foot prescriptions, with 3 on the Endolite Multiflex, 2 using the Endolite Javelin, 1 using an Endolite Dynamic response 2, 1 on an Ossur Re-flex VSP, 1 on a Seattle Lite-foot, 1 on the Endolite Elite Blade, and 1 using the Otto Bock 1D10.
To measure the differences in stance phase duration between the prosthetic and nonprosthetic foot, we used the Footwork Pro pressure plate from Mar Systems (see Figure 1). This is a 490 mm × 490 mm pressure plate, 4 mm deep, with 4096 individual capacitive sensors that take 200 readings a second across a pressure range of 1000 to 1200 kilopascals. The relatively small size of the plate and the fact that it runs off a laptop make it portable and practical for a functioning clinical setting and comparative technical assessment of the product has shown “high accuracy and precision.”22
The footwork pro software then collects pressure and timing data for each step. One limitation in the software is that it does not work out a hypothetical average but instead selects the mode, the actual physical step that the patient took that most closely matches the hypothetical average.
Before receiving the prosthetic foot with hydraulic ankle unit, each of the patients was asked, using their current prescription, to walk up and then back down a 6-m, flat section of the fitting room with the pressure plate located in the middle. The patient was asked to walk at self-selected “normal” walking pace and to attempt to ignore the plate. The patient was asked to do this for this for a total of 5 minutes to allow the pressure plate to capture a significant amount of “steps.” After these data were captured, the patient was then fitted with the Avalon/Echelon foot. When the patient was comfortable with the foot and the prosthetist was satisfied that the limb was aligned correctly with the unit set up to the optimum cylinder settings and safe for use, the patient would take the prosthesis away for a 4-week trial period. During this time, no other changes were made to the prescription. After this trial period, the patient would then return to the center and again walk for 5 minutes over the pressure plate, with the results recorded. It was requested that all the patients wear the same footwear as worn at the previous session to eliminate any potential discrepancy in results arising from sole units with a wider cross-sectional area.
The results were then to be analyzed statistically using a pair sampled t test. The author felt that this would be the most effective choice, as the paired t test calculates the difference within each before-and-after pair of measurements, determines the mean of these changes, and reports whether this mean of the differences is statistically significant.
Before their second appointment, 6 of the 22 patients were excluded from the results. One of the Echelon patients rejected the device and returned to his previous prescription (Endolite Elite Blade) before the 4-week trial period had ended; the patient preferred the increased “energy return” from this style of foot. Two patients returned wearing different footwear from the first visit, one Echelon (originally prescribed with Multiflex) and one Avalon patient. The stride length of three patients was not long enough for the plate to differentiate between the prosthetic foot and their contralateral foot; therefore, accurate data could not be captured (see Figure 2). These last three patients were all on the Avalon foot. Despite not being able to continue in the study, the patients with the short stride length and different footwear were all satisfied with the prescription change and continued using their new foot/feet.
Before the results could be analyzed, the data had to be cleared of any erroneous information. This involved checking through each of the steps recorded to eliminate any partial/incomplete steps where the user's foot was not fully contained within the border of the pressure plate (see Figure 3).
The amount of steps recorded per Echelon patient and the amount of completely clean hits are shown in Table 1. The average number of steps recorded by the 16 patients was 121.5 (range, 153–170) in the allotted time, with an average of 67.5 steps (range, 113–132), or 55.6% complete impressions. It was noted that the two patients with the lowest percentage of complete foot impressions were also the patients with the largest foot length (29 and 30 cm, respectively) and consequently the least likely to register a complete step.
The average number of steps recorded for the Avalon patients is shown in Table 2. This worked out at an average of 111.4 steps (range, 140–172), slightly less than the Echelon patients, which, given the difference in activity levels between the two categories of patient, is understandable. The average amount of eligible steps worked out at 69.4 (range, 109–123), or 62%.
When the results were amalgamated for the two groups, this worked out at an average total of 116.4 steps (range, 153–170) with an average of 68.4 eligible steps (range, 113–132), or 58.8%.
The results showed that the nonamputated foot stance phase of all the patients with unilateral amputations was longer than their prosthetic-side stance phase. All three of the bilateral patients also had a difference in time spent in stance phase between their left and right prosthetic feet. In the three cases, the patients expressed that dominant side (right in all three instances) was the side they spent longest on. Unsurprisingly, the largest difference measured was with the transfemoral patients, where the five largest differentials recorded belonged to these patients.
Looking at the results firstly by activity class, in six of eight of the K3/Echelon patients, there was a recorded decrease (improvement) in the difference between the left and right stance phase durations when the patients used the new foot (Table 3). One patient's stance phase difference remained the same and one patient showed a small increase in difference. There was no obvious correlation between the differing levels of amputation or if the patient was bilateral/unilateral.
With the K2 activity level/Avalon patients, the results were remarkably similar, with six patients showing a decrease (improvement) in the difference in stance phase durations when using the prosthetic foot with hydraulic ankle unit. As before, one patient had no alteration in difference and one had a slight increase. There was an increase in variance in these results, although the range was identical (Table 4).
Taking the results of the two groups together, there were many similarities. Both groups had the same breakdown in patients showing improvement, not showing improvement, or remaining static. Also, the average improvement for both groups came out the same at 21.3 milliseconds: For the Echelon group, it was P = 0.02, and in the Avalon group, P = 0.03, making both improvements statistically significant.
There are a number of limitations to this study. The sample group is relatively small and made up of a variety of activity levels and groups of persons with amputation. Also, although the K2 patients all started on the same foot, the K3 patients were all originally using differing prosthetic feet. However, the results across both activity levels indicate a positive improvement by reducing patients' gait asymmetry, which helps to explain the increased patient satisfaction levels recorded by patients using these components.
The purpose of this study was to assess in our patient group the asymmetry between the prosthetic foot and the nonamputated foot in the stance phase of the gait cycle and then the difference made with the inclusion of a foot containing a hydraulic ankle unit. This study is certainly not a definitive measure of the effect that prosthetic feet with hydraulic ankle units make to symmetry in gait but involves merely one aspect. Furthermore, more detailed work is indicated with more expansive gait analysis equipment to look further into the biomechanics. However, the results shown here do indicate that individuals with amputation using prosthetic feet with a hydraulic ankle unit do experience a more symmetrical stance phase with all the benefits that this entails, such as decreased load bearing through the sound side.
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