Rocker Profile Soles Can Restore Movement in Constrained Ankle

BOSTON — Most orthoses are typically designed to control motion in the ankle, but the motion control could have unintended effects on walking and biomechanics, especially when paired with footwear. Christopher Hovorka, MS, CPO, LPO, FAAOP, co-director of the MSPO program in the Georgia Tech School of Applied Physiology, addressed this problem at the American Orthotic and Prosthetic Association National Assembly.

“We know that major joints of the ankle and foot complex can collectively serve as rockers that actually provide rolling capabilities when the foot is on the ground as the person walks,” Hovorka said. “The motion control that the ankle-foot complex provides as a rocker mechanism varies considerably if any of those joints are somehow impaired through bone deformity or muscle weakness. Footwear designs may also influence these rocker mechanisms.”

According to Hovorka, there is little rigorous research demonstrating that motion controlling interventions, such as footwear, actually provide its desired effect, and this lack of literature stems from the difficulty to constrain motion of the ankle-foot complex in subjects during walking experiments.

To address this need, Hovorka designed a special ankle-foot orthosis (AFO) that could constrain movement of the ankle-foot complex in able-bodied subjects.

“To examine foot and ankle movement behavior during walking in response to lower limb orthotic motion control, we designed an AFO that could constrain movement of the ankle-foot complex and then designed a footwear combination that would restore some of the lost movement,” Hovorka said. “We added a footwear system consisting of a compliant heel, as well as a rigid midfoot section to allow the transference of forward momentum during of early and midstance phase. The forefoot component of our footwear system design included a fulcrum, a compliant zone and a toe ramp to allow the person to exit stance phase transition into swing phase.”

To test the AFO-footwear combination, Hovorka fitted 13 able-bodied subjects with the device. First, each subject walked in a gait analysis lab at comfortable speed on an instrumented dual belt treadmill for 15 minutes while wearing the same type of shoes as a control condition. Six high speed infrared cameras collected and recorded motion of the subject’s lower limbs. Instrumented force plates under each treadmill collected forces and moments of each limb and wireless EMG was used to collect activity of 14 lower limb muscles.

After a seated rest period between each condition, subjects resumed walking at a comfortable speed for 15 minutes but this time they used an AFO-FC with the ankle joint held in slight dorsiflexion and with the joint constrained, plantarflexion and dorsiflexion stop, on the right leg. Subjects wore the same shoe used in the control condition on the left leg. Data was synchronized, processed, analyzed and the preliminary results were reported for 1 of the 13 subjects.

“We wanted to evaluate healthy persons as they walked with constraint of the ankle-foot complex while wearing the device to determine how effectively they achieved stance rollover of the lower limb,” Hovorka said. “Our first discovery was that we did not see much difference in the way the person walked during constraint compared to control.”

Based on observational data and analyzed data, the researchers also found that there was little difference in rollover shape between the left and right legs while the subject walked with the orthotic constraint on the right leg and in the absence of constraint.

“There was not much of a spatial difference in step length, and there was no statistically significant difference in rollover shape,” Hovorka said. “We have preliminary data to support that the rocker profile we designed restored rollover shape, and we have raw video corroborating that story. Certainly additional subject data analysis is needed to confirm these encouraging preliminary results.”

Based on the small sample size of data collected, Hovorka preliminarily concluded that this symmetry was achieved by the footwear combination portion of the AFO-FC, and when designed correctly, a footwear combination as a rocker profile may restore ankle and foot motion despite ankle constraint.

“We partly achieved that by providing a compliant heel to restore the heel rocker,” Hovorka said. “If you adjust the stiffness and the shape of the heel cushion, you can tune the heel rocker phase of gait.”

Being able to tune the heel of a device will enable orthotists to control how quickly a person enters the initial stance phase of gait during loading response. According to Hovorka, using a rigid heel will reduce the amount of time it takes a person to roll over the heel which may impose knee instability, but a soft, compliant heel is recommended for unstable patients.

“We also know that by adjusting the stiffness and the shape of a midfoot sole, you can tune the ankle rocker,” Hovorka said. He told O&P Business News  that increased compliance (ie, softer material that deforms under loading) and decreased radius of curvature (ie, smaller radius) in the toe region of a rocker profile are likely to increase the velocity of rollover during terminal stance phase. As such, walking with a rocker profile containing soft material in the toe region is likely to elicit faster velocity of roll over at the end of stance phase. The clinical relevance of this preliminary interpretation is that the rocker profile design containing a soft forefoot is likely be indicated for persons who possess greater muscular control of proximal joints in order to prevent stumbling or falling.

If the patient is weak, Hovorka suggested using a less rounded midfoot to increase the radius of curvature so the patient can slowly progress through the gait cycle and control forward progression of the lower limb. He clarified to O&P Business News that using a less rounded (ie, flatter, rather than acutely rounded) contour in the midfoot section of a rocker profile will likely slow forward progression of the lower limb during midstance. This type of rocker profile design may be indicated for those who exhibit difficulty in controlling forward progression of the lower limb during stance phase of gait due to reduced muscle strength. — by Megan Gilbride

For more information:
Hovorka C. Movement patterns during walking are influenced by design features of a rocker profile sole. Presented at: The American Orthotic & Prosthetic Association National Assembly; Sept. 6-9, 2012; Boston.

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