Have an increasing number of anomalies led to legitimate challenges of classically taught principles of podiatric biomechanics? Suggesting that flaws in long-held tenets in biomechanics and an overreliance on static perceptions of a normal foot and gait have led to confusion and misperceptions, this author says an improved paradigm that reflects an evolving understanding of pathomechanics may lead to more practical applications for patient care.
In a classic essay, Kuhn used the term paradigm to describe the set of beliefs generally held by the majority of scientists in a particular field of study. What makes a paradigm useful is the predictions one can make from it.1 However, Kuhn proposed that science does not progress entirely in a linear manner but rather in repeated revolutions that are interspersed with periods of normal growth.
An example of an obvious revolution is the shift in astronomy from an early perception that the Earth was the center of the solar system to the eventual understanding that the sun is the center of the solar system. Prior to the work of Copernicus, astronomers followed Ptolemy’s teachings that the Earth was the center of the universe.2 New astronomers made additional observations and new instruments such as the telescope improved their ability to make these observations.
With continued observation, it became more and more difficult to reconcile new data with the old paradigm. Kuhn termed these new observations anomalies.1 Paradigms had to be altered or added to for them to make sense. Kuhn noted that increased recognition of anomalies precedes scientific revolutions as the current paradigm becomes less and less successful at describing nature. Kuhn called this buildup of anomalies a crisis.1
With this in mind, let us take a closer look at the paradigm of podiatric biomechanics and how it fits into Kuhn’s descriptions of how science progresses.
It is often hard to articulate an exact definition of podiatric biomechanics. However, it is important to try to articulate the paradigm of podiatric biomechanics in order to formulate research questions and assess usable concepts with which to make predictions on how to treat the foot. There are many different schools of thought on podiatric biomechanics that are marked by different ideas of how the foot works.3 Kuhn recognizes a proliferation of theories as another sign of an academic field being in crisis.1 When there are many variations on a basic theory, it means few agree on what the commonly accepted belief should be.
What Does Classic Biomechanics Tell Us About Lower Extremity Function?
For a starting definition of podiatric biomechanics, I will select the theories John Weed, DPM taught at the California College of Podiatric Medicine. One of the first things he taught was the biophysical criteria of normalcy. These criteria are a list of what he and his peers thought the ideal normal foot should be in static stance. This list appears to be something that Root, Orien and Weed invented as there is no citation of a source in in their classic text nor is there a citation in Dr. Weed’s lecture syllabus.4-6 Another of Weed’s podiatric biomechanical theories includes the idea that there is an ideal normal gait and motions different from that normal gait lead to pathology.4-6
After introducing the normal foot, Weed taught about deformities (e.g. rearfoot varus, forefoot valgus) that deviated from the ideal normal foot. These deformities caused compensations that supposedly lead to pathology.4-6 He also taught that abnormal pronation during gait leads to pathology. Weed and colleagues defined abnormal pronation as “when the amount of pronation, during any period of locomotion, becomes excessive or if any pronation occurs at a time when the foot should be supinating.”4 Saunders, Inman and Eberhart most closely matched this idea of normal gait in their observational study.7 Their work described these observations but no work went into determining whether gait that deviated from normal was more likely to have pathology, or if pathology could occur if someone was walking with an idealized normal gait.
The introduction of the concept of axis of rotation took place as a result of the discussion of idealized motions and position of the foot and joints.7 This is important because many of the corollaries of podiatric biomechanics are based on misconceptions about the axis of rotation. An axis of rotation is an imaginary line scientists and clinicians use to describe the occurrence of motion.8 An imaginary line cannot constrain motion. When Root, Orien and Weed presented the concept of the axis of rotation, they did so with a picture of a hinge with different angulations in space.4 A physical hinge has a pin and knuckles wrapped around the pin that apply forces that constrain motion. The screws that fasten the leaves of the hinge to the door jamb also apply forces that constrain motion.
In the foot, there are some joints that constrain motion better than others. One of the things that can constrain motion are forces applied at joint surfaces. A good example is when one looks at the anterior-posterior view of the ankle, the top of the talus looks flat. When compressing the ankle joint together by ground reaction force and body weight, any attempt to tilt the talus relative to the tibia would cause increased force on one side of the joint. However, on the sagittal view of the ankle joint, the top of the talus appears curved. One would not see similar compressive forces at the joint preventing motion in the sagittal plane.
Inman showed that the talar dome has the shape of part of a surface of a cone.9 Mathematically, a cone has an axis. Therefore, the surfaces can constrain motion about a single axis of motion. Cahill proposed that the surfaces of the subtalar joint are consistent with two cones pointed toward each other.10 One can expect both the ankle and subtalar joint to perform like a hinge, and have a fixed axis of rotation when the joint surfaces are compressed together by ground reaction force and body weight. In the case of the subtalar joint, there is a bundle of axes with roughly the same orientation.8 This is similar to what one would expect from a hinge of a door not tightly screwed to the door jamb.
Other joints do not have surfaces that can constrain motion around a fixed axis. The hip, talonavicular and first metatarsocuneiform joints have the surface shape of a sphere. In these joints, the joint capsule limits motion. Which part of the joint capsule limits the motion depends on which direction the joint is moved. Also, the axis of motion that one sees at these joints is dependent upon the forces used to move the structures. For example, when attempting to move the forefoot relative to the rearfoot in abduction and adduction, you will see motion that has a vertical axis different from the published axis of motion of the oblique or longitudinal axes of the midtarsal joint. In fact, the motion one sees at the midtarsal joint during gait is about an axis that is neither the longitudinal or oblique axis.11 Clinicians should expect this result if they remember that an axis of rotation is an imaginary line that describes the motion in a joint where the joint surfaces are not in a shape proficient at constraining motion.
Does Rigid Belief In A Paradigm At Times Limit An Evolving Understanding Of The Forces That Affect Motion?
Kuhn pointed out how a paradigm can limit what practitioners in a field accept as reality.1 I witnessed an example of how a rigid belief in a rigid axis limits understanding between practitioners during two recent virtual presentations from the
2020 Western Foot and Ankle Conference regarding first metatarsocuneiform and naviculocuneiform fusion of the first ray. One presenter cited multiple studies that said the first ray had an axis of motion that allowed simultaneous dorsiflexion, abduction and inversion, and therefore could not have a pronation-supination axis. Another presenter described how he positioned the metatarsal prior to fusion and showed a fluoroscopic video of him moving the metatarsal about an axis of motion. That axis included plantarflexion, adduction and inversion, which is motion about a pronation-supination axis.
One can reconcile these two conflicting observations by remembering that an axis of motion is an imaginary line that describes motion and the joint surfaces and ligaments of the first ray do not constrain motion to a single axis. Even though there are articles in the literature that acknowledge observation of a certain motion, it is still possible to have other motion. The motion one sees at both of these joints is dependent upon the forces applied to the bones on either side of the joint.
Therefore, when studying the cause of bunions, one should look at the forces that cause the first metatarsal head to move away from the second metatarsal head and look at the anatomical structures that can apply forces resisting that motion. Researchers have described and experimentally verified the forces that cause rotation of the metatarsal.12,13
Challenging Misperceptions About Axis Rotation And The Midtarsal Joint
Another area of podiatric biomechanics where misinterpretation of axis rotation as a rigid hinge is problematic is the explanation of why there is an increase in range of motion of the midtarsal joint with movement of the subtalar joint from a more supinated position to a more pronated position. The explanation involves the talonavicular and calcaneocuboid joint axes, but this theory has been thoroughly debunked by Van Langelaan.8 Previous teachings contended that this increase in range of motion makes the foot unstable and this instability leads to problems.4 Characterization of these problems include the foot becoming a loose bag of bones or the midtarsal joint unlocking.4 This is a good example of Kuhn’s description of an anomaly because new research calls into question one of the underlying assumptions of the paradigm.
An interesting analogy from the history of science is the early electrical research that led to the development of the Leyden jar. A Leyden jar is essentially a capacitor that can store an electric charge.14 At the time, researchers believed that electricity flowed like a liquid and tried to catch it in a jar. They were wrong about how electricity worked but they were successful in storing an electric charge. Even though they used a faulty paradigm, they still had success in making a discovery.
Weed taught that a functional foot orthosis would “support a forefoot deformity” and “resist abnormal intrinsic or extrinsic forces that would cause excessively medial or lateral distribution of weight into the rearfoot during the time the heel is bearing weight.”5,6 The idea underlying the paradigm is that pronation of the subtalar joint causes instability and the orthosis design should limit pronation and support deformities that could cause subtalar joint pronation. The flaw is in the underlying explanation of why pronation is a problem. Yet we see treatment successes with orthoses designed to attempt to supinate the subtalar joint and with wedges that support a deformity. Perhaps, this is like the aforementioned discovery of the Leyden jar. We have found success but for the wrong reasons.
Kuhn describes the period between scientific revolutions as one of normal science in which practitioners in the field make more measurements and observations that hopefully fit the current paradigm.1 Since the proposition of the idea that the subtalar joint being in a pronated position causes instability, we should have seen studies that looked at the position of the subtalar joint in symptomatic and asymptomatic populations. If pronation of the subtalar joint caused problems, we should see that in those studies. I believe there are no studies demonstrating pathology related to a pronated position of the subtalar joint. There is a difference between what a podiatrist would call a pronated foot and a foot with a pronated subtalar joint. When I was teaching, I had the opportunity to perform many stance measurements on asymptomatic podiatry students and I did not see much difference between them and the patients I saw in clinic.
Raising Issues That May Have Contributed To Misconceptions About What Constitutes A Pronated Foot
In addition to teaching about subtalar joint position, Weed discussed other factors that could lead to the need for more anti-pronation features in the orthosis.5,6 A lot of these factors (such as talar head adduction or excessive calcaneal eversion) are all things that led to the foot tending to sit more lateral to the leg than a foot that did not have these factors. This led to a common misconception among students who would describe these things as a pronated foot. A pronated foot is different from a pronated subtalar joint but these things got conflated in common usage.
I think Weed got the right idea with his analysis of the foot being lateral to the leg being likely to create more pronation. However, the flaw in his analysis was that he only assessed the rearfoot in a single frontal plane slice when the foot is a three-dimensional object. However, the focus on position of the joint makes one think of more pronation in terms of joint position. What the foot being lateral to the leg does is make the foot pronate harder. It takes a different way of looking at biomechanics to understand what pronating harder means.
The point here is that Weed looked at morphology, motion and joint position, but did not examine the forces that cause those motions. Biomechanics is the application of mechanical laws to living structures. For example, Newton’s 2nd law for angular motion says that moment is equal to moment of inertia times angular acceleration. In a list labeled “Medial Forces Causing Pronation,” Weed included rearfoot valgus and talar head adduction.5,6 These are not forces. They are positions.
There are available tools that one can use to look at the forces and moments applied to the foot. Clinicians may use the position of the center of pressure relative to the position of the subtalar joint axis to calculate the moment from ground reaction force.15 This is what one uses to calculate how hard a foot is pronating. Additionally, inverse dynamics and finite element analysis can help to calculate the forces on individual anatomical structures. Clinicians can use that information to design treatments to reduce those forces.16-20
Contemporary Applications Of A Changing Paradigm
I am not saying we should abandon something that is successful. What I am saying is that we should change how we explain why something is successful. The paradigm of comparing a foot to an idealized normal foot or someone’s gait to normal gait gave us some useful tools for treating patients. For example, we know that a forefoot valgus wedge can reduce tension in the plantar fascia.21 However, the paradigm limits the concepts and tools we use to treat the foot. Using the best paradigm is important because it provides better guidance in treating patients and choosing which research questions to ask.
There is a lot of talk of using biomechanics in surgical decision making. Take the example of a patient with diabetes who has had one metatarsal head resected and has osteomyelitis in another metatarsal head. Should you try to make the foot look more normal and just resect the infected metatarsal head? Alternatively, should you try to reduce stress on the distal aspect of the foot with a transmetatarsal amputation?
Researchers have looked at the motion of the foot during normal walking to see if people with frequent ankle sprains walk differently than those who don’t have ankle sprains. We don’t really want to know whether the patients’ gait is normal or not when they do not sprain their ankle. We want to know what happened when they sprained their ankle. The forces and moments at that instant are what caused the ankle to sprain.
We don’t need to decide whether a patient has a forefoot valgus but we do need to decide how much forefoot valgus wedge is the right amount to lower tension in the plantar fascia. This is a different way of looking at how to treat the foot. It just does not make sense to use a non-weightbearing measurement, made in a foot position different from what the patient will be in when standing, to determine the amount of wedge to use.
I have provided some examples of why I feel podiatric biomechanics is a paradigm in crisis. The paradigm is not providing enough useful answers to the questions that practitioners have. I feel there is much more promise in the tissue stress approach to treatment of the foot. Identify the injured structure. Design a treatment to lower stress on that structure to allow it to heal. Then reevaluate and modify treatment as necessary.22,23
Dr. Fuller taught biomechanics at the California College of Podiatric Medicine for 14 years. He is in private practice in Berkeley, Calif.
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13. Fuller EA. The windlass mechanism of the foot. A mechanical model to explain pathology. J Am Podiatr Med Assoc. 2000;90(1):35-46.
14. MIT Libraries. The Leyden jar. Available at: https://libraries.mit.edu/collections/vail-collection/topics/electricity/the-leyden-jar/ . Accessed July 28, 2020
15. Fuller EA. Center of pressure and its theoretical relationship to foot pathology. J Am Podiatr Med Assoc. 1999;89(6):278-291.
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19. Winter DA. Concerning the scientific basis for the diagnosis of pathological gait and for rehabilitation protocols. Physiotherapy Canada. 1985;37(4):245- 252.
20. Morlock M, Nigg BM. Theoretical considerations and practical results on the influence of the represeantation of the foot for the estimation of internal forces with models. Clin Biomech. 1991;6(1):3-13.
21. Kogler GF, Veer FB, Solomonidis SF, Paul JP. The influence of medial and lateral placement of orthotic wedges on loading the plantar aponeurosis. J Bone Joint Surg Am. 1999:81(10):1403-1413.
22. Kirby KA, Fuller EA. Subtalar joint equilibrium and tissue stress approach to biomechanical therapy of the foot and lower extremity. In: Albert SF, Curran SA, eds. Lower Extremity Biomechanics: Theory and Practice. Greenwood Village, CO: Bipedmed Publishing; 2013.
23. Fuller EA. A guide to orthotic prescription writing with the tissue stress theory approach. Podiatry Today. 2018;31(2):40-47.