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Prosthetic And Orthotic Science

Development of a Testing Apparatus for Structural Stiffness Evaluation of Ankle-Foot Orthoses

Polliack, Adrian A. PhD; Swanson, Christopher MSBME; Landsberger, Samuel E. ScD; McNeal, Donald R. PhD

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JPO Journal of Prosthetics and Orthotics: September 2001 - Volume 13 - Issue 3 - p 74-82
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Abstract

Analytical understanding of different ankle-foot orthosis (AFO) designs and their mechanical responses to both static and dynamic loads could aid in better prescription, affect rehabilitation and cost-effectiveness, and improve functional status for AFO wearers. Over the past three decades, several testing apparatuses and procedures have been developed to assess the mechanical characteristics of AFOs 1–3 (Table 1). All of the studies of these apparatuses have focused on flexibility of the AFO during plantarflexion and dorsiflexion. One of the earliest apparatuses was built by Rubin and Dixon 2; it measured the flexibility of an AFO using a tensiometer. The footplate of the AFO was bolted to a table, and a tensiometer was attached at the proximal edge of the AFO to assess the load-versus-deflection or the load-versus-angular properties at the ankle joint by applying a force parallel to the AFO footplate (in either the anterior or posterior direction). This information was converted into a chart to allow easy comparison of different AFO designs. This apparatus design was enhanced by Condie and Meadows, 1 who used an electronic measurement of the force and angle that was used in conjunction with a graph plotter. Further improvements were made when Yamamota et al 4 introduced an apparatus that involved the use of a human subject; the subject wore an AFO during the assessment procedure, making this test apparatus clinically relevant. Also, because medial and lateral forces were applied to the AFO, inversion and eversion characteristics could be measured. More recently, Klasson et al 5 constructed an apparatus that produced plantarflexion/dorsiflexion, inversion/eversion, and internal/external rotation about the AFO. As with the earlier devices, the footplate was fixed to a secure surface, but the shank that was placed into the AFO to simulate the lower limb was securely mounted and could be moved on all three axes. By using six dial indicators, movement in each direction could be measured simultaneously. Each of the apparatuses described above fixed the footplate to a table, applied static forces to the AFO, and measured the resulting deflection.

Table 1
Table 1:
Testing apparatuses and procedures.

Other devices have been created that apply dynamic loads to an AFO (Table 1). To ascertain the effect of fatigue, a device developed by Lundsford et al 6 applied cyclic loads that caused plantarflexion/dorsiflexion of the AFO. After a predetermined number of load cycles (200,000 to 1,000,000) had been applied, the AFO was removed from the cyclic loading machine and put into a custom-made device to compare the stiffness of the AFO before and after the cyclic loads have been applied. This custom device was similar in design to the devices described above in that this device also fixed the footplate of the AFO to the testing surface and applied loads through a dummy lower-limb shank.

Most of the test apparatuses used in the above studies fixed the footplate of the AFO to a solid base plate and then loaded, rotated, and/or pulled on the shank of the AFO. The loading was applied either directly to the AFO or through an interface with a dummy leg. When the footplate is locked, the only area that can be displaced is the neck region or the shank of the AFO. Studies have shown that during ambulation, the neck region of traditional polypropylene AFOs exhibits the highest stresses. 7,8

With the recent introduction of advanced composites and alternative materials to the orthotics industry, additional regions of the AFO have shown a tendency to fail and therefore need to be investigated. 9 Apart from the use of rigid composite orthoses, some full-length footplate AFO designs have had a tendency to fail along the metatarsal rocker, at either the medial or lateral border. During terminal stance, the force applied to the AFO by the floor is focused at the toe region at an average angle of 30° to the ground and at 1.3 times body weight. 10 This causes an isolated bending of the forefoot region of the full-length footplate of the AFO. It is during this stage of gait that failure along the metatarsal rocker is most likely to occur. Failure may also occur across the metatarsal rocker when walking up stairs or up an incline, which can also cause a large, concentrated dorsiflexion force on the forefoot region of the AFO. For this reason, tests were performed on both the entire AFO and forefoot region during terminal stance only. However, this apparatus can be easily reconfigured to simulate loading at other phases of gait, such as midstance and heelstrike.

MATERIALS AND METHODS

PURPOSE

The purpose of this study was to demonstrate the utility of a new AFO apparatus by quantitatively testing various AFO designs and materials. A low-cost test apparatus was developed to measure structural displacement (flexibility) and to examine either an entire AFO or a particular region when the AFO is subjected to loading conditions that occur at different gait phases. The apparatus is capable of both static and impact loading of an AFO at the heelstrike, midstance, and terminal stance phases of the gait cycle. The information gathered by using this apparatus could be of value to orthotic education programs, research and development centers, the orthotics industry, and large central fabrication facilities in evaluating and quantifying mechanical behavior of AFO designs. Orthotists can then use this information to better determine AFO design options and materials for their clients. This information may also be used to assess how new AFO designs meet certification guidelines (eg, Europe’s CE mark) compared with conventional designs.

APPARATUS

Three apparatus arrangements were used for three types of tests, with each arrangement designed to test either a specific region of an AFO or a different loading characteristic for a different phase of the gait cycle. For the purposes of this article, only terminal stance test results are presented. Test results for initial contact and midstance are not shown. The tests for terminal stance were as follows: 1) static test of an isolated forefoot region; 2) static test of an AFO attached to a shoe; and 3) impact test of an isolated forefoot region.

A single, master, plaster positive mold was used for all AFO testing. This positive mold was used to fabricate both polypropylene and composite AFOs. For tests 1 and 3, a solid, positive model was cloned via the computer-assisted design/controlled alignment method from the master plaster mold to create a wooden “dummy leg.” Also for these procedures, only a partial AFO was used for testing. This partial AFO consisted of the footplate and side supports and was termed the “foot-ankle complex” (FAC) (Figure 1).

Figure 1
Figure 1:
Carbon composite FAC attached to the wooden dummy leg with the wedge block and attachment lock.

For test 2, the AFO was loaded with a prosthetic leg (Figure 2). The prosthetic leg was made of standard prosthetic components. A single-axis, 22-cm right foot (Ohio Willow Wood Company, Mount Sterling, OH) was used without the ankle bumpers to simulate an uncontrolled ankle in plantarflexion and dorsiflexion. A firm foam cover (Otto Bock Industries, Minneapolis, MN) was shaped to approximate the shape of the positive mold used to make the AFOs. The interior load-bearing structures were standard endoskeletal modular components, including pyramid adapters and an aluminum pylon (Otto Bock). Additional prosthetic component adapters were used to attach the foot and ankle unit to the various testing fixtures.

Figure 2
Figure 2:
Carbon composite AFO attached to the prosthetic leg with the wedge block and attachment lock.

The testing apparatus, which loads the prosthetic leg with the attached AFO or the wooden dummy leg and the attached FAC, is a mechanical system only (Figure 3). It comprises 1) a steel frame, 2) dual linear guides with a ball-bearing sliding load block, 3) a weight bar, 4) a dial indicator, and 5) a cable cord running from the sliding load block through a pulley wheel to a socket ratchet and crank arm. The dimensions of the steel frame are 107 × 91 × 61 cm (42 × 36 × 24 inches); the frame supports the testing system and allows for attachment of all other components. The large frame allows easy access to the other components and easy adjustment of the assembly to for different tests (Figure 3).

Figure 3
Figure 3:
Static/impact testing apparatus with the prosthetic leg, attached AFO, a weight bar, weights, the dial indicator, and the cable cord running from the sliding load block up to the pulley wheel (not seen).

The dual linear guides comprise two parallel steel rods that are 13 mm (0.5 inch) in diameter and 71 cm (2 feet 4 inches) long; the rods are bolted to the frame supports. Sliding on the parallel rods is a steel load block that is 10 cm (4.0 inches) in height and 2.54 cm (1.0 inch) thick; the block slides through two sets of ball bearings (one set for each rod). The load block can travel up to 51 cm (2.0 feet) along the steel rods. Attached to the back surface of the load block is a weight bar that can support up to 133 kg (300 pounds). Attached to the front surface of the load block is a prosthetic pyramid adapter, which allows a horizontal arm (titanium prosthetic pylon) to attach to either the prosthetic leg and the affixed AFO or the wooden dummy leg and the attached FAC.

Actuation is measured using a dial indicator that measures up to 5 cm (2.0 inches) of displacement along the linear guides. The dial indicator is positioned perpendicular to and over the sliding load block to measure its movement during tests (Figure 4). When a dial indicator is used, the method of measuring the amount of deflection does not affect the testing procedure, because the instrument does not apply any significant amount of force to actuate the linear guides, while maintaining a measurement accuracy of ± 0.00254 cm (0.001 inch).

Figure 4
Figure 4:
The dial indicator unit consists of a 0- to 5-cm (0- to 2-inch) dial micrometer that is positioned before each test to measure the linear guide displacement.

The socket ratchet and crank arm mechanism allows for the wooden dummy leg and the attached FAC or the prosthetic leg with the attached AFO to be held at a chosen height before testing (Figure 5). When fully assembled, the FAC/dummy leg or prosthetic leg/AFO assembly is attached to the sliding load block with a 30-mm (1.2-inch) prosthetic pylon, which acts as a horizontal arm. Cantilever load tests showed no noticeable deflection of the arm for the range of loads used in the tests presented here. The angle of the dummy leg or prosthetic leg can be adjusted by use of a 30° wedge block. The block houses two prosthetic four-hole connectors with adjusting set screws for attachment to both the dummy or prosthetic legs and the load block (Figures 1 and 2). The 30° wedge is used to simulate terminal stance. A 0° wedge can be used to simulate midstance, and a −20° wedge can be used to simulate the heelstrike.

Figure 5
Figure 5:
The socket ratchet and crank arm unit locks and drives a cable cord that runs from the sliding load block through a pulley wheel to a rod that is attached to a socket ratchet and crank arm. The crank arm is used to control the placement of the AFO/prosthetic leg or the FAC/wooden dummy leg for initial setup before both static and impact loading tests.

METHODS

A step-by-step procedure to test the stiffness of each AFO design was developed. Before testing commenced, each AFO design was thoroughly documented to help ascertain the effects the design had on the experimental results. After the AFO design was documented, the AFO was fixed to the prosthetic leg and the tests were conducted to quantitatively determine stiffness.

APPARATUS SETUP

For tests 1 and 3, in which the wooden dummy leg was required, the FAC was fixed using three hose clamps with leather spacers to prevent damage to the FAC (Figure 1). Two hose clamps were placed 4 cm (1.6 inches) and 8 cm (3.1 inches) from the top edge of the FAC, and one was placed across the ankle from the dorsal cuboid to the calcaneus. For test 2, the AFO was clamped to the prosthetic leg using a Velcro strap at the proximal edge of the AFO and a shoe was placed on the distal end of the AFO and foot (Figure 2).

After the FAC or AFO was attached, a wedge was affixed to the proximal end of the dummy leg or prosthesis. The wedge block was machined so that it could be rotated for the different ground reaction loads experienced at the three gait phases (+30° for terminal stance, 0° for midstance, and −20° for heelstrike). The assembly of the FAC/dummy leg or AFO/prosthetic leg and wedge block is collectively referred to as the test assembly. For all tests, the weight of the test assembly was measured and recorded as the resting weight. The test assembly was then attached to the testing apparatus using the prosthetic four-hole connector, with set screws attached to the wedge block.

TESTS

Static Stiffness Tests

For the static stiffness tests (tests 1 and 2), the test assembly was raised using the socket ratchet and crank arm mechanism so that the FAC or AFO was no longer in contact with the base of the testing apparatus. A thin layer of Plexiglas lubricated with petroleum jelly was placed under the FAC or AFO to minimize friction and to allow the free edge to translate horizontally on the base surface. The socket ratchet and crank arm mechanism was then released to allow for a free axial load of the test assembly. The dial indicator was then positioned above the sliding load block to measure up to 5 cm (2.0 inches) of deflection. The initial height measurement was recorded and noted as the baseline.

Weights were added in 2.3-kg (5-pound) increments to the weight bar located on the back of the sliding load block. A 10-second equilibration period was allowed after each added weight before a reading from the dial indicator was recorded. The test assembly was loaded incrementally to 45 kg (100 pounds) beyond the resting weight.

Impact Tests

For the impact tests (test 3), the dial indicator was removed from the testing apparatus. The socket ratchet and crank arm mechanism was used to adjust the height so that the test apparatus was no longer in contact with its base but was still no more than 0.0254 cm (0.01 inch) off the ground. This was confirmed using sheets of paper. One sheet could slide between the floor and the test apparatus, but two sheets were too thick to pass through easily. Together, the two sheets of paper were 0.0254 cm thick. Weight was added to the weight bar up to 45 kg (100 pounds), and the indicator was instantaneously released to allow the test apparatus to be impact loaded. The FAC was removed from the test apparatus to determine whether the design had failed. If the FAC failed, the applied weight and the type, location, and magnitude of failure was noted. If the design did not fail, the FAC was tested again using more weight (added in 4.6-kg [10-pound] increments). Four different FAC designs (Table 2), custom made using the master cast, were tested. These designs were tested at terminal stance only.

Table 2
Table 2:
FAC designs.

RESULTS

Four different FAC designs were created and tested using this testing apparatus (Table 2). Two of the FACs were composed of conventional polypropylene AFO material (5 mm [3/16 inch] and 6 mm [1/4 inch] in thickness). In addition, two advanced-composite FACs were built and tested. The first composite FAC used carbon composite materials only (preimpregnated cloth and braid fabrics) and was approximately 4 mm (5/32 inch) thick. The second composite FAC was of a hybrid design, built using both carbon and Kevlar advanced composites (preimpregnated carbon cloth and braid, and Kevlar cloth fabrics) that were designed to replicate the footplate flexural behavior of polypropylene AFOs. This FAC was 3 mm (1/8 inch) thick.

Similar AFO designs were fabricated for test 2, except only three designs were used (Table 3). The decision to eliminate the solid carbon FAC design for full AFO assessment was due to the excessive stiffness of the design compared with polypropylene (Figure 6) and the poor performance under impact loading (Table 4).

Table 3
Table 3:
AFO designs for test 2.
Figure 6
Figure 6:
Load deflection results for static terminal stance for forefoot FAC tests: ▴, 5-mm (3/16-inch) polypropylene FAC; •, 6-mm (1/4-inch) polypropylene FAC; ♦, 4-mm (5/32-inch) solid carbon composite FAC; ▪, 3-mm (1/8-inch) articulating Kevlar/carbon composite FAC. Plots are mean responses of three tests conducted for each design.
Table 4
Table 4:
Impact loading performance.

TEST 1: STATIC STIFFNESS TEST OF ISOLATED FOREFOOT REGION AT TERMINAL STANCE

Each design described above was tested three times to test the repeatability of the testing apparatus. The average loads to deflection for the three runs for each design are presented in Figure 6. These results show the degree of material flexibility of each AFO forefoot design only. The 5-mm (3/16-inch)-thick polypropylene FAC showed the greatest flexibility, whereas the solid carbon composite FAC had the most rigid design.

TEST 2: STATIC STIFFNESS TEST OF FULL AFO, WITH SHOE, AT TERMINAL STANCE

Test 2 was similar to test 1. The average loads to deflection for the three runs are presented in Figure 7. These results show the degree of stiffness for each AFO design with the added internal resistance associated with the prosthetic foot and shoe used in the tests. As in test 1, the 5-mm (3/16-inch)-thick polypropylene AFO exhibited the greatest flexibility. The 3-mm (1/8-inch)-thick articulating Kevlar/carbon composite AFO exhibited a loading behavior similar to that of the 6-mm (1/4-inch)-thick polypropylene AFO.

Figure 7
Figure 7:
Load deflection results for static terminal stance for full-toe AFO tests: ▴, 5-mm (3/16-inch) polypropylene AFO; •, 6-mm (1/4-inch) polypropylene AFO; ♦, 3-mm (1/8-inch) articulating Kevlar/carbon composite AFO. Plots are mean responses of three tests conducted for each design.

TEST 3: IMPACT TEST OF ISOLATED FOREFOOT REGION AT TERMINAL STANCE

Each design was tested only once, since the impact test resulted in FAC failure. Three different categories of failure were created to describe the magnitude of the fractures that were observed: minor, major, and catastrophic.

Minor Impact

With minor failure, cracking of the FAC was seen or heard. Close inspection was required to see the failure site. Minor failure was seen in only a small portion of the orthosis. The orthosis could still be worn with little to moderate impact on performance or safety.

Major Impact

With major failure, cracking was seen or heard, and the failure area was easily seen with casual inspection. The orthosis could still be worn but with moderate to significant compromise to performance and wearer safety.

Catastrophic Impact

With catastrophic failure, cracking was seen, and the failure area was clearly visible from a distance. The performance of the orthosis was compromised, and it could no longer be worn because of loss of support and/or compromised safety.

The observed results for each design are given in Table 4. Both polypropylene designs performed well with no structural failure, but the 5-mm (3/16-inch) design did present with crazing along the medial-lateral edge of the footplate, and slight bowing occurred along the metatarsal rocker in the 1/4-inch design, after an impact load of 80 kg (176 lbs) for both FACs. There was major failure of the solid carbon FAC design at the medial metatarsal rocker after an impact load of 68 kg (150 lbs). The hybrid composite FAC design experienced minor failure at the medial and lateral metatarsal rocker region, after an impact load of 68 kg (150 lbs).

DISCUSSION

This article describes a series of stiffness and impact tests using a novel test apparatus. The test apparatus was designed and developed to assess stiffness and/or response to impact loading of AFOs. The AFOs, attached to a prosthetic leg, can be subjected to various loading conditions that occur at different stages of the gait cycle, namely at the heelstrike, midstance, and terminal stance phases. The AFO test apparatus, or information generated by its use, can be used by both industry and clinicians to better determine design and choice of materials and profile for a given patient. It can also be used to assess new AFO designs against the conventional standards required to meet certification guidelines (such as the CE mark).

The testing apparatus described here has several advantages. Its low assembly costs (less than $1,000 in materials and labor) and low-tech protocol allow for simple but functional testing of an AFO. Moreover, when the same prosthetic leg (or dummy leg) is used for all tests, the variability is isolated to changes in AFO design only. The apparatus is also versatile and capable of testing AFOs under varying load conditions with simple changes in the apparatus setup. Finally, the testing apparatus can be used to evaluate both an AFO and an isolated area of the footplate, if necessary. Previous testing apparatuses mostly fixed the footplate to the testing surface. This hinders or eliminates assessment of footplate and ankle-section behavior. The testing apparatus presented in this article is completely free, with no fixation of the footplate. It allows for static and impact loading of the AFO, which better simulates the AFO behavior experienced clinically.

The testing apparatus does have some associated limitations. The test protocol requires adding weights incrementally, which is cumbersome and time-consuming and poses a small safety risk during setup procedures.

The testing apparatus was able to show differences in mechanical behavior among the three different designs assessed in this study (Figure 6). The solid carbon design (design 3 from Table 2) did show greater rigidity than the other designs. This is not surprising, because advanced carbon-epoxy composite materials have a much higher modulus of elasticity (Young’s modulus) than polypropylene (carbon fiber: 136 to 149 GPa [20 to 22 msi]; polypropylene: 2 GPa [0.3 msi]). However, the advanced composite, hybrid carbon-Kevlar, articulating footplate AFO design showed mechanical behavior similar to that of a standard polypropylene AFO, as seen in Figure 7. This footplate design successfully matched the behavior of a conventional 6-mm (1/4-inch) polypropylene design, while achieving a lower weight (20% lighter) and a lower profile structure (a reduction of 3 mm [1/8 inch]).

This testing apparatus demonstrates its versatility in AFO stiffness and impact tests. Its ease of use and low cost may allow it to be used by industry for design evaluation and quality control.

ACKNOWLEDGMENTS

This study was funded by the National Institute of Disability and Rehabilitation Research (NIDRR), U.S. Department of Education (RERC on Technology for Children with Orthopedic Disabilities grant no. H13350006 and Mary Switzer Fellowship grant no. H133F980013-98).The authors thank Bryan Morrison, MSBME, for assembling the test apparatus, Ernest Meadows for machining the wood FAC and wedge assembly for the test apparatus, Carin Caves, CO, for fabrication of the prosthetic leg and all test orthoses, and Roger Weber, CPO, for clinical input.

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Keywords:

Ankle-foot orthosis; foot-ankle complex; advanced composites; orthosis stiffness; test apparatus

© 2001 Lippincott Williams & Wilkins, Inc.