Individual stiffness optimization of dorsal leaf spring ankle–foot orthoses in people with calf muscle weakness is superior to standard bodyweight-based recommendations

Abstract

Background In people with calf muscle weakness, the stiffness of dorsal leaf spring ankle–foot orthoses (DLS-AFO) needs to be individualized to maximize its effect on walking. Orthotic suppliers may recommend a certain stiffness based on body weight and activity level. However, it is unknown whether these recommendations are sufficient to yield the optimal stiffness for the individual. Therefore, we assessed whether the stiffness following the supplier’s recommendation of the Carbon Ankle7 (CA7) dorsal leaf matched the experimentally optimized AFO stiffness. Methods Thirty-four persons with calf muscle weakness were included and provided a new DLS-AFO of which the stiffness could be varied by changing the CA7® (Ottobock, Duderstadt, Germany) dorsal leaf. For five different stiffness levels, including the supplier recommended stiffness, gait biomechanics, walking energy cost and speed were assessed. Based on these measures, the individual experimentally optimal AFO stiffness was selected. Results In only 8 of 34 (23%) participants, the supplier recommended stiffness matched the experimentally optimized AFO stiffness, the latter being on average 1.2 ± 1.3 Nm/degree more flexible. The DLS-AFO with an experimentally optimized stiffness resulted in a significantly lower walking energy cost (− 0.21 ± 0.26 J/kg/m, p < 0.001) and a higher speed (+ 0.02 m/s, p = 0.003). Additionally, a larger ankle range of motion (+ 1.3 ± 0.3 degrees, p < 0.001) and higher ankle power (+ 0.16 ± 0.04 W/kg, p < 0.001) were found with the experimentally optimized stiffness compared to the supplier recommended stiffness. Conclusions In people with calf muscle weakness, current supplier’s recommendations for the CA7 stiffness level result in the provision of DLS-AFOs that are too stiff and only achieve 80% of the reduction in energy cost achieved with an individual optimized stiffness. It is recommended to experimentally optimize the CA7 stiffness in people with calf muscle weakness in order to maximize treatment outcomes. Trial registration Nederlands Trial Register 5170. Registration date: May 7th 2015. http://www.trialregister.nl/trialreg/admin/rctview.asp?TC=5170.

Design principles, manufacturing and evaluation techniques of custom dynamic ankle‐foot orthoses: a review study

“…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 – 50 ]. The energy absorbed/released by custom AFOs during gait has been seldom addressed in the literature [ 29 , 54 ].…”

“…For each paper, when present, it is reported the AFO type(s), the customization criteria, the materials, the functional data/parameters, and the main outcome. Comfort assessment or other subjective scores are also reported Authors/year Population (size) AFO type/ customization criteria Material Motor tasks Functional parameters Other scores Main outcome Waterval et al 2021 [ 56 ] unilateral plantar flexor weakness (9) dorsal leaf spring AFO Spring leaf Stiffness customizable energy cost optimized (Ankle7, OttoBock) carbon fiber walking spatio-temporal parameters GRFs hip, knee, ankle kinematics and kinetics peak vertical GRF of the contralateral leg significantly reduced and symmetry improved (AFO vs. no AFO) Waterval et al 2021 & 2020 [ 32 , 33 ] calf muscle weakness (34) dorsal leaf spring AFO Spring leaf Stiffness customizable (Ankle7, OttoBock)e carbon fiber walking spatio-temporal parameters hip, knee, ankle kinematics and kinetics energy cost reduction in energy cost (AFO optimized stiffness vs. non optimized) Kerkum et al 2021 [ 35 ] healthy subjects (12) dorsal leaf spring AFO Spring leaf Stiffness customizable (Ankle7, OttoBock) carbon fiber walking Ankle-foot kinematics work and power Total ankle-foot power increase with increasing footplate stiffness Lin et al 2021 [ 57 ] post-stroke drop-foot (12) 1. energy-Storage 3D Printed AFO 2. anterior-support AFO PLA + nylon+titanium thermoplastic walking spatio-temporal parameters pelvis, hip, knee, ankle kinematics (sagittal plane) Evaluation of satisfaction (QUEST) increased gait velocity and stride length (AFO1 vs. AFO2; AFO1 vs. barefoot) improved satisfaction (AFO1) Meng et al 2021 [ 58 ] post-stroke drop-foot (15) morphology PA2200 Somos NeXt PA12 NA NA …”

“…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, 30–33, 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.…”

“…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].…”

Carbon fiber ankle-foot orthoses in impaired populations: A systematic review

“…The body of literature is continually growing; therefore, a limitation of this study is the inability to capture relevant articles after conducting the search. [101][102][103] Furthermore, despite the purposeful language in the inclusion criteria, studies using devices primarily made of plastic with carbon fiber components may have been missed. 104 Additional limitations include the identification of articles exclusively in the databases evaluated.…”