Analysis of ankle kinetics and energy consumption with an advanced microprocessor controlled ankle foot prosthesis.

Transcription

1 Analysis of ankle kinetics and energy consumption with an advanced microprocessor controlled ankle foot prosthesis. D.Moser, N.Stech, J.McCarthy, G.Harris, S.Zahedi, A.McDougall Summary This study reports on several experimental investigations carried out to analyse the impact of adaptive control on the kinetic behaviour of ankle foot prosthesis in conjunction with locomotion energetics and biomechanical performance. The healthy limb has an amazing capability to adapt to the changing requirements of walking such as walking on varied inclined ground and at different walking speeds. Recently advances in prosthetics have sought to design prosthetic feet that have some form of adaptive capability combined with a deeper understanding of the effect of viscoelastic (hydraulic damping + elastic) foot behaviour. It is already now established that this functional concept provides greater stability within the socket, improved proprioception and lower stump interface pressures (Portnoy 2011). Crucially it has also been shown that more balanced distribution of loading takes place between pathological and intact limb thus reducing the risk of further pathologies developing and reducing healthcare costs. It is clear from biomechanics research that the rolling collision between foot and ground during locomotion is of paramount importance as biomechanical inefficiencies created at this interface are magnified and propagated up through the limb which may require greater compensatory effort on the part of the amputee. It seems logical that the changing environment and subtle changes between steps introduce a performance variable which must be properly understood if technology is to advance further. For further biomechanical optimization it seems logical that the mating foot in this collision must adapt in some way to the changing requirements to provide greater biomechanical optimization. Methods An advanced microprocessor controlled ankle foot system called elan (see figure 1) has been developed that has the capability to automatically alter and adjust how the foot interacts with the ground by controlling the amount of energy that is stored and released elastically by the foot in the stance phase of the gait cycle. The novel paradigm explored, is that active manipulation of the rolling collision that takes place between foot and ground can be used to optimize biomechanical performance for walking on various terrains and activities of daily living (ADL). In order to alter the characteristics and exchanges of energy between the body and ground changes in viscoelastic properties can be used to create more or less elastic effects and biasing effects to either assist or brake the fore aft exchange of energy as the body mass transitions over the ankle. The assist effect increases the hydraulic damping resistance in the PF direction whilst simultaneously the DF damping resistance is reduced. The net effect is that the heel spring becomes more elastic and the damping resistance in the DF is reduced to enable the greatest possible release of energy and minimize any hindrance at the ankle for anterior progression. The aim is to introduce this assisting effect advantageously when walking at faster speeds and up inclines to make the walking task less demanding. The brake effect reduces the PF damping resistance and increased the DF damping

2 resistance, the net effect is that the heel spring becomes less elastic and by increasing the DF resistance, motion and energy transfer in the anterior direction is braked. The aim is to introduce this braking effect advantageously when walking down inclines to create greater stability and safety and to reduce the demand on the residual musculature to provide the same braking effect. The design aim is that although only one set of carbon fibre spring is used, by adapting the hydraulics and this energy transmission through the springs virtual spring ratings can be created, in other words the springy effect of the springs can be adjust producing the same response as if the springs themselves were replaced with different spring constants. These virtual spring ratings change during the swing phase of the gait cycle so that at the point of heel strike the complete visco elastic system is fully optimized. Figure 1, Elan, microprocessor ankle foot system The experimental emphasis of the studies reported here were aimed towards developing a deeper understanding of how changing ankle foot properties via assistive and braking effects automatically for different walking environment and gait speeds influences the kinetics at the ankle and efficiency of locomotion produced by amputees. Pylon load cell, GRF were used to quantify ankle kinetics, and on board joint angle sensing data was user evaluated damped motion characteristics. High speed digital photography was used to quantify changes in spring defections. For each walking scenario/task the ankle foot properties (damping settings) were altered in each case to determine how the ankle foot kinetics/kinematics would change. Tests were contacted at various damping bias settings (assist) and (brake) and neutral (balanced PF/DF resistances) A wireless data acquisition system was used to collect the data and Matlab was used for data post process and statistical analysis of the results. Results and discussion Independent Research carried out at the Gait Analysis Lab of the University of Surrey, explored the improvements in the efficiency of walking when prosthetic feet with the introduction of visco elastic ankle movement in the sagittal plane were compared to the conventional, purely elastic type (Khadra, 2008). By measuring the heart rates of both Transtibial and Transfemoral amputees in realistic walking scenarios (level ground, up/down ramp, stairs), researchers were able to quantify via THBI differences in physiological cost energy between different when using prosthetic feet

3 designs required in walking. Compared to conventional energy return prostheses, the findings obtained from level, uphill, downhill, upstairs and downstairs extended walking trials showed that the new hydraulically assistive foot offered the amputees the ability to walk with up to 8.5% less effort. The changes in energy management with changes is damping settings, are presented in figures 2,3 and 5. With the neutral PF DF damping bias the range of movement at the ankle up to the point of heel rise is approximately evenly distributed (e.g. the ankle spends nearly the same time plantarflexing as dorsiflexing). With the brake settings in effect the PF motion duration is much shorting demonstrating that the ankle reaches a full flat and stable condition much earlier in the gait cycle. This is because DF motion cannot be created about the ankle until an external DF moment is created. This finding verifies the functional requirements that when walking down an incline more rapid movement in the PF direction is desirable to create a more stable base of support. With the brake mode in effect the response of the foot becomes less elastic, in other words it returns less energy thereby promoting stability and safety during stance phase. Comparison of PF and DF damped movement ranges assistive bias neutral bias brake bias 0% 20% 40% 60% 80% 100% PF DF Figure 2. Comparison between PF/DF resistance bias settings, of the damped range of movement expressed as a percentage of full range available.

4 Comparison of damped motion vs spring deflection ranges assistive bias neutral bias brake bias 0% 20% 40% 60% 80% 100% Range damped Range Elastic Figure 3. Comparison between PF/DF resistance bias settings, of the comparative distribution between damped movement and spring deflection expressed as a percentage of total available visco elastic system movements With the assistive mode setting enabled the results showed that the ankle spend more time plantarflexing and that less of the total ankle motion is being transmitted through the hydraulic system. This indicates that the ankle response during the collision is more elastic compared to other settings. Figure 4 shows the high speed photography results indicating clear differences between both in the positioning of the ankle unit relative to the lower carrier and the amount of heel spring deflection that is created in each mode setting. Figure 4. Comparison of the high speed photography results taken at the point local minimum vertical GRF data in mid stance, differences shown in terms of ankle and heel spring deflection for neutral and assist modes

5 The fore aft ground reaction force data (figure 6) results show that with the assist setting enabled there is greater reactive aft force been generated at the ground interface indicating a greater propulsive effect in comparison to the other mode settings. Interestingly also the fore shear force is reduces indicating less breaking in the collision at the ground interface. With the brake mode enables the aft shear is shown to be the least with proportionally the greatest fore force bias indicating a substantially more braked collision with the ground and energy management. The vertical ground reaction force data (figure 7) with the assistive setting enables shows a greater 2 nd peak indicating greater vertical acceleration off the force plate during late stance, suggesting a greater propulsive effect has been created. Summary of Key Findings By adapting the hydraulics many different virtual spring ratings and biases can be created enhancing the biomechanical characteristics of the foot system. With the brake mode enabled (down ramp) the stability, efficiency and safety of locomotion is increased: o The foot reaches the flat foot condition much earlier in the GC o The response from the foot is less elastic, and thus less reactive. o Proportionally more braking shear at the ground interface With the assist mode enabled (fast walking and up ramp) the efficiency of the locomotion task is improved: o More energy is directed through the spring the response becomes more elastic o Greater aft shear indicates more propulsion in the direction of progression o Greater 2 nd peak on the vertical ground reaction force indicates more vertical acceleration of the body mass during push off.

6 Figure 5. Comparison of ankle motion data with different damping settings the magnitude of the peaks and timing of the peaks provide some indication of changes to the mechanical response, shorter peaks indicate that more of the total ankle movement is being directed through the heel springs. Larger peaks indicate porportionaly more relative motion is being transmitted throught the hydraulic system. In the legend lower settings equate to lower damping resistance settings. Figure 6. Fore aft ground reaction force data with the each of the 3 mode settings enabled, the greatest aft shear is shown with the assistive mode enabled, whilst proportionally a greater fore shear indicates greater braking shear with the brake mode enabled.

7 figure 7, Vertical ground reaction force date with each of the 3 mode settings enabled. The larger 2 nd peak with the assist mode active suggests greater vertical acceleration of the body mass during push off, enhancing the push off and step to step transition dynamics Conclusions The conclusion from this study supports the view that future ankle foot systems should ideally have the capability to adjust to better optimize the gait tasks being undertaken. The consequences of pathological kinetics at the ankle we believe contribute greatly to the energetics of locomotion and the degree to which amputees have to adapt and compensate their gait. In more demanding locomotion tasks the degree of compensation appears to be much greater, this is evident in the existing THBI data and future studies already underway. We conclude that the interaction of the foot with the environment and varied loading requirements experienced by prostheses for different walking scenarios and conditions requires a step wise change in prosthetic design philosophy to normalise locomotion energetic as single mode device behaviour adds greatly to the costs and demands of prosthetic use. From our results to date we have demonstrated that it is possible to actively adapt the rolling collision of the foot with the ground interface in a way that has a positive effect biomechanically for different walking tasks. Future studies are underway to provide more insight into the muscular control effects on the part of the amputee and more widely into integrated multi joint prosthetic control systems. References Portnoy S, et al. (2011) Outdoor dynamic subject specific evaluation of internal stresses in the residual limb: Hydraulic energy stored prosthetic foot compared to conventional energy stored prosthetic feet. Gait & Posture, Volume 35, Issue 1, Pages

8 Khadra, H. (2008). Pilot Study for the Evaluation of a Novel Prosthetic Foot Design with Viscoelastic Properties. MSc thesis, University of Surrey, Guildford, United Kingdom. Moser.D et.al. (2009) Biomechanical analysis of a novel automatically self aligning ankle foot prosthesis Orthopädie Technik Quarterly, English edition III/2009 Moser.D et.al (2008) US Patent