Lateral wedge insoles for medial knee osteoarthritis: effects on lower limb frontal plane biomechanics.

Background: Lateral wedges reduce the peak knee adduction moment and are advocated for knee osteoarthritis. However some patients demonstrate adverse biomechanical effects with treatment. Clinical management is hampered by lack of knowledge about their mechanism of effect. We evaluated effects of lateral wedges on frontal plane biomechanics, in order to elucidate mechanisms of effect.

Methods: Seventy three participants with knee osteoarthritis underwent gait analysis with and without 5° lateral wedges. Frontal plane parameters at the foot, knee and hip were evaluated, including peak knee adduction moment, knee adduction angular impulse, center of pressure displacement, ground reaction force, and knee-ground reaction force lever arm.

Findings: Lateral wedges reduced peak knee adduction moment and knee adduction angular impulse (-5.8% and -6.3% respectively, both P<0.001). Although reductions in peak moment were correlated with more lateral center of pressure (r=0.25, P<0.05), less varus malalignment (r values 0.25-0.38, P<0.05), reduced knee-ground reaction force lever arm (r=0.69, P<0.01), less hip adduction (r=0.24, P<0.05) and a more vertical frontal plane ground reaction force vector (r=0.67, P<0.001), only reduction in knee-ground reaction force lever arm was significantly predictive in regression analyses (B=0.056, adjusted R(2)=0.461, P<0.001).

Interpretation: Lateral wedges significantly reduce peak knee adduction moment and knee adduction angular impulse. It seems a reduced knee-ground reaction force lever arm with lateral wedges is the central mechanism explaining their load-reducing effects. In order to understand why some patients do not respond to treatment, future evaluation of patient characteristics that mediate wedge effects on this lever arm is required.