Translation procedure
The forward-backward procedure was performed according to the guidelines from the study of Beaton et al. [10]. First, two native Dutch speakers independently translated the Pedi-IKDC and the KOOS-Child from English into Dutch. Second, two native English speakers independently and blinded for the original versions translated the preliminary Dutch versions back into English. These two translations were compared with the original English versions. All inconsistencies were discussed and adjustments were made. Finally, the second preliminary Dutch versions were evaluated by the Department of Patient Communication of the Maxima Medical Centre (Eindhoven-Veldhoven, The Netherlands). After adjustments, the pre-final Dutch versions were made. Subsequently, the questionnaires were pilot tested in a heterogeneous group of 10 children, aged 10–15 years, who received treatment for orthopaedic-related knee problems. Respondents were asked about item difficulty. In case of indistinctness or incomprehensibility adjustments were made. Final Dutch versions of the Pedi-IKDC and KOOS-Child were obtained. Patients of the pilot study were not included in the validation study.
Participants
Patients were consecutively recruited from the outpatient clinic of orthopaedics of the Maxima Medical Centre (Eindhoven-Veldhoven, The Netherlands) from February 2015 and May 2016. Patients were considered eligible if they were aged 10–18 years, had a knee injury and physical limitations. The study had the intention to include a heterogeneous patient population that was representative for children with a broad range of knee injuries. Patients were excluded if they had any comorbidity overriding their knee symptoms or had an inadequate command of the Dutch language. If necessary, parents or caregivers were allowed to assist their child in completing the questionnaires.
Study procedure
Eligible patients visiting the outpatient clinic were asked to complete the Dutch Numeric Rating Scale (NRS) Pain, Lysholm Knee Scoring Scale, EuroQol-5 Dimension (EQ-5D), Pedi-IKDC and KOOS-Child via Research Manager (baseline (T0)). Two weeks after baseline (T1) patients answered by e-mail the anchor question ‘did your knee complaints change over the course of the past two weeks after completing the last questionnaires?’ The time point of two weeks was chosen to prevent patients from remembering the previous answer and to avoid change in knee condition. The anchor question was used as an equivalent of possible change in knee condition of the patient during the past two weeks [11]. Consequently, only patients with stable knee conditions were asked to complete the Pedi-IKDC and KOOS-Child. One year after baseline (T2) all patients were asked by e-mail to complete an anchor question, Pedi-IKDC and KOOS-Child. This time point was chosen because clinical change of the patients could be expected. If needed, patients were reminded by e-mail and telephone.
Questionnaires
Pedi-IKDC
The Pedi-IKDC measures pain, symptoms and physical functioning during daily and sport activities in children and adolescents with knee-related problems [7]. A higher score represents less pain, less symptoms and a higher level of physical functioning. The Pedi-IKDC has been shown a valid, reliable and responsive questionnaire in children (aged 9–18 years) with a broad range of knee disorders [7, 9].
KOOS-child
The KOOS-Child evaluates short- and long-term symptoms, physical functioning during daily and sport activities and quality of life (QOL) in patients with a broad range of knee disorders. The KOOS-Child (LK 2.0) has 39 items, divided over the five subscales Symptoms, Pain, activities of daily living (ADL), Sport/play and QOL. A higher score indicates less knee-related problems. An aggregate score is not calculated since it is recommended to analyze and interpret the five dimensions separately. The study by Ortqvist et al. [12] showed good validity and reliability of the KOOS-Child in children with knee disorders.
NRS pain
The NRS Pain measures the level of pain at rest and during movement. The NRS Pain is a valid and reliable questionnaire in adult patients with knee osteoarthritis and a variety of musculoskeletal injuries. The NRS Pain showed strong correlations with the Faces Pain Scale Revised and Visual Analogue Scale in children, with bodily pain or postoperative pain. However, the psychometric properties of the NRS Pain in children with knee disorders have not yet been evaluated [13].
Lysholm knee scoring scale
The Lysholm Knee Scoring Scale measures symptoms of instability and was originally developed for patients who underwent or were about to undergo a knee ligament surgery. The Lysholm Knee scale is proven to be a valid and reliable questionnaire in adolescents (12–17 years) and adults with traumatic and non-traumatic knee problems [14].
EQ_5D
The EQ_5D measures health-related QOL by evaluating mobility problems, self-care problems, problems during usual activities and the level of pain and anxiety. The EQ_5D has shown highly valid and reliable in adults with knee osteoarthritis or patients undergoing arthroscopic knee surgery. In children the EQ_5D has shown valid for orthopedic problems, however not specifically for knee disorders [15] .
Psychometric properties
The psychometric properties were evaluated according to the COnsensus-based Standards for the selection of health Measurement Instruments (COSMIN) criteria [16,17,18].
Reliability is the degree to which a research instrument produces consistent results for patients who have not clinically changed. Based on the anchor question, patients with unstable knee condition between T0 and T1 –two weeks– were excluded from the analysis. Unstable knee conditions were defined when answered much better, better, worse or much worse. To assess the consistency of results, the test-retest reliability was measured using the two-way random effects model of Intraclass Correlation Coefficient (ICC) in absolute agreement. ICC coefficients of 0.70 or higher were considered excellent. The Standard Error of the Mean (SEM) (SEM = standard deviation (SD) difference/√number measurements) and the Smallest Detectable Change (SDC) (SDC = SEM × 1.96 × √2) were used as parameters of measurement error within the test-retest reliability.
Content validity is the degree to which the instrument is an adequate reflection of what it aims to measure. It was measured by evaluating the relevance of each item by orthopaedic surgeons/residents and patients. An item was defined as relevant if at least 75% of a subgroup found it to be relevant. The content validity of the Pedi-IKDC and KOOS-Child subscales was given a positive rating if 75% of the items were found to be relevant by both subgroups.
Construct validity is the degree to which scores of the instrument are consistent with hypothesis about the relationship to scores of other instruments. In this study, construct validity was measured by testing predefined hypotheses about the relationship of the KOOS-Child and Pedi-IKDC to other questionnaires (outlined in Table 6). A panel consisting of experts (specialist in clinimetrics and methodologist) in the field of outcome measurements in knee complaints formulated these hypotheses. These hypotheses were tested by calculating Pearson’s correlation coefficient between scores at T0 of the Pedi-IKDC, KOOS-Child subscales, NRS Pain, Lysholm scale and EQ-5D. Questionnaires measuring similar constructs were expected to correlate 0.5 or higher. In contrast, questionnaires focussing on different constructs were expected to correlate 0.35 or less. Direction of relation was taken into account by means of sign of expected correlation. The construct validity of the Pedi-IKDC and KOOS-Child subscales was considered good if at least 75% of the predefined hypotheses were accepted.
Interpretability is the degree to which one can assign qualitative meaning to a score. The interpretability was assed at T0 by examining the distribution of the Pedi-IKDC score and KOOS-Child subscale scores, including the mean, SD, minimum, maximum, median and floor and ceiling effects. Floor and ceiling effects were considered present if ≥15% of the patients scored the Pedi-IKDC or a KOOS-Child subscale with respectively 0–5 or 95–100. The interpretability of the Pedi-IKDC and KOOS-Child subscales was given a positive rating if floor or ceiling effects were absent.
Responsiveness is the ability of an instrument to detect change over time. The responsiveness was evaluated by comparing the change scores of the Pedi-IKDC and KOOS-Child subscales between baseline (T0) and 1 year (T2). Effect size (ES) was calculated as follows: (mean T2 – mean baseline scale)/SD of baseline scale. Standardized response mean (SRM) was calculated as follows: (mean T2 – mean baseline scale)/SD of change in scale. Hypotheses about the expected effect size and standardized response mean of the KOOS-Child and the Pedi-IKDC subjective were formulated (outlined in Table 6). Hypotheses were formulated separately for patients reporting no change and for patients who reported a difference in their experience knee condition (improvement or worsening). In creating the hypotheses, ES of ≥0.2 were interpreted as small, ≥0.4 as medium, ≥0.8 as large. Responsiveness was scored as adequate if minimally 75% of the hypotheses were confirmed.
A sample size of at least 50 patients is recommended for good test-retest reliability, construct validity, interpretability and responsiveness and > 10 respondents for excellent content validity [19]. All questionnaires were collected using the software program Research Manager Version 4.7.2.4. Data were analysed with SPSS version 22 (IBM SPSS Statistics). A significance level of p ≤ 0.05 was applied for all statistical analysis.
