Kevin A. Kirby, DPM

Kevin A. Kirby, DPM We provide the most advanced podiatric care to our patients with an emphasis on the biomechanics of the foot and lower extremity.
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Dr. Kevin Kirby graduated from the California College of Podiatric Medicine in 1983 and completed his first year surgical residency at the Veteran’s Administration Hospital in Palo Alto, California. He spent his second post-graduate year doing the Fellowship in Podiatric Biomechanics at CCPM where he also earned his MS degree. Dr. Kirby has authored or co-authored 27 articles in peer-reviewed jour

nals, has authored or co-authored five book chapters, and has authored five books on foot and lower extremity biomechanics and orthosis therapy, all five of which have been translated into Spanish language editions. He has invented the subtalar joint axis palpation technique, the anterior axial radiographic projection, the supination resistance test, the maximum pronation test and the medial heel skive and lateral heel skive orthosis techniques. He has also created and developed the Subtalar Joint Axis Location and Rotational Equilibrium Theory of Foot Function and has co-developed the Subtalar Joint Equilibrium and Tissue Stress Approach to Biomechanical Therapy of the Foot and Lower Extremity. He has lectured internationally on 33 separate occasions in China, Spain, Belgium, New Zealand, Australia, England, Dominican Republic and Canada over the past 23 years on foot and lower extremity biomechanics, foot orthoses, and sports medicine. He has also lectured extensively throughout the United States. Dr. Kirby is a member of the editorial advisory board for the Journal of the American Podiatric Medical Association and a manuscript reviewer for the Journal of Biomechanics, Journal of Foot and Ankle Surgery, Medicine and Science in Sport and Exercise, Journal of Foot and Ankle Research and Journal of Sports Sciences. He is currently an Adjunct Associate Professor in the Department of Applied Biomechanics at the California School of Podiatric Medicine and has a full time podiatric biomechanics and surgical practice in Sacramento, California.

08/07/2026

Ganglion Aspiration Without Anesthetic or Syringe

I performed this ganglion aspiration on a 49 year-old female patient who has had a ganglion recur on and off over the past five years on the dorsal aspect of her forefoot. The ganglion was about 3.0 cm x 3.0 cm in area and about 8 mm thick, filled with about 3-4 cc of ganglionic fluid.

In this procedure, no syringe or local anesthetic is necessary. After sterile prep with a povidone-iodine solution, ethyl chloride is sprayed onto the ganglion for about 5-6 seconds to temporarily numb the skin over the ganglion. Then an 18-gauge hypoderemic needle is used to puncture the lateral aspect of the gangion. The ganglionic fluid is then fully expressed out of the ganglion using manual pressure on the top and sides of the ganglion. A sterile 2x2" gauze and 3" coban wrap is then used for a compression dressing which is kept on for 24 hours.

Total procedure time for this alternative ganglion aspiration technique is about 5 minutes. I have used this ganglion aspiration technique now for the past 40 years and have never had any problems with the procedure. Patients are pain-free the next day after the procedure and are also able to perform full activities the day following the procedure.

The Biomechanics of Tissue StressThe tissue stress approach to mechanical foot therapy is based on the concept that the ...
08/04/2026

The Biomechanics of Tissue Stress

The tissue stress approach to mechanical foot therapy is based on the concept that the location, type and magnitude of stresses that cause mechanical symptoms of the foot and lower extremity should be strongly considered when specific treatments are being designed for a patient. The ideas inherent in the tissue stress approach should not be foreign to any clinician who routinely treats mechanically based symptoms of the foot and lower extremity. For example, if a patient has pain plantar to the second metatarsophalangeal joint (MPJ), the tissue stress approach simply states that treatment should be directed toward reducing any abnormal stresses in the injured structures of the plantar second MPJ so that healing may be accelerated and a normal gait pattern reestablished.

This type of approach to mechanical foot therapy has a different emphasis than that advocated by the proponents of the Subtalar Joint Neutral (SJN) theory in which mechanical treatment is directed at preventing “abnormal compensations” for “deformities” in the foot and/or lower extremity. When prescriptions for custom foot orthoses are considered, the SJN theory places much more emphasis on the concept of "subtalar joint neutral" and much less emphasis on the anatomical site and type of pathological stresses which are causing the symptoms and/or injury in the patient than does the tissue stress approach to mechanical foot therapy.

In order for the clinician to become more familiar with the idea of tissue stress, it is important that the basic mechanical concepts of loading forces and stresses first be reviewed. The external forces that cause stresses to occur within a structure may be classified by the way they tend to deform the structure upon which they are acting. Axial loading can occur in either one of two ways: compression and tension.

A structure is defined as being under compression if the axial loading force tends to make it shorten (see my illustration below). If the axial loading force tends to make a structure elongate, then it is defined as being under tension. Another form of loading on a structure is called shear loading. Shear loading tends to produce horizontal sliding of one layer of a structure over another. Tension and compression forces are commonly called normal forces while shearing forces are commonly called tangential forces (Ozkaya, N., Nordin, M.: Fundamentals of Biomechanics: Equilibrium, Motion and Deformation. Van Nostrand Reinhold, New York, 1991, p. 271).

Any tissue in the body, when subjected to an external loading force, will develop an internal resistance to that load, which is called stress. If a tissue tends to have a large capacity to resist external loading forces, then the internal load, or stress, it can develop under load will be relatively large. However, if an anatomical structure has only a small capacity to resist the loading forces, then the internal load, or stress, it can develop under load will be relatively small. For example, a steel cable is able to develop higher stresses within its structure when compared to a rubber band since the steel cable has a larger capacity to resist tensile loading forces than does the rubber band (Whiting, W.C., Zernicke, R.F.: Biomechanics of Musculoskeletal Injury. Human Kinetics, Champaign, IL, 1998, p. 67).

Depending on the nature of the external loading force acting on it, a structure may develop compression stress, tension stress and/or shear stress. In order to develop a compression stress, the structure must develop an internal loading force that tends to make it resist being pushed together (e.g., the tibia develops compressive stress in response to supporting the mass of the body during weightbearing activities). In order to develop a tension stress, the structure must develop an internal loading force that tends to resist being pulled apart (e.g., the Achilles tendon develops tension stress in response to ankle joint dorsiflexion). Any forces acting parallel, or tangential, to the applied external loading force creates a shear stress. Shear stress is commonly developed in structures where torsional forces are applied to that structure. For example, the tibia will develop shear stresses directed perpendicular to its long axis when torsional forces are applied that tend to cause internal rotation of the tibia relative to the foot (Whiting, Zernicke, 1998, pp. 67, 75).

The magnitude of stress within a structure is calculated by dividing the magnitude of the axial load, F, by the cross-sectional area over which the load is distributed, A. Axial, or normal, stress is commonly denoted by the Greek letter, sigma (σ). Therefore, the formula for determining axial stress is as follows: σ = F/A. Forces that act tangential to the applied load create shear stress which is commonly denoted by the Greek letter, tau (τ). Therefore, the formula for determining shear stress is as follows: τ = F/A. The standard unit used for the measurement of stress in biomechanics is the Pascal (Pa), which is defined as one Newton [N] distributed over one square meter (1 Pa = 1 N/m2). One Newton is equivalent to 0.225 pounds (Whiting, Zernicke, 1998, p. 67).

Since the magnitude of stress acting on a biological tissue is determined by the magnitude of loading force divided by the cross-sectional area over which the load is distributed, then when tissue stress is a prime consideration, the cross-sectional area of the tissue is just as important as the absolute force acting on the tissue. For example, if a tension loading force of 300 N is applied to a posterior tibial tendon with a cross-sectional area of 0.25 cm2, then the stress on the posterior tibial tendon would be as follows: σ = 300 N/0.0025 m2 = 120,000 Pa.

However, if the same loading force of 300 N is applied to a posterior tibial tendon where half of the tendon fibers have been ruptured, so that the cross-sectional area of the tendon is now 0.125 cm2, then the magnitude of stress acting on the remaining tendon fibers would be increased by two-fold as follows: σ = 300 N/0.00125 m2 = 240,000 Pa. See my illustration below.

Therefore, the loss or rupture of a percentage of fibers in a tendon can greatly increase the stress on the remaining tendon fibers that, in turn, will increase the likelihood of tendon rupture even under normal magnitudes of tensile loading forces on the tendon. These same concepts related to tissue stress also apply to other structural biological tissues such as bone, ligament and cartilage.

[Reprinted with permission from: Kirby KA: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 15-16.]

Subtalar Joint Neutral versus Tissue Stress Approach to Mechanical Foot TherapySubtalar Joint Neutral Theory, proposed b...
08/03/2026

Subtalar Joint Neutral versus Tissue Stress Approach to Mechanical Foot Therapy

Subtalar Joint Neutral Theory, proposed by Dr. Merton Root and colleagues at the California College of Podiatric Medicine, is based on the premise that the structural components of the foot and lower extremity can be accurately measured so that any deviation from an ideal or a “normal” structure would be considered to be a “deformity”. Using the Subtalar Joint Neutral Theory as the basis for mechanical foot therapy, foot orthoses are designed to “prevent compensation for deformities” with the orthosis prescription being based on the “deformities” which are determined during the biomechanical examination of the patient.

Proponents of the Subtalar Joint Neutral Theory do not necessarily change the prescription variables of foot orthoses when patients are encountered with different injuries to different parts of the foot and/or lower extremity. It was assumed by those Subtalar Joint Neutral Theory proponents that by simply “preventing compensation for deformities”, more normal gait function will occur and the injured structure will eventually heal.

Within the podiatric biomechanics community during the past 30+ years, there has been a gradual shift away from using the Subtalar Joint NeutralTheory as a theoretical basis for mechanical foot therapy. One of the reasons why many podiatrists have moved away from the Subtalar Joint NeutralTheory is due to some of the inherent problems and inconsistencies with this theory of mechanical foot therapy.

One large problem with the Subtalar Joint Neutral Theory relates to the reliability of the measurement procedures used within the standard biomechanical examination techniques proposed by Root et al over thirty years ago (Root, M.L., W.P. Orien, J.H. W**d and R.J. Hughes: Biomechanical Examination of the Foot, Volume 1. Clinical Biomechanics Corporation, Los Angeles, 1971). These examination techniques have been found to have only fair intertester reliability and, therefore, can not be considered reliable from one examiner to another (McPoil, T.G. and G.C. Hunt: Evaluation and management of foot and ankle disorders: Present problems and future directions. JOSPT, 21:381-388, 1995.)

Another criticism of the Subtalar Joint NeutralTheory is that the criteria for normalcy proposed by Root et al are not clinically practical since they are so restrictive that few individuals have “normal” foot and lower extremity structure (Root et al, 1971). In addition, the idea of Root et al that the subtalar joint should supinate through neutral position during the midstance phase of walking gait has been questioned by research by McPoil and Cornwall on 100 healthy, asymptomatic feet in which the subjects were more likely to have a rearfoot motion pattern which correlated to their resting calcaneal stance position than to their neutral calcaneal stance position (McPoil, T.G. and M.W. Cornwall: The relationship between subtalar joint neutral position and rearfoot motion during walking. Foot Ankle Intl., 15:141-145, 1994.)

McPoil and Hunt have provided an excellent review of the problems associated with the SJN approach to mechanical foot therapy, including those listed above, and 31 years ago proposed a new model, the Tissue Stress Model, for the approach to mechanical foot therapy (McPoil and Hunt, 1995).

McPoil and Hunt have chosen to use the Tossue Stress Model “as the basis for developing an examination and management paradigm for treating individuals with foot disorders”. They claimed that the tissue stress model is not a novel idea since it is based on the same ideas that are already in current use in the treatment of parts of the body other than the foot and lower extremity. In addition, one of the benefits claimed for the tissue stress model is that it doesn’t rely on the use of the “unreliable measurement techniques” currently in use within the podiatric profession (McPoil and Hunt, 1995).

There have also been others that have also advocated the use of the tissue stress approach to mechanical foot therapy. 26 years ago, Eric Fuller, DPM, previously described the effects of rearfoot and forefoot wedging and how he uses the tissue stress approach in the clinical setting as a basis for mechanical foot therapy (Fuller, E.A.: Reinventing biomechanics. Podiatry Today, 13:(3), December 2000). Dr. Fuller has also reviewed the concept of tissue stress and how computerized gait evaluation techniques along with the concept of modeling of the foot and lower extremity can help predict the stress in a specific anatomical structure (Fuller, E.A.: Computerized gait evaluation. pp. 179-205, in Valmassy, R.L. (editor), Clinical Biomechanics of the Lower Extremities, Mosby-Year Book, St. Louis, 1996).

In addition, in two articles on future directions for podiatric biomechanics, I have also described the important concept of modeling of the foot and lower extremity and how modeling can be used to predict the loading forces, or stresses, which occur in the structural components of the foot and lower extremity during weightbearing activities (Menz, H.B. (moderator), Kirby, K., Cornwall, M., Rome, K., Tinley, P., Murphy, N., Keenan, A.: Clinical measurement of the lower extremity-where to from here? Australasian J. Pod. Med., 31 (3):95-99, 1997; Kirby, K. A.: What future direction should podiatric biomechanics take? Clinics in Podiatric Medicine and Surgery, 18 (4):719-723, October 2001).

Previous to the time that I first heard the concept of the “tissue stress model” in a lecture given by Tom McPoil, PhD in 1997 at the American Academy of Podiatric Sports Medicine Annual Meeting in Bellevue, Washington, I had independently developed a similar thought process and approach to mechanical foot therapy that I called “thinking like an engineer” (Kirby, K.A.: Thinking like an engineer. March 1992 Precision Intricast Newsletter. In Foot and Lower Extremity Biomechanics: A Ten Year Collection of Precision Intricast Newsletters. Precision Intricast, Inc., Payson, Arizona, 1997, pp. 267-268).

In the newsletter, I described how it is more important for the podiatrist to focus on the internal loading forces, or stresses, which cause injury when treating mechanically related pathology than to just focus on determination of externally-apparent “foot and lower extremity deformities”. I also described how a structural engineer might use a similar approach when analyzing the stresses within the structural components of a building or bridge. I am publishing that newsletter from March 1992, in full, in another post today.

The tissue stress model is another way of stating the idea that podiatrists would be more effective at treating their patients if they would only use some of the basic mechanical concepts that have already been used for decades by structural engineers. The model is based on the concept that any mechanical therapy designed for the patient should be based not only on the specific anatomical site of injury of the patient, but also on the nature of the pathological loading forces that are causing the injury and how to most effectively design a mechanical therapy program to reduce these pathological loading forces so that healing may be optimized.

Podiatrists who use the more logical and biomechanically-sound approach to mechanical foot therapy inherent in the tissue stress model are much more likely to efficiently and effectively heal the mechanically based pathology of their patients. The podiatrist that only uses the concepts advocated by the proponents of the Subtalar Joint Neutral Theory, where treatment of externally apparent “deformities” guides the design of the mechanical foot therapy, likely will be less effective at treating the wide range of foot and lower extremity pathology that can be treated with foot orthoses.

[Adapted from: Kirby KA: Tissue Stress Approach to Mechanical Foot Therapy, February 2002 Precision Intricast Newsletter, in Kirby KA.: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 13-14.]

Does Functional Hallux Limitus Cause a Low-Arched Foot?Functional hallux limitus (FnHL) occurs when a foot has normal ha...
08/02/2026

Does Functional Hallux Limitus Cause a Low-Arched Foot?

Functional hallux limitus (FnHL) occurs when a foot has normal hallux dorsiflexion in the non-weightbearing examination but has limited hallux dorsiflexion during standing and weightbearing activities. Functional hallux limitus was first described in 1972 by the late Patrick Laird, DPM. Dr. Laird claimed that FnHL was due to to excessive rearfoot pronation which increased first ray dorsiflexion (Laird PO: Functional hallux limitus. The Illinois Podiatrist. 9:4, 1972).

Then, 14 years later, in 1986, Dananberg described how FnHL could lead to flattening of the medial longitudinal arch (MLA), "Lowering of medial longitudinal arch is related to the inability of the hallux to extend." Dananberg claimed that FnHL causes MLA flattening whereas Laird believed that MLA flattening precedes and causes FnHL. There is no research evidence to date to support the idea that FnHL caused MLA flattening. However, mechanical modelling of the foot does strongly support that idea that flattening of the MLA can cause FnHL.

In addition, arthrodesis procedures of the 1st metatarsophalangeal joint (MPJ), which are now a very common surgical procedure in podiatric patients, do not seem to cause arch flattening over time. If lack of hallux dorsiflexion, in fact, somehow mechanically causes medial longitudinal arch flattening over time, then why doesn't completely eliminating hallux dorsiflexion during gait with a 1st MPJ arthrodesis procedure cause arch flattening?

My answer to this question is that FnHL occurs due to arch flattening which, in turn, causes an increase in plantar fascial tension which limits hallux dorsiflexion. The lack of hallux dorsiflexion is not the cause of arch flattening, but is the biomechanical result of arch flattening.

In my illustration below, a model of the foot with the plantar fascia attaching to the plantar heel and hallux is presented. In the upper illustration, the MLA of the foot is higher than in the lower illustration. Static analysis of this mechanical system shows that the plantar fascia will have reduced tension in a foot with a higher MLA and the plantar fascia will have increased tension force in a foot with a lower MLA.

A similar mechanical model was first described by John Hicks in his classic paper on foot biomechanics from 1961(Hicks JH: The Three Weight Bearing Mechanisms of the Foot. In F.G. Evans (ed): Biomechanical Studies of the Musculoskeletal System. C.C. Thomas Co., Springfield, Ill., pp. 161-191, 1961). The other illustration below is from this classic paper by Hicks from 65 years ago.

Since tension within the plantar fascia produces a hallux plantarflexion moment, then increased plantar fascia tension will produce increased hallux plantarflexion moment, which will lead to increased tendency to cause FnHL. Lower MLA height will increase plantar fascia tension, increase hallux plantarflexion moment and will decrease the likelihood that ground reaction force plantar to the hallux during propulsion will result in hallux dorsiflexion.

As a result, feet with lower MLA height have a greatly increased risk of developing FnHL. It is highly unlikely that FnHL is the cause of feet developing a reduced MLA height. Rather, it is very likely that reduced MLA height and the resulting increase in tension force within the plantar fascia are the main cause of FnHL.

References:
Kirby KA: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 139-152.

Accommodating Plantar Fibromas in Custom Foot OrthosesPlantar fibromatosis, or Ledderhose disease, is a relatively uncom...
08/01/2026

Accommodating Plantar Fibromas in Custom Foot Orthoses

Plantar fibromatosis, or Ledderhose disease, is a relatively uncommon, benign hyperproliferative fibrous tissue disorder resulting in the formation of nodules within the plantar fascia. These nodules, or plantar fibromas, can vary in size and shape and occur predominantly within the medial band of the plantar fascia. Even though smaller plantar fibromas may be asymptomatic, larger plantar fibromas may result in patients have difficulty walking and performing other weightbearing activities without pain (Young JR et al: The etiology, evaluation, and management of plantar fibromatosis. Orthopedic Res Rev, 11:1-7, 2019).

The best clinical examination method for determining the location and size of plantar fibromas within the plantar fascia involves having the patient sit or lie on an examination table or chair, and then the examiner using their thumb to push under the first metatarsal head while the index finger of the same hand pushes upwards on the plantar hallux of the patient’s foot. The foot is next dorsiflexed at the ankle joint so that the Achilles tendon and plantar fascia are placed under tension loading force. Then, the thumb of the other hand of the examiner is used to feel along the length of the medial band of the plantar fascia in order to palpate for any irregularities of contour within the plantar fascia.

Of course, visual examination of the plantar foot may also detect larger plantar fibromas, while many smaller plantar fibromas may only be detected by the palpation technique described above.
Smaller plantar fibromas are commonly asymptomatic since they are only subjected to the physiologic tension forces which occur within the plantar fascia during daily weightbearing activities. However, as plantar fibromas become larger in size, they will more likely press against the arch supports of shoes which, over time, will cause repetitive abnormal compression forces on the fibroma and lead to increased pain and tenderness in the plantar fibroma.

Plantar fibromas may also become so large that they may press against the ground while barefoot and become painful. My observation of hundreds of patients with plantar fibromas of varying sizes over my 41 years of clinical practice leads me to believe that abnormal compression forces from a shoe insole, foot orthosis or even the ground (i.e. for very large fibromas) are the cause of the vast majority of symptoms within plantar fibromas. However, plantar fascia tension forces, by themselves, seem to rarely result in plantar fibroma symptoms.

When a patient has been determined to require custom foot orthoses in order to relieve their mechanically-based foot and lower extremity symptoms, I will always perform a clinical palpation examination of the plantar fascia of both feet, as described above, to determine whether there are any plantar fibromas present, or not. If even a small plantar fibroma is noted, then special modifications may need to be made to the custom foot orthosis to improve the comfort of the orthosis. In addition, if the plantar fibroma is noted to be so large that it is becoming a weightbearing structure during barefoot activities, then I am much less likely to initiate custom foot orthosis therapy for the patient until I can use other treatments, such as intralesional cortisone injections, in order to try to reduce the size of the plantar fibroma.

Since the key mechanical factor which causes plantar fibromas to become symptomatic is repetitive compression forces acting on the plantar aspect of the fibroma from the shoe insole, foot orthosis, or even the ground, then custom foot orthoses will need to be modified for most patients with plantar fibromas (see my illustration below). The only exception to this clinical guideline is when the plantar fibroma is extremely small which will greatly reduce the chance that plantar fibroma irritation will result from the dorsal shell of the orthosis.

The most common orthosis modification for symptomatic plantar fibromas is to grind an accommodation into the dorsal aspect of the orthosis so that the compression force from the orthosis onto the fibroma is reduced or eliminated. In order the grind an accurate plantar fibroma accommodation into a preexisting foot orthosis, the boundaries of the plantar fibroma should be marked on the plantar foot with an ink pen, or lipstick, so that when the orthosis shell is pressed congruently against the foot, the location of the plantar fibroma relative to the orthosis shell can be determined.

Then, using the edge of the drum on a grinder, or a hand rotary tool (e.g., Dremel), the dorsal aspect of the orthosis is ground so that a shallow groove or furrow is made in the dorsal orthosis shell to exactly match the plantar location, thickness and surface area of the plantar fibroma. One clinical pearl for performing this orthosis grinding procedure for plantar fibromas is to assume that the fibroma will always shift 2-3 mm distally relative to the orthosis once the patient starts standing or walking on the orthosis. In other words, when grinding a plantar fibroma accommodation into an orthosis, I always start my grind 2-3 mm distal to the marks made on the dorsal orthosis plate for the plantar fibroma accommodation.

If, however, the patient is noted to have a plantar fibroma (i.e. symptomatic or asymptomatic) of medium to relatively large size, and the patient needs to have a new pair of custom foot orthoses made for them, then the best way to accommodate the plantar fibroma is to have the orthosis laboratory add a “plantar fibroma accommodation” into the dorsal shell of the orthosis. In this case, the plantar fibroma is marked on the foot with an ink pen or indelible pencil so that when a plaster cast is taken or a scan is made of the foot, the plantar location of the fibroma will be easily seen within the cast or scan.

The area of the intended plantar fibroma accommodation is next marked on the cast 2-3 mm anterior to the marks transferred to the negative cast from the foot so that the resultant plantar fibroma accommodation will be in the proper location once the patient starts walking on the orthosis. The clinician needs to also note the thickness of the fibroma accommodation on the orthosis order form so that the proper depth of fibroma accommodation can also be constructed into the orthosis by the lab. Careful consideration and performance of these orthosis modification procedures will invariably lead to consistently better orthosis outcomes for patients with plantar fibromas.

[Reprinted with permission from: Kirby KA: Precision Intricast Newsletter, Precision Intricast, Inc., Payson, AZ, August 2020.]

Prescribing Better Foot Orthoses: Knee Osteoarthritis and Genu Varum and Genu Valgum DeformitiesOsteoarthritis (OA) of t...
08/01/2026

Prescribing Better Foot Orthoses: Knee Osteoarthritis and Genu Varum and Genu Valgum Deformities

Osteoarthritis (OA) of the knee is a relatively common pathology that affects many individuals and is especially prevalent in those who have had previous meniscal or ligamentous injuries of the knee. Either the medial or lateral compartment of the knee may be affected by OA and is associated with thinning or narrowing of the articular cartilage in the affected compartment. Common symptoms may include swelling, stiffness and pain in the knee with standing and walking. On clinical examination, patients may often demonstrate edema and tenderness at the affected knee compartment, palpable marginal joint osteophytes, a lack of full knee joint extension and an antalgic gait pattern.

One of the most important concepts regarding the biomechanics of knee OA is that the narrowing of the knee joint cartilage that occurs with its progression will also tend to cause an increase in abnormal frontal plane angulation of the knee. For example, the gradual thinning of the medial compartment cartilage that occurs over time in patients with medial knee OA will cause progressive varus angulation of the tibia and the development of a genu varum deformity over time. Additionally, in lateral knee OA, the gradual loss of lateral compartment knee cartilage will cause development of a valgus tibial alignment and the progression of genu valgum deformity (Maquet, Paul G.J.: Biomechanics of the Knee. Springer-Verlag, New York, 1984).

The frontal plane angulation of the knee also has a significant mechanical effect on the compression forces within medial and lateral compartments of the knee. In a knee with normal frontal plane angulation, both the medial and lateral compartments will share fairly equal compression loads. When the upward directed ground reaction force (GRF) vector acting on the plantar foot is directed close to the center of the knee joint, a normal distribution of medial and lateral compartment compression forces will occur.

However, with genu valgum deformity, since the foot is located more lateral relative to the knee, the GRF vector acting on the plantar foot will be directed more lateral, causing increased lateral compartment compression loads. Alternatively, with genu varum deformity, since the foot is located more medial to the knee, the medially directed GRF vector will cause increased loading of the medial compartment (see my illustration below).

These increased intra-compartmental compression forces in either the medial or lateral compartments may lead to further narrowing of the joint cartilage over time in the affected compartment, leading to further progression of the knee OA. As a result of these mechanical interrelationships between knee cartilage thickness, frontal plane knee angulation and intra-compartmental compression forces, medial knee compartment OA not only can be caused by genu varum deformity, but genu varum deformity can be also be caused by medial knee OA. The inverse applies in that lateral knee OA can be caused by genu valgum deformity and genu valgum deformity can be caused by lateral knee OA.

Because a lateral location of GRF relative to the knee joint can cause an increase in lateral knee joint intra-compartmental forces and a medial location of GRF can cause an increase in medial knee joint intra-compartmental forces, then any in-shoe modifications that can cause lateral or medial shifting in the plantar location of GRF may also reduce the load on the affected knee joint compartment. The therapeutic effects of in-shoe frontal plane wedges for medial and lateral knee OA have been known for the last few decades.

For example, orthopedic surgeons commonly use valgus insoles in the shoes of patients with medial compartment knee OA (Sasaki T, Yasuda K. Clinical evaluation of the treatment of osteoarthritic knees using a newly designed wedged insole. Clin Orthop, 215:181, 1987). Recent research has confirmed the positive changes in knee mechanics and knee symptoms that can occur with appropriate application of these wedges (Van Raaij TM et al: Medial knee osteoarthritis treated by insoles or braces: A randomized trial. Clin Ortho Rel Res, 468:1926-1932, 2010).

One of the most obvious complications that can occur with the use of valgus or varus-wedged insoles in treating patients with knee OA is that, in an attempt to alter the loading forces within the knee, these insoles may also cause the undesirable effect of placing abnormal frontal plane forces on the foot and ankle. For example, a valgus-wedged insole used to treat medial knee OA may cause increased pronation of the foot while increased supination of the foot may occur with use of a varus-wedged insole to treat lateral knee OA. Therefore, the clinician that uses these valgus and varus-wedged insoles must also be aware of any foot pathologies that may result from their use over time.

I have been treating patients with medial and lateral compartment knee OA now for the past 30 years with valgus-wedged or varus-wedged custom foot orthoses and have found them to be highly successful at reducing knee pain and increasing overall mobility in these patients. For patients with medial knee OA, a 3/16” polypropylene orthosis is commonly ordered along with a 2-3 mm lateral heel skive, a 3-5 degree everted balancing position, a flat rearfoot post, extrinsic lateral plantar arch filler and a valgus forefoot extension to effectively shift GRF more laterally on the plantar foot and decrease the pain within the medial knee compartment during their daily activities.

For patients with lateral knee OA, a 3/16” polypropylene orthosis may be ordered along with a 3-4 mm medial heel skive, minimal medial arch fill, a 3-5 degree inverted balancing position, and a standard rearfoot post to effectively shift the GRF more medially on the plantar foot and decrease the patient’s weightbearing pain in the lateral knee. Not only do these specialized foot orthoses tend to work extremely well at nearly immediately easing knee pain with ambulation in patients with medial or lateral knee OA, they also create a minimum of other mechanical issues within the foot and lower extremity during weightbearing activities.

[Reprinted with permission from November 2010 Precision Intricast Newsletter published in: Kirby KA: Foot and Lower Extremity Biomechanics IV: Precision Intricast Newsletters, 2009-2013. Precision Intricast, Inc., Payson, AZ, 2014. pp. 105-106.]

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