By Jordana Bieze Foster
The mechanism by which ankle foot orthoses (AFOs) reduce drop foot in patients with Charcot-Marie-Tooth disease (CMT) involves proximal effects as well as the distal changes with which clinicians are more familiar, according to research from the UK.
In 14 adults with CMT, investigators from University College London found that AFO use was associated with significant decreases in hip flexion during swing phase, in addition to significant increases in ankle dorsiflexion, compared with wearing shoes alone. The findings were published in the October issue of Muscle & Nerve.
Previous studies have demonstrated that CMT patients compensate for dorsiflexor weakness by increasing hip flexion during swing phase to improve clearance, which can lead to hip flexor fatigue. By improving hip flexion, AFOs not only help normalize gait mechanics but also can reduce the risk of hip flexor fatigue, said Gita M. Ramdharry, MSc, PhD, clinical lecturer and specialist neuromuscular physiotherapist at the Centre for Neuromuscular Diseases, University College London Institute of Neurology, and senior lecturer at St George’s School of Rehabilitation Sciences, also in London.
“Foot clearance doesn’t just happen at the ankle. The knee and hip are also contributors. The AFOs increased dorsiflexion angle and reduced hip flexion angle though foot clearance was still achieved,” Ramdharry said. “The clinical implication is that proximal muscle fatigue could be attenuated, as the requirement for the compensatory hip flexion is reduced when the foot is supported in a more dorsiflexed position.”
The 14 CMT patients in the study, all of whom had significantly less ankle dorsiflexion than 12 normal controls, walked at a self-selected speed with shoes only and with three different prefabricated AFOs. The AFO styles tested included an ankle cuff with detachable elastic strap and plastic flange to tie into shoelaces; an ankle-high preformed foam brace covering the medial, posterior, and lateral aspects of the ankle and extending under the calcaneum; and a plastic polymer posterior leaf AFO that allowed the ankle to move into dorsiflexion during stance.
All three AFOs significantly decreased hip flexion and increased ankle dorsiflexion compared to the shoe-only condition. However, there was no significant correlation between the reduction in hip flexion and the increase in ankle dorsiflexion. The posterior leaf AFO appeared to reduce ankle power generation at preswing and increase time spent in double support compared to the preformed foam brace and the shoe-only condition; the authors suggest these findings may have been related to the fact that none of the participants had experience with the posterior leaf device.
The findings have significant clinical implications for the CMT population, said Joshua Burns, PhD, associate professor of physiotherapy at the University of Sydney in Australia.
“Foot and ankle muscle weakness is the single most debilitating problem in CMT,” Burns said. “3D gait analysis of different types of orthoses for people with CMT is very useful in order to understand the features responsible for reducing foot drop and alleviating abnormal proximal compensatory action. Larger and long-term studies are now needed to evaluate the effect of different AFOs and footwear types on comfort, pain, function, and deformity.”
The study findings may also have implications for other neuromuscular populations in whom AFOs can be used to treat drop foot, but key differences between conditions must be considered, Ramdharry said.
“There is little published work in people with neuromuscular diseases, yet they are a group that can respond well to a careful prescription and consideration of the biomechanical effects of braces,” she said. “This a group with a lower motor neuron condition, so the effects may differ from upper motor neuron conditions such as stroke, multiple sclerosis, or cerebral palsy. People with increased tone would not necessarily have this benefit as the lightweight devices [tested] would not be sufficient to control spasticity, if that is contributing to foot drop.”
Ramdharry GM, Day BL, Reilly MM, Marsden JF. Foot drop splints improve proximal as well as distal leg control during gait in Charcot-Marie Tooth disease. Muscle Nerve 2012;46(10):512-519.