Introduction: Survivorship of total joint replacement is limited by infection or mechanical failure. While acute infection is obvious low grade infection is sometimes difficult to prove.

Regarding to the literature the management in revision surgery I terms of differentiate between septic and aseptic loosening is still a matter of debate.

Material and Methods: From 1997 to 2004 we investigated 228 revision total knee replacements prospectively as far as bacterial colonisation is concerned. If there was pre- or intraoperative evidence of infection the loosening was quoted as septic. So 40 two stage revision in cases of suspicion and 188 one stage revision had been performed.

Specimen of joint fluid, of membranes from the metal-cement and cement-bone interfaces were collected an together with all retrieved components they were investigated through a specific microbiological protocol. All isolated bacterias have been specified and categorised with biochemical methods.

Results: As a result of this specific protocol 65 of the primarily assessed as aseptic cases proved to be infected. As far as spectrum is concerned coagulase-negative Staphylococcus was found in the majority of the cases.

These bacteria are known to produce a film of exo-polysaccharide around implants which leads to resistance against immune competent cells as well as to antibiotics, hi all these cases pre and intraoperative screening tests were negative in regards to infection.

Conclusion: Conclusively only the characteristic medical history with a short term pain free period followed by persisting pain or discomfort is suspicious for low grade infection. Our investigation raises the question if coagulase-negative Staphylococcus is a reason for aseptic loosening in clinically bland cases and if low grade infection is more frequent than expected.

We present a case in which the growth of an intraosseous cyst arising from the proximal tibiofibular joint appeared to have been increased by polyethylene wear particles from a medial unicompartmental knee replacement. Histological examination of the cyst wall showed a histiocytic response associated with numerous polyethylene wear particles. This case demonstrates that there is a direct communication between the joint cavity and the cyst. Such communication is probably through openings in the articular cartilage large enough to allow the passage of these particles.