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Technical Note: Fabrication of a Dual-Axis Articulated Ankle-Foot Orthosis

Elizabeth L. Lawrence, PhD
Vern L. Houston, PhD, CPO

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ABSTRACT

Custom and prefabricated ankle-foot orthoses (AFOs) can either augment or hinder patients' gaits, depending upon how they are made and aligned. To optimize patient function, it is necessary to accurately and effectively fit and align orthoses to individual patients' anatomy and biomechanical characteristics. Unlike traditional AFOs, such as the posterior leaf spring AFO and the articulated ankle AFO with metal stirrup and dual uprights, the dual-axis AFO allows both the anatomical ankle and subtalar joint axes to function naturally and contribute to the motion of the lower limb and body throughout the gait cycle. The dual-axis AFO design follows a design originally proposed by Campbell et al. [Campbell J, Henderson W, Patrick D. UC-BL dual axis ankle-control system: casting, alignment, fabrication and fitting. Bull Prosthet Res 1969;10 –11:184 –214], incorporating a floating yoke containing an articulated ankle joint. The yoke and ankle joint are supported by struts mounted on an articulated subtalar joint attached to a stirrup on the shoe. The ankle and subtalar joints can be made with flexion/extension (inversion/eversion) stops and/or spring assists as required by the respective patient. The dual-axis AFO design allows a more natural, stable, energy-efficient gait. (J Prosthet Orthot. 2006;18:68–71.)

In designing and fitting an ankle-foot orthosis (AFO), to avoid creation of a pseudo-arthrosis between the orthosis and patient's leg and foot, which can lead to chafing and abrasion, and destabilizing movement of the brace on the patient's limb during ambulation, it is necessary to determine and align the orthosis joint(s) with the patient's "effective" ankle and subtalar joint axes' locations and orientations. In theory, there are several methods that may be used; however, in clinical practice this is most often accomplished through palpation and trial and error. The statistical average values from cadaveric studies performed by Inman ¹ suggest an initial assumption that the ankle joint passes 5±3 mm below the distal tip of the medial malleolus, 3±2 mm below the distal tip of the lateral malleolus, and 8±5 mm anterior to the distal tip of the lateral malleolus. Similarly, the subtalar joint (created by the articulation of the calcaneus with the talus) rotates about the axis of Henke, which can be envisioned as entering the posterolateral tuberosity of the calcaneus and running anterosuperomedially, exiting through the medial neck of talus. The cadaver studies' mean angle of projection of the subtalar joint onto the sagittal plane is 42° ±9°, and onto a horizontal plane is 23° ± 11°. ¹ Conventional AFO designs incorporating just an ankle joint assume that the anatomical ankle joint axis passes through the tips of the malleoli, and the subtalar joint is ignored.

The UC-BL dual-axis AFO created by Campbell et al. ² incorporated single-facet ankle and subtalar joints aligned to the mean cadaveric anthropometric axes' orientations. Because the standard deviations in the respective axes' orientations and locations are very large, problems were often encountered fitting the UC-BL AFOs. In addition, the design was structurally insufficient for active users. The orthosis design and manufacturing method described here is based on the tenets that 1) the direction of the joint axis can be accurately determined from three points along the path of motion of the medial tibial plateau as the leg rotates about the ankle joint and subtalar joint, respectively; and 2) all points on the medial tibial plateau are equidistant from the effective ankle/subtalar joint center of rotation. ³ In the work reported here, a MacReflex Qualisys Motion Analysis System (Qualisys, Inc., East Winsor, CT) was used to determine the spatial locations at three points along the trajectories of the centers of photoreflective markers on the leg, as it rotated about the ankle and subtalar joints. The resulting data from which were then used to calculate the joint locations and orientations. A set of three or more synchronized digital cameras positioned around the patient could just as easily have been used. Having determined the location and orientation of both ankle and subtalar joint axes for an individual patient, a time-efficient and cost-effective method of fabricating an AFO with corresponding anatomically aligned ankle and subtalar joints was sought.

FABRICATION METHOD

The locations of the respective joint axes were marked on the test subject's right limb with indelible ink, and a negative plaster of Paris mold was made of the subject's right foot and tibial limb segment. Dental plaster was poured into the cast, creating a positive mold of the lower leg and foot, after which the plaster of Paris bandage was peeled off, leaving the positive mold of the leg/foot with the locations of the joint axes preserved. Holes were then drilled through the cast along the ankle and subtalar axes as marked by the indelible ink, and 8% brass rods inserted through the holes along the respective axes ( Figure 1 ).

The design for the dual axis ankle-foot orthosis consisted of a shoe attachment stirrup with an articulated subtalar joint, supporting a floating yoke with an articulated ankle joint, with or without dorsiflexion/plantar flexion stops and spring assists. ⁴ Matte board patterns for the yoke and stirrup were made and adjusted to fit onto the plaster model. Definitive components were then fabricated ( Figure 2 ). For the prototype, the yoke was fabricated using brass; however, Type 304 stainless steel (18% chromium and 8% nickel, with 35 ksi yield strength and 170 Brinell hardness) would be used when making a brace for a patient. Type 304 stainless steel has a lower carbon content than that of type 303, and thus is more easily welded; it also has good formability and corrosion resistance. The shape and dimensions of the yoke design were checked with the Matte board pattern on the plaster cast, ensuring that it fit around the foot, the joint axes were properly aligned, and there was adequate space on all sides, allowing the yoke to rotate about the subtalar and ankle axes without binding and without impinging on the subject's foot/ankle. The yoke was designed to interface with conventional dorsiflexion/plantar flexion spring-assisted ankle joints and/or adjustable limit stop joints. If adopted by commercial orthotic suppliers, the joints, yoke, and stirrup components could be made available in a range of shapes and sizes to match a majority of the population and could then be custom assembled and aligned to individual patients' ankle and subtalar joint axes, and to their respective foot and ankle dimensions.

To attach the ankle yoke to the subtalar joint and to secure the orthosis to the subject's shoe, a stirrup with an anteromedial strut and posterior flange was made from steel ( Figure 2B ). Holes (21/32-inch) were drilled into the posterior flange and into the end of the anteromedial strut of the stirrup. Steel ball joints with 21/32-inch external diameter, 0.25-inch internal diameter, and 11/32-inch thick, were press fit into the respective holes in the stirrup for the subtalar joint. Three 0-80 holes were drilled and tapped in the outer borders of the stirrup around the holes, into which 0.125-inch long, 0-80 set screws were inserted to secure the ball joints. A centerline was drawn down the center of the base plate of the stirrup to facilitate alignment with the subtalar joint axis of the foot, upon attachment to the shoe. A posterior strut was welded to the yoke for support and clearance for the foot/ ankle, and attached to the balljoint at the subtalar articulation on the stirrup's posterior flange with two 8-32, 0.25-inch long machine screws and an 8-32, 0.5-inch long, stainless steel, internally threaded bushing, with a 0.25-inch outer diameter ( Figure 3 ).

Klenzak joints were then attached to aluminum uprights and secured with screws ( Figure 4 ). With the use of bending irons, the stirrup, yoke, and back strut were bent about the cast so that the anatomical joints, as indicated on the cast, coincided with the mechanical joint centers of the orthosis components ( Figure 5 ).

The distance between the inner and outer surfaces of the base of the shoe was measured, and the base of the flat surface of the stirrup was kept at that measured distance from the bottom surface of the foot of the cast. The contra- and ipsilateral faces of the ankle joint and the subtalar joint were made parallel by placing the respective joint faces in an alignment jig,² and torquing them until they were perpendicular to the plane of the respective joint axis. The proximal part of the orthosis was designed to be a conventional patellar tendon bearing-type cuff, supported by dual aluminum uprights. This design allows varying degrees of weight bearing to be achieved with the orthosis, depending upon how tightly the cuff closure straps are pulled. A polyethylene foam liner was thermoformed on the cast, and a sheet of polypropylene thermoformed over that and trimmed appropriately to produce the cuff. The cuff was then cut off the cast, and the borders sanded to the patellar tendon bearing trimlines and smoothed. A tongue running the length of the posterolateral seam of the cuff was cut from 1-mm thick polyethylene. With the cuff, liner, and the ankle-foot assembly (yoke, posterior strut, and stirrup) positioned on the cast, the two aluminum uprights were bent to match the contours of the leg. Dacron Velcro closure straps were then sewn and riveted onto the cuff ( Figure 6 ).

The stirrup was aligned with the midline of the base plate coincident with the projection of the subtalar axis onto the sole of the shoe and secured to the shoe with four 6-32, 0.5-inch screws. A heel was cut out and attached to the shoe to match the height of the contralateral (left) shoe. The orthosis was then fit on the subject, and the tension in the ankle joint dorsiflexion assist springs adjusted to provide adequate clearance of the subject's toe during the swing phase of gait.

CONCLUSIONS

Advances in technology now enable new instrumentation and procedures to be developed that in turn facilitate design and production of better fitting, more functional orthoses that more closely match the characteristics and fulfill the needs of a broader spectrum of patients. Provision of better fitting, more functional orthoses can also alleviate patients' reports of discomfort, including chafing and skin abrasion caused by movement of the orthosis on the leg attributable to malalignment of anatomical and orthotic joints. Although most of the orthosis components used in this study were custom fabricated, premanufactured components could be made available for custom assembly and fitting. The improvements in function, stability, and support afforded by dual-axis orthoses versus conventional AFOs can mean the difference between mobility and self-sufficiency and confinement to a wheelchair for numerous patients, especially those with hemiplegia.

Correspondence to: Elizabeth L. Lawrence, PhD, New York University School of Medicine, Department of Veterans Affairs, New York Harbor Healthcare System, 423 East 23rd Street, Room 14045-W, New York, NY 10010; e-mail: .

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ELIZABETH L. LAWRENCE, PhD, is affiliated with the New York University School of Medicine and the Department of Veterans Affairs, New York Harbor Healthcare System, New York, New York.

VERN L. HOUSTON, PhD, CPO, is affiliated with the New York University School of Medicine and the Department of Veterans Affairs, New York Harbor Healthcare System, New York, New York.

References:

1. Inman V. The Joints of the Ankle. Baltimore: The Williams & Wilkins Company; 1976.
2. Campbell J, Henderson W, Patrick D. UC-BL dual axis anklecontrol system: casting, alignment, fabrication, and fitting. Bull Prosthet Res 1969; 10–11:184–214.
3. Lawrence EL, Houston VL. Computerized method to determine the location and orientation of the ankle and subtalar joint axes of rotation of the human ankle/foot complex. Proceedings of the American Society of Mechanical Engineers 2005 Summer Bioengineering Conference, June 22–26, 2005.
4. Lawrence EL. Design and analysis of a dual-axis, articulated ankle-foot orthosis [dissertation]. The City University of New York, 2004.

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