Diederik Hentenaar

116 Chapter 5 compensated for these potential unusable samples (24%), yielding a sample size of 56 implants (42.5/0.76). According to the assumption that each patient has on average more than two implants with peri-implantitis (De Waal et al. 2013), a sample size of 28 patients was chosen (56/2, 14 patients per group). Statistical analysis For the analysis of the primary outcome variable and the secondary microbiological outcome variable linear regression analysis was performed. The implant was taken as the statistical unit. Total anaerobic bacterial loads at baseline (T pre and T 0) were distributed normally after logarithmic transformation. Baseline values were included in the regression model. For the comparison of the number of culture-positive implants after the decontamination period the chi-square test was used. The secondary clinical outcome variables were analyzed using a two-level hierarchical random intercepts model. The two-levels of analysis were implant-level and patient-level. With the crude analysis, the effect of the intervention was determined, while controlling for baseline value. Because a previous study (De Waal et al. 2016) has shown that mean bone loss at baseline and smoking are prognostic indicators for the outcome of resective peri- implantitis treatment, these factors were additionally included in the model (adjusted analysis). Descriptive data and data regarding the microbiological outcome variables were analyzed using IBM SPSS Statistics 22 Version 22.0 (IBM Corp. Armonk, NY: IBM Corp.). Multilevel models were analyzed using MLwiN version 2.12 (Centre for Multilevel Modeling, University of Bristol, Bristol, UK). RESULTS The progress of patients throughout the different phases of the study is illustrated in Figure 1. Table 1 depicts the baseline demographic patient and implant characteristics. The included patients had a total of 128 implants of which 53 implants showed signs of peri-implantitis. Different implant brands and types with different implant surfaces were present, including Straumann (Straumann AG, Basel, Switserland; SLA® and SLActive® surface), Nobel Biocare (Nobel Biocare AB, Göteborg, Sweden; TiUnite® surface), Biomet 3i (Biomet Inc., Warsaw, Indiana, USA; OSSEOTITE® surface), Frialit-2, (Dentsply Friadent, Mannheim, Germany; FRIADENT® plus surface) and Pitt-Easy (Sybron Implant Solutions GmbH, Bremen, Germany; Puretex® surface). Three patients with each one implant with peri-implantitis were lost to follow-up (2 patients from control group, 1 from test group).