DEVELOPMENT OF A ONE PIECE MULTIAXIAL PROSTHETIC FOOT SYSTEM
Eric Rubie, Nate WIlliams, Springlite, Inc. Salt Lake City, UT
The objective of this paper will be to describe the design evolution and critical performance parameters pursued in the development of a one-piece, multi-axial composite foot system. The development of this device has evolved from existing Springlite prosthetic systems and has built upon Springlite's unique elastomeric bonding technology. Through an identification of the significant performance parameters of available prosthetic systems, both multi-axial function and dynamic response were defined as critical success factors for the design. KEY WORDS: Polyurethanes, Prosthetics, Medical
Over the past decade many advancements have been made in the design and functionality of prosthetic foot devices. In the not so distant past, an amputation often resulted in a drastic change in an amputee's lifestyle due to the limited function of available prosthetic devices. The amputee was often encumbered by the weight and lack of dynamic response provided by available foot systems. However, many present devices provide the amputee with previously unheard of levels of performance and comfort, allowing them to regain the physical capabilities lost due to amputation.
Prosthetic devices related to this discussion may be grouped into two basic categories: dynamic response and multi-axial. Each of these foot types possess specific advantages and disadvantages which can be traced to their inherent design principles. The discussion will begin with a review of the reasons for and benefits of each system. The paper will conclude with a review of the design principles and critical factors pursued in the development of a foot system which incorporates the desired features of each of the subject categories.
2. PRESENT SYSTEMS
2.1 Dynamic Response Systems The dynamic response foot as it relates to this discussion can generally be defined as possessing a carbon fi-ber/epoxy L-shaped upper section in combination with a carbon/epoxy cantilevered plate which is attached beneath the upper section. The forward sweep of the upper section defines the forefoot or toe portion of the device. The lower plate defines the heel section of the foot. In the example shown in Figure 1 , the lower plate is permanently affixed to the upper section through the use of an elastomeric polyurethane resin. This polyurethane layer acts to not only attach the two composite components together, but also provides an integral shear surface allowing the upper and lower plates to move relative to one another.
Dynamic response feet generally provide the amputee with maximum performance at the lightest weight. These two factors allow the amputee to expend less energy during ambulation and to achieve the greatest degree of athletic achievement. Upon heelstrike (the first stage of the gait cycle) these feet store energy as the amputee's weight and momentum tend to deflect the heel member. As the gait cycle progresses, the deflected heel member begins to rebound to its unloaded state, returning energy in the process. This process tends to drive the amputee's tibia forward, thus assisting in the achievement of a more natural gait cycle. As the amputee's weight shifts onto the forefoot of the device the upper member is deflected along with the forward portion of the lower member again storing energy. The strong reactionary force created by the deflected components drives the amputee's leg upward and forward, thus completing the gait cycle.
As may be inferred through an analysis of this foot's design, the major limitation of this device is a lack of motion in the "ankle" region of the foot. In order to accommodate the high levels of stress induced upon the foot, the ankle region must be reinforced substantially. In addition, these designs were developed to provide reactionary forces in only one rotational plane (anterior/posterior). Therefore, these designs provide minimal motion capability in medial/lateral (side to side) and rotational planes.
2.2 Multi-Axial Systems The multi-axial systems, as their name implies, provide motion capability about more than one rotational plane. Figure 2 shows an example of a successful multi-axial design. These systems provide controlled motion through the incorporation of compressible elastomeric bumpers, bushings, or a combination thereof. The loading and functional characteristics of these devices in the longitudinal plane are very similar to the dynamic response feet. However, in contrast to the high degree of energy return provided by the dynamic response feet, a significant amount of the energy stored within the compressed elastomeric members is dissipated.
While, multi-axial systems may not provide the high degree of energy return that is characterized by dynamic response systems, the additional capability provided by their greater functionality provides a high degree of comfort for the amputee. Rotational capability about an axis transverse to the foot provides excellent "terrain conformance" thus reducing the stresses transferred to the amputee's residuum when an oddly shaped obstacle or obstruction is encountered. This rotational capability also helps to maintain a greater portion of the foot in contact with the ground, thus providing a more stable base for the amputee.
Rotation about a vertical axis is also provided by many multi-axial foot designs. This rotational capability allows torsional forces to be absorbed and dissipated, further contributing to the comfort of the amputee.
In order to achieve the high degree of function described above, current multi-axial foot systems tend to be relatively complex in design. The systems consist of many parts working together to provide the desired function. One drawback of this design approach is that numerous components are often prone to wear which may cause these systems to become very maintenance intensive.
The approach taken to combine the attributes of these two dissimilar systems focused on the development of a novel method of composite construction. The design intent focused on the elimination of all mechanically fastened joints. The primary goal was to replace the fastened joints with a compliant elastomeric interface.
It was envisioned that this interface would both bind the composite components together (as in the dynamic response foot described above) while also providing the desired multi-axial functionality.
After extensive materials testing, a "hybrid" composite design was developed. In this application, hybrid composites are defined as a series of laminated composite members which are joined by a compliant interface to form a singular structure. This design philosophy eliminates the complex multi-axial mechanisms which tend to wear and require frequent maintenance. Also, by joining the load bearing structures with a substantially thick elastic core, the upper and lower components are allowed to move independently of one another while still providing the necessary structural rigidity.
A multi-axial prosthetic foot was developed based upon the hybrid composite design as shown is Figure 3. In order to enhance the adhesive strength between both the composite structure and the polyurethane elastomer, a proprietary bonding process was developed.
The unique physical characteristics of the polyurethane resin utilized in this design make the system feasible. The elastomeric material selected for this application is a thermoset product which possesses a tensile strength of 3000psi, a modulus of elasticity of 300psi, and provides an ultimate elongation of 700%. The high elongation coupled with the low modulus provide a fluid interface upon which the composite structures can translate nearly independent of one another. When subjected to an off-axis load, the upper component of the prosthetic foot provides free range of motion in all planes. However, the amount and characteristic of this motion can be controlled and modified through an adjustment of either the physical properties of the elastomer, the geometry of the elastomer, or various combinations of the two.
The design concept can best be described through an analysis of the loading characteristic of the foot as illustrated in Figure 3 . Upon heel loading, the ground reaction force causes both the composite heel member to deflect and the posterior section of the elastomer to compress. In order to counteract the rotational moment induced by the compression of the posterior section of the elastomer, the forward portion elongates, thus slowing the progression of the load. As the amputee continues through the gait cycle, the composite heel section returns energy as the elastomeric interface dissipates shock and vibration. As the amputee's weight is transferred from the heel to the toe of the device, the urethane layer provides a fluid interface which contributes a very smooth transition from heel strike to toe-off. As the toe portion of the device is deflected, the forward section of the elastomer is compressed and the rear section elongated. As the amputee's gait continues, the composite footplate returns energy to the amputee propelling the amputee's leg upward and forward.
As off-axis loads are encountered by the amputee, the elastomeric layer allows rotation about the transverse axis thus providing multi-axial function. As described above, the amount and characteristic of this multi-axial capability can be modified through a variation in the elastomer material properties or through changes in the geometry of this layer. In addition to the capability of this device to absorb and counteract transverse loads, rotational loads are also accommodated within the elastomeric interface. Therefore, a compromise between desired functionality's (anterior/posterior stiffness, transverse rotation, and vertical rotation) must be evaluated and defined. As the stiffness of the device is modified in one plane (through variations in material or geometry) the stiffness in another plane will be affected. In this application, dynamic response was preferred over extreme multi-axial capability; therefore, the amount of motion of the device has been tailored to allow energy storing capability with comfort provided by the elastomeric interface.
5.1 Structural Testing Structural testing consisted of both static and cyclic loading. The ISO 10328 testing specification was used to evaluate and proof the design of this device. Cyclic testing was performed through alternately loading the toe and heel of the device to 1,400 N for a duration of 2 million cycles. Upon completion of the cyclic loading phase of the test cycle, each device was subjected to a proof load of 2350 N and an ultimate static load of 4500 N. All test results indicate that the foot is structurally sound for the loads applied. No failures within the polyurethane elastomer were noted during development of the device. The extensive testing demonstrates that the materials maintain their structural integrity under sustained loading and effectively work to distribute the induced stresses over the entire bonded surface. Throughout the testing phase of development. there were no bond failures, which indicates that very high bond strength has been achieved.
5.2 Beta Testing Patient testing was performed over a one-year period on forty trans-tibial and trans-femoral amputees having low. moderate, and high activity levels. Patient feedback indicates that the device provides significant improvements over the patient's previous device in terms of both functionality and comfort. General comments suggest that the device provides a very smooth rollover and "fluid" feel similar to what is experienced in the human ankle. Limited gait analysis was performed on selected beta test subjects to assess the ground reaction forces (GRF) of the device as compared to other prosthetic systems. The results of these tests indicate that the device has normal ground reaction force characteristics as compared to the other systems.
The Springlite Low Profile Advantage prosthetic foot design described in this discussion utilizes hybrid composite construction. The simplicity of the design provides motion in multiple planes with few moving parts, thus reducing the weight and eliminating the need for future maintenance. The device allows motion similar to the natural pronation and supination of the human ankle joint during ambulation. Finally, the system allows a generous range of motion in the lower ankle section without a significant reduction in the energy storing capabilities of the composite components.
- L.K. English, "Fabricating the Future With Composite Materials", Materials Engineering, October 1990, pp. 41-45.
- A. Vickland. "Advances in Biomedical Composites", Advanced Composites, July/August 1993, pp. 22-26.