04/08/2024
Why the Term "Overpronation" is Inaccurate and Meaningless
In 2012, a friend and podiatric colleague of mine from the UK, Ian Griffiths, wrote a very thought-provoking article on the term overpronation (Griffiths I. Overpronation: Accurate or parachronistic terminology. SportEx Dynamics. 32:10-13, 2012). In this article, Griffiths describes the reasons why the term "overpronation" is an inaccurate and meaningless term and why it should no longer be used by podiatrists. The purpose of this newsletter is to expand on many of the points made in his excellent article.
Pronation, as a type of foot motion, was first described within the English scientific literature 135 years ago by Johnson Symington (1851–1924) who was a British anatomist, zoologist and physician (https://en.wikipedia.org/wiki/Johnson_Symington). In his 1884 paper, “Anatomy of Acquired Flat-foot”, Symington very clearly describes the motions of the talus relative to the calcaneus and the “overpronation” that occurs at this and other joints of the foot to create a flatfoot deformity (Symington J: Anatomy of acquired flat-foot. Journal of anatomy and physiology. 19(Pt 1):82-93, 1884).
In more modern times, the term "pronation" refers to a tri-planar movement of the foot where a distal part dorsiflexes, abducts and everts relative to a proximal part of the foot. In the first textbook published by Dr. Merton Root and coworkers in 1971, these authors define pronation as “A complex triplane motion consisting of simultaneous movement of the foot or part of the foot in the direction of abduction, eversion and dorsiflexion. The axis of this motion passes through the foot from posterior, lateral and plantar, to anterior, medial and dorsal (Root ML, Orien WP, W**d JH, RJ Hughes: Biomechanical Examination of the Foot, Volume 1. Clinical Biomechanics Corporation, Los Angeles, 1971, p.10).
At the subtalar joint (STJ), pronation means that the calcaneus dorsiflexes, abducts and everts relative to the talus, and supination means that the calcaneus plantarflexes, adducts and inverts relative to the talus. Unlike many other pedal joints, the STJ is a relatively constrained joint, meaning that the calcaneus will tend to rotate on the talus in a relatively predictable direction, regardless of the magnitude, direction and point of application of the external force applied to the foot. This high amount of joint constraint seen within the STJ is likely due to the thick, tight ligamentous structures deep within the STJ which hold the three articulating facets of the STJ in close apposition to each other. As a result, pronation and supination are still very meaningful terms to describe STJ rotational motions (Huson A: “Functional anatomy of the foot.” In Jahss MH (ed): Disorders of the Foot and Ankle. W.B. Saunders Co., Philadelphia, 1991, pp. 409-431).
However, at the midtarsal joint (MTJ) and midfoot joints, the term "pronation" is more problematic since the motions at these joints are less constrained than at the STJ. The reason that MTJ motion is relatively less constrained is due to the fact that the ligaments holding the talo-navicular and calcaneo-cuboid joints together are not thick ligaments deep within the joint, but rather are ligaments that only exist at the joint margins. As a result, the MTJ has a relatively large range of motion (ROM), with the forefoot tending to move in the direction of the external force applied to the forefoot, rather than always tending to move in the same direction no matter how the external force is applied to the forefoot (Huson, 1991).
In other words, for the MTJ and midfoot joints, due to their relative loose ligamentous restraints, these joints do not always undergo triplane pronation-supination motions. Depending on how much external force is applied to a foot and in what direction that force is applied to the foot, the MTJ or midfoot joints may rotate in any number of directions, and not just in dorsiflexion-abduction-eversion (i.e. pronation) and in plantarflexion-adduction-inversion (i.e. supination). This fact creates significant issues when one attempts to use the terms "pronation" and/or "supination" accurately and meaningfully to describe motion at any pedal joint other than the STJ.
Another big problem with using the term "overpronation" to describe abnormal motion or position of the foot, is that we first must define and agree upon what normal actually means. Unfortunately, the podiatric term "normal", is not the same normal used by the rest of the medical profession. The podiatric belief of what normal means probably comes from the ideas of Root and colleagues who defined their normal foot as needing to adhere to all of their Eight Biophysical Criteria for Normalcy. These eight criteria, which includes structural alignment and joint position parameters for the leg and foot, were meant to represent “the ideal physical relationship of osseous segments of the foot and leg for the production of maximum efficiency during static stance and locomotion” (Root et al, 1971, p. 34). Therefore, Root et al’s idea for normal meant an ideal structure and alignment, not an average foot structure or alignment in an asymptomatic population.
If we needed to bring podiatry back to how mainstream medicine views normal, we would first need to have a large normative database which accurately describes the normal ROM of the STJ for us to confidently say that a foot has "overpronation". However, a normative database such as this does not currently exist. If we did have such a database on the STJ pronation ROM, for example, from a study of 10,000 subjects, we might end up with what is called in statistics, a “normal curve”. If we determined “normal” to be one standard deviation from the mean value of STJ pronation with this “normal curve”, then that would include 68% of the population of feet. If we determined “normal” STJ pronation to be two standard deviations from the mean, then that would include 95% of the population of feet measured had "normal" pronation (Moore DS, McCabe GP: Introduction to the Practice of Statistics, 3rd ed., W.H. Freeman and Co., New York, 1999, pp. 70-79).
For most laboratory values in the medical world, the “normal values”, or “reference range”, are defined to be 95% of the population, or within two standard deviations of the mean (Boyd JC. Defining laboratory reference values and decision limits: populations, intervals, and interpretations. Asian J Andrology, 12(1):83-90, 2010). In other words, values ranging from 0 to 2.75% of the 10,000 subjects measured for STJ pronation ROM would be considered abnormally low pronation values and values ranging from 97.5-100%, would be consider abnormally high pronation values, or possibly, "overpronation". Only once we have developed this type of database, could we confidently say that an individual suffered from "overpronation". Since we aren’t even close to being there yet in podiatry, and for the other reasons mentioned above, the term "overpronation" remains an ill-defined, inaccurate and meaningless term which should not be used either within or outside the podiatry profession.
{Reprinted with permission from: Kirby KA, Precision Intricast Newsletter, Precision Intricast, Inc., Payson, Arizona, May 2019.]
