Article

Solid ankle-foot orthoses (AFOs) are designed to immobilise the ankle but
numerous studies have measured a considerable ankle range of motion (ROM) in AFO
users. Measurement of ankle kinematics may be affected by soft-tissue artefact
(STA) of the knee marker, deformation of the AFO or tibial movement within the
AFO. A new model based on the Conventional Gait Model (CGM) was developed to
calculate these effects. Although movement of the AFO within the shoe should not
affect the measured ankle joint angle the model also allows an estimation of this
movement. Seven children (13 limbs) with spastic diplegic cerebral palsy were
assessed to present the benefits of the new model compared to the CGM. STA of the
knee marker was estimated to result in a 1.5 degrees overestimation of total
ankle ROM (from 8.2 degrees to 9.7 degrees ). STA error was strongly related to
angle of knee flexion (r=0.82) with an average maximum error of 3.8 degrees . AFO
deformation contributed approximately two thirds of the ankle ROM (6.0+/-4.3
degrees ) with the remaining third from tibial movement relative to the AFO
(2.8+/-0.9 degrees ). Movement of the AFO within the footwear was very small
(1.8+/-0.8 degrees ). A strong positive relationship (r=0.9) was found between
body mass (kg) and AFO deformation which was statistically significant (p<0.001).
This is the first model to attempt to quantify different contributions to ankle
dorsiflexion measured during gait analysis of people wearing AFOs.
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