All the implants with peri-implantitis came from the HP group, not the H group, thus supporting the view that a history of periodontitis is a risk indicator for peri-implantitis. In the diagnosis of periodontitis within the HP group, it is notable that peri-implantitis increased as periodontitis become more severe. This supports the findings of a previous study that reported that the survival rate of implants is inversely proportional to the severity of past periodontitis [25]. In our study of long-term maintenance patients, the findings that the prevalence of peri-implantitis was 10.8% at the implant level and 10.9% at the patient level were similar to previous findings. It was suggested that the criteria for peri-implantitis that we adopted were reasonable. The low prevalence of peri-implantitis in this study (approximately 10%) may be related to the fact that all the subjects received maintenance therapy [26].

The average mPI was about 0.2 for both groups, indicating little plaque adhesion. One characteristic of patients in this study is that they understood the importance of maintenance and they received oral hygiene instruction. Although poor oral hygiene has been reported to be a risk indicator for peri-implant disease [3], our results suggest that the oral hygiene was maintained in all patients including peri-implantitis patients.

Among all the subjects, 11.5% of the total implant sites had an average PPD of more than 4 mm. Our results were comparable with previous studies, which have reported peri-implant PPDs ranging from 2.52 to 3.8 mm [27,28,29]. However, we believe that this study was more thorough because of its long-term follow-up. PPD in the HP group was 3.33 ± 1.07 mm, which was significantly greater than that in the H group (2.87 ± 0.48 mm). Interestingly, we found that PPD was greater in patients with a history of periodontitis than in those without this history. It has been reported that worsening periodontal disease of natural teeth affects the pocket depth of adjacent implants [30]. However, although in natural teeth there is a criterion of critical probing depth (4 mm or more) [31], there are no such reference values for implants because the site and placement depth differs and the biological width of the implant is not constant [32].