A Closer Look At The Principles Of Fluid Dynamics As They Relate To Orthoses
The fluid support effect on foot function suggests that stability at the midtarsal joint complex is primary and stability at the subtalar joint complex is secondary. Once the midtarsal joint is fully loaded and stable, rearfoot pronation is already limited. This concept has support in a 2008 study about invasive in vivo measurement of rear-, mid- and forefoot motion during walking.3 The study showed that “the movement at the talonavicular joint was greater than at the talocalcaneal joint and motion between the medial cuneiform and navicular was greater than expected.”
Sustaining arch stability in the sagittal plane throughout mid-stance directly affects pronation at the midtarsal joint as well as the subtalar joint and forefoot. A study of “the midtarsal joint locking mechanism” demonstrated that motion in the forefoot is influenced by the hindfoot position through the midtarsal joint.4 There is no need for cupping the heel to aid in the control of rearfoot motion. Additionally, McPoil and Cornwall did not support rearfoot posting to control pronatory forces in a study comparing a posted orthosis and non-posted orthosis.5
A traditional shell orthotic fits by cupping the heel to just behind the metatarsal heads. Pronation control is static under the midtarsal joint complex from heel contact throughout midstance phase until forefoot contact. Then, the forward progression of the plantar surface of the foot momentarily halts, as GRF of the forefoot increases and decelerates against the ground.
Now, the transference of the forward momentum and sagittal force on the foot coupled with the weightbearing and pronatory forces of the midtarsal joint complex against the position and surface of the orthosis at the end of midstance, disengages the metatarsophalangeal joint (MPJ) complex, just enough to allow for forefoot pronation, through the casted orthotic position.
This last phase of intrinsic pronation at the forefoot continues through the moment of heel-off under the weightbearing and pronatory forces of the midtarsal joint complex and destabilizes to increase the flexibility of the foot’s structure. Considering that pronation movement has a retrograde effect that occurs in all three segments (forefoot/midfoot/rearfoot), if any segment even partially unlocks or destabilizes, re-supination to a stable-neutral position is questionable. I will refer to this as the optimal position.
Assessing The Effects Of Fluid Mechanics And Biomechanics
My clinical experience and observations comes with the use of the Dynamic Pedobarograph in the early years and later with Tekscan/Matscan (Tekscan, Inc.) tests taken at marathons and triathlons.
The following is my clinical assessment of the fluid mechanical effects and biomechanics. When an individual steps onto the silicone dynamic orthotic, there are four areas in which biomechanical loading occurs.
1. The silicone orthosis has its initial effect on rearfoot pronation movement at the moment of heel contact with the orthotic. Silicone near the back of the template immediately dampens heel strike and begins reducing the velocity of pronation as the foot continues its forward progression.
Can an orthosis control motion of the foot? Kinematic studies on the “hindfoot” have demonstrated that “reduction in maximal velocity of pronation may be the key to biomechanical alteration to result in 60 to 70 percent satisfaction rate from patients who wear orthoses.6
2. At the beginning of mid-stance, as silicone volume and pressure builds under the mid-tarsus, the fluid displaces under pressure from its most posterior position in the template under the tarsus and along the medio-longitudinal column, loading to fill the arch.
3. At forefoot contact, the position of the first metatarsal loads by the pressure of the fluid into a plantarflexed state. The fluid forms as a dynamic fulcrum under the midtarsal joint complex as silicone simultaneously fills under the transverse arch, loading the tarsus and tarso-metatarsus.