Introduction Content validity is the most important property of PROMs. The COSMIN guidelines are often referred to as gold standard to evaluate PROM properties. The aim of this study was by use of the COSMIN checklist to evaluate the content validity of five PROMs, all highly relevant in musculoskeletal research; the modified Harris’ Hip Score (mHHS), the Copenhagen Hip and Groin Outcome Score (HAGOS), the International Knee Documentation Committee Subjective Knee evaluation Form (IKDC-SKF), the Knee injury and Osteoarthritis Outcome Score (KOOS) and the Knee Numeric-Entity Evaluation Score ACL (KNEES-ACL).

Materials and Methods Development articles were identified in PubMed and SCOPUS. A secondary literature search identified studies assessing content validity of the PROMs. Missing information was obtained from the five developers after direct request. To evaluate the quality of the development studies and rate the content validity, the COSMIN Risk of Bias checklist was applied to all relevant studies by two independent researchers.

Results The development of mHHS, IKDC-SKF, and KOOS was rated inadequate, and these PROMs possess insufficient content validity. KOOS was in particular inappropriate to evaluate patients with ACL injury, but it is, despite this, the primary outcome in the Scandinavian ACL-reconstruction registries. The development of HAGOS was rated inadequate, although the insufficiency aspects can be regarded as minor. KNEES-ACL possessed sufficient content validity.

Conclusion Out of five highly relevant orthopaedic PROMs, only KNEES-ACL possessed sufficient content validity according to COSMIN guidelines. There is an urgent need in musculoskeletal research for condition-specific PROMs developed with adequate methods.