The impact of regression towards the mean on probing changes in studies on the effect of periodontal therapy

Three-year randomized study of manual and power toothbrush effects on pre-existing gingival recession

AIM

To compare long-term effects of brushing with an oscillating-rotating power toothbrush or an ADA reference manual toothbrush on pre-existing gingival recession.

MATERIALS AND METHODS

In this controlled, prospective, single-blind, parallel-group study, healthy subjects with pre-existing recession were randomized and brushed with a power toothbrush (n = 55) or an ADA reference manual toothbrush (n = 54) for a 3-year study period. Subjects were required to brush their teeth twice daily for 2 min. using a standard fluoride toothpaste. During the study, subjects were assessed for clinical attachment loss and probing pocket depths to the nearest mm at six sites per tooth by the same calibrated examiner. Gingival recession was calculated at pre-existing sites as the difference between clinical attachment loss and probing pocket depths. Hard and soft oral tissues were examined to assess safety.

RESULTS

After 35 ± 2 months, mean gingival recession did not differ significantly between groups, but was significantly reduced from baseline (p < 0.001), from 2.35 ± 0.35 mm to 1.90 ± 0.58 mm in the power and from 2.26 ± 0.31 mm to 1.81 ± 0.66 mm in the manual group.

CONCLUSIONS

Gingival recession in subjects with pre-existing recession was significantly reduced after 3 years of brushing with either a power or manual toothbrush.

Defining and predicting outcomes of non-surgical periodontal treatment: a 1-yr follow-up study

This study reports on 1-yr outcomes of non-surgical periodontal therapy and compares predictive models resulting from different definitions of treatment success. A total of 149 participants, 30-70 yr of age, provided clinical periodontal data and data on sociodemographic status, health status, symptoms, and oral health-care behaviors at baseline. One week later, clinical attachment level and probing pocket depth were recorded again in 148 patients. Participants underwent non-surgical periodontal therapy, including scaling and root planing, during three to four clinical sessions. Three and 12 months later, clinical attachment level, probing pocket depth, and bleeding on probing (BOP) were recorded in 141 and 137 participants, respectively. Using test-retest data, patients were classified as having 'downhill', 'stable', or 'improved' results on three clinical attachment level and three probing pocket depth outcomes, and their classification was found to vary considerably according to outcome. Although the predictors of treatment outcome varied depending on the variable chosen to represent the treatment outcome, some predictors were more commonly noted as predicting improvement, namely a high baseline percentage of sites with subgingival calculus and the presence of suppuration at baseline. The latter was, however, also predictive for tooth loss during the study. Our findings underline the need for uniformity in defining the outcomes in trials of periodontal therapy.

Effect of Supragingival Plaque Control in Smokers and Never-Smokers: 6-Month Evaluation of Patients With Periodontitis

BACKGROUND

The effect of supragingival plaque control on clinical signs of periodontitis is controversial, particularly when smoking habits are considered. This study evaluated the clinical effects of supragingival plaque control on clinical signs of periodontitis in smokers and never-smokers.

METHODS

The following data were collected for 25 never-smokers and 25 smokers at baseline and 30, 90, and 180 days: visible plaque index (VPI), gingival bleeding index (GBI), bleeding on probing (BOP), periodontal probing depth (PD), and clinical attachment loss (CAL). After baseline examinations, supragingival scaling was performed. Oral hygiene practices were reinforced and reevaluated weekly during the experimental period. Linear models adjusted for clustering of observations within individuals were used for statistical analysis.

RESULTS

Reductions in VPI were significant for both groups, with no intergroup differences. GBI at baseline was similar between groups, and at 30, 90, and 180 days, smokers had a lower GBI than never-smokers. Significant reductions were observed in PD for shallow (1 to 3 mm), moderate (4 to 5 mm), and deep sites (> or = 6 mm) in both groups. CAL was significantly greater in smokers throughout the study, but gains in attachment were similar for both groups (0.71 to 1.00 mm). BOP reductions were similar in both groups.

CONCLUSIONS

Supragingival plaque control resulted in significant changes in clinical parameters associated with gingivitis and periodontitis. Smoking did not affect results, regardless of initial PD.

Trends and plot methods in MLR studies

Regression toward the mean effects are presented within the field of quantitative structure-activity relationship modeling and in situations in which multilinear regression techniques are considered for model building. The concept is related to the graphical aspect of some scatter plots (experimental vs fitted and fitted vs experimental values). These graphs demonstrate how the point cloud is not always symmetrically distributed along the so-called "ideal" or "desired" line, that is, the bisector of the first and third quadrants. The deviation from the ideal line is fixed, and it is also related to the coefficient of determination. An extrapolation of these regression effects is also discussed within the context of property predictions obtained via the leave-one-out cross-validation technique.

The Application of Multilevel Modeling in the Analysis of Longitudinal Periodontal Data –Part II: Changes in Disease Levels over Time

BACKGROUND

The aim of this study was to investigate the longitudinal relationships between the outcome measurements of changes in lifetime cumulative attachment loss (cLCAL) and changes in probing depth (cPD) in relation to potential risk factors or other risk markers for periodontal disease progression from a cohort of 100 young males. In order to account for the hierarchical data structure, and to explore explicitly the site, tooth, and subject levels simultaneously, multilevel modeling was undertaken.

METHODS

The analyses were undertaken in two parts. Within a previous article, the absolute levels of disease were analyzed in relation to potential risk factors; within this article, changes in disease are analyzed in relation to these factors. Each analytical approach yielded substantively different insights.

RESULTS

Subject-level risk factors had limited predictive value for cLCAL/cPD throughout the 30-month observation period. Tooth position demonstrated a near linear relationship for both outcomes, with disease increasing from anterior to posterior teeth. Supragingival plaque had no significant effect on cLCAL/cPD, while subgingival calculus and bleeding on probing were negatively associated with cLCAL/cPD. In contrast to the outcomes LCAL/PD, supragingival calculus had no significant protective effect on cLCAL/cPD. There was no significant influence of smoking in this cohort.

CONCLUSIONS

This study provides, for a relatively young cohort, considerable insights into the factors associated with longitudinal patterns of early-life periodontal disease at all levels of the natural hierarchy of sites within teeth within subjects. Furthermore, it is demonstrated how multilevel modeling can provide considerable insight into some of the inconsistencies and controversies found in the previous periodontal literature.

Perceived oral health: changes over 5 years in one Swedish age-cohort

OBJECTIVES

The purpose of this study was to investigate if a change in the social gradients in perceived oral health occurred over a 5-year period, 1992-97, using a cohort population from two Swedish counties.

METHODS

In 1992, a cross-sectional mail questionnaire was sent to all 50-year-old persons in two counties in Sweden, Orebro and Ostergötland, and altogether there were 8888 persons. In 1997, the same population was sent a new questionnaire. The cohort, comprising the same respondents from 1992 and 1997, was of 5363 persons. An index of perceived oral health was constructed out of three questionnaire variables: satisfaction with teeth, chewing ability and the number of remaining teeth. This index value was set as a dependent variable in a regression model. Reports of toothache were investigated in a separate logistic regression model.

RESULTS

There were obvious social gradients in the perceived oral health index both in 1992 and in 1997. Marital status, foreign birth, education and occupation were all substantially related to the perceived oral health. The change in perceived oral health was analyzed. Almost half of the cohort (47.4%) showed no change at all. Those with increased and those with decreased health were rather evenly distributed on both sides, with 22.0% with better health in 1997 and 30.6% with worse health. Gender and education were related to toothache experience.

CONCLUSION

Changes have been moderate in the perceived oral health in this cohort, despite the rather drastic changes in the remuneration of dental care during this study time. On the other hand, this also means that the social differences remain, despite the official goals of increased equity.

Unification of the "burst" and "linear" theories of periodontal disease progression: a multilevel manifestation of the same phenomenon

Previously, burst and linear theories for periodontal disease progression were proposed based on different but limited statistical methods of analysis. Multilevel modeling provides a new approach, yielding a more comprehensive model. Random coefficient models were used to analyze longitudinal periodontal data consisting of repeated measures (level 1), sites (level 2), teeth (level 3), and subjects (level 4). Large negative and highly significant correlations between random linear and quadratic time coefficients indicated that subjects and teeth with greater-than-average linear change experienced decelerated variation. Conversely, subjects and teeth with less-than-average linear change experienced accelerated variation. Change therefore exhibited a dynamic regression to the mean at the tooth and subject levels. Since no equilibrium was attained throughout the study, changes were cyclical. When considered as a multilevel system, the "linear" and "burst" theories of periodontal disease progression are a manifestation of the same phenomenon: Some sites improve while others progress, in a cyclical manner.

Is reduction of pocket probing depth correlated with the baseline value or is it "mathematical coupling"?

Previous studies using correlation or regression analysis have showed that treatment effects measured by the change in clinical parameters are often associated with baseline values of the same parameters. These studies, however, have a methodological weakness. Correlation/regression between baseline measures and the derived change variable invalidates the statistical procedures of testing the null hypothesis: that the coefficient of correlation/regression is zero. This is due to the phenomenon of mathematical coupling. To investigate the impact that this has on the observed correlation/regression coefficient when in reality this is zero, we used random simulations of hypothetical data to model the treatment of periodontal pockets. Results showed a strong probability of obtaining statistically significant correlation/regression coefficients. To separate this artificial effect of mathematical coupling from the true underlying biological relationship, one must apply appropriate analytical strategies to re-evaluate previous evidence within the periodontal literature.

Is loss of attachment due to root planning 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 probing depths. However, these effects are also consistent with a statistical phenomenon referred to as regression towards the mean. This principle suggests that extreme values will moderate the next time they are recorded. The purpose of this report was to estimate the effect that regression towards the mean has on perceived changes in attachment level after root planing and scaling.

METHODS

During the initial examination, 2 different investigators conducted 2 full-mouth probings. Two quadrants were randomly selected to be root planed 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 before and after root planing and scaling, measurements before and after a period of 4 to 6 weeks of no therapy, and duplicate measurements at the beginning of the study.

RESULTS

Using the repeat examination when no true change could occur, shallow sites (< or =3 mm of probing depth) showed average negative differences between repeat attachment level measurements (-0.23 mm), which mimicked loss of periodontal attachment. Deep sites, (>6 mm) showed average positive values (0.40) mimicking gain in attachment level. These results 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 exhibit regression towards the mean, but it explains the majority of perceived loss of periodontal attachment after scaling at sites that have minimal probing depth.

CONCLUSIONS

These results suggest that the majority of perceived loss of attachment due to scaling at sites of minimal probing depth that have been reported in many studies may be due a statistical phenomenon called regression towards the mean.

Issues in the evaluation of clinical trials of periodontitis: a clinical perspective

The goals of clinical trials designed to establish claims for equivalency or superiority of treatment for periodontitis must be clearly stated and defined in terms of measurable and meaningful response variables. It is suggested that these clinical trials use designs that compare new treatment methods to basic periodontal therapy which consists of thorough scaling and root planing, oral hygiene instruction, and regular maintenance care. The primary response variable should be clinical attachment level. It is important to document changes in probing depth since this is a meaningful measure to many clinicians. Gingival inflammation and bleeding should be used as secondary response variables because these are not necessarily indicative of progressive periodontal destruction. Radiographic measures of disease may be useful as primary response variables if safe, reproducible and valid methods of measuring change are utilized. Microbiological monitoring should be a secondary response variable because of numerous questions concerning sampling methodology, quantitative expression of data, and meaningful interpretation in terms of relevance to disease activity. The length of periodontitis trials should be set at a minimum of 9 months if claims of superiority or equivalency are made compared to basic periodontal therapy. Calibration trials should be included and measurement error should be expressed in terms that are meaningful to the clinician while retaining statistical validity. Statistical methods for determining change should take into account site and subject heterogeneity, bursts of change and gradual change over time. It is important that statistical techniques be used that detect change as early as possible and that appropriate consideration be given to the clinical implications of the type, magnitude and duration of change in outcome variables.

Clinical effects of root debridement in molar and non-molar teeth. A 2-year follow-up

12 patients were studied longitudinally to monitor the effects of basic periodontal therapy in molar and non-molar teeth. Periodontal sites were grouped into molar furcation sites, molar flat-surface sites and non-molar sites. Clinical measurements were taken at baseline and directly followed by full mouth root debridement. Subsequently, measurements were taken every 3rd month until 24 months. At each of these appointments, the patients were monitored for their oral hygiene performance and given supragingival prophylaxis. The mean results indicated that initially moderately deep and deep molar furcation sites responded less favorably to therapy compared to non-molar sites and molar flat-surface sites of similar probing depth. Initial improvements in probing measurements for moderately deep and deep molar furcation sites were limited and also tended to revert during the observation interval. Identification of individual sites with probing attachment loss disclosed that 25% of molar furcation sites lost probing attachment as compared to 7% for non-molar sites and 10% for molar flat-surface sites. These results corroborate previous findings and call for additional or alternative treatment regimens for periodontal furcation pockets.