Pressure, Equilibrium, and Lever Arms: Principles of Orthosis Design and Fabrication

Pressure, Equilibrium, and Lever Arms: Principles of Orthosis Design and Conventional metal orthoses have not changed significantly over the past 150 years. Aluminum, stainless steel, and titanium are used in the fabrication of these devices along with a wide selection of ankle and knee joint components. Patients with lower-limb neurological conditions or weaknesses, or who have paresis or paralysis of their lower limbs may benefit from the use of a conventional metal AFO or KAFO. Fabrication of these devices, however, not only requires hand skills, but a certain amount of knowledge of anatomy, biomechanics, and orthotic principles. In this article, we will review basic principles of metal bending that may help improve clinical outcomes and share some tips and tricks for fabrication. An orthosis describes a device that is used to support or correct a deformed or weak body part, to restrict motion, or to eliminate motion. A metal orthosis can be fabricated from aluminum, which is lighter in weight. The use of aluminum should be considered when fabricating a metal orthosis for patients with marked lower-limb weakness or flaccid paralysis. Patients with spasticity or the need for rigid support may require stainless steel in the fabrication of the orthosis. The choice of material should be determined by the clinician. A metal orthosis should never be fabricated based on the technician's abilities or metal preference, but rather on the user's needs. A well-designed orthosis with the appropriate material selection can improve the patient's mobility as well as quality of life. From the perspective of patients who use orthoses, there are two main determinants for success: the comfort of the device and whether the device meets their needs and helps them reach their goals. The four Cs are the most important principles a clinician and technician should consider in the design and material selection: control, comfort, cosmesis, and cost. 1 In the fabrication of a conventional metal orthosis, several principles need to be considered, such as pressure, equilibrium, and lever arms. It is important to remember these principles when designing and fabricating an orthosis. The cuffs need to cover the greatest area for patient comfort and padding needs to be adequate to prevent skin irritation. The total force acting on the involved segment must be evenly distributed with balanced pressure throughout the orthosis. The length of the orthosis also needs to be sufficient to provide an adequate moment that creates the desired corrective forces or support needed for the patient's ambulation. The farther the point of force is from the joint, the greater will be the moment arm and the smaller the magnitude of forces is required to produce a given torque at the joint. This is why most lower-limb orthoses are fabricated with long metal uprights that are the length of the segment. 2 In a KAFO, or three-point control system, a proximal and distal force is applied in the same direction and is countered by a third force in the opposite direction that is someplace in between. Each plane and motion the orthosis attempts to control is a three-point loading system. The technician also needs to consider the force application of the orthosis and the patient's body segment, or the amount of pressure applied. Too much pressure over a long period of time can lead to breakdown or tissue deformation. Forces transferred to the body from the orthosis need to be distributed over as much of the limb as possible to keep pressure applied by the orthosis within levels that are comfortable an d tolerable to the patient. Technicians need to have a good working knowledge of the materials that are being used. Strength is the most important criterion for a lower-limb orthosis. Other factors to consider are the stress and strain, or stiffness and durability of the material being used. It is important that a good tracing with circumferential, M/L, and linear measurements is provided. When fabricating a metal orthosis, tibial torsion should not be overlooked and should be fabricated into the orthosis for best outcomes. Special attention needs to be made for knee and ankle alignment, tibial torsion, and toe-out. To determine the tibial torsion or external rotation, the midpoints of the anterior and posterior malleoli in the anterior and posterior plane can be used as a reference point (Figure 1). With the patient's knee and ankle placed at 90 degrees, measure the distance from the back of the heel to the medial and lateral perpendicular marks. Using the difference between the medial and lateral malleoli measurements provides the technician with the amount of offset required. For example, if the medial malleoli measurement is three inches and the lateral measurement is two inches, the offset would be one inch. Therefore, the medial ankle joint would be offset anteriorly by one inch plus 1/8 inch to allow for proper ankle clearance (Figure 2). If the difference is an inch or less, the offset should be done on the medial upright as the medial malleoli is anterior to midline. If the measurement exceeds one inch, the lateral upright would then be deflected posteriorly to accommodate for the excess amount. Once the ankle joints have been offset and re-squared, toe-out can then be accommodated for in the orthosis (Figure 3). Using the measurements provided, the thigh bands and calf bands can be cut to length and contoured. To determine band length, divide the circumference by two and add 4cm or 1.57 inches. 4 In our Fillauer labs, our technicians use a band bender mounted in a table vise for ease of contouring (Figure 4). You can bend your bands with two bending forks, however, there is a greater tendency to torque the band. If you torque the band, you can use a hammer to hammer out the kinks. Leave about an inch on both sides as you do not need a radius in this area. Check the band to the tracing, making sure to contour it evenly throughout the entire radius. The band may need to be twisted to match up with the tracing (Figure 5). Do not place your band in the center of the vise but offset it and then make the correction (Figure 6). Once the bands have been contoured and squared, the uprights can then be contoured. Use the Fillauer Universal Alignment Fixture, or similar, to keep the knee joints in alignment and to allow for ease of contouring (Figure 7). Make sure your uprights are square throughout the process. If there are any twists in the uprights the bands will be out of alignment. Once the uprights have been contoured and squared, the bands can then be attached. Measure and mark the bottom and top of the distal upright to stirrup (Figure 8). Make sure there are no bends in the upright where it fits into the channel of the stirrup as this may lead to a weak spot where the orthosis can break. Continue to check that the uprights remain square (Figure 9). When fabricating a KAFO, care must be taken to maintain the neutral attitude of the knee and ankle joints. Tibial torsion must be built into the orthosis for the orthosis to function appropriately and for good clinical outcomes. In our lab, we use the bar-contouring device built into our bench (PN 220103) to use the benchtop to keep our upright true when deflecting the bar (Figure 10). Upon completion of fabrication, the mechanical knee and ankle joints must be squared. A joint alignment tool can be used to verify the joints are in good alignment (Figure 11). The ankle and knee joints should move freely. Your orthosis is now ready to be fitted. Jackie Valdez, MEd, CO, BOCP, FAAOP, attended Century College and was trained at Gillette Children's Hospital where she began her P&O career as a technician. References Lusardi, M. M. and C C. Nielsen, eds. Orthotics and Prosthetics in Rehabilitation. 2nd ed. St. Louis: Saunders Elsevier, 2007. Anderson, M. 1977. A Manual of Lower Extremities Orthotics. 2nd ed. Springfield: Charles Thomas. Coppard, B. H. Lohman, H. 2014. Introduction to Orthotics: A Clinical Reasoning & Problem-Solving Approach, 4th ed., Mosby. Lusardi, M. 2016. "Principles of Lower Extremity Orthoses." In Orthotics and Prosthetics in Rehabilitation, 219-65. Physical Medicine & Rehabilitation.

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