Participants
We included children and adolescents with brain or peripheral nerve damage resulting in functional limitations of at least one hand. Both in- and out-patients, between 6 and 18 years of age, were recruited at the Swiss Children's Rehab clinic. The participants should be able to sit for an hour and understand the tasks of the study protocol. Children or adolescents who were not able to actively flex either their shoulder or their elbow against gravity (manual muscle testing (MMT) score < 3) [15, 16] were excluded from the study. All participants and their legal guardians provided verbal consent to participate in the study: parents and adolescents aged 14 years and older provided written consent.
Age, sex, most affected hand, dominant hand, and handedness were recorded to characterize the participants. We also noted whether the children had received Botox injections in the upper limbs during the six months before the study. Participants were characterised according to six standard clinical assessments comprising the Manual Ability Classification System (MACS), MMT, Selective Control of the Upper Extremity Scale (SCUES), Hypertonia Assessment Tool (HAT), modified Ashworth Scale (MAS), and Functional Independence Measure for Children (WeeFIM). Trained therapists performed the clinical assessments, except for the WeeFIM, which was scored by trained and certified nurses in the center.
The MACS reliably classifies whether and how children handle objects in everyday life. Classifications vary between level I, where children handle objects effortlessly and successfully, and level V, where children do not handle objects at all [17].
MMT was applied to rate upper extremity muscle strength from 0, i.e., no contraction visible, to 5, i.e., movement over the full range of motion against gravity and severe resistance [15, 16]. If the patient can perform the movement against gravity covering the whole range of motion, the MMT is 3. The test protocol included standardized starting positions, a demonstration of the test by the therapist, and the active execution of the movement by the participant. Shoulder and elbow flexion as well as wrist and finger extension were tested.
The SCUES measures upper limb selective voluntary motor control [18], which is defined as the ability 'to selectively activate muscles independently of each other in the context of the requirement for voluntary movement or posture' [19]. Shoulder abduction and adduction, elbow flexion and extension, pro- and supination, wrist flexion and extension, and finger flexion and extension are tested. Each movement is scored on an ordinal scale from 0, indicating no selective motor control, to 3, reflecting normal selective motor control.
The HAT differentiates between the hypertonia categories spasticity, dystonia, and rigidity (or mixed) and consists of seven items [20]. Items 1, 2, and 6 indicate dystonia, 3 and 4 spasticity, and 5 and 7 rigidity. Each limb is scored separately.
Spasticity severity was measured with the MAS that scores the speed-dependent resistance of moving a joint [16]. In this study, the therapist assessed the MAS of the wrist and finger joints by first moving the joint covering the full passive range of motion at a slow pace, followed by a faster movement. The ordinal scale varies from 0 (i.e., no resistance during passive movement) to 4 (i.e., the affected section is rigid in flexion or extension).
The WeeFIM is a valid and reliable instrument assessing the degree of independence on a seven-level scale [21]. The functional assessment includes 18 items covering self-care, mobility, and cognition. The participants were characterized with the WeeFIM total and particularly the self-care score, as the latter contains items reflecting upper limb use in daily activities.
Assistive hand exoskeleton PEXO
The detailed design of the pediatric assistive hand exoskeleton PEXO was presented previously [12], and an overview of PEXO components is shown in Fig. 1A. In short, PEXO assists full-hand grasping in children with neuromotor impairment by actively supporting flexion and extension of the four fingers (index, middle, ring, and little finger) combined and the thumb separately, using a soft three-layered spring blade mechanism [22]. The thumb of the exoskeleton can be moved to opposition using a passive slider, allowing the users to perform different grasp types relevant for activities of daily living (e.g., power grasp, precision pinch, and lateral grasp). The hand exoskeleton provides sufficient force to grasp objects weighing up to 0.5 kg and closes and opens within 1 s. PEXO consists of a hand module (i.e., the actual exoskeleton) and a back module. The sleek hand module (weight < 105 g, maximum 1.5 cm added height on the back of the hand) is donned on a user's hand using a Velcro glove to fixate the exoskeleton on the fingers. Two straps around the wrist and one strap around the palm securely fix the exoskeleton. The back module (weight 492 g) contains the electronics, motors, and battery to power the hand module via a cable-based transmission system [23]. This design reduces the weight carried on the hand. The entire hand exoskeleton system is fully wearable since the back module can be worn as a backpack or attached to a wheelchair, allowing the user to move around freely (see also Fig. 1B, C). While the hand module of PEXO was explicitly optimized for the application in children in terms of size, weight, design, and functionality [12], the back module remained unchanged from the prior developed RELab tenoexo for adults with neurological hand impairment after stroke or spinal cord injury [13]. This commonality, combined with the possibility of detaching the transmission system from the hand module, allowed for using hand modules of different sizes with only a single back module. Hand modules were prepared in three different sizes for the left and right hand, covering the hand sizes of children aged 6 years, 7 to 8 years, and 9 to 12 years based on anthropometric data. The hand module of the adult RELab tenoexo was used by adolescents between 13 and 18 years of age. Large-diameter pushbuttons were used in this study to trigger the opening and closing movement of PEXO. An additional control unit allowed therapists or other caregivers to adjust the supporting force exerted by the hand exoskeleton.
Measurement procedures and assessments
The measurements took about two hours and were paused for a break to avoid fatigue of the participants. An experienced occupational therapist (JL) and a research engineer (JD) conducted the measurements. The order of the tests and the instructions were standardized.
First, the patient descriptors (MMT, HAT, MAS, and SCUES) were assessed (without PEXO). To determine the most appropriate PEXO size, participants had to place their hands on wooden stencils, which were created in accordance with the available hand module sizes based on age-appropriate standard anthropometry data of children. Subsequently, the participants were asked to put on the back module, the glove, and PEXO hand module as independently as possible. We recorded the time needed and whether the children needed assistance to put the separate components on. Next, the participants chose a location on the table or the body (e.g., see Fig. 1C) that was easy for them to reach, where the pushbutton to close and open PEXO was placed. Then, while wearing PEXO, the participants performed a standardized assessment with the Smart Pegboard (Neofect, Munich, Germany). This instrumented pegboard is usually used therapeutically to practice reaching, grasping, and transporting movements and fine motor skills. The pegboard includes animated games on an electronic perforated plate with light signals (Fig. 1B). Patients have to insert pegs, which can be of different dimensions, in the holes that are illuminated. For this study, we used pegs (dimensions: length 4 cm, diameter 5 mm) with a knob (diameter 10 mm) on top, allowing a lateral or tip pinch. The time needed by the participant to insert a maximum of eleven pegs was measured, with the maximum test duration being set to 120 s. The number of pegs positioned in the appropriate hole [x/11] was recorded if the participants could not insert all pegs within 120 s. Afterward, the participants had the opportunity to practice the use of PEXO on the pegboard. The practice time was recorded. Then, the pegboard assessment was repeated, once with and once without PEXO.
The grip strength and lateral pinch strength were measured with and without PEXO using the Jamar dynamometer and the finger closure gauge [15, 16, 24]. Reference data for typically developing children and adolescents are available for comparison [15, 16, 24].
We then investigated whether participants could perform various hand movements (i.e., lateral pinch, tip pinch, and fist closure). When assessing the ability to perform various hand movements, the child manually repositioned the PEXO thumb if possible. For those children who were unable to do so, the therapists assisted the child. Furthermore, they performed two functional assessments, the Assisting Hand Assessment (AHA) and the Box and Block Test (BBT), with and without PEXO. The kids-AHA is a test procedure for children between 18 months and 12 years of age and assesses how effectively a child uses its impaired upper extremity (assisting hand) in bimanual tasks. For the analysis, the participant is videotaped in a play situation. Afterward, a trained and certified occupational therapist assesses the spontaneous use of the assisting hand for 20 items. Each item is scored on a scale from 1 to 4 (1—does not do, 2—ineffective, 3—somewhat effective, and 4—effective). Rated items are, for example, whether participants initiate the use of the assisting hand themselves, open a bottle (Fig. 1C), stabilize objects or whether they reach for objects with the assisting hand [25]. The AHA provides raw scores but also scaled scores (0–100), which are derived from Rasch-analysis and are interval-scaled.
The BBT is a capacity test measuring unimanual gross dexterity of the arm and hand. Within 60 seconds, the participants need to move as many blocks as possible from one compartment of the box to the other. Age-appropriate norm values exist for children and adolescents [26, 27].
In line with the International Classification of Functioning, Disability, and Health-Children and Youth Version (ICF-CY), strength as assessed by the Jamar dynamometer is a body function, while the pegboard and the BBT are capacity measures (activity domain), and the kids-AHA is a performance measure (activity domain) [28, 29].
After completing the tests, the therapist supported the participants in doffing PEXO. The participants were asked to rate six statements concerning the training with PEXO on a Likert Scale from 1 (not at all) to 5 (very much). The statements are listed in Table 3 (P1 to P6). Additionally, the participants were questioned regarding pressure points while wearing PEXO, potentially leading to discomfort. If the participants experienced discomfort, these areas were located and the participants were asked to rate the intensity of the caused pain on a Visual Analogue Scale from 0 (no pain) to 10 (worst imaginable pain) [30]. Finally, the participants were asked to give feedback on what they liked and disliked about the therapy with PEXO.
The therapist filled in a custom-made questionnaire consisting of five statements (T1 to T5 in Table 3) and answered three open questions “If the child was not able to perform a goal-oriented training, please specify why this was not possible”, “What was your general impression of training with PEXO for this specific child?”, and “Were there any technical problems? If yes, please describe them in detail and indicate their numbers.” We rated the technical errors by their number of occurrences and severity, comparable to a retrospective failure mode, effect, and criticality analysis (FMECA) [31, 32]. The following severity levels were defined:
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1
Negligible issue not influencing performance or functionality.
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2
Marginal issue allowing successful task completion, leading to a small delay (< 1 min) and/or requiring additional action/adjustment by the user.
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3
Issue allowing successful task completion, leading to a major delay (> 1 min) and/or requiring support from a caregiver.
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4
Critical issue requiring intervention by the study coordinator to avoid potential harm to the participants and/or preventing task completion due to total failure of the device requiring technical maintenance
Finally, we noted any PEXO-related adverse events.
Outcomes and statistical analyses
Appropriateness We quantified bimanual hand performance with the AHA scaled score and hand capacity with the BBT. Due to the small sample size, the non-parametric Wilcoxon signed-rank test was performed to determine differences between the conditions with versus without PEXO. The Z-statistic value of the Wilcoxon signed-rank test and the p-value were reported.
As a first step in identifying children who could improve bimanual performance and unimanual hand capacity when using PEXO, the non-parametric Spearman's correlations (ρ) were calculated between the differences in AHA scaled scores (i.e., with PEXO minus without PEXO) and various patient characteristics and functional measures. We interpreted the magnitude of the correlation coefficients as follows: 0–0.25 (no or little relationship), 0.25–0.50 (fair degree), 0.50–0.75 (moderate to good relationship), 0.75–1.00 (very good to excellent).
In addition to the correlation analyses, we calculated a dichotomous variable indicating an improvement in bimanual performance yes/no. Based on the standard error of measurement calculated for the intra-rater reliability (raw score: 1.2 points), we estimated the smallest detectable change (2.77 × 1.2 = 3.3) and made a conservative estimation of the smallest detectable change for the scaled scores (i.e., 7 points) using transformation curves published by the authors of the AHA [33], i.e., we interpreted an improvement of 7 points or more when wearing PEXO as a conservative estimate of improved bimanual performance. To identify characteristics and functional measures that differed between the children who could improve bimanual hand performance when wearing PEXO or not, chi-square tests were used to determine differences in dichotomous measures and Wilcoxon signed-rank test to determine differences in ordinal or interval-scaled measures. Furthermore, Receiver Operating Characteristics (ROC) analyses were performed to determine the level of sensitivity and specificity with which the ordinal and interval-scaled measures could distinguish between participants who performed better when wearing PEXO (≥ 7 points improvement in scaled AHA scores) and those who did not.
To investigate familiarization with using PEXO, data from the Smart Pegboard was used (number of correct placements from 11 pegs and time needed to accomplish the task). Differences in the pegboard scores were compared between pre- to post-practice time. The post-practice conditions with versus without PEXO were further compared. While α was generally set at 0.05 for all comparisons, we set it at 0.025 for these multiple comparisons.
Practicability Time needed for donning the components of PEXO and whether this could be done independently by the participant. For the time needed to don the back module, the glove, and the hand module, the median and interquartile range (IQR) were calculated and the minimum and maximal values were reported.
Furthermore, the number and nature of technical and safety issues were described.
Acceptability The descriptive values of the Likert scores that participants and the therapist provided to the various questions were reported.