Osteoarthritis (OA) of the knee is defined as an inflammatory process leading to loss of protective cartilage which results in pain, disability and loss of function. Around 25% of the population between 55 and 84 years is affected by OA of the knee. It can be graded using various classification, one of the most used being the Kellgren-Lawrence-Score (KL) ranging from 1 (early) to 4 (end stage). The treatment of choice for end-stage three compartmental knee OA (KL 4) is a total knee arthroplasty (TKA), whereby the damaged/degenerated parts of the knee are removed and replaced by a prosthesis. However, in patients with early to mid-stage OA (KL 1-3), which are usually younger patients, a conservative treatment is indicated. The goal of this conservative treatment is to relieve symptoms and at best slow progression of the diseases.

In medial knee OA (when mainly the medial compartment of the knee is affected by OA), one approach is to reduce the medial compartment contact force (MCF) by shifting load from the medial (disease affected) to lateral (disease free) compartment. Thereby lateral wedge insoles have been promoted as cost effective and simple solution. The biomechanical idea of these insoles is a shift in load distribution through a change in lower limb alignment. The lateral wedge of the insoles leads to a change in foot alignment (varus hindfoot) and consequently to a change in knee alignment (valgus knee = "knock-knees") and load distribution. Biomechanical studies confirmed a reduction in external knee adduction moment (EKAM) and knee adduction angular impulse (KAAI), which are surrogate measures to estimate knee joint load distribution (e.g. MCF). However, clinical trials report conflicting results, with some reporting no benefit while others reported a small decrease in knee pain in at least some patients.

In the light of this, the connection between knee OA and foot deformities has been reassessed. One study found patients with medial knee OA to have a more pronated foot type (i.e. tendency to plano-valgus deformity) compared to an age-matched control group without OA. Other investigations found more rigid foot types in patients with medial knee OA and a more internally rotated tibia during gait as well as an inversion of the forefoot. This seems important because the hindfoot axis has a significant influence on the mechanical axis of the lower limb and thereby on the load distribution in the knee. Half a degree valgisation of the hindfoot axis leads to one-degree varisation of the mechanical axis. Furthermore, a plano-valgus foot morphology was found to lead to internal rotation of the tibia which increases shear forces in the medial knee compartment.

In summary, patients with medial knee OA tend to have a plano-valgus foot deformity which is associated with a varus lower limb alignment and an internal rotation of the tibia. Both have been associated to an increased loading of the medial compartment. Consequently, some authors proposed the correction of plano-valgus deformity with customized foot deformity correcting insoles instead of lateral wedge insoles as conservative treatment option for early to mid-stage medial knee OA. Until today, only one study investigated the effect of foot deformity correcting insoles (e.g. insoles with customized medial arch support) as alternative to lateral wedge insoles and reported encouraging results. However, they used lateral wedge insoles with and without medial arch support and did not explicitly only correct the foot deformity. Furthermore, they did not perform any biomechanical analysis, which seems important since some previous studies found significant differences in KAAI and EKAM among patients.