Effect of length on laterally‐wedged insoles in knee osteoarthritis

Introduction Knee osteoarthritis (OA) is a prevalent musculoskeletal condition (1). It imposes a significant economic burden and is associated with considerable pain, disability, and loss of quality of life. Although the causative mechanisms of knee OA are not entirely clear, increased joint loads during walking have been associated with the initiation and progression of the disease and its symptoms (2). The external knee adduction moment, which reflects the dynamic load borne on the medial tibiofemoral joint compartment, has become a biomechanical marker for the risk of progression for medial compartment OA (2). Accordingly, much research is focused on devising and evaluating interventions that may reduce the peak adduction moment with a view to alleviating symptoms and reducing the risk of disease progression in knee OA. Laterally-wedged insoles are an in-shoe orthotic recommended to manage medial knee OA (3). Lateral wedges are hypothesized to reduce the moment arm of the ground reaction force vector relative to the knee center during walking (4), leading to reduced knee pain. Biomechanical studies have demonstrated that lateral wedges can reduce the peak adduction moment in patients with OA and healthy subjects (4–7). However, this finding is not consistent across all studies, with some showing no effect of the wedge on the adduction moment (8,9). Although differences in study samples and measurement methodologies may account for the conflicting findings, it is possible that the varying results may be attributable to differences in the designs of the lateral wedges tested. The most apparent difference in wedge design that is evident across studies relates to the length of the lateral wedge. Most studies that have demonstrated a beneficial effect of the wedge on the adduction moment have tested a full-length insole that wedged the foot from the heel to the forefoot (4–7). In contrast, the studies that demonstrated no effect on knee loading tested insoles that wedged only the rearfoot (8,9). The only clinical trial to evaluate the efficacy of lateral wedges for knee OA used rearfoot wedges and found no significant effect on symptoms or disease progression over time (10,11). Excluding the degree of wedging (which can mediate the effect of a wedge on the adduction moment) (5,7), the design features of laterally-wedged insoles are not considered to influence their efficacy. Evidence suggests that wedging of the entire foot is necessary for a beneficial effect, yet no study has evaluated the influence of wedge length on the adduction moment. The aim of this laboratory study was to compare the effect of a full-length, laterally-wedged insole on the adduction moment with that of a rearfoot wedge in patients with medial knee OA.