A Closer Look At The Principles Of Fluid Dynamics As They Relate To Orthoses
Fluid technology may enhance foot alignment and dynamic stability for walking and running. This author draws on 20 years of empirical experience to provide insights on fluid dynamic orthosis technology, including silicone dynamic orthotics.
Approximately 20 years ago, the late Martin Krinsky, DPM, originated his concept of externally replacing the plantar fat pad that had atrophied from pathological states. His thought was to fill the tarsus completely with a “cushion” of viscous fluid silicone, just enough to allow a minimum but necessary amount of pronation/supination movement. There would be no compartments in the template so the fluid would move under the various loads of the weightbearing and pronatory forces. The volume of fluid would be just enough to enable the primary weightbearing points of the foot during stance phase to have full contact with the ground and full enough to support the arch.
Pressure mat tests first became available (Dynamic Pedobarograph) in 1990 and one was able to see the registered “ground reaction forces” (GRFs) and loading that patients produced barefoot (see figure 1 at right). Krinsky subsequently calibrated a volume of fluid and asked a test subject to repeat a gait cycle wearing the silicone dynamic orthotic in a tight fitting sock to hold it in place so he could see the same barefoot test with orthotic support (see figure 2 at left).
This procedure confirmed a redistribution of the GRF under the total surface area of the foot including the tarsus. Some areas of higher pressure decreased while other areas of lower pressure increased. The average pressure gradient at heel strike was lower, the GRF under the tarsus was very low and evenly distributed, and the GRFs for the entire forefoot were more evenly distributed and essentially reduced. The resulting pressure pattern of the test subject with the silicone dynamic orthotic produced a more balanced gait picture. Additionally, each metatarsal was peaking uniformly closer to the same time in stance.
Krinsky interpreted these changes as more efficient functioning of pronation movement forces. It appeared to Krinsky that in order to produce the resultant balance, the fluid was hydrodynamically supporting the planes of motion at the tarsus.
Krinsky repeated this procedure over a period of six months with a variety of people from all walks of life, sedentary and active, with and without lower extremity problems. In each case, they demonstrated a change in gait efficiency.
Researchers presented a study of the silicone insert (based on Krinsky’s model) at the 21st Annual Meeting of the American Society of Biomechanics.1 The study corroborates Krinsky’s observation and interpretation of the pedobarograph readings. The silicone dynamic orthotic reduced rearfoot pronation movement in both time and pressure at the rearfoot and forefoot as well. “These results suggest that these silicone gel insoles have beneficial effects for managing elevated plantar pressure,” notes the study.
A Closer Look At The Literature
One of the tenets of Merton Root, DPM, (the father of podiatric biomechanics) is that the midtarsal joint needs to “resupinate after pronation” to lock the tarsus in the sagittal plane to allow for efficient propulsion.2 Since we are talking dynamic motion, rather than the term “lock,” I will substitute the term “stabilize.”
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.
4. The fourth area of biomechanical loading occurs with the transference of motion in a forward movement coupled with the displacement of the fluid under the weightbearing and pronatory forces of the midtarsal joint complex. Fluid loads up against the contact of the metatarsals as ground reaction forces increase and decelerate against the ground, loading each metatarsal into a hydrodynamic alignment position, contributing to the loading of the tarsometatarsal joints.
Computerized gait tests with the silicone dynamic orthotic show that the forces under the MTJ complex measured by the ground reaction forces are less than the combined ground reaction force of the heel, lateral column and forefoot together. Increasing or decreasing the load under the tarsus has a direct correlation with the biomechanical loading measured in ground reaction force. As a result, this forces the fluid under the entire tarsus and forefoot (proximal to contact), and the fluid is not displaced laterally under weightbearing and pronatory forces. However, anomalies such as rigid or neurologic flatfoot would not be suitable for this technology.
At mid-stance, hydrodynamic pressure loads and self posts the midfoot and forefoot to an equilibrium state of stability concurrent with the subtalar joint optimal position.7 This contributes to the “lever complex” as it were for propulsion while redistributing GRF of the forefoot efficiently. There is no “drop off” edge to the orthotic.
Heel-off and release of vector force allows for supination of the rearfoot. The forward progression of the rearfoot as it pivots onto the metatarsal heads and downward force at the forefoot, coupled with the weightbearing and pronatory forces of the midtarsal joint complex, now displaces fluid back to the rearfoot, momentarily prolonging the equilibrium state of stability under the forefoot and midfoot.
Supination of the midtarsal joint and metatarsal lift off, fluid support has unloaded back to its most posterior position of the rearfoot in preparation of heel strike again.
During the entire stance phase of the gait cycle, this fluid technology is in constant biomechanical loading or unloading contact with the plantar surface of the foot, resulting in a greater surface area and reduction in “force per unit area.”
Additional Considerations With Fluid Technology And Orthoses
Since one can predict what the fluid support effects will probably do mechanically, the practitioner can predict, via the fit of the devices and the patient’s comfort, if the biomechanical outcome is correct.
Knowing exactly what to expect allows the practitioner the opportunity to evaluate the biomechanical prescription from a “criteria of the fit” point of view over the patient’s subjective complaints. If necessary, one can make incremental adjustments by adding 3 to 6 mg of fluid, which raises the talonavicular joint and supinates the planes of motion at the tarsus. Reducing fluid volume does just the opposite in cases of over-correction.
What makes the fluid technology so simple to use is that for the majority of biomechanical issues attributable to aging, physical activities and generalized wear and tear, orthotic correction is just a matter of adding or reducing the volume of fluid to make a difference between optimal position and the available range of motion of the midtarsal joint for functional efficiency.
For other complex issues and anomalies we see, modifications with other materials (e.g. cutouts, posts or foam) would allow for some creativity to supplement comfort if necessary.
What makes the technology challenging is assessing whether the patient’s subjective complaints are pain due to healing, prescription or transitioning to the orthosis.
1) The patient should feel support as the device is best as full or snug.
2) The patient should feel that the orthosis is stable with or without slight motion front and back while ambulating. Lateral motion or feeling supinated is critically not a good fit.
3) The orthosis should be comfortable to wear all day once the patient has fully transitioned.
The term “overpronation” is controversial. For others, descriptions such as excessive pronation or excessive pronation movement, abnormal pronation or hyperpronation may be more suitable. Working with the silicone dynamic orthotic has made me confident that overpronation (in whatever form) represents the entire range of motion of the midtarsal joint in any gait cycle, which results in maximum or partial instability during locomotion of the kinetic chain.
What The Research Shows About Traditional Custom Orthoses
Traditional orthotics have been based on concepts of foot control by Root, Weed and others. The level of efficacy in clinical studies comparing over-the-counter and prefabricated products with custom functional orthotics have demonstrated results that are questionable in my opinion, considering this medical technology is about 60 years old.
The Landorf study is well known for its conclusion that there is little difference as far as beneficial effects between custom and prefab orthotics, not only in the short-term but in the long run as well.8
A 1999 study involving 236 patients concluded that, “when used in conjunction with a stretching program, a prefabricated shoe insert is more likely to produce improvement in symptoms as part of the initial treatment of proximal plantar fasciitis than a custom polypropylene orthotic device.”9
One of the greatest problems custom orthotics have faced in terms of clinical efficiency and long-term effects is that there is zero published research indicating that orthotics have any long-term preventative value in terms of “wear and tear” or other musculoskeletal problems. There are two reasons for this. First, there is no follow-up to adjust the prescription orthosis as the foot undergoes physiologic and biomechanical changes of its "optimal" position on the ground after one or two years. (Patients can experience several prescription changes up to 10 years.) Second, there is too much inconsistency in duplication of pronation control in both new and existing patients with traditional manual technology.
Reducing and minimizing the newer increased pronation movement can occur by using the silicone dynamic orthotic with as little as 2 mg of fluid to make a difference in articular apposition and symptoms. Due to the measurement of fluid weight in grams, one can consistently repeat or change a prescription precisely. The blend of fluid mechanics and biomechanics will always work the same if administered with a custom prescription.
The siliconedynamic orthotic has been in clinical use for over 20 years for the treatment of most lower extremity biomechanical inflammatory conditions and pathological ulcers of the feet with the exception of neuropathic disorders such as spastic flatfoot.
The ability of the silicone dynamic orthotic to enhance quantitative biomechanical foot alignment and dynamic stability for walking and running can add greatly to the arsenal of orthotic technologies available today.
Dr. Kiper has specialized in sports biomechanics for over 30 years and is one of the nation’s top practitioners utilizing fluid technology in orthoses. He has published over a dozen articles on biomechanics for magazines, periodicals and online. Dr. Kiper has disclosed that he markets the silicone dynamic orthotic.
1. Quesada PM, Sawyer FD, Simon SR. Temporal and gel volume and effects on plantar pressure relief with use of silicone gel-filled insoles. Presented at the 21st Annual Meeting of the American Society of Biomechanics. Clemson University, South Carolina, September 24-27, 1997.
2. Root ML, Orien WP, Weed JH. Normal and abnormal function of the foot. Clinical Biomechanics, 1977.
3. Lundgren P, Nester C, Liu A, Arndt A, Jones R, Stacoff A, Wolf P, Lundberg A.
Invasive in vivo measurement of rear-, mid- and forefoot motion during walking. Gait Posture. 2008; 28(1):93-100.
4. Blackwood CB, Yuen TJ, Sangeorzan BJ. The midtarsal joint locking mechanism. Foot Ankle Int. 2005; 26(12):1074-80.
5. McPoil TG, Cornwall MW. The effect of foot orthoses on transverse tibial rotation during walking. J Am Podiatr Med Assoc. 2000; 90(1):2-11.
6. Baxter D. The ideal running orthosis. Biomechanics. 1996; 3(3):42.
7. Information from William A. Taylor, PhD, Emeritus Professor of Physics, Department of Physics and Astronomy, California State University, Los Angeles.
8. Landorf K, Keenan AM. Effectiveness of foot orthoses to treat Plantar Fasciitis. Arch Intern Med. 2006; 166:1305-1310.
9. Pfeffer G, Bacchetti P, Deland J, et al. Comparison of custom and prefabricated orthoses in the initial treatment of proximal plantar fasciitis. Foot Ankle Int. 1999; 20(4):214-21.