Introduction Evaluating treatment outcomes for children with anterior cruciate ligament (ACL) deficiency relies on patient-reported outcome measures (PROMs). The existing knee-specific pediatric PROMs lack content and construct validity for children with ACL injury. This study aimed to develop a preliminary version of a new PROM for children with ACL injury.

Materials and Methods The development adhered to ‘COSMIN’ guidelines for PROM development. Informants were children with ACL deficiency, purposively sampled based on age, sex, and treatment method. Semi-structured interviews were guided by an interview guide within the ICF model and continued beyond data saturation. New themes and items emerged by thematic analysis and probing items from the adult KNEES-ACL. Content coverage, relevance, and understandability were continuously evaluated. All interviews were recorded and transcribed verbatim. The NVivo 12 software was used for data analysis and coding of items. All content was tested in its final form.

Results There were substantial differences in the psycho-social impact between adults and children. The children experienced a more considerable negative psycho-social impact caused by a loss of participation in sports, lower self-confidence, and a loss of social networks. This resulted in four domains; “School”, “Friends”, “Family” and “Mood and self-confidence”. The physical aspects were quite similar, with few exceptions. 41 of 55 items from KNEES-ACL were endorsed; however, all required rewording to ensure understandability.

Conclusion A preliminary version of ‘KIDS-KNEES-ACL’ containing 60 items across nine domains was developed. Modifications based on subsequent psychometric analysis will ensure adequate measurement properties of a final version.