title: Outcome variables in periodontal research: means and threshold-based site changes.
abstract: BACKGROUND
  The exclusive use of mean measurements in periodontal research might at times be misleading, as changes in different sites in the same individual might nullify each other. The purpose of the present study was to compare disease progression and response to periodontal therapy using both individual site activity with thresholds and mean patient changes.
  
  
  METHODS
  Seventy-nine (79) subjects with established periodontitis were monitored for 1 year (no treatment [NTx] group); 108 subjects who received scaling, root planing, and quarterly prophylaxis were observed in a similar time interval (treatment [Tx] group). Probing depth (PD), attachment level (AL) and alveolar crestal height (ACH) were measured at baseline and 1 year using pressure-sensitive probes and computer-assisted image analysis of radiographs.
  
  
  RESULTS
  Mean reduction in PD (0.50 mm) was observed in the Tx group compared to a small increase (-0.04 mm) in the NTx group (P= 0.0001). Treatment resulted in mean AL gain (0.44 mm) compared to net AL loss (-0.21 mm) in the NTx group (P= 0.0001). Subjects in the Tx group had twice as many sites with AL gain (beyond the threshold) compared to NTx subjects (16.64+/-1.07% versus 8.11+/-0.68%) which was highly significant (P = 0.0001, Student t test). Similarly, although in the opposite direction, percentage of sites with AL loss beyond threshold was 6.21+/-0.47% (Tx group) and 14.02+/-1.15% (NTx group) which was also highly significant (P = 0.0001, Student t test). Change in ACH was minimal for the Tx group (-0.07+/-0.03 mm) while NTx subjects experienced greater bone loss throughout the year (-0.16+/-0.02 mm); percentage of sites with ACH loss was similar for both groups, while the Tx group had 3 times the number of sites with ACH gain (11.76% versus 3.42%), suggesting that the reduction in mean bone loss in the Tx group did not result from a reduction in losing sites, but rather from an increase in gaining sites. The use of both means and binary data seems to supplement the information regarding the nature and mechanism of disease progression and arrest.
  
  
  CONCLUSIONS
  The characteristics of periodontal disease with its multifactorial patient, local, and site-related etiology support the use of both mean and site-based changes in clinical studies of periodontal disease and treatment.
authors.affiliation: Department of Oral Biology, School of Dental Medicine, State University of New York at Buffalo, USA.
authors.forename: E E
authors.initials: EE
authors.lastname: Machtei
authors.forename: M
authors.initials: M
authors.lastname: Schmidt
authors.forename: E
authors.initials: E
authors.lastname: Hausmann
authors.forename: S
authors.initials: S
authors.lastname: Grossi
authors.forename: R
authors.initials: R
authors.lastname: Dunford
authors.forename: G
authors.initials: G
authors.lastname: Davies
authors.forename: J
authors.initials: J
authors.lastname: Chandler
authors.forename: R J
authors.initials: RJ
authors.lastname: Genco
mesh_terms: D000328:Adult; D000368:Aged; D016301:Alveolar Bone Loss / Q000145:classification / Q000000981:diagnostic imaging / Q000628:therapy; D000539:Alveolar Process / Q000000981:diagnostic imaging; D000704:Analysis of Variance; D003777:Dental Prophylaxis; D012534:Dental Scaling; D018450:Disease Progression; D005500:Follow-Up Studies; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008875:Middle Aged; D017622:Periodontal Attachment Loss / Q000145:classification / Q000628:therapy; D010510:Periodontal Diseases / Q000145:classification / Q000628:therapy*; D010514:Periodontal Pocket / Q000145:classification / Q000628:therapy; D010517:Periodontics / Q000295:instrumentation; D010518:Periodontitis / Q000145:classification / Q000628:therapy; D011859:Radiography; D012107:Research Design; D016745:Root Planing; D012907:Smoking / Q000503:physiopathology; D016896:Treatment Outcome