The German version of the KOOS-Child questionnaire (Knee injury and Osteoarthritis Outcome Score for children) shows a good to excellent internal consistency and a high test–retest reliability in children with knee problems

PURPOSE: The Knee Injury Osteoarthritis Outcome Score for children (KOOS-Child) is a self-administered, valid and reliable questionnaire for children and adolescents with knee disorders such as Osgood Schlatter disease, anterior knee pain, and patella dislocation. This study aimed to cross-culturally adapt the German version of the KOOS-Child questionnaire and test the reliability in two groups of children, one treated conservatively and the other surgically. METHODS: A forward–backward translation of the original questionnaire into the German language was conducted. Children and adolescents between 10 and 18 years of age with knee disorders were included. Two groups were compared: sample one consisted of 24 participants with knee pain [20.8% boys; mean age = 13.4 (1.8) years treated conservatively. These participants completed the KOOS-Child questionnaire twice within two weeks to assess test–retest reliability. The second sample included 23 subjects (21.7% boys; mean age = 15.3 (1.9) years] treated surgically due to a knee disorder. They completed the questionnaire before surgery and six months postoperatively. Test–retest reliability and internal consistency were assessed using Spearman’s rank correlation and Cronbach’s alpha. RESULTS: All subscales showed a good to excellent internal consistency at both measurement points in both groups (conservatively treated group: a = 0.88–0.95; surgery group a = 0.80–0.91), with the exception of the subscale knee problems (conservatively treated: a = 0.60 and 0.52; surgery: α = 0.77 and 0.66). Test–retest reliability was between r = 0.85 and 0.94. CONCLUSION: The predominantly good to excellent internal consistency and the high test–retest reliability justifies the use of the German adaptation of the KOOS-Child questionnaire as a reliable multidimensional instrument for measuring health status and therapeutic effects in adolescents’ knee disorders. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00167-022-07074-4.