T.E. #5: Effect of Foot Orthoses on Sinus Tarsi Compression Force
In Thought Experiment #5, a foot with a medially deviated STJ is illustrated (left) along with the same foot, now with a foot orthosis underneath its plantar arch (right). The foot has a medially deviated STJ axis so that the medial weightbearing surface of the foot is 3 cm medial to the STJ axis and the lateral weightbearing surface is 5 cm lateral to the STJ axis. As in Thought Experiments #4, included within this foot model is a buttress of bone on the foot that is 2.0 cm lateral to the STJ axis in the area of the sinus tarsi of the foot. This buttress of bone on the foot acts as a "bumper" to prevent further STJ pronation past the maximally pronated position and is meant to model the approximate anatomical location of where the lateral process of the talus abuts against the floor of the sinus tarsi of the calcaneus when the maximally pronated position of the STJ is reached.
Both the medial and lateral weightbearing surfaces of the foot have a ground reaction force of 200 N in the foot without an orthosis (left). In the foot with a foot orthosis (right), the orthosis has been designed to shift the reaction forces on the plantar foot to a more medial location. The orthosis reaction force (ORF) can be thought to act with a magnitude of 200 N at a point 0.9 cm medial to the STJ axis (i.e. the center or pressure of the orthosis is located 0.9 cm medial to the STJ axis). Note that since the foot orthosis is now bearing half of the plantar forces from the foot, the medial and lateral weightbearing surfaces of the foot now each have 100 N of GRF (right).
The vertically-directed compression force acting through the STJ axis is F1 and the vertically-directed compression force acting through the sinus tarsi is F2 in the foot without an orthosis (left). In the foot with an orthosis acting on its plantar aspect (right), the vertically-directed compression force acting through the STJ axis is F3 and the vertically-directed compression force acting through the sinus tarsi is F4. Both feet are given to be in translational and rotational equilibrium.
Here are my questions for Thought Experiment #5:
1. What is the magnitude of compression forces acting at the STJ axis (F1) and sinus tarsi (F2) in the medially deviated STJ axis foot without an orthosis (left)?
2. When a foot orthosis is added to the plantar aspect of the foot with a medially deviated STJ axis (right) so that it shifts the reaction force on the plantar foot in a medial direction, what is now the magnitude of compression forces acting at the STJ axis (F3) and at the sinus tarsi (F4)?
3. Why does the foot orthosis cause a change in the compression forces acting at the sinus tarsi (F2 and F4)?
4. Why does the foot orthosis cause a change in the compression force acting across the STJ axis (F1 and F3)?
5. Has the foot orthosis altered the rotational position of the STJ axis of this medially deviated STJ foot (i.e. has the foot moved out of the maximally pronated STJ position as a result of putting an orthosis underneath it)?
6. If the foot orthosis has not caused STJ supination (i.e. has not moved the foot out of its maximally pronated position), how can it possibly cause a change in the magnitude of compression forces within the sinus tarsi and possibly reduce the symptoms within the sinus tarsi in a patient that has a medially deviated STJ axis?
7. What does this mechanical analysis seem to indicate regarding the common notion among clinicians and past and present biomechanics researchers that attempt to use solely foot joint rotational position or foot joint motion (i.e. kinematics) in an attempt to predict injury within the human foot?
8. What do clinicians and researchers miss in regard to understanding mechanically-based foot and lower extremity injuries when they don't take into account the magnitude and locations of external forces acting on the plantar foot relative to spatial location of joint axes in the human foot?
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
107 Scripps Drive, Suite 200
Sacramento, CA 95825 USA
Voice: (916) 925-8111 Fax: (916) 925-8136