Lipopolysaccharide was found in periprosthetic tissue from all four patients with inflammatory arthritis who had aseptically loosened implants even in the absence of any clinical or microbiologic signs of infection. Periprosthetic tissues from patients with inflammatory arthritis also likely contain gram-positive endotoxin-like molecules because peptidoglycan was found in synovial lining obtained at the time of primary arthroplasties in patients with rheumatoid arthritis.42 It is interesting to speculate that lipopolysaccharide and peptidoglycan may contribute to the elevated rate of aseptic loosening in rheumatoid arthritis reported in some studies.37,40,43 Our findings support the possibility that endotoxins derived from gram-negative bacteria may contribute to aseptic loosening in patients with inflammatory arthritis. Alternatively, loose prostheses may be more prone to subclinical infections related to chronic tissue irritation from intermittent loading-induced movement. This alternative may be especially likely in patients who are immunocompromised. Another possibility is the patients with positive lipopolysaccharide assays had been infected at an earlier time and, although the bacteria subsequently were eradicated, the endotoxin remained in the periprosthetic tissue. We consider this last alternative to be unlikely because there was no evidence of prior infection in the patients' histories. Most importantly, in any of the above-described alternatives, endotoxins from gram-negative bacteria may contribute to aseptic loosening. This is possible whether the endotoxins precede the initial stages of loosening or vice versa and is supported by the extensive cell culture and animal studies showing endotoxin increases the biologic activity of orthopaedic wear particles.2,6-10,13,19,22,26,44
Our study has several limitations, including the small number of patients studied, the spectrum of diseases in the inflammatory arthritis group, and the levels of lipopolysaccharide in the periprosthetic tissue. It is unknown whether these levels are high enough to be biologically important. It also is unknown whether the lipopolysaccha-ride is adherent to the wear particles or is in the surrounding tissue.29 However, this limitation is of relatively minor importance because, in either case, the lipopolysaccharide would be expected to act together with the wear particles to increase the inflammatory response. A final limitation is the possibility that the lipopolysaccharide might be attributable to contamination of the periprosthetic tissues during preparation for the assays. However, because the samples from patients with osteoarthritis rarely contained detectable lipopolysaccharide, it is unlikely contamination would preferentially occur in samples from patients with inflammatory arthritis. Moreover, we were unable to detect endotoxin in saline that was added to specimen cups in the operating room and then processed identically to the tissue samples.
The bacterial biofilm found on many aseptically loose implants is unlikely to be the source of the lipopolysaccharide because the biofilm usually consists of gram-positive bacteria,19,47 and therefore would not produce lipopolysaccharide. An alternative source is lipopolysaccha-ride adherent to the implants before surgery.36 Unless clearance of this lipopolysaccharide is profoundly reduced in inflammatory arthritis, it probably would be cleared after implantation because lipopolysaccharide is rapidly cleared in animal models.44 Therefore, lipopolysaccharide initially adherent to the implant surfaces is more likely to inhibit early osseointegration rather than to increase osteolysis.
Lipopolysaccharide was detected in only one of the six patients with osteoarthritis who had aseptic loosening. Nevertheless, bacterially derived endotoxins may contribute to aseptic loosening in patients with osteoarthritis for at least three reasons. First, we cannot exclude the possibility that periprosthetic tissue from patients with osteoarthritis contains lipopolysaccharide at levels below the sensitivity of our methods. The second reason bacterially derived endotoxins may contribute to aseptic loosening in patients with osteoarthritis despite our findings comes from a study of greater than 40,000 total hip replacements.16 That study found including antibiotics in bone cement reduces the frequency of aseptic loosening in patients with osteoarthritis, suggesting a role for subclinical levels of bacteria.16 In addition, the number of doses of antibiotics administered systemically during the initial 3 days after hip replacement surgery dose dependently reduced aseptic loosening.16 The long-term effect of early antibiotic administration, in the PMMA cement or administered systemically, is likely because of the race for the surface of the implant between the patient's osteoblasts and biofilm-forming bacteria.21 The winner of the race at each portion of the implant is thought to establish a colony that inhibits adherence of the other cell type and therefore has long-term effects on implant stability.21 An additional mechanism that may account for the long-term effects of antibiotics in the PMMA cement is that antibiotic elution from the cement occurs for a much longer time in patients than in vitro. Therefore, substantial antibiotic concentrations are found in cement recovered 5 years after implantation.30 The third reason bacterially derived endotoxins may contribute to aseptic loosening in patients with osteoarthritis despite our findings is because periprosthetic tissue from these patients likely contains endotoxin-like molecules such as lipoteichoic acid and peptidoglycan derived from the gram-positive bacterial biofilm found on many aseptically loose implants.19,47 In support of this possibility, gram-positive bacteria secrete a highly active form of lipoteichoic acid.24 Also, patients with infected implants produce antibodies specific for this secreted form of lipoteichoic acid,35 indicating it activates the patients' immune system. In addition, because peptidoglycan is detectable in the synovial lining of patients with osteoarthritis,42 it is likely that peptidoglycan also exists in periprosthetic tissue surrounding aseptically loosened implants in these patients.
We showed that lipopolysaccharide exists in periprosthetic tissues from patients with inflammatory arthritis who have clinical and laboratory findings of aseptically loose implants. In contrast, periprosthetic tissue from most patients with osteoarthritis who have aseptically loose implants does not contain detectable levels of lipopolysaccharide. Therefore, lipopolysaccharide is more likely to contribute to aseptic loosening in patients with inflammatory arthritis. Although the relative importance of lipopolysaccharide and other bacterial endotoxins in aseptic loosening compared with wear particles per se or other factors is undetermined, this topic merits additional investigation, especially in patients with inflammatory arthritis and compromised immunity.
We thank Dr. Donald Goodfellow, Dr. William Petersilge, and Dr. Roger Wilber for providing additional tissue samples; and Dr. Andrew Islam, Renata Kadlcek, Dr. James Ninomiya, and Dr. Matthew Smith for helpful discussions during preparation of the manuscript. We also thank Dr. Ronald Berzofsky (BioWhit-taker), Dr. Marilyn Gould (Associates of Cape Cod), and Dr. Thomas Novitsky (Associates of Cape Cod) for advice on endotoxin assays and Associates of Cape Cod for donation of Endospecy assay kits; and Barbara Calabro, Patricia Conroy-Smith, Christine Mullins, Cathy Ostrander, and Rebecca Thomas for assistance in retrieving demographic information.
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