Recently, a meta-analytic pooling of randomised control studies investigating the use of lateral wedge shoe insoles showed that they have little effect on pain or function in medial knee osteoarthritis.
The clinical effectiveness of lateral wedges may depend on specific patterns of foot and knee function. Identifying patients with foot/ankle characteristics that predict response to this treatment will be important.
Reducing Medial Loading
During weight bearing, the medial knee compartment experiences greater stress than the lateral knee compartment. This increased loading can lead to a bowed knee, or valgus, which increases the load on the inside knee compartment. Over time this can cause a build-up of cartilage damage in the medial compartment that results in joint space narrowing and bone on bone knee osteoarthritis.
Lateral wedge insoles can help reduce this internal load by transferring some of the weight onto the lateral knee during walking. However, how this happens is not well understood and this paper aims to investigate the biomechanical effects of this treatment using gait analysis.
The study included 12 trials with a total of 885 participants; 502 received the lateral wedge inserts. The studies were randomized and controlled. 7 trials used a neutral insole as their control, 4 included no insert at all and 1 trial compared the lateral wedge to a subtalar strap.
The pooled standardized mean difference (SMD) in knee pain was -0.47 (95% CI, -0.80 to -0.14). This is a small effect size that could have clinical significance. When the analysis was grouped by the type of control used, the neutral control subgroup showed no significant association with knee pain, suggesting that the lateral wedge may not be able to significantly decrease knee pain compared to a neutral insole.
Preventing Patellofemoral Loading
Several studies report that use of lateral wedge insoles decreases knee pain during walking compared to neutral insoles. However, the effect size for this primary outcome is small, and may not be clinically significant in most persons with medial OA.
Furthermore, in the subset of trials comparing lateral wedges with a neutral insert, the standardized mean difference (SMD) for pain reduction was only 0.07 on a 0-10 scale; this is a trivial reduction and not likely to be clinically meaningful.
In these trials, participants were prescreened to exclude those with patellofemoral disease (since wedge inserts do not reduce the load in this compartment and might actually make it worse), or with tricompartmental knee OA (in which case decreasing EKAM will probably make symptoms worse). In addition, the biomechanical response of the person to the wedge insert was measured to determine whether they showed a reduction in the loading of their lateral cartilage surface.
Those who responded to the lateral wedge insoles were defined as “biomechanical responders” and showed a statistically significant reduction in pain intensity with the lateral wedge insole compared to the neutral insole. The biomechanical response was characterized by a decrease in the knee adduction moment and an increase in the mediolateral force during walking. These changes in gait dynamics have been shown to lead to reduced patellofemoral loading, which could explain the observed knee pain relief.
Reducing Medial Pain
Few effective therapies exist for painful medial knee osteoarthritis, and many persons who have the condition have pain on both sides of the joint. We tested whether lateral wedge shoe insoles, which reduce loading across the affected medial knee compartment, also decrease pain on the contralateral knee in persons with symptomatic knee osteoarthritis.
Participants with medial knee OA were recruited from orthopedic clinics and from television ads. They were assigned to wear a 5deg lateral wedge insole or a neutral insole for 8 weeks, followed by a washout period (8 weeks) and then the other type of shoe insert. They were evaluated for pain, knee stiffness, and functional outcomes on a 0 to 10 scale. Results showed a modest reduction in pain with lateral wedges (difference, 0.7 on a 0-10 scale; 95% CI 0.1, 1.2), but no other clinically important effect.
Lateral wedge insoles reduce peak external knee adduction moment (EKAM) on the affected side by about 5%. However, this reduction in load may not be enough to reduce pain, as other factors (eg, the sagittal force and muscle co-contraction) contribute significantly to medial knee loading.
We conducted a meta-analysis of trials that reported data on both the affected and the contralateral knee, and we found no significant association between lateral wedges and pain in comparison with a control treatment using a neutral insert. Furthermore, when we restricted our analysis to only those trials that used a neutral insole as the control, the association with pain was still not statistically significant.
Identifying Biomechanical Non-Responders
The peak external knee adduction moment during walking is thought to be an important contributor to medial compartment osteoarthritis [1]. Reducing this load has been advocated as a non-surgical treatment for medial knee OA. However, lateral wedge insoles (LWI) do not reduce this loading in all persons. This is thought to be because the LWI shifts load laterally, potentially worsening patellofemoral loading and pain.
To reduce the impact of this, some studies have prescreened participants to remove those with patellofemoral OA (Kellgren & Lawrence Grade 2-4). Unfortunately this does not eliminate all those that do not benefit from lateral wedging. A recent randomised controlled trial found that lateral wedge insoles were only associated with a small reduction in overall knee pain compared with neutral insoles (SMD of 0.03 on the WOMAC pain subscale, which is a clinically trivial reduction).
Further analysis from this study showed that if the foot was in a supinated position during the stance phase of walking then the LWI did not influence the peak external knee adduction moment or the knee-ground reaction force lever arm. This was also true if the heel wedge was used instead of the lateral insole. This indicates that a bowed knee or ‘varus’ in the foot will not respond to lateral wedging, and therefore may be more likely to develop medial knee osteoarthritis. In these individuals it may be preferable to do a high tibial osteotomy surgery, which involves cutting a triangle wedge out of the tibia, thus straightening the leg and therefore reducing the load on the inside knee compartment.