Abstract
[Purpose] We previously confirmed that a plastic ankle-foot orthosis with a joint having a built-in T-strap could correct ankle joint supination in able-bodied participants during stance phase without initial contact. In the present study, we examined the adaptability of a newly prototyped T-strap made of soft thermoplastic resin (plastic ankle-foot orthosis T-strap) by evaluating the pressure exerted on the center of the lateral malleolus. These results were compared to results obtained with a conventionally used leather T-strap. [Participants and Methods] We compared the pressure exerted on the center of the lateral malleolus by the plastic ankle-foot orthosis T-strap and a leather T-strap on one leg of each of eight able-bodied participants. Each leg was cast to fabricate eight custom ankle-foot orthoses. [Results] The applied pressure was significantly reduced with the plastic ankle-foot orthosis T-strap than that with the leather T-strap in all three phases—loading response, mid-stance, and terminal stance. [Conclusion] The plastic ankle-foot orthosis T-strap displayed better adaptability than did the leather T-strap. The tested plastic ankle-foot orthosis T-strap may be used as a more reliable alternative to a conventional leather T-strap.
Keywords: Stroke, Ankle-foot orthosis, T-strap
INTRODUCTION
The Japan Stroke Society Guideline 2021 for the Treatment of Stroke recommends the use of ankle-foot orthoses (AFOs) in patients with stroke hemiplegia with pes equinovarus in order to improve walking function1). Nakamura stated that orthoses must be able to adapt to not only static conditions but also dynamic changes in form, allowing force to be transmitted while maintaining adaptive conditions in which pressure is dispersed2). A questionnaire survey of convalescent hospital wards across Japan conducted by Hirayama et al. showed that the most commonly prescribed AFOs were those of the shoehorn type (shoehorn brace: SHB)3). Typically, we use metal strut AFO with leather T-straps for correcting ankle joint inversion. Kimura have reported that SHB is more satisfactory about outlook than metal strut AFO4). In a previous study, we prototyped a SHB with leather T-strap in able-bodied participants and found that it was effective in correcting ankle joint adduction in the standing position5). Meanwhile, in a subsequent study, we prototyped a plastic AFO with an ankle joint having a soft plastic T-strap with free dorsiflexion and restricted plantarflexion in able-bodied participants, and confirmed that this AFO was effective in correcting ankle joint inversion during gait6). Watanabe et al. have reported that in patients with severe spasm of the gastrocnemius muscle, the ankle joint of the AFO should be adjusted to restricted plantarflexion or increased braking of plantarflexion with free dorsiflexion7). Due to the structural limitations of the SHB, increasing plantarflexion braking also limits dorsiflexion. Therefore, in patients with increased muscle tone in the plantarflexion muscles, the use of an AFO with a T-strap that enables free dorsiflexion is considered preferable, as it does not restrict the range of motion of ankle joint. Metal strut AFO has been known effectiveness of correcting ankle joint inversion. A survey by Hirayama et al. found that the most commonly prescribed ankle joint-equipped plastic AFOs were those with a deflecting ankle joint, such as the Tamarack Flexure Joint and the Gillette Ankle Joint3). These deflecting ankle joints are used in a setting where they can move almost freely in the dorsiflexion direction, but movement in the plantarflexion direction is restricted. Because the lower margin of the lower leg portion behind the ankle joint is fixed with the plastic part of the upper margin of the AFO, making them highly effective for patients with increased muscle tone in the plantarflexion muscles. For the T-strap that we prototyped (plastic ankle-foot orthosis T-strap: PATS), however, the suitability of the lateral malleolus during gait has not been verified.
The objective of the present study was to prototype a plastic AFO equipped with Tamarack Flexure Joint ankle joint and incorporating the PATS (hereafter, PATS-AFO), which was designed to allow free dorsiflexion and to restrict plantarflexion, in able-bodied patients for an adaptability evaluation of PATS. The results were compared those obtained with a leather T-strap. The results of this study demonstrate that, even for patients with severe ankle joint inversion, a lightweight plastic AFO with an ankle joint, which is easier to wear with shoes, can be used instead of an AFO with a metal strut, and suggest that the use of a T-strap can correct ankle joint inversion in patients who experience pain in the lateral malleolus, thereby contributing to improved quality of life for orthosis users.
PARTICIPANTS AND METHODS
This study involved 8 unilateral legs of able-bodied men and women (height 168.9 ± 9.9 cm, weight 57.4 ± 6.3 kg, age 20.3 ± 2.1 years). The 8 legs were cast with an initial dorsiflexion angle of 0° to fabricate 8 PATS-AFOs.
The procedure for fabricating the PATS-AFO was as follows. The casting and positive model modifications were carried out in the same way as for a normal plastic AFO with a Tamarack Flexure Joint, and vacuum molding was done with a PATS (made of soft thermoplastic with high flexibility even at ordinary temperatures [Clear Sheet 3 mm; Keiai Orthopedic Appliance, Tokyo, Japan]) attached to the outer part of the ankle joint of the positive model, in order to prevent adaptation failure due to the thickness of the PATS. After vacuum molding using polypropylene, a 30-mm-wide hole was made posterior to the medial ankle joint to allow the PATS or leather T-strap band to pass from the inside to the outside of the orthosis (Fig. 1). In the current prototyping, in order to standardize the PATS, a reference pattern paper (Fig. 2) was made by taking into account the ankle joint form of all 8 legs, and the 8 PATS and leather T-straps were cut to the same shape using the pattern paper. Before plastic vacuum molding, a process was added to make the PATS to conform to the shape of each participant’s lateral malleolus by applying heat to the PATS and attaching it to the lateral malleolus part of the positive model, which had been modified by building up (Fig. 3). During plastic vacuum molding, silicone spray was applied to prevent the PATS from sticking to the vacuum molding stockings, and plastic molding was carried out with the PATS placed over the positive model. This process softens the PATS with the residual heat of the polypropylene, allowing the PATS to be molded to the shape of the participant’s lateral malleolus. Since part of the PATS overlapped the trimming line, vacuum molding was done with a 5-mm-thick polyethylene foam placed over the PATS for the purpose of protecting the PATS. The leather T-straps were fabricated using the conventional method. All leather T-straps were newly made for this study and thus were not adapted to the shape of each participant’s lateral malleolus. In this orthosis setting, 4-mm-thick polypropylene was used. The trimming line was created so that the upper margin was 2–3 cm distal to the lower end of the head of fibula on the frontal plane, and the MP joint was made to cover half of the head and match the form of the toe tip. On the sagittal plane, the trimming line was made to cover two-thirds of the maximum diameter of the lower leg. In the case of swelling of the foot, we can adjust the circumference of the ankle by doing up its belt.
Fig. 1.
Appearance of prototyped orthosis (plastic ankle-foot orthosis T-strap (PATS)- ankle-foot orthoses (AFO))
Fig. 2.
Pattern paper of plastic ankle-foot orthosis T-straps (PATS).
Fig. 3.
Vacuum molding of plastic ankle-foot orthosis T-straps (PATS).
For measurement, inertial sensors (AMWS020B; ATR-Promotions, Kyoto, Japan) were attached to the lower leg of the PATS-AFO-wearing limb using an adhesive elastic bandage. For measurement, 2 pressure sensors (Flexi Force; Tekscan, Norwood, MA, USA; maximum measurement load 440N) connected to the inertial sensors via external extension connectors (3 V, 10 mA, 12-bit AD) were attached directly to the center of the lateral malleolus and the center of the heel on the participant’s body. The pressure applied to the lateral malleolus was calculated using data analysis software (Sensor Data Analyzer; ATR-Promotions, Kyoto, Japan). Before the actual measurement, calibration was carried out by placing weights on the pressure sensors and comparing the measured values (mV) with the pressure applied by the weights (kPa). The pressure values were measured in kilopascals in accordance with the SI unit system. Based on the results of the calibration, the raw data obtained from the pressure sensors was calibrated using the function f(x): y=0.0543704x for the range of measured values from 0 to 297.85 mV, and the function g(x): y=0.080703x − 7.843038 for the range of measured values exceeding 297.85 mV, where x represents measured value (mV) and y represents the pressure value (kPa). From the recorded data of 10 consecutive steps, the initial contact point was identified from the spike-shaped waveform of the vertical acceleration obtained from the inertial sensors in the lower leg and the pressure applied to the center of the heel, thereby extracting 3 consecutive gait cycles. The time of each of the 3 extracted gait cycles was normalized to 100%, and the average value of the 3 gait cycles was calculated. From the 1 averaged gait cycle, the average values of the pressure applied to the lateral malleolus in each of the following phases were calculated: initial contact (0–2%), loading response (2–12%), mid-stance (12–31%), terminal stance (31–50%), pre-swing (50–62%), initial swing (62–75%), mid-swing (75–87%), and terminal swing (87–100%)8).
Each able-bodied participant wore an AFO on their lower limb and walked straight for about 10 m with their non-AFO limb barefoot. Participants were instructed to walk at a comfortable pace, after pausing for 3 s with the ankle joint in the neutral position. Measurements were taken first with the PATS attached to the PATS-AFO, and then with the leather T-strap attached to the PATS-AFO. The gait was recorded after 3 practice sessions for each setting. When the orthosis was worn, the tension of all bands was standardized to 19.6 N using a spring scale.
For each orthosis, the pressure applied to the center of the lateral malleolus during each phase of the gait cycle was averaged, and the data obtained from 8 PATS-AFOs were compared between the PATS and leather T-strap using the Wilcoxon test. Statistical analysis was performed using JSTAT software (ver. 16.1) with a significance level of 1%.
This study was conducted after obtaining approval from the ethics committee at Niigata University of Health and Welfare (approval number: 18936-221123) and obtaining written consent from the participants following an explanation of the details of the study using instructions.
RESULTS
The results are summarized in Table 1. The pressure applied was significantly reduced with the PATS compared with the leather T-strap in three phases of gait cycle, namely loading response, mid stance, and terminal stance. No significant difference was observed between PATS and leather T-strap during initial contact, pre-swing, initial swing, mid-swing, and terminal swing (also cf. Fig. 4).
Table 1. Pressure of center on lateral malleolus (unit: kPa).
| Gait cycle | Item | Median | IQR | p-value |
| Initial contact (0–2%) | PATS | 25.00 | 58.99 | |
| Leather | 51.25 | 44.18 | ||
| Loading response (2–12%) | PATS | 35.57 | 35.94 | * |
| Leather | 60.97 | 53.45 | ||
| Mid-stance (12–31%) | PATS | 12.98 | 19.99 | * |
| Leather | 43.53 | 50.03 | ||
| Terminal stance (31–50%) | PATS | 2.16 | 7.89 | * |
| Leather | 24.47 | 34.30 | ||
| Pre-swing (50–62%) | PATS | 11.87 | 39.38 | |
| Leather | 37.78 | 59.95 | ||
| Initial swing (62–75%) | PATS | 18.11 | 79.38 | |
| Leather | 50.74 | 66.25 | ||
| Mid-swing (75–87%) | PATS | 11.94 | 33.64 | |
| Leather | 31.87 | 35.89 | ||
| Terminal Swing (87–100%) | PATS | 29.75 | 63.71 | |
| Leather | 43.69 | 46.45 |
*p<0.01 (Wilcoxon test). IQR: interquartile range; PATS: plastic ankle-foot orthosis T-strap.
Fig. 4.
Pressure of the center of the lateral malleolus (average of all participants).
DISCUSSION
From the measurement results, it was found that the PATS of the PATS-AFO is better adapted to the lateral malleolus during the loading response, mid-stance, and terminal stance phases than the leather T-strap. The fabrication of PATS involves a process of softening the material by applying heat and then molding it to the shape of the lateral malleolus of the positive model before plastic vacuum molding. Furthermore, attaching the PATS during plastic vacuum molding ensures a more suitable form that covers the lateral malleolus of each participant more widely. Therefore, compared with the leather T-strap fabricated in a plane, the PATS allows the pressure to be distributed over the entire lateral malleolus, reducing the pressure applied to the center of the lateral malleolus. Nakamura suggests that better pressure dispersion can be achieved by matching the shape of the material to that of the user’s body2).
The PATS provides better pressure dispersion probably because it has a shape that is more adapted to the user’s body than the leather T-strap. Our previous study demonstrated that an AFO with an ankle joint with a T-strap with free dorsiflexion and restricted plantarflexion was effective in correcting ankle joint inversion during gait in able-bodied participants6). In the present study, the PATS was better suited to the lateral malleolus than the leather T-strap, with a reduced pressure applied to the center of the lateral malleolus during the loading response, mid-stance, and terminal stance phases. Our next goal is to verify the effectiveness of the PATS in correcting ankle joint inversion and its suitability to the lateral malleolus in actual stroke patients. The present study was able to verify its suitability to the lateral malleolus of able-bodied participants.
Limitations of the study include that it included only able-bodied participants. In patients with excessively increased muscle tone due to ankle joint inversion, the plastic part of the PATS-AFO ankle joint may be deflected, rendering it less effective in correcting ankle joint inversion. In addition, although leather is known to adapt to the shape of the user’s body over time, the present study used newly made leather T-straps, which differed from the real-world situation. Regarding the resolution of the pressure sensors used in this study, the manufacturer’s user manual states that drift of less than 3% per log hour may occur, which may have affected the measurement results. Further studies are needed to verify the corrective effect of the PATS-AFO compared with metal strut-equipped AFOs, and to evaluate its usefulness in clinical patients with stroke hemiplegia with excessive muscle tone due to ankle joint inversion.
Funding
This study received no funding.
Conflict of Interest
The authors declare no conflicts of interest.
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