The Comprehensive Aachen Knee Score: Development and validation of a new patient-reported outcome measure for patellofemoral pathologies

Background Existing patient-reported outcome measures (PROMs) that assess knee pain and symptoms show insufficient quality regarding their psychometric properties. We developed and validated a new PROM assessing knee pain and especially patellofemoral pathologies, the COMPrehensive AaChen Knee Score (COMPACK). Methods We followed a three-phase process of development and validation. (1) Scale development: we conducted a systematic literature research to collect and analyse the most commonly used PROMs for knee and patellofemoral pathologies to identify appropriate items. (2) Pilot testing: we assessed the content validity of the COMPACK based on interviews with patients and health experts regarding relevance, comprehensibility and comprehensiveness of the used items. (3) Field testing: we used the COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN) guidelines to evaluate the structural validity, internal consistency and test–retest reliability of the COMPACK. Results The COMPACK scale showed good structural validity, with two subscales measuring pain intensity and frequency. Further, scores of the COMPACK correlated positively with those obtained from the International Knee Documentation Committee Subjective Knee Form (IKDC) (r = 0.64) and the 12-item Short Form Health Survey subscale of physical function: (r = 0.43). Both the total scale and the two subscales showed good internal consistency (Cronbach’s α > 0.82) and good to excellent test–retest reliability (Cronbach’s α > 0.82). Conclusions The final COMPACK score was shown to be a valid and reliable instrument and thus meets the most important psychometric quality criteria. In its development, we specifically focused on its content validity, structural validity and applicability.