Abstract
Prosthetic loosening and periprosthetic osteolysis have been debated for decades, both in terms of the timing and nature of the triggering events. The hypothesis of wear-particle-induced loosening states that wear particles cause a foreign-body response leading to periprosthetic osteolysis and ultimately to late prosthetic loosening, i.e., that the osteolysis precedes the loosening. The theory of early loosening, on the other hand, postulates that the loosening is already initiated during or shortly after surgery, i.e., that the osteolysis is secondary to the loosening. This commentary focuses on the causal relationship between prosthetic loosening and periprosthetic osteolysis.