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.”