A Closer Look At The Principles Of Fluid Dynamics As They Relate To Orthoses
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.