The effect of a single session of scaling and root planing on the subgingival periodontal flora of 14-adult human subjects was investigated by darkfield microscopy. At baseline, one randomly selected periodontally diseased site in each subject was assessed for GI and PlI scores, probing depth and the percentage distribution in subgingival debris of coccoid cells, spirochetes, motile cells and other microorganisms. Following a single full mouth scaling and root planing session, these criteria were reevaluated at other initially diseased sites, one per subject and time interval. The intervals tested were days 3, 7, 14, 21, 28, 35, 42, 49, 56, 70 and 90. The PlI and GI scores tended to decrease during the first 14 days, returning to baseline around days 21-28. After another transient decline around 35-42 days the values stabilized around baseline levels till the end of the experiment. Probing depth decreased below baseline during the first 7 days and with the exception of day 28, remained below baseline level till the end of the experiment. Coccoid cells increased from 25.1 % at baseline to 76.1 % on day 3. Return to the baseline level occurred by day 21. Spirochetes did not return to baseline imtil day 42. The percentage of motile cells decreased significantly from baseline on day 3 only (14.8 % to 3.8 %).The results indicated that a single session of scaling and root planing is capable of disturbing the proportions of certain bacterial forms in the subgingival periodontal flora, and that it may require approximately 42 days for the proportions to return to baseline levels. Probing depth was significantly decreased by the debridement throughout most of the 90-day experimental period. The proportion of coccoid cells was negatively correlated with both GI and PlI scores, while the percentage of spirochetes was positively correlated with GI and PlI scores as well as probing depth measurements.
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