Clinically, ankle-foot orthoses (AFOs) are commonly prescribed to control foot drop and improve the gait pattern of patients with movement disorders such as stroke, cerebral palsy, and spinal cord injury.1–8 Solid AFOs (SAFOs) are one of the most popular orthotic designs because of the ease of fabrication and satisfactory gait stabilizing effectiveness. The major function of a SAFO is to constrain the ankle joint around the neutral position for stance phase stability and reduce the effort of patients to produce toe clearance during swing phase.
Since its introduction, the SAFO has been made with a wide range of materials (e.g., thermoplastic and thermoset resins, carbon fiber composites). Besides various materials, SAFOs can be made with different reinforcement techniques, such as straight (without any modification), corrugated (with a hollow ridge posterior to the ankle joint axis), and reinforcement (e.g., Becker’s CompCore®). Mechanical properties of SAFOs such as rigidity are closely related to their materials and reinforcement techniques. Practitioners pick the appropriate materials and reinforcement techniques to meet a wide range of needs. For example, when foot-ankle motion restriction is the utmost demand, the materials and reinforcement techniques have to be selected to maximize the rigidity by taking into account both the patient size and ambulatory level.
Thermoplastic resin is the most used material owing to its low cost, light weight, and ease of fabrication. In clinical practice, polypropylene is the primary choice because of its good mechanical properties such as stiffness, impact performance, and resistance to fatigue. There are two major forms of polypropylene: homopolymer and copolymer. The former has pure polypropylene resin, whereas the latter has a small percentage of polyethylene as an alloying agent, which improves resistance to cold fracturing. Recently, discontinuous carbon fibers have been added into a polypropylene resin base as a prepreg composite to improve fabrication and clinical performance. Furthermore, the material properties of the thermoplastic could be altered when subject to heat treatment (e.g., annealing). Annealing has been extensively examined and applied in the plastics industry; however, its application in orthotics especially in AFOs is rare.
Practitioners will benefit from a study in which the material properties of SAFOs with various materials, reinforcement techniques, and heat treatment are quantitatively investigated, as the outcome will provide meaningful guidance regarding SAFO fabrication. The preliminary study is therefore conducted to provide quantitative outcomes regarding the mechanical properties of a SAFO.
FABRICATION OF SOLID ANKLE-FOOT ORTHOSES
Eight SAFOs were fabricated with three types of thermoplastic materials and two reinforcement methods (Table 1). The SAFO design followed the total contact concept pioneered by Simons et al.9 with a mid-malleolus trimline at the ankle for stiffness.10 Thermoplastic materials were used based on the technique first established at Rancho Los Amigos Hospital by Murray and Greenfield.11 One reinforcement technique incorporated a corrugation technique initiated by Clark and Lundsford12 at Rancho. The second reinforcement technique was developed and patented by Bedard and is currently available from Becker Orthopedic as Compcore AnkleForms®,13 which is a polypropylene/glass/carbon prepreg composite reinforcement coupon. The coupon is designed to bond to the host orthosis during the fabrication process. The three thermoplastic extruded sheet materials used in the fabrication included natural copolymer polypropylene, natural homopolymer polypropylene, and a new material, which is a carbon-infused homopolymer polypropylene. The sheets were all in a 3/16-in gauge to reduce the complexity of testing multiple wall thickness variables in this preliminary study. The trimline of AFOs can significantly affect the bending stiffness.14 To eliminate the trimline effects as much as possible, all of the AFOs were fabricated carefully using the same polyethylene mask, which was obtained from the first finished test AFO. The mask was vacuum formed over a finished AFO on a positive model to match the trimline. The mask was then placed on the outer surface of each subsequent unfinished AFO for trimline transfer. Precautions were taken to limit the variables of the vacuum thermoforming process for the eight orthoses. All the SAFOs were fabricated by the same seasoned practitioner to ensure consistency.15–18
A conventional oven was used for annealing (Gruenberg Oven Company, Inc, Williamsport, PA, USA). The oven is equipped with a Watlow Series SD ¼ Din PID controller (Watlow Electric Manufacturing Company, St. Louis, MO, USA), which can be set with a ramping profile with controlled rate (Figure 1). A thermometer (Atkins Technical Inc, Gainesville, FL, USA) with a bare tip probe was clipped on the SAFO to monitor the temperature. Two homopolymer SAFOs were heat treated following the procedure: 1) place the SAFO in the oven at room temperature; 2) ramp up the temperature at 50°F/h until 185°F; 3) leave the SAFO in the oven for 2 hrs; and 4) reduce the temperature at the same rate (i.e., 50°F/h) until the room temperature is reached.
EXPERIMENTAL SETUP AND PROCEDURE
An AFO plaster positive model was segmented at the ankle joint for use as a surrogate leg in the test apparatus. The mandrel extending from the surrogate leg was attached to a mounting jig with adjustable components, which allowed precise positioning in both horizontal and vertical directions (Figure 2). A computer-controlled motorized device with an inline torque sensor (Transducer Techniques Inc. Temecula, CA, USA) and an optical encoder developed in an earlier study19,20 was used to move the AFO within the prescribed range of resistance torques. The servomotor was controlled by a motor drive under speed mode in which both the speed and direction of the motor rotation were modulated. A National Instrument PCI card was used for data acquisition and motor control, and a custom graphical interface written in LabVIEW™ (National Instrument Inc, Austin, TX, USA) was used to set testing parameters including speed and ankle resistance torques and display registered signals. The data were sampled at a rate of 1000 Hz.
The SAFO was donned on the surrogate leg, inserted in a modified postoperative shoe, which was firmly attached to the apparatus footplate, and secured via Velcro™ bands (Velcro Comp, Manchester, NH, USA) (Figure 2). The center of the motor shaft represented the anatomical surrogate leg ankle joint axis. The SAFO was passively moved by the motor at a prescribed speed of 5°/s, with peak resistance torques of 20 Nm in both dorsiflexion and plantarflexion. The resistance torque and corresponding angular position were registered via the uniaxial torque sensor and motor encoder at a rate of 1000 Hz. Each test lasted 120 seconds and was repeated six times for each sample.
The torque and position signals were digitally filtered using a fourth-order, zero-lag, low-pass Butterworth filter with cutoff frequency set at 5 Hz. The torque component attributed to the footplate due to gravity was subtracted from the original torque signal. The torque-angle relationship was plotted to establish the hysteresis loops (Figure 3). Index of hysteresis was calculated as the ratio of area within the loop to the area below the ascending limb and is presented in percentage.21,22 Each hysteresis loop was further divided into an ascending limb (toward plantarflexion) and a descending limb (toward dorsiflexion) (Figure 3). The multiple ascending and descending limbs were pooled and averaged respectively. As shown in Figure 3, the loading curves across cycles were very consistent with little deviation. The following mechanical properties of the SAFOs were calculated: range of motion (ROM) (a combination of dorsiflexion and plantarflexion within 20 Nm resistance torques), quasi-static stiffness at selected resistance torques (e.g., 5, 10, and 15 Nm), and index of hysteresis related to the viscoelastic properties of the SAFOs. Specifically, the stiffness was obtained as first derivative (i.e., steepness of the slope) along both the loading and unloading curves.
To analyze the response variables including the ankle ROM, stiffness, and index of hysteresis, t-test was used. The significance level was set at 0.05. All statistics were conducted in MATLAB® statistics toolbox (MathWorks, Inc, Natick, MA, USA).
EFFECTS OF MATERIALS AND REINFORCEMENT TECHNIQUES
RANGE OF MOTION WITHIN PRESCRIBED RESISTANCE TORQUE LIMITS
Both materials and reinforcement techniques have effects on the SAFO ROM. In general, the carbon-infused group has smaller ankle ROM compared with either the copolymer or the homopolymer group. For example, for the straight SAFO, the mean (SD) ankle ROMs were 10.21° (0.56°), 10.88° (0.7°), and 10.81° (0.8°) for carbon-infused, copolymer, and homopolymer polypropylene, respectively (Table 2). Specifically, carbon-infused SAFO with CompCore reinforcement showed the smallest ROM of 9.27° (0.74°), whereas the copolymer corrugated SAFO allowed the largest motion, 11.18° (0.88°).
In addition, the ROMs were generally reduced when the reinforcement technique was either the corrugated or the CompCore for both carbon-infused and homopolymer groups. For instance, the mean (SD) ROMs for the carbon-infused group were 10.21° (0.56°), 9.87° (1.01°), and 9.27° (0.74°) for straight, corrugated, and CompCore reinforcement, respectively (Table 2). For the copolymer group, however, the corrugated reinforcement showed a slightly larger ROM compared with the straight AFO, but the difference did not reach statistical significance level (p = 0.265).
STIFFNESS AT PRESCRIBED RESISTANCE TORQUES IN PLANTARFLEXION
In general, when the SAFOs are loaded into plantarflexion, the stiffness increases with the increment of resistance torque. For example, for straight carbon-infused SAFO, the stiffness was increased from 4.20 (0.19) Nm/deg at 5 Nm to 5.18 (0.09) Nm/deg at 15 Nm (Table 3).
Overall, the carbon-infused group showed relatively higher stiffness at a resistance torque of 15 Nm toward plantarflexion. For example, the stiffness of corrugated reinforcement was 5.60 (0.05), 5.09 (0.21), and 5.14 (0.20) Nm/deg for the carbon-infused, copolymer, and homopolymer groups, respectively. The copolymer group showed relatively lower stiffness compared with the other two groups. For example, straight copolymer AFO had a stiffness of 5.08 (0.10) Nm/deg, whereas the stiffness was 5.18 (0.09) and 5.18 (0.15) Nm/deg for the carbon-infused and copolymer groups, respectively. In addition, corrugated reinforcement showed mixed effects across groups. For the carbon-infused group, corrugated reinforcement significantly improved the stiffness across various loading levels (e.g., at 15 Nm, from 5.18 [0.09] to 5.60 [0.05] Nm/deg; p < 0.05). For the copolymer group, the effect was subtle, and for the homopolymer group, it even slightly reduced stiffness across loading levels.
Similarly, when the SAFOs were loaded into dorsiflexion, the stiffness increased with the increment of resistance torque, with few exceptions. For instance, the stiffness of carbon-infused SAFO with compcore reinforcement was 3.59 (0.33) Nm/deg at a resistance torque of 5 Nm and dropped to 3.47 (0.08) Nm/deg at a resistance torque of 10 Nm. Overall, stiffness on the loading curve toward dorsiflexion was significantly lower compared with that on the loading curve toward plantarflexion at prescribed resistance torque. For example, at a resistance torque of 15 Nm, the stiffness of homopolymer SAFO without reinforcement toward dorsiflexion and planar flexion was 3.19 (0.17) and 5.18 (0.15) Nm/deg, respectively. The carbon-infused group showed the highest stiffness across the reinforcement techniques and prescribed resistance torques. However, compared with plantarflexion, the reinforcement techniques showed differential effects on stiffness when loading toward dorsiflexion especially for carbon-infused and copolymer groups. Specifically, corrugated reinforcement led to a reduction in stiffness compared with SAFO without reinforcement when the resistance torque was equal and higher than 10 Nm.
INDEX OF HYSTERESIS
Hysteresis, as an indicator of energy loss during loading-unloading cycle, was systematically changed with various materials and reinforcement. Overall, the copolymer group showed the lowest hysteresis (e.g., 8.81% [0.22%] for straight), whereas the homopolymer group showed the largest hysteresis across the reinforcement techniques, with the exception of compcore reinforcement. Reinforcement techniques also affected the hysteresis. There was a trend that the hysteresis decreased when reinforcements were introduced. In addition, the effects of reinforcements were related to the materials. For example, for carbon-infused SAFO, the compcore reinforcement led to increased hysteresis compared with straight and corrugated ones.
EFFECTS OF HEAT TREATMENT
RANGE OF MOTION WITHIN PRESCRIBED RESISTANCE TORQUE LIMITS
The same homopolymer SAFO after annealing treatment showed significant improvement in restricting ankle ROM, and ROM was decreased from 10.81° (0.80°) to 10.03° (0.27°) (i.e., a 7.2% reduction). The annealing treatment showed superior performance (i.e., 10.03° [0.27°]) compared with the corrugated reinforcement (i.e., 10.17° [0.35°]), although the difference did not reach statistical significance level. Overall, compcore reinforcement demonstrated the best performance in reducing the ROM (i.e., 9.58° [1.10°]). However, there was no statistical difference between the two reinforcement techniques and annealing treatment (Figure 4).
STIFFNESS AT PRESCRIBED RESISTANCE TORQUES IN PLANTARFLEXION
Heat treatment showed effects on the stiffness of SAFO, and the effect was also related to the loading phases (e.g., either on the loading or unloading curve). As the homopolymer SAFO was loaded into plantarflexion (i.e., on the loading curve), there was a slight improvement in stiffness after annealing, with an exception at 10 Nm resistance torque. For example, the stiffness was increased from 5.18 (0.15) to 5.29 (0.09) Nm/deg when the resistance torque was set at 15 Nm, although the difference did not reach statistical significance (Table 5). However, on the unloading curve, the stiffness of SAFO after annealing decreased especially at the higher resistance torques. For instance, the stiffness was reduced from 7.70 (0.32) to 7.08 (0.09) Nm/deg at a resistance of 15 Nm. The reduction in stiffness was further decreased when the resistance dropped to 10 Nm. At 5 Nm, the stiffness of SAFO after heat treatment was even slightly higher than before treatment.
INDEX OF HYSTERESIS
As shown earlier, the reinforcement techniques (either corrugated or compcore) slightly reduced the index of hysteresis, with compcore showing the lowest hysteresis, 9.53% (1.36%). Figure 5 is an extension of Table 4 with the addition of the heat treatment effect. As illustrated, annealing showed the lowest hysteresis index in the group, and the index was decreased from 10.39% to 9.18% after heat treatment (p < 0.05).
In summary, in this preliminary study, we have systematically evaluated the effects of materials, reinforcement technique, and heat treatment (i.e., annealing) on the material properties of SAFOs. Unlike most of the previous studies on AFO stiffness, we report stiffness at prescribed resistance torque and on both loading and unloading curves. Both materials and reinforcement techniques show strong influence on the SAFO properties, such as ROM restriction, quasi-static stiffness, and hysteresis. In addition, annealing shows positive effects in improving the overall mechanical properties of homopolymer SAFO.
The thermoplastics used in the study are chosen by popularity and availability. Homopolymer polypropylene is the ubiquitous choice in the United States for fabrication of lower limb orthoses. Copolymer polypropylene is included in the study for its good performance in resistance to cold fracturing and easier fabrication characteristics. In this study, for the first time, we quantitatively investigate the properties of a recently developed carbon-infused polypropylene. Our results show that carbon-infused polypropylene outperforms the other two materials in ROM restriction and stiffness. Copolymer polypropylene allows the largest ROM. Compared with homopolymer polypropylene and copolymer polypropylene, carbon-infused polypropylene reduces the ROM up to 5% and 7%, respectively. Although carbon composites have been widely used in fabricating SAFOs, especially dynamic ones that are aimed at reducing weight and improving energy storage and return, application of carbon-infused polypropylene in SAFOs and related quantitative study have not been reported. Most likely, similar to carbon composites, although the carbon fibers infused in polypropylene are short and randomly oriented, they can still improve the overall material strength because of carbon’s high strength-to-weight ratio.
The higher flexibility of copolymer polypropylene is attributed primarily to the addition of polyethylene (typically 6% to 8%). Adding polyethylene also lowers the glass transition temperature in the copolymer material, which improves its resistance in cold fracturing in a cold climate. It is commonly claimed that copolymer polypropylene leads to about a 25% decrease in stiffness when compared with the book values of homopolymer polypropylene. Our results, however, show a slightly lower decrease in stiffness (e.g., 20% reduction in stiffness at 15 Nm resistance torque). The discrepancy could be attributed to the test protocol. The stiffness quantified in our study represents the resistance to rotation moment per unit angular displacement in SAFO, and the published physical characteristics are commonly obtained following ASTM (American Society for Testing and Materials) D4101, whereby the thermoplastic test coupons are injection molded and then annealed to optimize the crystalline structure of the resin. Therefore, physical test coupons formulated under ASTM D4101 represent the idealized physical characteristics of the resin formula but may not be representative of the actual molecular structure found in the extruded thermoplastic sheet or the finished product.
The primary function of SAFO is to offer foot-ankle stability, which is tightly related to its stiffness or rigidity. As one of the major mechanical properties of SAFO, stiffness has been widely quantified with methods ranging from manual loading23,24 to computer-controlled testing.19,20,25–28 In most of the previous studies, a posterior leaf-spring type of SAFO was typically tested and stiffness was obtained via linearly fitting the whole loading curve.29 Because the trimline was placed more posterior to the malleolus, the resultant stiffness was relatively low (e.g., around 1–2 Nm/deg).30,31 In the current study, the trimline of SAFO is placed more anterior to the malleolus, which leads to significantly higher stiffness (e.g., ranges from 4.01 [0.22] to 5.69 [0.14] Nm/deg on the loading curve within resistance torques of 5–15 Nm). In addition, as shown in the current study and previous studies,19,20,27,28 both the loading and unloading curves of thermoplastic AFOs are not straight lines, which indicate that the steepness of the slope along the curves (i.e., the stiffness) is not constant. The mechanical property of thermoplastics is characterized as viscoelasticity, which typically shows hysteresis loop during the loading cycle. The index of hysteresis serves as a quantification of relative energy loss.22 Owing to the existence of the hysteresis loop, the loading and unloading curves are separated and demonstrate different stiffness at either the same resistance torque or the same joint position. For example, the stiffness on the unloading curve is typically higher than that on the loading curve, as shown in our results. The viscoelastic characteristic of SAFO indicates that stiffness be more appropriately quantified and reported on both loading and unloading curves at either prescribed resistance torque or joint position.
Reinforcement techniques also have strong influence on the mechanical properties of tested SAFOs. In addition, the two reinforcement techniques show differential effects that appear to be related to the materials. Corrugated reinforcement technique is most effective in improving mechanical properties, including ROM restriction and stiffness, for carbon-infused polypropylene. The effects of corrugated reinforcement on the other two groups, however, are subtle (e.g., copolymer) or even reversed (e.g., homopolymer). Corrugated reinforcement seems cost-effective in clinical practice because it is relatively simple to implement and there is no extra cost incurred. However, its positive effect on polypropylene SAFO is not well supported by our preliminary results. CompCore reinforcement technique, on the other hand, shows consistent improvement in mechanical properties across materials groups. The CompCore AnkleForms are a thermoplastic prepreg consisting of continuous carbon and glass fibers imbedded into homopolymer polypropylene resin. The AnkleForms are precut with a CNC water jet. The machine precut shape also provides consistency across the test SAFOs and eliminates the need of handcraft and related variance.
Annealing is commonly used in the thermoplastic industry to relieve modeled-in stresses due to either molding or extrusion process. In polymer science, studies have shown that annealing of polypropylene could alter the crystalinity percentage and improve its mechanical properties such as elastic modulus.32,33 Our preliminary results on annealing are in agreement with previous findings. The homopolymer polypropylene SAFO after heat treatment has shown significantly improved restriction in ROM and slightly improved stiffness on the loading curve. However, on the unloading curve, the stiffness after annealing drops significantly at high resistance torque (e.g., 15 Nm) and increases slightly at lower resistance torque (e.g., 5 Nm). In addition, the hysteresis (i.e., the energy loss) is dramatically reduced. Overall, annealing at 185°F (i.e., 85°C) has significantly improved the mechanical properties of the homopolymer polypropylene SAFO. As shown in an early study, annealing temperature is critical, and an increase in annealing temperature leads to improved mechanical properties (e.g., elastic modulus).32 For example, the increase in elastic modulus is from 3.6% to 24.3% when the annealing temperature is increased from 194°F to 320°F. Our annealing temperature was close to the lower bound, and most likely, we might be able to improve the stiffness of homopolymer polypropylene SAFO by increasing the annealing temperature.
Limitations of the study are acknowledged. There was only one SAFO fabricated in each tested condition, and a larger sample size needs to be used in future study to improve the statistical power. In this study, one single movement speed at 5°/s was selected. To better approximate the average speed during ambulation, a large range of speeds need to be investigated in future study. The tension of the strap might affect the tested results. In the current study, all the Velcro straps were pulled as hard as possible to secure and an objective monitoring on tightness might help produce more consistent results. In addition, the AFO was donned directly on the plaster mold, which lacks the soft tissue in human subject. The corrugated reinforcement technique implemented in the current study involves only one single corrugation. Considering that corrugated reinforcement is more cost-effective compared with the other techniques, the effects of multiple corrugations deserve to be investigated in the future study. Only homopolymer polypropylene SAFO without reinforcement is annealed at one relatively low temperature. In a future study, we will investigate the effects of temperature and its interaction with reinforcement techniques. Furthermore, annealing has also been applied on copolymer polypropylene and shows improvement in both impact strength and rigidity.33 We will plan to apply annealing to copolymer polypropylene SAFO as well. Finally, one should be cautious to extend the results of our bench test directly to functional performance (e.g., gait) while wearing SAFOs. Future study will be conducted in a clinical gait laboratory and focused on investigating clinical outcome measures especially gait pattern.
Our preliminary results show that materials, reinforcement techniques, and heat treatment affect the mechanical properties of the fabricated SAFOs to different levels. Among the materials used, carbon-infused homopolymer polypropylene outperforms the other two popularly used polypropylenes. The compcore reinforcement technique is more favored than the corrugated reinforcement technique. Lastly, annealing has shown promising effects on improving the overall mechanical properties of homopolymer polypropylene SAFOs. Physical testing of a finished orthosis in a computer-controlled bench test apparatus offers the closest approximation to clinical application of a particular material as compared with pure physical material testing. The various physical characteristics demonstrated in a range of materials and reinforcement techniques will be useful for practitioners to improve the criteria of both design and prescription.
1. Lehmann JF. Biomechanics of ankle-foot orthoses—prescription and design. Arch Phys Med Rehabil
1979; 60: 200–207.
2. Stills M. Thermoformed ankle-foot orthoses. Orthot Prosthet
1975; 29: 41–51.
3. Fatone S, Hansen AH. Effect of ankle-foot orthosis on roll-over shape in adults with hemiplegia. J Rehabil Res Dev
2007; 44: 11–20.
4. Wolf SI, Alimusaj M, Rettig O, Doderlein L. Dynamic assist by carbon fiber
spring AFOs for patients with myelomeningocele. Gait Posture
2008; 28: 175–177.
5. Leung J, Moseley A. Impact of ankle-foot orthoses on gait and leg muscle activity in adults with hemiplegia: systematic literature review. Physiotherapy
2003; 89: 39–55.
6. Burdett RG, Hassell G. Effects of three types of ankle-foot orthoses on the gait and bicycling of a patient with Charcot-Marie-Tooth disease. J Prosthet Orthot
2004; 16: 25–30.
7. Middleton EA, Hurley GR, McIlwain JS. The role of rigid and hinged polypropylene
ankle-foot-orthoses in the management of cerebral palsy: a case study. Prosthet Orthot Int
1988; 12: 129–135.
8. Gok H, Kucukdeveci A, Altinkaynak H, et al. Effects of ankle-foot orthoses on hemiparetic gait. Clin Rehabil
2003; 17: 137–139.
9. Simons B, Jebsen R, Wildman L. Plastic short leg brace fabrication. Orthop And Pros Appl J
1967; 21: 215–218.
10. Lehmann J, Esselman P, Ko M, et al. Plastic ankle-foot orthoses: evaluation of function. Arch Phys Med Rehabil
1983; 64: 402–407.
11. Murray W, Greenfield J. The cosmetic below-knee brace. Orthot Prosthet
1970; 24: 27–30.
12. Clark D, Lunsford T. Reinforced lower-limb orthosis—design principles. Orthot Prosthet
1978; 32: 35–45.
13. Bedard GG. Compcore reinforcement of thermoplastic orthopedic devices with autobonding carbon fiber composites. Proceedings of the AOPA National Assembly
; 1992; Orlando, FL.
14. Bielby S, Warrick T, Benson D, et al. Trimline severity significantly affects rotational stiffness of ankle-foot orthosis. J Prosthet Orthot
2010; 22: 204–210.
15. Bedard GG. Temperature process controls in the heating of thermoplastic sheet: a review of methods and equipment. Proceedings of the AOPA National Assembly
; 1994; Washington, D.C.
16. Bedard GG. Guidelines for thermoforming equipment: transfer of production line standards to single event drapeforming. Proceedings of the AAOP Annual Meeting and Scientific Symposium
; 1994; Nashville, TN.
17. Bedard GG. Optimizing the thermoforming process in orthotic and prosthetic fabrication. Proceedings of the ISPO World Congress
; 1998; Amsterdam, the Netherlands.
18. Bedard GG. Polypropylene: polymer alchemy or fool’s gold: a review of the material. Proceedings of the AOPA National Assembly
; 2001; Dallas, TX.
19. Gao F, Carlton W, Kapp S. Development of a motorized device for quantitative investigation of articulated AFO misalignment. 2010 4th International Conference on Bioinformatics and Biomedical Engineering (iCBBE)
; June 18–20; Chengdu, China; 2010: 1–4.
20. Gao F, Carlton W, Kapp S. Effects of joint alignment and type on mechanical properties of thermoplastic articulated AFO
. Prosthet Orthot Int
2011; 35: 181–189.
21. Gao F, Ren Y, Roth EJ, et al. Effects of repeated ankle stretching on calf muscle-tendon and ankle biomechanical properties in stroke survivors. Clin Biomech (Bristol, Avon)
2011; 26: 516–522.
22. Kubo K, Kanehisa H, Kawakami Y, Fukunaga T. Influence of static stretching on viscoelastic properties of human tendon structures in vivo. J Appl Physiol
2001; 90: 520–527.
23. Bregman DJ, De Groot V, Van Diggele P, et al. Polypropylene
ankle foot orthoses to overcome drop-foot gait in central neurological patients: a mechanical and functional evaluation. Prosthet Orthot Int
2010; 34: 293–304.
24. Takahashi K, Stanhope S. Estimates of stiffness for ankle-foot orthoses are sensitive to loading conditions. J Prosthet Orthot
2010; 22: 211.
25. Cappa P, Patane F, Di Rosa G. A continuous loading apparatus for measuring three-dimensional stiffness of ankle-foot orthoses. J Biomech Eng
2005; 127: 1025–1029.
26. Kobayashi T, Leung AK, Akazawa Y, et al. Design of an automated device to measure sagittal plane stiffness of an articulated ankle-foot orthosis. Prosthet Orthot Int
2010; 34: 439–448.
27. Yamamoto S, Masahiko Ebina CPO, Mitsuo Iwasaki CPO, et al. Comparative study of mechanical characteristics of plastic AFOs. J Prosthet Orthot
1993; 5: 59.
28. Major RE, Hewart PJ, MacDonald AM. A new structural concept in moulded fixed ankle foot orthoses and comparison of the bending stiffness of four constructions. Prosthet Orthot Int
2004; 28: 44–48.
29. Bregman DJ, Rozumalski A, Koops D, et al. A new method for evaluating ankle foot orthosis characteristics: BRUCE. Gait Posture
2009; 30: 144–149.
30. Novacheck TF, Beattie C, Rozumalski A, et al. Quantifying the spring-like properties of ankle-foot orthoses (AFOs). J Prosthet Orthot
2007; 19: 98–103.
31. Sumiya T, Suzuki Y, Kasahara T. Stiffness control in posterior-type plastic ankle-foot orthoses: effect of ankle trimline. Part 1: a device for measuring ankle moment. Prosthet Orthot Int
1996; 20: 129–131.
32. Frontini PM, Fave A. The effect of annealing temperature on the fracture performance of isotactic polypropylene
. J Mater Sci
1995; 30: 2446–2454.
33. Ito J, Mitani K, Mizutani Y. Annealing of polypropylene
–poly(ethylene-co-propylene) blends. I. Thermal and physical properties of blends. J Appl Polym Sci
1984; 29: 75–87.