A Journey in Subtalar Joint Biomechanics

I have been fascinated by the biomechanics of the subtalar joint since I was a podiatry student in the early 1980s. As podiatry students, we learned that the subtalar joint had a fixed, hinge-like axis that was angulated 16º from the sagittal plane and 42º from the transverse plane. We also learned that since the subtalar joint axis was angulated to all the cardinal body planes, pronation and supination would result in a “triplane motion” of the foot relative to the leg.

Subtalar joint biomechanics seemed fairly straightforward when I was a podiatry student. Little did I know at the time how things would change for me over the coming years regarding my appreciation of the biomechanics of this important pedal joint.

There were two significant events in my Biomechanics Fellowship at the California College of Podiatric Medicine (CCPM) (1984–1985) that piqued my interest in the subtalar joint. First of all, in 1984, much to my delight while perusing new releases in the CCPM library, I came across an amazing PhD thesis by an orthopedic surgeon from the Netherlands, Evert Van Langelaan, MD, PhD.¹ His thesis demonstrated that nearly all I had been taught about that subtalar joint and its biomechanics had been wrong.

In his ground-breaking research, Dr. Van Langelaan inserted small metallic beads into the tibia, talus, calcaneus, navicular, and cuboid in 10 foot-leg cadaver preparations, and mounted these cadaver preparations in a motor-driven apparatus that rotated the tibia on the foot. Then, using two X-ray units angulated 90º to each, Dr. Van Langelaan was able to very precisely determine how the subtalar joint axis moved in space for each 5º of transverse plane rotation of the tibia.

Instead of the fixed, hinge-like axis of the subtalar joint that we learned as podiatry students, Dr. Van Langelaan eloquently demonstrated in his landmark research that the subtalar joint was not fixed in space but instead rotated and translated to a new spatial location with each small increment of pronation and supination motion, resulting in a “bundle” of discrete joint axes piercing anteriorly through the dorsal neck of the talus. These research revelations from 39 years ago, which have been confirmed by later research by Benink, Lundberg and Svensson,^2,3 showed there is much more complexity to subtalar joint biomechanics than what we learned in podiatry school.

In other words, if podiatrists still teach or think that the subtalar joint axis functions as a fixed, non-moving hinge in space, then they are, simply, quite wrong.

The second significant event during my Biomechanics Fellowship came when I had the opportunity to talk with John Weed, DPM, one of my former biomechanics professors. My question to him one day during the latter part of 1984 was to ask how he determined which feet would need to have extra “pronation-control” measures such as deeper heel cups or longer rearfoot posts added to his custom foot orthoses. I remember that Dr. Weed replied that he would simply press on the bottom of the heel of his patient with his thumb and that if he had to press very far medially on the patient’s plantar heel to supinate the subtalar joint, then he would order extra “pronation-control” measures into that patient’s custom foot orthoses.

Dr. Weed’s “heel-pushing technique” was totally new to me, since no one had taught us this clinical examination technique of pushing on the plantar heel to determine where subtalar joint supination occurred. I distinctly remember immediately going next door into the biomechanics clinic and trying this technique on a number of the podiatry students. Later on, in the next few months of experimentation, I modified Dr. Weed’s technique by moving further anteriorly on the plantar foot with my manual pushing force to determine the plantar location of the subtalar axis. Therefore, due to Dr. Weed’s simple suggestion of his clinical technique to me one day, a new technique for clinical determination of the subtalar joint axis resulted.

These personal stories from nearly four decades ago indicate that, sometimes, all it takes is a little inspiration from one or two individuals to light the fire in a young researcher. For me, the influence that Drs. Van Langelaan and Weed had on my research interests over the next few decades were very important. My hope is that other young podiatrists will also have the fire lit inside of them in the coming years to research the complexities of foot and lower extremity biomechanics. Their profession and their patients will definitely all benefit.

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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References

1.    Van Langelaan. A kinematical analysis of the tarsal joints: an X-ray photogrammetric study. Acta Orthop Scand. 1983; 54(suppl):204.
2.     Benink RJ. The constraint mechanism of the human tarsus. Acta Orthop Scand. 1985; 56(suppl):215.
3.     Lundberg A, Svensson OK. The axes of rotation of the talocalcaneal and talonavicular joints. Foot. 1993; 3:65.
4.     Kirby KA. Methods for determination of positional variations in the subtalar joint axis. J Am Podiatr Med Assoc. 1987; 77:228–234.