Finding the Right Balance

Research & Presentations

Finding the Right Balance

Both prosthetic knee and ankle-foot technologies influence standing balance outcomes

By David Moser, PhD, BEng, BSc, and Mike McGrath, PhD

Background: Standing

The act of standing still and maintaining balance may seem quite simple. However, despite this apparent simplicity, it is quite literally a complex balancing act and a marvelous feat of biomechanical control. If you reach forward to pass an object or simply point to something, your body’s center of mass (COM) will move, and whether you realize it or not, you’ll make subtle motor adjustments that affect balance to allow for this change. Next time you are standing in a crowd, observe those around you and you will realize that most people don’t stand still for very long. You will notice that they are continuously moving, swaying slightly, and shifting their weight around.

For an individual with limb loss, this is much more challenging and often hazardous, as the body COM is decentralized due to the missing limb mass. Compromised sensory feedback and a loss of active motor control at the knee and ankle, both of which are used to recover from unbalanced situations, add to the difficulty and risks when standing, particularly on uneven surfaces.

A key problem that arises is a lack of adaptation within the prosthetic limb to adjust for variations in changing ground inclines, which affects limb loading. The consequence is often pain and discomfort at the socket interface, which can result in an unloading compensation strategy to alleviate the discomfort. A vicious circle is created because the

resulting uneven inter-limb loading then further compromises balance stability, safety, and clinical outcomes.

Why Alignment and Combined Component Selection Matter

When considering normal biological control of multiple joints during standing, it is interesting to observe that we rarely stand with the hip, knee, and ankle joint at the full extent of motion. When standing on level ground, only the knee is close to a fully extended position; both the hip and ankle joints

Figure 1

will be positioned well within the available range of motion. When static, the ground-reaction force is projected from the center of pressure (COP) and passes ahead of the ankle joint (creating an external flexion moment), while at the same time passing slightly ahead of the knee joint, and through or slightly behind the hip joint. Upright stability is created with very little muscular effort in the form of a closed isokinetic chain by the interplay of passive tension created within the limb. Therefore, when considering the combined effect of multiple

Illustration showing the effect changes to ankle joint AP alignment can have on limb stability. The red line represents the ground-reaction vector. Large changes to AP ankle position can cause excessive flexion and extension tendencies, which require physical compensation.

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Research & Presentations

prosthetic joints, the most basic considerations relate to joint position and the mechanism that creates resistance to movement. The position or alignment of a prosthetic joint is important since this will determine the combined external moments acting on the leg, and thus the forces at each of the joints involved in the control of stability when standing.

During our research and development of hydraulic ankles, we observed that this was an essential consideration in achieving a good outcome. More specifically, it became clear that there was a direct correlation between ankle alignment and compensatory balance control mechanisms that would occur when standing. Figure 1 illustrates these compensatory mechanisms. Figure 1(a) shows the tendency for unstable flexion, which results when the ankle position is set excessively posteriorly relative to the residual joints. This is caused by an abnormally large external flexion moment acting about the ankle. As the ankle tends to flex, the ground-reaction vector may fall behind the knee, causing an external flexion moment about the knee. With this alignment, patients typically report a feeling of instability and a sensation of falling forward. To remain upright, and to overcome this forwardflexion tendency, the user may add compensatory effort from the hip and knee extensors to sustain limb extension. Clearly, this is undesirable due to the instability and extra muscular effort required. In contrast, as shown in Figure 1(c), when the ankle is set too posteriorly, the external flexion moment about the ankle becomes much smaller. This, combined with an extension moment about the knee, causes a tendency for the limb to over-extend and the COP to move posteriorly, causing a feeling of falling backward. A corrective and undesirable anterior trunk lean compensates and helps move the COM forward to restore balance stability. A stable alignment position, as shown in Figure 1(b), is one that is comfortable, requires little

effort, and allows postural adjustments that mimic natural biological control. Hence, we refer to this as a biomimetic alignment.

The Effect of Prosthetic Foot Components

As stated earlier, aside from alignment, the design of the ankle-foot resistance mechanism also plays a key role in the control of standing stability. An ankle-foot design that is largely elastic in nature will have an unloaded equilibrium point and a natural tendency to release energy whenever the elastic elements of the foot are loaded. The system can be modeled as a pair for springs, as shown in Figure 2(a). When aligned on level ground, fewer problems are encountered since the internal joint moments created are minimal and are largely balanced. However, when the ground incline changes, the loading applied to the foot can become considerably biased toward hind- or forefoot (depending on incline) during static standing weight bearing. This, in turn, causes an unbalancing of the internal forces created at the ankle, which can lead to discomfort and instability since these will act through the socket interface.

In contrast, a foot design that includes a hydraulic actuator in series

Figure 2

Figure 3

Illustration of Elan MPF standing support mode. When standing situations are detected, hydraulic resistance is automatically increased to provide higher levels of limb support. A small amount of adjustment is preserved to facilitate natural postural movements.

with a spring, as shown in Figure 2(b), becomes viscoelastic and will actively dissipate force within the range of hydraulic movement. This has the effect of eliminating the internal joint moments that are created whenever loading is applied to the foot. Overall, the system has a tendency to self-align, that is, to rotate to a position where the joint becomes

The equivalent mechanical models of (a) a rigid, energy-storage-and-return ankle-foot, Esprit, and (b) a hydraulic ankle-foot device, Echelon.

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Research & Presentations

fully compliant with the ground interface, providing a high degree of stability. When standing situations are detected using microprocessor foot (MPF) control, it is possible to further enhance stability by applying a higher level of hydraulic resistance while maintaining an ability to adjust upright standing body posture. This enhanced standing function is illustrated in Figure 3 and acts as a brake on the ankle joint.

Methods

A recent study in our biomechanics lab sought to examine how various changes to prosthetic limb components influence standing performance on slopes. Five adult participants with transfemoral (TF) and four adults with transtibial (TT) limb loss took part in the study; all were K3 ambulators. A control group of five adult subjects with no lower-limb impairments also participated in the study.

Each participant stood, facing down a slope on an instrumented ramp, for a period of at least 15 seconds. At least three trials were recorded for each intervention. During this time, joint angles were measured using a 3-D motion capture system. The chosen gradient of the slope was 5 degrees. This complies with guidelines set out in the Americans With Disabilities Act of 1990 (ADA) 5 . Ground-reaction forces (GRF) and COP movement were recorded using Kistler force plates embedded within the ramp as indicators of inter-limb loading distribution and balance control.

All the TF participants were fitted with an Endolite Orion3 microprocessor knee (MPK). This MPK has an enhanced standing support function, which can be switched “on” or “off” by the programming clinician and was tested in both conditions. With each of the knee conditions the TF participants

Figure 4

Changes in standing posture of a transfemoral participant using Orion3 when standing with (a) an Esprit foot and (b) an Echelon hydraulic ankle-foot.

tested an Esprit and Echelon foot (Endolite). The TT participants tested Esprit, Echelon, and Elan feet (Endolite).

Results and Discussion

One of the chosen outcome measures of the study was to measure the groundreaction force under each limb while

Figure 5

Ground-Reaction Force

Distribution Between Limbs

60% 58% 56% 54% 52% 50% 48% 46% 44% 42% 40%

TransfemoralTranstibial

standing, and to calculate the distribution between the prosthetic and sound sides as a percentage of the total. The results showed that the control group presented with a slight degree of loading bias, with increased loading on their dominant limb (50.5 percent of total load ± 2.2 percent standard deviation). The prosthetic loading outcomes were considered with this finding in mind.

When testing the Esprit foot, the lack of compliance to the inclined ground surface generated compensations at the knee and hip with increased levels of joint flexion. This would tend to offset the knee joint center, resulting in an external flexion moment about the knee. The MPK knee stance control was unaffected despite these kinematic changes, and did not alter weight bearing support provided by the knee. Changes to the resulting standing posture are illustrated in Figure 4, which contrasts the effect of foot selection. Result (a) shows the Esprit and (b) shows the Echelon. Both TF and TT subjects presented similar compensations to standing posture.

Bar chart showing the mean degree of asymmetry between the prosthetic and sound limb ground-reaction forces for each of the three prosthetic conditions.

Prosthetic/ non-dominant side

Sound/ dominant side

Espirit, MPK standing support off (TF)Echelon, MPK standing support off (TF)Echelon, MPK standing support on (TF)Control mean + S.D.Espirit (TT)Echelon (TT)Elan, MPF standing support on (TT)

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Research & Presentations

Figure 6

Example, inter-limb ground-reaction force loading, TF (top row), TT (bottom row), for each of the tested prosthetic conditions. Sound limb loading shown blue and prosthetic limb loading shown red. Control data (bottom right) is included for comparison.

In terms of loading distribution between limbs, the Esprit condition proved to be the most asymmetrical for both TF and TT subjects, with the sound limb bearing 54 percent of the load, on average. This fell outside of the “dominant limb” range of variation. Echelon improved limb load distribution for both levels of limb loss, exhibiting loading levels comparable with able-bodied participants. The addition of MPK standing support for the TF subjects and MPF standing support for TT subjects produced results nearly indistinguishable from the able-bodied participants, with average intact limb loading of between 49.9 and 51.0 percent. Under these conditions, prosthetic side loading was occasionally greater than the sound side. A breakdown of the results is presented in Figure 5.

The inter-limb loading can be visualized using 3-D Pedotti diagrams, which show direction, magnitude, and temporal variation of the ground-reaction forces. Figure 6 illustrates some sample

results from the study. The top row shows TF subject results; the magnitude of the force under the sound limb (blue) is clearly greater than that under the prosthetic limb (red) for the Esprit condition. The spread of the vector lines at the ground represents the COP movement, which is visibly smaller for the Echelon condition with MPK standing

Figure 7

Percentage Change in COP

Movement from Control Mean

100 80 60 40 20 0 -20 -40 -60

Transfemoral

Less Stable

More Stable

Transtibial

support on, indicating improved balance. Similarly, for the TT subject shown, the Esprit condition displays a loading asymmetry with increased weight bearing on the sound limb. The Echelon conditions present more even load magnitude distributions, occasionally with more weight bearing evident on the prosthetic side in some cases. The addition of Elan MPF standing support mode reduces the spread of the vector lines at the ground, indicating less COP movements and more stability of posture.

The other outcome measure in this study related to balance ability was COP fluctuation. The COP movement gives a good indication of how well balanced a person is and provides some indication of whole body COM motion and control activity. For each prosthetic condition, this value was calculated relative to the baseline for the able-bodied control participants. The results are shown in Figure 7. For TF subjects, Esprit was once again the poorest performing condition, with an 80 percent increase in movement compared to able-bodied measurements. The prosthetic condition that was closest to the control data was the combination of a hydraulic ankle and MPK standing support. For the TT subjects, subjects using Esprit presented a mean increase in COP movement of 27 percent, compared to able-bodied participants. For the two hydraulic ankle conditions, however, this

Espirit, MPK standing support off (TF)Echelon, MPK standing support off (TF)Echelon, MPK standing support on (TF)

Espirit (TT)Echelon (TT)Elan, MPF standing support on (TT)

Mean percentage change in center-of-pressure movement, compared to the mean value for able-bodied control participants, for each of the three prosthetic conditions.

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Research & Presentations

measure was significantly reduced, indicating greater stability than the controls. The least COP fluctuation, indicating a high degree of stability, was achieved with Elan, the combination of a hydraulic ankle and MPF standing support.

Conclusions

It is well known that individuals with lower-limb loss are particularly susceptible to falls. This vulnerability and safety issue is partly attributable to the level of stability provided by the prosthesis, the activity being performed, and the ground surface. Changes to anklefoot component selection can have a significant effect on inter-limb loading and its variation 1,2 . It is therefore critical that multiple prosthetic joint types are considered in combination with each other since component features can act cumulatively to either hinder or assist standing balance stability. This is most evident with hydraulic ankles, such as Echelon, and MPF, such as Elan,

which have standing support features to improve standing balance and promote greater weight bearing on the prosthesis. Devices like these should be considered in view of reducing socket discomfort, the risk of falls, and severity of secondary complications, such as lower back pain and osteoarthritis, that are related to poor posture and long-term asymmetrical limb loading 3,4,6 .

David Moser, PhD, BEng, BSc, is head of research at Endolite, part of the Blatchford Group. Mike McGrath, PhD, is clinical research scientist at Endolite, part of the Blatchford Group.

References

1. De Asha AR, Munjal R, Kulkarni J, Buckley JG. Walking Speed Related Joint Kinetic Alterations in Transtibial Amputees: Impact of Hydraulic ‘Ankle’ Damping. J Neuroengineering Rehabil. 2013;10(1):1.

2. Moore R. Effect on Stance Phase Timing Asymmetry in Individuals

With Amputation Using Hydraulic Ankle Units. JPO J Prosthet Orthot. 2016;28(1):44–48.

3. Portnoy S, Kristal A, Gefen A, Siev- Ner I. 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. 2012;35(1):121–125.

4. Johnson L, De Asha AR, Munjal R, Kulkarni J, Buckley JG. Toe Clearance When Walking in People With Unilateral Transtibial Amputation: Effects of Passive Hydraulic Ankle. J Rehabil Res Dev. 2014;51(3):429.

5. ADA.gov homepage [Internet]. [cited 2017 Dec 8]. Available from: https:// www.ada.gov/

6. Gailey R. Review of Secondary Physical Conditions Associated With Lower-Limb Amputation and Long- Term Prosthesis Use. J Rehabil Res Dev. 2008;45(1):15.

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