"Flexure Joint Technology for Orthotics: Theory and Practice"
J. Martin Carlson, CPO and Lisa P. Vervena, MS. Orthopädie-Technik, May, 1999.
Abstract
Flexures have long served a variety of hinging functions as an alternative to the "pin" type joint. Recent flexure design innovations combine the characteristics of a high tensile strength fiber cable with the bending flexibility of a short, cylindrical flexure member. That composite design creates a unique orthotic joint component which combines fabrication efficiencies with extraordinary durability. A variation on that freely bending flexure design yields an interchangeable motion assist unit. These composite flexure joints can be used in a variety of applications, including thermoplastic ankle, wrist, elbow and some pediatric knee orthoses. Proper anchoring of the flexure joints will enhance precise function, so molding blanks are used to create snug cavities to receive and contain the flexures. The composite flexure joint development process utilized a durability test machine. Objective, comparative test data, even in the absence of standards, can be a very valuable tool for making design, material and assembly choices. Plastic shell technology is fundamentally different from metal bar-and-band structures. Custom formed plastic shells offer a new level of intimate orthopedic support with a fraction of the weight. However, we must take care to use this technology correctly to maximize the benefits for our clients.
Background
Flexures have been used since prehistoric times when rawhide was pierced and pegged to form hinges for shelter doors and container lids. They have effectively served humankind down through the ages to the present day in applications as prosaic as fishing tackle boxes and cabinets to the sophisticated, exotic needs of the aerospace industry. Flexures are fundamental engineering design tools with an amazing range of capabilities. Their configurations vary greatly depending on the requirements of each application. This paper focuses on the service of flexures in what we might call the "thermoplastic era" of orthotics. In North America that era began in about 1970 and usage is still expanding. Several of the early plastic AFO designs allowed some ankle motion by means of a somewhat flexible posterior "leaf" portion between the foot and calf sections of the orthosis. Posterior leaf AFO designs continue to be practical in certain applications where requirements for good orthotic joint axis alignment and precise motion control are not important.
In the 1970s, thermoplastic knee and ankle flexure components were manufactured for use in fracture orthotics. The limitations of these "long" flexures quickly became apparent. They buckled easily when subjected to even small compressive or transverse loads commonplace in orthoses. Both experience and the principles of deformable body mechanics clarify that only "short" flexures can approach the stability and control capabilities of "pin" joints. The active bending length of a flexure compared to its transverse thickness (in the direction of bending) is, in fact, an extremely important ratio with fundamental ramifications for all aspects of performance. The shorter and better supported a flexure is, the more it can withstand compressive and transverse forces while preserving original alignments. One-piece molded polypropylene hinges of a "short" flexure design were already replacing some metal hinges on kitchen cabinets in the early 1970s.
Polyurethane Flexure Design for Orthotics
Experimentation with the design and use of short polypropylene flexures for pediatric AFO ankle joints began in 1974 at Gillette Children's Hospital1. The polypropylene joints were extremely durable for very small children, but were not reliable for some older, stronger clients. In time, the Gillette designers turned to another thermoplastic-polyurethane. Polyurethane had the right combination of toughness and elasticity to allow the flexure to be more robust and formed with a circular cross section. The cylindrical flexure configuration has a very important advantage during orthosis fabrication. Since bending (flexing) occurs with equal ease about all transverse axes (see Figure 1), a pair of such flexure joints requires little attention or care as to how they align with each other in any respect. The orthotic joint axis of rotation will be the line connecting the centers of the flexure joint pair. Eventually, those experiments and design improvements led to the introduction of the Gillette polyurethane ankle joint through Becker Orthopedic2 in 1988. Unfortunately, the low modulus of elasticity of polyurethane, which is necessary to achieve low bending resistance for that "short" flexure configuration, allows the flexure to stretch too easily when subjected to shear/torsion or tension loading. That presents a problem in many applications. Joint elongation of as little as 3 mm on an adult AFO represents plantarflexion motion of about four degrees beyond a desired stop. Therefore, those free motion ankle flexure joints, molded of a single material, are not sufficient when there are significant varus/valgus, torsional or plantarflexion moments imposed on the orthosis.
Composite Flexure Design Reduces Elongation
Tamarack Habilitation Technologies3 undertook further flexure joint design development in the early 1990s. The engineering challenge was to refine flexure design in a way which preserves the bending flexibility and self-aligning characteristics but eliminates the elongation. The key to accomplishing that was to embed a strong but flexible tensile load bearing element (TLBE) along the longitudinal centerline of the flexure. All neutral bending surfaces of the flexure pass through its longitudinal centerline, so locating the TLBE there will not significantly alter the bending flexibility of such flexures (see Figure 2). An orthotic flexure joint which utilizes that composite design was introduced by Tamarack Habilitation Technologies through Becker Orthopedic in the spring of 1995.
Design of a flexible but very tough tension load bearing element (TLBE) with great fatigue resistance proved to be a challenge. As development of the composite flexure proceeded, a comparative durability test machine provided the most important design performance feedback (see Figure 3). The foot or stirrup section of an AFO is mounted to a horizontal bar. In-line (1 kg) and laterally off-center (2 kg, offset 12.5 cm) weights are attached to the calf cuff 21 cm proximal to the ankle joint axis. The bar is driven by an electric gear motor to rotate the AFO backward through a circular path at approximately 12.5 rpm. The AFO is allowed, on its upward path, to go into dorsiflexion of 25-30°. As the AFO rotates over the top of the circle, the weights begin to fall, causing rapid plantarflexion of the AFO ankle until the motion stop at 90° is reached. In this manner the machine subjects AFO ankle joints to cyclical motion and shock loading. The shock loadings are severe enough to generate complete fractures through the stainless steel stirrup of conventional metal AFOs in less than 200,000 cycles. Three pairs of free motion steel ankle joints designed for use with plastic shell AFOs were tested and lasted 506,150, 34,369 and 72,671 cycles. Failure of those units was due to gradual wearing away of metal until spontaneous disassembly occurred. The variation in the wear rate for the metal joints was probably related to how precisely each pair was aligned on a common axis of rotation.
The tension load bearing elements in the earliest composite flexure designs lasted only a few thousand cycles on the durability test machine before breaking. As subsequent material and design changes were considered, analysis of cycle test results guided design choices and material selection very effectively. The final composite flexure joint design, introduced in 1995, will withstand over 3,000,000 cycles on the durability test machine.
Proper Fixation of Flexures is Enhanced with Use of Molding Blanks
It is part of the nature of short flexure joints that the precision and reliability of their performance is enhanced by proper anchoring, including the correct amount of enclosing coverage by the thermoplastic shell. For that reason, a simple fabrication system utilizing molding blanks was introduced along with the new composite flexure design. The slightly altered length and configuration of the molding blanks create vacuum-formed cavities in the AFO shell which more snugly and completely enclose and anchor the installed flexure joints. The end result is a flexure joint which approximates the precision and control of traditional orthotic joints, but with significant durability, cosmesis and fabrication advantages (see Figure 4).
Motion Assist Option
It was explained earlier how strategic alignment of the tensile load bearing element (TLBE) along the neutral bending plane nullifies its effect on bending flexibility. Conversely, if the TLBE is aligned well away from the neutral plane, it will have a very strong effect on flexure bending. The configuration shown in Figure 5 has the TLBE "bowstrung" across the concave side of the flexure, which is molded at a pre-flexed angle of 45°. When these flexures are installed in an AFO with a straight flexure cavity molded into it, they tend to hold the foot in dorsiflexion. As the foot section of the AFO is moved through neutral into plantarflexion, these flexures store energy to return the foot to the dorsiflexed position. They provide a surprising amount of dorsiflexion assist power in a very small package. This is accomplished by the virtual inextensibility of the offset TLBE converting the polyurethane column from a simple bending flexure to a columnar compression spring (see Figure 6).
The dorsiflexion assist flexures are also tested on the durability test machine shown in Figure 3. The testing action is a bit different, however, since they are not generally used in opposition to a plantarflexion stop. They are therefore loaded with just enough weight to pull them into 15-20° of plantarflexion on each cycle, alternating with 30° of dorsiflexion. What happens is very interesting. At about 1,000,000 cycles, a small crack develops in the anterior third of the polyurethane. Crack propagation then appears to stop. The amount of motion assist is only slightly reduced by the crack and the TLBE seems unaffected. We know that 1,000,000 cycles is many times the number of cycles that the compression spring in a metal joint can withstand.
The main factors affecting the magnitude of dorsiflexion assistance are the durometer or "hardness" of the flexure material and the size of the flexure joint. A larger size, molded of a higher durometer material, will generate stronger assistance. For each size (large, medium and pediatric), the free motion flexures and the two durometers of dorsiflexion assist joints manufactured by Tamarack use the same molding blanks and are thus interchangeable. This interchangeability currently gives the orthotist six possible levels of motion assist for each size.
Dorsimeter Facilitates Flexure Joint Selection
How should an orthotist know which combination of flexure joints is most appropriate for the dorsiflexion assistance needs of each client? One may estimate which combination of components will work, try that combination, then change components if necessary. This process may or may not be efficient. A device called a dorsimeter (see Figure 7). has been designed to help orthotists be more objective and deliberate regarding component selection. The dorsimeter is a very simple clinical device which mimics the function of a dorsiflexion assist AFO and provides a quick measurement of how much assist a client requires.
The dorsimeter is fixed in place in less than one minute then adjusted as necessary to provide for safe, toe-clearing gait. The whole process requires less than five minutes and tells us what components the client needs. The dorsimeter may also be used by physicians or physiotherapists to measure the moment of assist needed. They may find it a useful tool to objectively monitor and chart one aspect of their clients' status and progress.
Fabrication Overview
To create a flexure joint fixation cavity in a plastic shell orthosis, position the molding blanks on the plaster cast so that the midpoint of the flexure will be located near or on the desired axis for joint motion. Use small nails to secure the molding blanks to the plaster cast (see Figure 8). Pull a nylon stockinette over the cast and vacuum form the thermoplastic shell. After removal of the shell from the plaster cast, pry out the molding blanks (these may be re-used up to 10 times depending on thermoforming temperatures). With a narrow kerf saw, make the cut separating the foot and calf sections from each other. Do only minimal clean-up burr removal on that separation cut. To achieve free dorsiflexion motion when using free-motion flexures, remove a small "V" section from the anterior half of each cavity (see Figure 9).
For proper fit of dorsiflexion assist flexures, a small "U" section must be removed from the anterior half of each cavity. It is common with the assist joints to allow about 20° of plantarflexion beyond neutral. To do that, remove a 20° wedge of plastic posterior to the cavity centers.
Mark the location of flexure joint fixation screws and drill or punch the holes for the screws to pass through the plastic. A special hand punch simplifies that step. Slip the flexure joints and flange nuts into the cavities and secure with the mounting screws. The final assembly of a set of free-motion flexure joints should have a snug, well-covered fit as in (see Figure 4).
For those applications where a solid ankle AFO is being replaced with an articulating ankle, it may be possible to retrofit the original solid ankle with flexure joints. In a matter of minutes, the flexure cavity impression tool (see Figure 10) allows you to create perfectly formed cavities for flexure joint installation. To do this, gently heat the inside surface of the orthosis at the location you want the flexure joints to fit, until the plastic is almost clear. Do not overheat it. Then, place the tool into position and gently squeeze the handles until they lock Make sure the depth of the die is adjusted on the Vise Grip™ so it compresses to a point slightly thinner than the thickness of your plastic. This will assure a strong cavity.
Conclusion
This paper has related flexure joint technology to orthotic ankle joint applications because that has been the design focus and most common usage of this type of joint. However, flexure joint technology is also very useful for wrist, elbow and some pediatric knee joint applications (see Figure 11). In particular, the options this technology opens up for wrist motion assist offer excellent cosmetic advantages over the use of rubber bands and springs.
Thermoplastic materials and processes are providing the means for more intimate orthotic support and lighter, more cosmetic orthoses. These materials also bring new challenges regarding joint motion componentry. Joint components must be compatible with both the fabrication processes and the appearance of those newer materials. We lack durability test standards in this field. However, tests can be designed and performed that give us valuable, objective information of a comparative nature.
Gillette Children's Hospital |
Becker Orthopedic |
Tamarack Habilitation Technologies, Inc. 1471 Energy Park Drive St. Paul, Minnesota 55108 USA | |
Reference
- Carlson JM, Day B, Berglund G. Double Short Flexure Type Orthotic Ankle Joints. Journal of Prosthetics and Orthotics 2(4):289-300, 1990
