High Ankle Carbon Fiber Energy Storage Foot

A total of 242 papers were found. 75 of these complied with the inclusion criteria and were included in the review. Some of these papers covered more than one macro-topic specified in Material and Methods. The number of papers covering each topic follows:

1. a)

   16 papers addressed and collected patient geometry of the shank and foot;

2. b)

   14 papers reported AFO customization criteria other than those based on foot and leg morphology;

3. c)

   19 papers reported the production techniques;

4. d)

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   16 papers investigated characterization of mechanical properties;

5. e)

   33 papers reported the functional evaluation of patients/subjects.

A summary of the main results from the literature review on each topic is summed up in the following subsections.

1. a)

   Scanning technologies and geometry acquisition

Custom AFOs are traditionally modelled by hand by the orthotist via thermal molding on models of the patient’s foot and leg. Traditionally, the plaster model is obtained by filling the negative impression of the patient’s cast with liquid plaster. The custom AFO is then manufactured over the positive model. This process, however, is time-consuming and highly operator-dependent. Therefore, in the last 10 years, new technologies to obtain a 3D digital replica of the patient’s geometry have been used to create a solid model of the foot and leg: laser-based scanners [5,6,7,8,9,10] (6 out of 16 studies); structured-light scanners [11,12,13] (3/16); computer tomography [5, 14,15,16] (4/16); 3D coordinate digitizer to acquire landmark positions [17, 18] (2/16), and photogrammetry [19] (1/16). According to recent reviews [20, 21], 3D scanning, computer tomography and optical motion capture systems all represent valid and reliable alternatives to traditional casting methods to obtain a solid model of the patient’s foot and leg geometry.

1. b)

   Customization criteria

According to the present review, PD-AFOs are usually customized on the patient’s lower limb morphology. Few studies used a commercial customizable PD-AFO — the modular Intrepid Dynamic Exoskeletal Orthosis (IDEO) — featuring a posterior strut, the stiffness of which can be customized to the patient’s ankle ROM, the type and level of activities, body mass and load carriage requirements [22,23,24,25]. A similar modular design featuring a variable stiffness rod in relation to the patient’s degree of impairment was proposed [26]. However, no indications are provided on the weight and the direction (towards stiffer or more compliant) of each parameter on the strut rigidity. AFO stiffness optimization based on the minimization of knee angle and energy cost of walking was reported for children with cerebral palsy [27, 28]. A combination of the following parameters has also been used as input data to set the stiffness of the custom AFOs: the patient’s prior experience; visual observations of patient’s gait; body weight; muscle strength; severity of ankle deformity [29,30,31,32,33]. Only one study customized the AFO stiffness according to the natural ankle pseudo-stiffness [34]. The majority of the studies optimized the stiffness of the calf shell. Only one study reported the effect of footplate stiffness on ankle joint power in gait [35].

1. iii)

   Production techniques

Additive manufacturing is becoming widely used in orthopaedics, since it allows to obtain complex shaped devices made with a number of different materials [20]. The present review, in agreement with two recent studies [36, 37], has shown that most 3D-printed PD-AFOs are manufactured via Selective Laser Sintering (SLS) [5, 6, 8, 14, 15, 18, 25, 26, 38,39,40,41,42] and Fused Deposition Modeling (FDM) – also known as Fused Filament Fabrication (FFF) – [10, 15, 17, 43, 44]. SLS works with a high-power laser to sinter polymer powders, while FDM adds melted thermoplastic filaments in consecutive stratified layers to create the object. Stereolithography (SLA) [7, 11] and Multi Jet Fusion (MJF) [11] are less frequently used to produce custom AFOs. In SLA, a UV laser induces polymerization of a photopolymer to obtain the object; in MJF, a fusing agent is deposited on layers of heated powder where the particles are fused together.

1. iv)

   Mechanical testing

This section is reporting only studies related to the experimental analysis of custom-made PD-AFOs. Whenever the AFO type was not clearly defined as “dynamic”, it was decided to include only the manuscripts which reported the force/deformation properties, providing evidence of a dynamic behavior of the orthosis. Three review studies were found which reported stiffness values for a variety of AFOs — custom and off-the-shelf — and the testing methods [3, 45, 46]. Most of these studies investigated the stiffness properties in plantar-dorsiflexion in the range 20 deg plantar- to 30 deg dorsiflexion. Only one study assessed the AFO’s mechanical properties outside the sagittal plane [47].

Most studies assessed the stiffness properties of the strut component, i.e. the long, flexible part of the calf shell [17, 41, 47,48,49,50,51]. Fewer studies investigated the mechanical properties of other components, such as the foot plate [50], or isolated parts of the AFO [52]. Displacements during AFO deflection were assessed in two studies [49, 53], while only one study performed a fatigue test [44]. A few papers [17, 49, 52] reported the mechanical testing of dynamic AFOs which were customized on a healthy subject’s leg or on other geometrical models of the lower limb and not for drop-foot patients were included in this review. In general, the AFO foot plate is fixed, and bending moments/forces or displacements are applied to the calf shell, simulating ankle dorsiflexion. The reported bending stiffness of the strut, in terms of resistance to dorsiflexion moment, ranged between 0.12 and 8.9 N*m/deg across these studies [15, 17, 33, 41, 48,49,50]. The energy absorbed/released by custom AFOs during gait has been seldom addressed in the literature [29, 54].

Custom PD-AFOs have also been tested in-silico via FEA [17, 42, 48, 52,53,54]. Boundary conditions were generally consistent with those used for the experimental mechanical tests, when present. In addition to stiffness [17, 42], FEA allowed to estimate the maximum Von Mises stresses [52, 55] and displacements [53] of the analyzed AFOs. Only one study assessed the maximum Von Mises stress against the material yielding [52], and reported the safety factor of each component in simulated jogging and downhill walking tasks.

1. e)

   Functional evaluation

Table 1 sums up the outcome of the literature review in relation to the functional evaluation of custom dynamic AFOs. Thirty-three papers published from 1999 to 2021 were retrieved and found relevant to the topic. In terms of populations investigated, custom AFOs were used for post-stroke patients (n = 6) [11, 34, 57, 58, 62, 64], for generic drop-foot and muscles weakness (n = 13) [8, 24, 30,31,32,33, 39, 40, 44, 50, 51, 56, 59], for lower limb reconstruction (n = 4) [22, 23, 25, 60], for cerebral palsy (n = 4) [27, 28, 61, 66], for Charcot–Marie–Tooth (n = 1) [29], in children with hemiplegia (n = 2) [63, 65], and in normal/healthy subjects (n = 3) [7, 35, 43]. Posterior Leaf Spring (PLS) are the most common types of AFOs functionally evaluated and were compared to solid and hinged AFOs, and/or to shod/barefoot conditions. Carbon-fiber was found to be the most used material; plastic (nylon and polyamide) and thermoplastic (polypropylene and polyurethane) were also used due to their favorable manufacturing process and compatibility with current 3D printing technology. In terms of functional evaluation, gait analysis during walking at comfortable speed was by far the most common motor task investigated. Three studies reported on stair ascent/descent, and two studies reported on walking over an inclined ramp or treadmill. In one study, the AFOs were evaluated in a static balance test. Spatio-temporal parameters and lower limb joint kinematics and kinetics (mainly in the sagittal plane) were usually recorded and analyzed. Two studies also reported on surface EMG of the main lower limb muscles. Six studies reported on other qualitative scores such as comfort or ease of use (donning and removing). In terms of spatio-temporal parameters, while it is difficult to compare the functional outcome of PD-AFOs customized and produced for different populations with ankle weakness, 8 studies reported improved gait velocity and stride length in custom AFOs with respect to solid AFOs or shod/barefoot conditions. Due to the flexibility of the calf shell, custom PD-AFOs can absorb and release energy during walking. The two studies that assessed this parameter reported a reduction in the energy cost of walking while wearing the optimal stiffness AFOs with respect to other AFOs.

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