**Conflict of interest**

Jörg Lützner has received research grants from Aesculap, Link, Mathys, Smith & Nepehw and ZimmerBiomet and honoraria for lectures from Aesculap, Mathys, and Pfizer. Ana Laura Puente Reyna, Brigitte Altermann and Thomas M. Grupp are employees of Aesculap AG, Tuttlingen, a manufacturer of orthopedic implants.
