Is loss of attachment due to root planing and scaling in sites with minimal probing depths a statistical or real occurrence?

Background: Following root planing and scaling many studies have implied an association between a loss of clinical attachment at sites with initially shallow pockets (1 to 3 mm) and gains in attachment level for deeper probin g depths. However, these effects are also consistent with a statistical phe nomenon referred to as regression towards the mean. This principle suggests that extreme values will moderate the next time they are recorded. The pur pose of this report was to estimate the effect that regression towards the mean has on perceived changes in attachment level after root planing and sc aling. Methods: During the initial examination, 2 different investigators conducte d 2 full-mouth probings. Two quadrants were randomly selected to be root pl aned and scaled until the root surfaces were smooth by tactile touch of an explorer. The 2 remaining quadrants were not treated. At 4 to 6 weeks after treatment, another full mouth probing was done. An examiner who was blind to the quadrants that had been scaled measured attachment level and probing depth after therapy. This study design provided periodontal measurements b efore and after root planing and scaling, measurements before and after a p eriod of 4 to 6 weeks of no therapy, and duplicate measurements at the begi nning of the study. Results: Using the repeat examination when no true change could occur, shal low sites (less than or equal to3 mm of probing depth) showed average negat ive differences between repeat attachment level measurements (-0.23 mm), wh ich mimicked loss of periodontal attachment. Deep sites, (>6 mm) showed ave rage positive values (0.40) mimicking gain in attachment level. These resul ts suggest that regression towards the mean is a significant effect in this data set. Both shallow non-scaled and scaled sites had similar differences in repeat measures (-0.28 mm, -0.25 mm) which were also similar to and not statistically different from changes after therapy for both non-scaled (-0 .21 mm) and scaled sites (-0.08 mm). Thus not only does this data set exhib it regression towards the mean, but it explains the majority of perceived l oss of periodontal attachment after scaling at sites that have minimal prob ing depth. Conclusions: These results suggest that the majority of perceived loss of a ttachment due to scaling at sites of minimal probing depth that have been r eported in many studies may be due a statistical phenomenon called regressi on towards the mean.