Introduction
ACL injuries, ACL, osteoarthritis, various knee conditions/injuries, various knee problems
OSTEOARTHRITIS
The International Knee Documentation Committee Subjective Knee Form (IKDC) is a region-specific 18-item questionnaire that collects information regarding symptoms, function, and sports activities, in patients with various knee conditions, reflected in one overall score [The IKDC results in little patient burden, with studies reporting adequate internal consistency reliability, test–retest reliability, construct validity, and responsiveness, with no floor or ceiling effects across patients with various knee conditions [A systematic review of psychometric properties of PROMs used for ACL injuries indicated moderately positive evidence of test–retest reliability and hypothesis testing validity for the IKDC in this specific patient population [Developers of the IKDC hypothesized that the underlying factor structure of the IKDC would be unidimensional, which was confirmed during its development through factor analysis with polychoric correlations, in a sample of patients with various knee conditions/injuries [Higgins and colleagues proposed an alternative shortened two-factor model of the IKDC, with 15 of the 18 original items, using over 1000 patients with various knee problems [It is important to understand what scoring system (i.e., a single overall score, or two or more subscale scores), will be most meaningful and appropriate when interpreting IKDC outcomes in patients with ACL tears. These patients tend to be quite different than those with other knee conditions such as meniscal tears or osteoarthritis (often younger and more active in specific cutting/pivoting sports). Therefore it is possible that questionnaire structure might be different for these respondents, and that their outcome should be scored or interpreted differently. Based on the relative lack of evidence for structural validity of the IKDC for respondents with ACL injuries [We chose to focus on investigating structural validity at one time point, as the proposed structure should be confirmed in other datasets before tackling other psychometric properties reliant on the factor structure such as measurement invariance across time, internal consistency, or cross-cultural validity.