A septic loosening is loosening at the interface of the implant and the bone, or it could be loosening at the interface between the cement and the bone. With time, there is wear of the plastic, and the plastic can eat into the bone causing osteolysis. If the osteolysis is significant, implant fixation is compromised. There are certain uncemented implants which have poor track record, and those implants loosen out earlier than cemented knee replacement surgeries that have had a longer track record.
There are also certain designs of knee replacement surgery, which allow more percolation of the debrided plastic into the bone, and there’s a thought that there is more osteolysis associated with certain designs of knee implant. It is therefore necessary to get annual radiographs and a physical examination to see if there is significant osteolysis (bone erosion), after a knee replacement.