The effect of pronation on insoles used for the treatment of medial knee osteoarthritis
Beardsley, Michael Steven
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Lateral wedged orthotics, a non-surgical treatment for medial knee osteoarthritis, reportedly shift the center of pressure of the ground reaction force laterally to reduce medial knee forces, as quantified by the knee adductor moment. However, lateral wedged orthotics' reduction of the knee adductor moment and pain is inconsistent. Since the ground reaction force is transferred through the subtalar joint to the knee, the forces acting at the knee can be modified by the subtalar joint. Subtalar laxity has been shown to alter kinematics of the lower body kinetic chain. To date, no previous study has stratified subjects by foot-type to investigate the influence of the subtalar laxity on the efficacy of lateral wedge orthotics. Therefore, the effect of subtalar laxity on kinematics and kinetics while accommodating a lateral wedged orthotic was studied. 24 healthy adults were stratified based on presence of subtalar laxity and issued both a lateral wedged (treatment) and a neutral wedged (control with equal average thickness) custom orthotic. Subjects walked overground at 100 steps/min while lower body kinematics and kinetics were measured using a motion capture system and two floor embedded force platforms. Key variables measured included 1 st and 2 nd peak knee adductor moments, knee adductor moment impulse, frontal plane ground reaction force magnitude and knee adduction moment arm, and rearfoot, knee and hip kinematics and kinetics. Although the knee adductor moments (1 st and 2 nd peak, and impulse) were not statistically different between groups nor between orthotics, there were significant differences in frontal plane kinematics between groups and between orthotics. At the knee, the subtalar lax group was more adducted than the stable group while wearing the control (p<0.05); the lax group's adduction was reduced with the treatment orthotic (p<0.05). At the hip, the subtalar stable group was abducted while the subtalar lax group was adducted with the control (p<0.05) and treatment orthotics (p<0.05). Adaptation of the lower-body kinetic chain to lateral wedged orthotics is affected by subtalar laxity. Subtalar laxity altered the frontal plane orientation of the lower body. These differing frontal plane alignments were magnified by the lateral wedged orthotic. The equivalent knee adductor moments across groups and conditions may be attributable to the target group recruited. While our hypothesis that subtalar laxity would evoke altered lower extremity kinematic profiles between the subtalar stable and lax groups was supported, none among the cohort of participants had clinical symptoms of knee OA. Perhaps inclusion of persons with medial knee OA may elicit the expected differences in knee adductor moments dependent on the presence of subtalar laxity. Therefore, further study is warranted to determine if the observed kinematic differences significantly alter knee adductor moments due to the greater knee adduction and knee adductor moments present in subjects with medial knee OA.