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Effect of a Prosthetic Foot with a Hydraulic Ankle Unit on the Contralateral Foot Peak Plantar Pressures in Individuals with Unilateral Amputation

Moore, Raymond BSc, Pros/Orth

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Journal of Prosthetics and Orthotics: July 2018 - Volume 30 - Issue 3 - p 165-170
doi: 10.1097/JPO.0000000000000196
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The dangers to a diabetic foot are well documented, including the increased chance of ulceration,1–3 the threat of amputation,4–6 the cost of treating these conditions,7–9 and the increased healing time in this patient group.10–12 Many studies have documented the incidence rate of a second amputation occurring to the remaining sound limb of an individual with unilateral amputation. A study by Pandian et al.13 found that 55% of patients with diabetes would go on to have an amputation performed on their sound side within 2 to 3 years. The Amputee Coalition estimates this figure to be between 28% and 51%,14 although some of the studies this figure was based on are dated. Similarly, when Anderson studied individuals with dysvascular amputation, she documented that almost a third would go on to have an amputation to their contralateral limb within 2 years.15 Unsurprisingly, these studies found that the patient’s life expectancy was significantly shortened after the amputation to the second leg. Robbins et al.16 demonstrated that the 5-year mortality rates for persons with dysvascular amputation were higher than for patients with breast, colon, or prostate cancer. Another well-documented area is the role played by external forces acting upon the foot in gait,17,18 and within this, the role that elevated plantar pressures play in the onset of ulceration leading to eventual breakdown and amputation.17,19–22 It is therefore of great importance to patients and clinicians that these external forces be managed in order to help prevent further amputations with all their associated problems and costs. One of the reported key attributes found in previous studies involving prosthetic feet with hydraulic ankle units has been the beneficial effect their inclusion has had on gait asymmetry of patients with amputation and the reduced time that the patient spends in stance phase on the contralateral (remaining) foot.23 The aim of this study was to determine whether this reduced time also translated into a reduction in the stance phase peak pressures acting on the contralateral foot, perhaps producing a significant long-term benefit.

Since the introduction of prosthetic feet with integrated hydraulic ankle units, many patients have tried and adopted these components 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.24 Since then, many design variations have been created with additional features such as an integrated shock module,25 a device with integrated dorsiflexion assist,26 a lighter single dual function hydraulic cylinder version with three-quarter length keel specifically designed for lower-activity K2 patients,27 and even a microprocessor-controlled version.28 Many benefits have been recorded over and above the ability of the foot to adjust to uneven slopes and surfaces.29 The work by Kristal et al.30 has shown that the foot can help in descending and ascending stairs, and Johnson demonstrated a reduced likelihood of falling by the foot aiding toe clearance in swing phase.31 These novel prosthetic components also provided biomechanical advantages such as smoother center of pressure transition,32 reduced interface pressures,33 and an increased prosthetic stride length.34,35 A recent presentation at an Annual Meeting of the UK Member Society of the International Society for Prosthetics and Orthotics (ISPO UK) from the prosthetic team at the West of Scotland mobility and rehabilitation center has also shown that the inclusion of a foot with hydraulic ankle to a prosthesis increased the users’ maximum walking distance by over 50% as well as a 15% increase in socket comfort score.36,37 Importantly, from a users’ perspective, Sedki and Moore38 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 pattern39 when compared to those prosthetic feet that work along more conventional means.

Given the significance of the effect of elevated plantar pressures in the health of the remaining limb of the persons with amputation and the effect the Echelon foot has had on gait asymmetry, the purpose of this case series is to assess whether the inclusion of a prosthetic foot with a hydraulic ankle unit has any effect on the peak plantar pressures acting on the contralateral foot.


Fourteen patients were chosen to be fitted with the Echelon foot. The multidisciplinary team settled on the Echelon foot from Chas. A. Blatchford. Of the other options, the team felt that the similarly priced Kinterra did not remain in as dorsiflexed a position through swing and consequently would not match the Echelon’s better swing phase ground clearance characteristics. The patient 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.

The first determinant as to whether the patient was suitable candidate for an Echelon foot was his/her activity level. For this, the manufacturer’s guidelines were used, these being based on the commonly used K-code activity levels40,41 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 Medicare42 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 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 further list of prescription criteria for the trial which were as follows:

  • An activity level of K3 (as per the manufacturer’s guidelines).
  • SIGAM (special interest group in amputation medicine) mobility grade of D/b (able to walk 50 m or more on level ground in good weather with one stick/crutch or higher43).
  • Unilateral amputation.
  • No current prosthetic socket fitting problems.
  • No other component changes required.
  • Patient body weight under the manufacturer’s recommended 125 kg weight limit.44

The patient demographic was 13 male patients and 1 female patient, comprising 9 individuals with unilateral transtibial amputation and 5 individuals with unilateral transfemoral amputation. To measure the peak pressures, 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 10 to 1200 kPa. The relatively small size of the plate and fact it runs off a lap top make it portable and practical for a functioning clinical setting and comparative technical assessment of the product has shown “high accuracy and precision.”45

Figure 1
Figure 1:
Amcube pressure plate.

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 which most closely matches the hypothetical average.

Before receiving the prosthetic foot with hydraulic ankle unit, each of the patients was asked, using his or her 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 his or her self-selected “normal” walking pace and to attempt to ignore the plate. The patient was asked to do this for a total of 5 minutes to allow the pressure plate to capture a significant number of “steps.” After these data were captured, the patient was then fitted with the 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. Apart from the foot and consequent alteration in prosthetic pylon height, no other changes were made to the prescription. After this period, the patient would then return to the center and again walk for 5 minutes over the pressure plate, with the second set of 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 paired sampled t-test; it was felt by the author 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 his second appointment, one of the patients rejected the device and returned to his previous prescription; the patient preferred the increased “energy return” from his previous blade-style foot. The other 13 patients all preferred their new foot and remained in the trial. Before the results being 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 2).

Figure 2
Figure 2:
Example of incomplete pressure reading.

The amount of steps recorded per patient and the amount of completely clean hits are shown in Table 1. The average number of steps recorded by the 13 patients was 143 (range, 164–113) in the allotted time, with an average of 80 “complete” steps (range, 115–38), or 59% (Table 1). It was noted that the 2 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.

Table 1
Table 1:
Total steps recorded vs eligable steps recorded

All the patients assessed showed a reduction in contralateral foot peak plantar pressures when walking with hydraulic ankle unit included in their prosthesis (Table 2). This averaged out at 48 kPa with a range of 1 to 165 kPa and was found to be statistically significant with a P value of 0.002 when analyzed using the paired sampled t-test.

Table 2
Table 2:
Peak plantar pressures by patient


Notably, all 13 patients showed a decrease in peak plantar pressures when using the prosthetic foot with hydraulic ankle unit. When assessed by amputation level, the average reduction found at transtibial level was 26 kPa and at transfemoral level was 84 kPa. Notably, the two largest single reductions in pressure were both found to be transfemoral patients at 165 kPa and 129 kPa, respectively. These two sizeable decreases may partially be explained by the fact that both these patients changed from a prosthetic foot that had a three-fourth length keel to a one with a full-length keel; however, if this solely was the case, then we would expect to see the reduction in pressure take place on the patients’ contralateral heel in early stance. This was not the case as when the individual pressure readings were examined; the maximum peak pressure areas were both located at the forefoot under the metatarsal heads, indicating that the patients were generating these excessive pressures late in stance phase due to vaulting. As great care was taken to make sure no alteration was made to the overall height of the prosthesis when changing the foot, it can be expected that the reduction was as a result of the prosthetic foot adopting a dorsiflexed position in swing phase, helping the patients to clear the ground more easily and not requiring them to compensate with the contralateral limb.

One of the questions arising from these findings is why these patients’ contralateral foot showed a significant reduction in plantar pressures. When viewed in conjunction with previous studies, which proved an increase in gait symmetry for patients using prostheses with hydraulic ankle units,23,46 it does appear that having a prosthetic ankle unit at the same height as the patient’s own allows the patient’s normal foot to act in a more natural manner. Another question which currently can only be guessed at is if this decrease in peak plantar pressures and increase in gait symmetry provides further benefits such as reduced risk of osteoarthritis or other degenerative joint conditions shown to be more prevalent in persons with amputation.47,48

There are a number of limitations to this study. The sample group was small, comprising only 13 patients, and the amputation level was not uniform, because the group included individuals with both transtibial and transfemoral amputations. With time, hopefully larger patient groups in both levels can be assessed independently to give greater insight into amputation level-specific outcomes. Second, the pressure measurement was taken externally through the shoe and not with an in-shoe system; this did not take into account the effect of shear forces acting on the contralateral foot. Again, hopefully further work using different measurement apparatuses can be used to explore the effects of the components when assessed with these.


As prosthetists, particularly in academic systems where students are dual-trained in orthotics, the importance of maintaining the remaining contralateral limb is already viewed as an issue of great priority. Previously, this maintenance has revolved around applications specifically to this limb with orthotic devices such as total contact or pressure-relieving insoles, custom footwear, or adaptations to the patient’s existing footwear. Now it has been shown that the components of the prostheses on the amputated side, specifically with the inclusion of a prosthetic foot with hydraulic ankle, can directly play a statistically significant part in the health and longevity of the remaining limb by reducing the peak plantar pressures acting upon it.


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prosthetics; hydraulic ankles; plantar pressure; prosthetic feet

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