The International Knee Documentation Committee Subjective Knee Form (IKDC) is the most highly recommended patient reported outcome measure for assessing patients with anterior cruciate ligament (ACL) injuries and following ACL reconstruction (ACLR) surgery. The IKDC was developed as a unidimensional instrument, however the structural validity of the IKDC has not been definitively confirmed for the young athletic ACL population. Structural validity would be demonstrated if factor analyses confirmed that the IKDC consists of a single (i.e., unidimensional) factor, as the questionnaire developers intended.
Determine whether a unidimensional or multidimensional structure of the IKDC is most appropriate for use in young, active patients with ACL tears.
A cross sectional retrospective secondary data analysis of the Stability 1 baseline IKDC data was completed to assess the structural validity of the IKDC using exploratory and confirmatory factor analyses. In total, 618 patients were randomized into the Stability 1 trial. Patients between 14 and 25 years old, with factors deeming them at high risk of re-injury were included. A cross sectional retrospective secondary data analysis of the Stability 1 baseline IKDC data was completed to assess the structural validity of the IKDC using exploratory and confirmatory factor analyses. Factor analyses were used to test model fit of the intended unidimensional structure, a previously proposed two-dimensional structure, and an alternative bifactor structure (i.e., a combination of a unidimensional factor with additional specific factors) of the IKDC, in a dataset of young active ACL patients.
Of the trial participants, 606 patients (98%) had complete baseline IKDC questionnaire data available for this analysis. The simple unidimensional and two-dimensional structures of the IKDC displayed inadequate fit in our dataset of young ACL patients. A bifactor model provided the best fit. This model contains one general factor (symptoms, function, and sports activity) that is strongly associated with all items, plus four secondary, more specific content factors (symptoms, activity level, activities of daily living, and sport) with generally weaker associations to subsets of items. The bifactor model supports the unidimensionality of the IKDC when covariance between items with similar linguistic structure, response options, or content are acknowledged.
Overall, findings of a bifactor model with evidence of a reliable general factor well defined by all items, lends support to continue interpreting and scoring this instrument as unidimensional. Clinically, the IKDC can be represented by a single score for young active patients with ACL tears. A more nuanced interpretation would also consider secondary factors such as sport and activity level.