Effect of an Ankle Foot Orthosis on Roll-Over Shape in People with Hemiplegia

Introduction: Hemiplegic gait is characterized by slow, labored and uncoordinated limb movements [1]. Variability in diagnosis, area of lesion, and period of recovery contribute to differences in gait among people with hemiplegia. Residual muscle weakness, abnormal movement synergies, and spasticity result in altered gait patterns and contribute to poor balance, greater risk of falling and increased energy expenditure during walking [2]. People with hemiplegia have poor single-limb balance and difficulty controlling forward progression [3]. Gait is asymmetrical in timing and magnitude of motion and walking speed is significantly reduced. Gait abnormalities result not only from the inability to selectively control movement but also from the slow speed of movement.

Limited hip, knee and ankle motion resulting in a stiff-legged gait is frequently reported. There is often an equino varus deformity present that compromises heel strike during walking [4]. Perry [5] described normal function of the foot and ankle as the combination of three sequential rockers: the heel, ankle, and forefoot rockers. She suggested that during the stance phase of able-bodied subjects, progression over the supporting foot is assisted by the actions of these three rockers. It has been shown that the ankle-foot complex of able-bodied persons creates an effective roll-over shape during normal gait that is invariant to added weight to the torso, walking speed, and footwear [6,7,8]. Roll-over shape is defined as the geometry the anklefoot complex effectively conforms to between initial contact and opposite initial contact [7] and represents the integrated effect of the ankle-foot rockers described by Perry [5] that occur during the same period.

When pathologies such as hemiplegia are present, ankle-foot function is disrupted and an Ankle Foot Orthoses (AFO) may be worn in an attempt to restore function. AFOs have been reported to improve toe clearance during swing and ankle position at initial contact (IC) [4,9]. Improved ankle-foot kinematics may also lead to improved roll-over shape. This seems quite feasible as a goal since the non-disabled ankle-foot complex adapts to various conditions such as walking speed so as to maintain a consistent roll-over shape orientation. We believe that rollover shape may improve with an AFO by increasing center of pressure excursion. The purpose of this study was to investigate the effect of an AFO on roll-over shape in people with hemiplegia following stroke.

Methods: Kinematic and force data were recorded from 12 people with hemiplegia and 12 age-matched controls using an 8-camera real-time motion capture system (MAC, Santa Rosa, CA) in conjunction with 6 AMTI (Watertown, MA) force-plates embedded flush in the floor. A standard Helen Hayes marker set was used. Because the AFO obscured the landmarks required for identification of the physiologic ankle joint axis, markers were screwed into the mechanical ankle joint of the AFO. Participants, using the same shoes, walked at a normal selfselected speed with and without a custom, thermoplastic, articulated AFO with 90° plantarflexion stop and full length foot-plate. The Northwestern University Institutional Review Board approved this study and informed consent was obtained from each individual prior to their participation. Ankle-foot roll-over shapes were found by transforming the center of pressure (COP) of the ground reaction force into a shank-based coordinate system [7]. The Mann- Whitney Test was used to compare independent groups (hemiplegic versus control subjects) and Wilcoxon Signed Ranks Test was used to compare dependent groups (No AFO versus AFO).

Results: Hemiplegic subjects ranged in age from 43 to 58 years (mean age of 50.25±5.45 years) with a mean time since stroke of 8.33±4.72 years. Able-bodied subjects had a mean age of 57±8 years. For ‘normal' self-selected walking speed and regardless of AFO use, the control subjects walked significantly faster than the hemiplegic subjects (p=0.000) (Figure 1a). However, compared to the hemiplegic subjects ‘normal' speed, there was no significant difference in walking speed when control subjects walked at a ‘very slow' self-selected walking speed (p=0.908 without AFO and p=0.729 with AFO). At the ankle, the AFO (compared to No AFO) significantly changed the plantarflexion angle to neutral at initial contact (p=0.002) and significantly changed the angle at mid-swing from plantarflexion to slight dorsiflexion (p=0.006) (Figures 1b and 1c). Without an AFO, step length was shorter on the sound side compared to the involved side but this asymmetry decreased significantly when an AFO was worn (p=0.01) due to a significant improvement in sound limb step length (p=0.034 for the sound limb and p=0.136 for the involved limb) (Figure 2). The durations of gait cycle phases were unaltered by the AFO, but step width was significantly reduced (p=0.015) (see Figure 3).

Figure 1 - Mean (SD) walking speed (a), ankle angle at initial contact (b) and ankle angle at midswing (c) for the involved limb.

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Figure 2 - Mean (SD) sound side step length (a), involved side step length (b) and step length symmetry index (c) for the hemiplegic subjects (symmetry index for control subjects is 0). Symmetry index calculated as sound limb minus involved limb.

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Figure 3 - Mean (SD) gait cycle phase durations (a) and step width (b). Phase durations for control subjects are for "very slow" walking speed only.

Bilateral, mean roll-over shapes for all subjects and each condition are shown in Figure 4. For the involved limb, the AFO (compared to No AFO) significantly increased the roll-over shape arc radius and length (p=0.015 and p=0.003, respectively) and significantly altered the sagittal plane location of the first center of pressure (COP) point, moving it posterior to the ankle joint (p=0.002) (Figures 5a and 5b). Mean arc length was significantly less than control subjects when walking with an AFO (p=0.004) but the arc radius, though greater, was not significantly different (p=0.453). While the AFO condition resulted in the first COP point moving posterior to the ankle joint (p=0.002), it remained further anterior compared to the control subjects, although this was not significant (p=0.198) (Figure 5c).

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Figure 4 - Mean (SD) roll-over shapes.

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Figure 5 - Mean (SD) arc radius (a) and arc length (b) of the roll-over shape and (c) mean sagittal plane (A-P) location of the first COP point with respect to ankle joint for the involved limb.

Discussion: It appears that the AFO tested in this study improved roll-over shape, in particular arc radius and the sagittal plane location of the first COP point, but did not completely normalize it for the subjects tested, since arc length remained shorter than normal. When walking without an AFO (shoes only), initial contact by the hemiplegic subjects was with the entire foot at once, or fore-foot first and then the heel, rather than the ‘heel-toe gait' exhibited by able-bodied ambulators. In this situation, the arc length of the roll-over shape was shortened and the center of pressure moved back and forth beneath the foot during the first half of stance rather than progressing anteriorly in an uninterrupted manner. With the AFO there was still a perturbation in the center of pressure progression during mid-stance, although less pronounced, that implies that forward progression of the body over the foot continued to be disrupted despite significant increases in roll-over shape arc length and radius.

Walking is a process of getting from one point to another as safely and efficiently as possible and fast enough to function in society. Despite significant increases in roll-over shape arc length and radius with an AFO we did not see any changes in walking speed. So having a roll-over shape that was closer to normal did not impact the ability to get from one point to another any more quickly. An appropriate roll-over shape may improve stability: Wisse and van Frankenhuyzen [10] showed that a mechanical model with a zero radius rocker could not tolerate disturbances without falling down while mechanical models with rockers could tolerate increasingly larger disturbances as the radius of the rocker increased. If step width is considered to indicate stability, then the significant decrease in step width with an AFO might suggest improved stability.

Roll-over shape provides us with a method to quantify the ankle-foot rockers described by Perry [2] and explore the contribution of ankle-foot function to walking. However, at present there are some limitations to applying the roll-over shape analysis to data from hemiplegic subjects: Our ability to compute meaningful radii and arc lengths is compromised for flat shapes and concave-down shapes (as demonstrated by some hemiplegic subjects who were excluded from this study) because flat shapes have an infinite radius and our routine was intended for concave-up shapes as found in able-bodied subjects. As currently defined, roll-over shape describes function of the ankle-foot complex only while it is ‘rolling over' and not when it is being unloaded during the double support phase of terminal stance. Giuliani [1] suggested that the greatest loss of motor control of the hemiplegic limb occurs at phase transitions, such as during the transfer of weight from one limb to the other. For this reason, further investigation of COP during unloading of the affected limb may be warranted and additional investigation is needed to see if we can further improve roll-over shape arc length by altering AFO design.

References

[1] Giuliani, C. (1990) in G. Smidt (Ed.) Gait in Rehabilitation, Churchill Livingstone Inc: NY. p. 253- 66. [2] da Cunha Jr, I.T., et al. (2002) Arch Phys Med Rehabil, 83:1258-65. [3] Perry, J. (1969) Clin Orthop, 63:23. [4] Hesse, S., et al., (1999) Stroke, 30(9):1855-61. [5] Perry, J. (1992) Gait Analysis: Normal and Pathological Function. McGraw-Hill: NY. [6] Hansen, A. and D. Childress (2005) J Rehab Res Devel, 42(3)381-90. [7] Hansen, A.H., et al. (2004) Clin Biomech, 19(4):407-14. [8] Hansen, A.H. and D.S. Childress (2004) J Rehab Res Devel, 41(4):547-54. [9] Weiss, W., et al. (2002). Gait & Posture, 16(Suppl. 1): S2. [10] Wisse, M. and J. van Frankenhuyzen (2003) Proc AMAM Conf Adaptive Motion of Animals and Machines, Kyoto, Japan.

Acknowledgements:

This material is based upon work supported by the Office of Research and Development (Rehabilitation R&D Service), Department of Veterans Affairs.