title: The application of multilevel modeling in the analysis of longitudinal periodontal data--part II: changes in disease levels over time.
abstract: 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.
authors.affiliation: Department of Periodontology, Leeds Dental Institute, University of Leeds, Leeds, UK.
authors.forename: Yu-Kang
authors.initials: YK
authors.lastname: Tu
authors.forename: Mark S
authors.initials: MS
authors.lastname: Gilthorpe
authors.forename: Gareth S
authors.initials: GS
authors.lastname: Griffiths
authors.forename: Ian H
authors.initials: IH
authors.lastname: Maddick
authors.forename: Kenneth A
authors.initials: KA
authors.lastname: Eaton
authors.forename: Newell W
authors.initials: NW
authors.lastname: Johnson
mesh_terms: D000293:Adolescent; D000328:Adult; D000704:Analysis of Variance; D015986:Confounding Factors, Epidemiologic; D003627:Data Interpretation, Statistical; D018865:Dental Research / Q000379:methods*; D018450:Disease Progression; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008889:Military Personnel; D015233:Models, Statistical; D010510:Periodontal Diseases / Q000453:epidemiology*; D012307:Risk Factors; D013997:Time Factors; D006113:United Kingdom / Q000453:epidemiology