Effect of pH on endotoxin affinity for metal-ceramic alloys

Statement of problem. Crevicular pH may modify bacterial endotoxin affinity for high-noble metal-ceramic alloys. Purpose. Porphyromonas gingivalis lipopolysaccharide (LPS) affinity for 3 metal-ceramic alloys at 3 different pH levels was compared in vitro by measuring adsorption to and release from the alloy surface. Material and methods. Metallographically polished disks were fabricated from Pd-Ag-Sn, Au-Pd-Ag-Sn-In, and Au-Pd-In-Ga alloys. Clean disks were placed individually into 1 mL at pH 6.5, 7.0, or 7.5 phosphate-buffered saline solution containing 0.9 endotoxin units per square millimeter tritiated LPS (n = 3 disks per alloy-pH group). The disks were incubated for 24 hours at 37°C before being transferred to LPS-free buffer and incubated, again for 24 hours at 37°C, to evaluate elution. This transfer continued at 24-hour intervals up to 96 hours total elution incubation. Lipopolysaccharide adsorption to and elution from disks was determined through liquid scintillation spectrometry. Adsorption data were evaluated with a 2-way analysis of variance (α=.05) and the post hoc Tukey honestly significant difference test. Results. Lipopolysaccharide adsorption values ranged from 0.48 ± 0.04 EU/mm² for the Au-Pd-Ag-Sn-In alloy at pH 7.5 to 0.75 ± 0.04 EU/mm² for the Pd-Ag-Sn alloy at pH 6.5. Alloy type (P=.0001) and environmental pH (P=.0001) significantly influenced adsorption. Adsorption to the Pd-Ag-Sn and Au-Pd-In-Ga alloys at pH 6.5, 7.0, and 7.5 were similar and decreased with increasing pH. In contrast, adsorption to the Au-Pd-Ag-Sn-In alloy was significantly less than to other alloys at pH 6.5 but did not differ at other pH levels. Lipopolysaccharide release from the alloy surface could not be detected. Conclusion. P. gingivalis LPS affinity for metal-ceramic alloys was modified by environmental pH. The degree of LPS adsorption depended on the composition and surface chemistry of each alloy.