At the time of optional maintenance, the following parameters were measured for each implant. Data collection and analysis were conducted by two operators who were blind to the research purpose and methods. One operator statistically analyzed the final data set.

Plaque adhesion (modified Plaque Index: mPI) on the superstructure of the implant was measured on a 4-point scale (0, no plaque; 1, plaque seen with probing of the tooth surface; 2, moderate accumulation of soft deposits visible with the naked eye; 3, abundance of soft deposits on the tooth). We calculated the average score of four surfaces (buccal, lingual, mesial, and distal aspects).

To standardize the probing force (0.15 N), the operators used a precision scale to calibrate their force with repeated measurements. The same operator inserted a periodontal pocket probe (11 Colorvue® Probe Kit; Hu-Friedy, Chicago, IL, USA) into the pocket around the implant with a force of approximately 0.15 N. PPD was measured in 1-mm increments with the 6-point method (at the mesial and distal angles and at the center of the buccal and lingual aspects of the implant). We calculated the average of the six points for each implant.

Bleeding on probing (BoP) was evaluated 10 s after the 6-point probing as negative (0, no bleeding) or positive (1, bleeding). We calculated the average of all six points obtained. The minimum BoP score was 0 and the maximum was 1.

Data analysis was performed with statistical software (GraphPad Prism 5.0; GraphPad Software Inc., San Diego, CA, USA). Quantitative analysis of differences between the H and HP groups was performed with the Mann–Whitney U test. Clinical parameters were analyzed with Spearman’s rank correlation coefficient (R _s ). A significance level of 95% (p < 0.05) was considered statistically significant.