BackgroundAcute ankle sprains represent one of the most common traumatic injuries to the musculoskeletal system. Many individuals with these injuries experience unresolved symptoms such as instability and recurrent sprains, leading to chronic ankle instability (CAI), which affects their ability to maintain an active lifestyle. While rehabilitation programs focusing on sensorimotor, neuromuscular, strength and balance training are primary treatments, some patients require surgery when rehabilitation fails. A critical analysis of the patient-reported outcome tools (PROs) used to assess CAI surgical outcomes raises some concerns about their measurement properties in CAI patients, which may ultimately affect the quality of evidence supporting current surgical practice. The aim of this research is to develop and validate a new PRO for the assessment of ankle instability and CAI treatment outcomes, following recent methodological guidelines, with the implicit aim of contributing to the generation of scientifically meaningful evidence for clinical practice in patients with ankle instability.MethodsFollowing the COnsensus-based Standards for the selection of Health Measurement Instruments (COSMIN), an Ankle Instability Treatment Index (AITI) will be developed and validated. The process begins with qualitative research based on face‒to‒face interviews with CAI individuals to explore the subjective experience of living with ankle instability. The data from the interviews will be coded following an inductive approach and used to develop the AITI content. The preliminary version of the scale will be refined through an additional round of face‒to‒face interviews with a new set of CAI subjects to define the AITI content coverage, relevance and clarity. Once content validity has been examined, the AITI will be subjected to quantitative analysis of different measurement properties: construct validity, reliability and responsiveness.DiscussionThe development of AITI aims to address the limitations of existing instruments for evaluating surgical outcomes in patients with CAI. By incorporating patient input and adhering to contemporary standards for validity and reliability, this tool seeks to provide a reliable and meaningful assessment of treatment effects.Trial registrationNot applicable.