Data sources

The Danish National Patient Register (DNPR) contains information on all in- and outpatient contacts with Danish private and public hospital departments (10). The completeness and validity of the clinical data in DNPR vary, but the registration of orthopedic procedure codes is considered to have the highest validity and accuracy of all clinical specialties. The data reported in the DNPR is of 2 types: administrative data including the CPR number, date, and time of activity; and clinical data including diagnosis and surgical procedures (10). The surgical procedures have been coded with high validity according to the Danish version of Nordic Medico-Statistical Committee Classification of Surgical Procedures (11). The Danish Register of Causes of Death and the Danish Population Statistic Register from Statistics Denmark contributed information on age, sex, and death (12, 13).