Can Foot Orthoses Have An Impact For Knee Osteoarthritis?

Given the common nature of knee osteoarthritis, this author discusses the link with progressive genu varum deformity and shares insights from the literature as well as his clinical experience on the benefits of valgus-wedged insoles and orthoses for this patient population.

Over the past three decades, there has been a steady increase in research on the biomechanics and therapeutic effects of foot orthoses. Numerous scientific studies have clearly demonstrated the beneficial therapeutic effects of foot orthoses in the treatment of mechanically-based foot and lower extremity pathologies that cause significant pain and disability.

Research has shown that foot orthoses are helpful in treating painful foot and lower extremity pathologies caused by plantar fasciitis, posterior tibial tendon dysfunction, rheumatoid arthritis, juvenile idiopathic arthritis, hemophilia A and patellofemoral syndrome.^1-14 Likewise, researchers have shown that foot orthoses reduce the incidence of medial tibial stress syndrome, lower extremity stress fractures and chronic exertional compartment syndrome; reduce plantar pressures in neuropathic diabetic feet; and also reduce plantar pressures and pain in patients with metatarsalgia.^15-22

However, over the past five years, medial compartment knee osteoarthritis (OA) has been the subject of the most research in regard to the ability of foot orthoses to treat abnormal biomechanics and painful symptoms associated with this condition.

Osteoarthritis of the knee is a relatively common pathology appearing radiographically in approximately one-third of older adults with an increasing incidence of knee osteoarthritis occurring in older age groups.²³ Knee osteoarthritis affects women more than men with risk factors including obesity, prior knee injury, and occupational bending and lifting.²⁴ Common symptoms of knee OA may include pain, swelling, impairment in function, morning stiffness and joint crepitus. Common physical examination findings in knee OA include palpable effusion, tenderness, bony enlargement of the joint margins, an antalgic gait pattern, joint crepitus and reduced range of motion.²⁵

When Knee OA Is Associated With Genu Varum Deformity
Even though osteoarthritis may affect the medial compartment, lateral compartment or the patellofemoral joint of the knee, the medial compartment is much more likely to develop osteoarthritis than the lateral compartment or patellofemoral joint.^26,27 In more advanced cases of medial compartment knee OA, a significant genu varum deformity may develop over time.²⁸

In theory, the main causative factor behind the onset and progression of medial compartment knee OA is the higher compression forces within the medial compartment of the knee due to the line of action of the ground reaction force (GRF) vector passing medial to the knee joint center during gait.^29,30 When the GRF vector, which arises from the mechanical interaction of the foot with the ground, passes medial to the knee joint center, this creates an external knee adduction moment. The increased external knee adduction moment increases the loading of the medial compartment of the knee and decreases the loading within the lateral compartment of the knee.^31,32 Researchers also believe an increase in external knee adduction moment is associated with an increased incidence of medial compartment knee OA.³³

In medial compartment knee OA, the cartilaginous surfaces of the medial knee compartment gradually thin due to the deterioration of the medial meniscus and/or the joint cartilage of the femur and tibia within the medial compartment.³⁴ The gradual narrowing of the medial knee cartilage that occurs over time in medial compartment knee OA will cause a gradual increase in genu varum deformity, which will in turn cause an increase in the pathological compression loads within the medial compartment of the knee.³⁵ In addition, the gradual thinning and destruction of the medial compartment cartilage may lead to other medial compartment pathologies such as bone marrow lesions, which appear to be strongly correlated to the genu varum deformity often arising in medial compartment knee OA.³⁶

The genu varum deformity often associated with medial compartment knee OA affects the frontal plane angulation of the knee and lower extremity. The frontal plane angulation of the knee in turn has a significant mechanical effect on the compression forces within both the medial and lateral compartments of the knee.²⁸ In a knee with normal frontal plane angulation, both the medial and lateral compartments will share relatively equal compression loads.³⁷ With genu varum deformity, since the foot is located more medial to the knee, the more medially directed GRF vector will cause increased loading of the medial compartment and less loading of the lateral knee compartment.^28,38 The inverse is also true in that a genu valgum deformity will cause a lateral positioning of the foot relative to the knee, which will increase the lateral compartment compression loads and decrease the medial compartment loads within the knee.³⁰

Any progression of genu varum deformity within the lower extremity will further increase the medial knee compartment compression loading forces.²⁸ Due to the medial positioning of the foot relative to the knee in a genu varum deformity, the point location of the GRF vector acting on the plantar foot, known as the center of pressure, will also become more medially positioned relative to the knee in comparison to a normally aligned lower extremity.³⁹

As the foot becomes increasingly more medially positioned relative to the knee with increasing degrees of genu varum deformity, the GRF vector that normally passes just slightly medial to the center of the knee becomes even more medially positioned relative to the knee. The greater the medial positioning of the GRF vector relative to the knee, the greater the increase in external knee adduction moment. The basic mechanical effect of an increased magnitude of external knee adduction moment is an increased tendency of the tibia to rotate into a varus position (i.e. an increased genu varum position) relative to the femur.⁴⁰  
Researchers have long thought the increased external knee adduction moment caused by an increased genu varum deformity causes an increase in compression forces within the medial compartment of the knee and a decrease in the compression forces within the lateral compartment of the knee.³⁵ The increase in medial compartment loading forces within the knee, suspected to occur due to an increased external knee adduction moment, has had confirmation in vivo through the use of instrumented knee joint implants that surgeons have inserted into patients, who subsequently had post-surgical study with three-dimensional video and force plate analysis while walking.^31,32

Therefore, the overall effect of any increase in genu varum deformity is an increase in external knee adduction moment and an increase in the medial compartment compression forces within the knee. Over time, the increased medial compartment compression forces acting on the articular cartilage and subchondral bone at the medial tibiofemoral joint will tend to increase the likelihood for medial compartment knee OA to occur.²⁴ In addition, the progressive narrowing of the medial compartment cartilage of the knee further increases the genu varum deformity and the external knee adduction moment.⁴¹ All of these factors, acting together and reinforcing each other, will cause larger magnitudes of medial compartment compression forces, higher magnitudes of medial knee compartmental pressures and likely further worsening of the medial compartment knee OA over time.

What The Literature Reveals About Valgus-Wedged Insoles For Medial Compartment Knee OA
From the above biomechanical analysis, it becomes evident that the medial shift in center of pressure acting on the plantar foot relative to the knee within the frontal plane is largely responsible for the abnormal biomechanics of the lower extremity that eventually leads to the development of medial compartment knee OA. Therefore, it makes good biomechanical sense that if one can use an in-shoe insert, such as a valgus-wedged insole or foot orthosis, to shift the center of pressure laterally on the plantar foot, this may lessen the magnitude of external knee adduction moment and may also reduce the magnitude of medial compartment loading forces. In this regard, for nearly three decades, clinicians have been empirically using various types of valgus-wedged insoles to reduce the pain of medial compartment knee OA in patients.^42-44

Numerous research studies have demonstrated that valgus-wedged insoles reduce the magnitude of the external knee adduction moment during walking.^45-54 Previous research has also shown that valgus-wedged insoles cause a lateral shift in the center of pressure acting on the plantar foot, which mechanically correlates with a reduction in the external knee adduction moment.^47,55-57 In addition, there are now a number of research studies that have confirmed the positive changes in knee mechanics and knee symptoms that can occur with appropriate application of various types and degrees of valgus-wedged insoles and/or foot orthoses.^{40,42,43,54-56,58-63} For patients with medial compartment knee OA, the changes in painful symptoms that routinely occur with valgus-wedged insoles and orthoses are likely related to the decreased magnitude of medial tibiofemoral contact forces. Computer modelling techniques such as finite element analysis have confirmed these changes and, as I mentioned earlier, instrumented in-vivo knee implants have experimentally verified these findings.^32,37

However, one of the obvious problems with placing valgus-wedged insoles into the shoes of patients with medial compartment knee OA is that the valgus-wedged insoles or orthoses will shift loading forces from one structural component of the foot and lower extremity to another, possibly causing a new injury in a different anatomical location. Valgus-wedged insoles and orthoses, due to their ability to decrease the load on the medial knee compartment by redistributing the load onto the lateral knee compartment, have reportedly caused painful but transient bone marrow edema within the lateral femoral condyle.⁶⁴ In addition, research has shown that valgus-wedged insoles placed within the shoes of healthy people increased the external subtalar joint pronation moment even though these insoles also decreased the external knee adduction moment during walking.⁵⁶

In effect, even though valgus-wedged insoles or foot orthoses can be quite effective at reducing medial compartmental knee pain in patients with medial compartment knee OA, these same in-shoe valgus-wedged devices may cause new pathologies in other areas of the foot and lower extremity over time.

It is important for the podiatrist to remember that foot orthoses, or any in-shoe insert for that matter, produce their mechanical effects on the structural components of the foot and lower extremity by altering the magnitudes, temporal patterns and plantar locations of GRF acting on the plantar foot during weightbearing activities.⁶⁵ Since foot orthoses can alter the external forces acting on the plantar foot that occur due to GRF, foot orthoses can therefore also alter the internal forces and moments acting within the foot, and within any of the other joints of the lower extremity.  

How Tissue Stress Theory Can Guide Orthotic Prescription
Given the known abilities of foot orthoses to redirect external and internal loads on the foot and lower extremity, the emerging concepts of tissue stress theory can help guide the design of these orthosis prescriptions and improve their therapeutic efficacy.^66-68 The treatment goals of foot orthoses using tissue stress theory are to: reduce the pathological loading forces on the injured structural components of the foot and lower extremity, optimize gait function, and prevent other injuries from occurring.

The use of valgus-wedged insoles and orthoses to treat medial compartment knee OA is an excellent example of how one can design foot orthoses with much greater therapeutic efficacy by using the concepts of tissue stress theory versus the older theoretical podiatric concepts of designing foot orthoses to “prevent compensation for foot deformities,” “hold the subtalar joint in neutral position” and/or “lock the midtarsal joint” that professors have taught for decades in podiatric medical colleges in the United States and internationally.^68,69

Tissue stress theory recognizes the fact that since foot orthoses can change how the plantar foot mechanically interacts with the ground, foot orthoses also have the ability to reduce the pathological loading forces on the bones, ligaments, muscle, tendons, fascia, cartilage and skin of the foot and lower extremity. In this regard, tissue stress theory also recognizes that foot orthoses redirect loading forces away from the injured structural components of the foot and lower extremity, and toward other structural components that may over time experience new injuries.
In other words, tissue stress theory emphasizes that by altering the magnitudes, plantar locations and temporal patterns of the external forces acting on the plantar foot, we must design foot orthoses to “rebalance” the internal loading forces acting within all the structural components of the foot and lower extremity not only to allow the patient to heal the current injury but also to prevent the patient from developing new injuries as a result of foot orthosis therapy.

Relatively distinct foot orthosis designs, such as the valgus-wedged orthosis used in treating medial compartment knee OA, may be problematic for some patients due to their unique construction, which is intended to unload a specific injured structure within the foot and lower extremity by redirecting loading forces elsewhere. As noted earlier, this change in internal loading patterns has the potential to cause excessive external subtalar joint pronation moments, abnormal gait patterns and even new pathologies as time progresses.

However, podiatrists who are comfortable using and practicing foot orthosis therapy using the principles of tissue stress theory — which aids one in designing foot orthoses to reduce stress acting on the injured structural components of the foot and lower extremity without causing other injuries — will be able to easily understand and successfully employ the concepts of the valgus-wedged foot insole and/or orthosis used for treating medial compartment knee OA in their patients. One important consideration in this regard, and also an inherent part of tissue stress theory, is that the podiatrist prescribing relatively unique foot orthosis designs needs to perform more frequent post-dispensing monitoring of patients so the risk of injury to their patients is minimal.  

Clinical Observations On The Impact Of Valgus-Wedged Insoles And Orthoses For Medial Compartment Knee OA
Clinically, I have been using valgus-wedged insoles and prescription foot orthoses for my patients who have mild to moderate degrees of medial compartment knee OA to reduce their medial knee pain for the past two decades with good to excellent results. Anecdotally, I have had little to no success in reducing medial knee pain in patients with more severe degrees of medial compartment knee OA with these same valgus-wedged insoles and foot orthoses.  

Early on in my experimentation with valgus-wedged insoles and orthoses in treating medial compartment knee OA, it became clinically evident that the valgus wedging needed to be plantar to the rearfoot, midfoot and forefoot, and not just plantar to the rearfoot to be therapeutically effective. It also became evident that one needs to titrate the amount of valgus wedging with each patient in order to produce the best therapeutic effects. Over the years, I also observed that reduced medial longitudinal arch support improved the short and long-term comfort of the orthosis while too much medial longitudinal arch support tended to negate the therapeutic efficacy of the orthosis.

Supporting my clinical observation that valgus wedging is more important than medial longitudinal arch support for orthosis treatment of medial compartment knee OA, research has demonstrated that orthoses with medial longitudinal arch support and without valgus wedges do not seem to reduce either the external knee adduction moment or knee pain in patients with medial compartment knee OA.⁷⁰
Currently, my clinical preference for treating medial compartment knee OA with custom foot orthoses is to use a full-length orthosis that incorporates valgus wedging under the rearfoot, midfoot and forefoot portions of the orthosis, but which also has reduced medial longitudinal arch height. Effective orthosis modifications for treating medial compartment knee OA include a 2-4 mm lateral heel skive modification in which one forms a valgus-wedged heel cup into the orthosis to shift the orthosis reaction force (i.e. the reaction force from the orthosis acting on the plantar foot) more laterally on the plantar rearfoot.^71-73 I also use a flat rearfoot post with no rearfoot post motion to shift the orthosis reaction force further laterally on the plantar rearfoot.

I combine the lateral heel skive with extra medial arch fill within the positive cast to further decrease the medial longitudinal arch height of the orthosis and prevent excessive medial shifting of the orthosis reaction force within the midfoot portion of the orthosis. In addition, I add a forefoot valgus extension to the sulcus into the orthosis to effectively shift the orthosis reaction force more laterally on the plantar forefoot.⁷⁴

If I determine that the patient does not need a custom-casted foot orthosis, I initially add a simple heel-to-sulcus valgus wedge to the bottom of an over-the-counter insole with strips of adhesive felt. This full-length, valgus-wedged insole will effectively shift the center of pressure laterally on the plantar foot and decrease the external knee adduction moment. I eventually replace the adhesive felt wedge with a more durable EVA or rubberized cork that I grind into a plantar valgus wedge at a subsequent office visit. The use of such a “temporary orthosis” over the past two decades has worked very well in many patients for up to six to 12 months, but lacks the durability of the custom-casted, valgus-wedged foot orthosis used for the treatment of medial compartment knee OA.

In Conclusion
Medial compartment knee OA is associated with a progressive genu varum deformity due to the gradual thinning and destruction of the medial knee compartment cartilage. The increased genu varum deformity causes a relative medial shift of the plantar foot relative to the knee, which in turn causes an increase in external knee adduction moment and a concomitant increase in medial knee compartment loading forces. The scientific research to date very clearly demonstrates that appropriately designed valgus-wedged insoles and/or orthoses can be very effective at reducing the external knee adduction moment, reducing the medial compartment loading forces and thus reducing the medial knee pain in patients with mild to moderate medial compartment knee OA.  

The unique valgus-wedged design of these insoles and orthoses, along with proper and timely modification of these in-shoe devices, will allow podiatric clinicians to provide their patients with maximum therapeutic benefit and a minimum risk of new injury and dysfunction. Podiatrists who are familiar with the concepts of tissue stress theory will be better able to understand the biomechanical concepts and design principles of valgus-wedged insoles and orthoses, which in turn will improve one’s ability to effectively and safely treat patients who suffer from the pain and disability of medial compartment knee OA.

Dr. Kirby is an Adjunct Associate Professor within the Department of Applied Biomechanics at the California School of Podiatric Medicine at Samuel Merritt University in Oakland, Calif. He is in private practice in Sacramento, Calif.

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