Why The Term ‘First Ray Hypermobility’ Should Be Extinct

Over 80 years ago, Dudley J. Morton, MD, first coined the term “hypermobility of the first metatarsal segment” in his classic book on functional disorders of the human foot.¹ In his book, Morton described that “if the plantar ligaments of any segment are slack when the head of its metatarsal bone lies on the same plane as the others whose ligaments are taut, that segment will fail to share in the carriage of body weight” and will become “ineffective as a weightbearing structure” with “an unusual amount of motion” occurring “between the medial and middle cuneiform bones.”

In 1971, Merton L. Root, DPM, coauthored his first book on foot and lower extremity biomechanics, and defined the term “hypermobility” as “any motion occurring in a joint, in response to gravitational force, at a time when that joint should be stable under such force.”² More specifically in regard to first ray biomechanics, Root and coauthors defined hypermobility of the first ray in 1977 as “a state of abnormal first ray instability that occurs while the forefoot is bearing weight.”³

Ever since Dr. Root and colleagues expanded on Morton’s description of first ray hypermobility, the podiatric profession has done little to refine or further clarify that definition for future generations of podiatrists. Like many clinical specialists, podiatrists tend to pass on ideas, techniques and terms from one generation of practitioners to another, sometimes with little thought being given as to whether those ideas, techniques and terms are based on good research evidence and/or are consistent with modern medical and scientific principles.

Unfortunately for the modern podiatrist, within the international engineering and biomechanics communities, the term “hypermobility” does not describe the mechanical or biomechanical characteristics of an object. Why? “Hypermobility” is an imprecise and unquantifiable term that one should not use to describe the load versus deformation characteristics of any object, let alone the first ray of the foot.

The term that engineers and biomechanists around the world use to describe the load versus deformation characteristics of an object is “stiffness.” Stiffness is defined as the amount of force required to produce a given amount of deformation.⁴ The greater the force required to deform an object a given amount, the greater the stiffness of that object. Accordingly, instead of using the clinical term “first ray hypermobility,” a more accurate and mechanically quantifiable term to describe the load versus deformation characteristics of the first ray is “reduced first ray dorsiflexion stiffness.”⁵

The term “stiffness” is not an unusual or esoteric scientific term that few people use in everyday language. In fact, people in non-podiatric circles use the word “stiffness” much more commonly than the term “hypermobility.” People may say that the shock absorbers on one vehicle are stiffer than those on another vehicle, causing a harsher ride. Podiatrists may even talk about how a shoe sole is stiffer than another sole or may discuss how one foot orthosis material is stiffer than another orthosis material. We don’t say that our shock absorbers, shoe soles or foot orthoses are “hypermobile.” Do we?

Regardless of these facts, podiatrists still routinely use “hypermobility” to describe the load versus deformation characteristics of the first ray. By continuing to use “hypermobility” to describe first ray biomechanics in their articles, books and lectures, podiatrists are ignoring the use of standard scientific terminology to describe load versus deformation mechanics. In other words, by not replacing an outdated, inaccurate clinical term that was invented when the cost of a loaf of bread was 8 cents and the cost of a gallon of gas was 10 cents, our podiatric profession is simply being intellectually lazy.

In conclusion, “first ray hypermobility” is an invalid and unquantifiable term, which podiatrists still use to describe how much force is required dorsiflex the first metatarsal. It is a dinosaur of a term that is left over from when almost no foot and lower extremity biomechanics research was being published, and when our understanding of foot and lower extremity biomechanics and foot and ankle surgery was very primitive. For this reason, for the good of the podiatric profession, the term “first ray hypermobility” should become as extinct as the brontosaurus.

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.

References

1. Morton DJ. The Human Foot: Its Evolution, Physiology and Functional Disorders. Columbia University Press. Morningside Heights, New York, 1935, pp.187-195.
2. Root ML, Orien WP, Weed JH, RJ Hughes. Biomechanical Examination of the Foot, Volume 1. Clinical Biomechanics Corporation, Los Angeles, 1971, p. 24.
3. Root ML, Orien WP, Weed JH. Normal and Abnormal Function of the Foot. Clinical Biomechanics Corp., Los Angeles, CA, 1977, pp. 350-351.
4. Nigg BM, MacIntosh BR, Mester J (eds.). Biomechanics and Biology of Movement. Human Kinetics, Champaign, IL, 2000, p. 155.
5. Kirby KA, Roukis TS. Precise naming aids dorsiflexion stiffness diagnosis. Biomechanics. 2005;12 (7): 55-62.