Definition of infection.

Acute periprosthetic infections occur either early, due to perioperative contamination, or late and haematogenous, and are caused by virulent bacteria. Late chronic or delayed periprosthetic infections produce implant loosening or symptoms of articular pain, and are caused by low-virulence bacteria. The intension of the definitions characterising the virulence of bacteria are more important than exact periods (delayed from 4 weeks to 2 years, early before and late after this period).