Discussion
ACL, pain
SECONDARY
Overall, our analyses of several alternative models provide support for unidimensional scoring of the IKDC with some qualifications, in a population of young active patients with ACL tears.Based on exploratory analyses of the IKDC, it appears as though two distinct factors may be present when used with ACL patients, in contrast to the intended unidimensional structure. However, eigenvalues from the exploratory analysis indicated similar strength of the first factor in this study and the initial IKDC development study [However, when confirmatory analyses were conducted, both the single and two-factor structure of the IKDC required too many modifications to obtain adequate fit for us to have confidence in the utility of these models for our patient population. Accepting the one- or two-factor structures of the IKDC with the required modifications would complicate scoring and convolute interpretability of IKDC in young active patients with ACL tears. The pattern of item correlations suggested by modification indices during the CFA process made it clear that items worded similarly or sharing the same number of response options tended to group together. This is intuitive and may indicate clustering of items based on linguistic factors rather than constructs of the questionnaire. This analysis outlines potential issues with multiple rating scales in the same unidimensional outcome as a consideration when developing future measures. When creating new orthopaedic PROMs, it may be helpful to standardize the response options and linguistic style of questions to avoid such item clusters.There were four content factors created: symptoms (items 2, 3, 4, 6), activity level (items 1, 5, 7, 8, 10b), ADLs (items 9a-f), and sport (items 9 g-i), and a four factor model was tested based on these theoretical relationships between items. Finally, a bifactor model was run to test the hypothesis that there is indeed one overall general factor of “symptoms, function and sports activity” as intended, with residual associations between subsets of items similarities in with linguistics, response options, or content. The symptoms factor covers patients’ pain, stiffness, and locking/catching of the knee. The activity level factor consists of questions about the activity level a patient can participate in without various symptoms. The ADL and sport factors are distinct in the types of activities being inquired about. We believe that this distinction is important for our population of interest and a consideration that was missing from the previously proposed two-factor structure.The four-factor and bifactor model showed acceptable fit indices without modifications, unlike the previously tested structures. The fact that some items’ loadings decreased when moving from the four-factor model without the general factor to the bifactor model suggests that these particular items overlap more strongly with the general factor. Additionally, the fit comparison of these nested models indicated improved fit with addition of the general factor (bifactor specification). Each IKDC item showed adequately strong (≥ 0.35) associations with the general factor, and generally weaker and inconsistent associations with the secondary, more specific content factors.Importantly, measures of explained systematic, reliable, and common variance (ωTherefore, in the context of young active ACL patients, we propose a bifactor model structure for the IKDC whereby one general factor best represents the items as intended, but item clusters caused by differences in content and item construction (wording and response options) are accounted for to obtain adequate fit. Clinically, this means that clinicians and researchers can continue to use the IKDC as it was designed, as a unidimensional instrument for young active patients with ACL tears. What the bifactor model adds is acknowledgement of item clusters depicting symptoms, activity level, ADLs, and sport. Clinicians and researchers can look at these clusters of items more closely to determine what secondary factor may be driving a high or low score, and patient progress or lack thereof. Importantly, these factors should not be treated as subscales, and scoring of the IKDC has been preliminarily validated in this population to remain as is. Forming subscales in addition to the total score creates a multi-factor correlated model, with resultant scores which would be highly correlated and redundant, because they would include a mix of both the general and secondary factors. The bifactor model therefore does not change use or scoring of the IKDC in young active patients with ACL tears but increases confidence in the utility of one total score and adds further nuance to the interpretability of the scale for these patients.