This study enrolled 55 patients (25 men, 30 women; mean age, 63.53 ± 10.51 years) who visited Nihon University School of Dentistry Dental Hospital for implant maintenance from April to September 2016. The inclusion criteria were treatment at the same hospital with two-stage implant placement before 2015, and ongoing maintenance at 3–6-month intervals after superstructure placement. Patients whose superstructure had been in place for less than 3 months were excluded. The dental implants used were Replace Select™/Steri-Oss® implant system (Novelbiocare, Switzerland), Novel Replace® (Novelbiocare, Switzerland), OsseoSpeed™ (ASTRA TECH Implant System, Switzerland), OSSEOTITE® XP (BIOMET 3i, Indiana, USA), and Brånemark system® Mk III (Novelbiocare, Switzerland); all implants had a rough surface (Table 1). This study was conducted with the approval of the Nihon University School of Dentistry Ethics Committee (Permit Number: 2013-15).

Patients with chronic periodontitis were diagnosed based on the latest AAP Classification of Periodontal Diseases and Conditions at the first visit. Clinical attachment level (CAL) was examined for each individual tooth. A CAL of 1–2 mm was defined as mild periodontitis, 3–4 mm was moderate periodontitis, and 5 mm or more was severe periodontitis. The range of the disease was considered to be localized when 30% or less of all teeth were affected, and generalized when more than 30% of teeth were affected [16]. Cases of aggressive periodontitis were not included in this study. Implant therapy was performed after active periodontal treatment with confirmed improvement of the periodontal tissue. We classified patients into two groups based on their medical records: a healthy group without a history-of-periodontitis (H) and a history-of-periodontitis group (HP).

We recorded the following information from the medical chart: patient sex and age, number of natural teeth at first visit, final number of natural teeth at the beginning of implant treatment, total number of extracted teeth and installed implants, and average maintenance period.

At the time of maintenance, the status of the peri-implant tissue was evaluated. Peri-implantitis is defined as a chronic infection of peri-implant tissue, with resorption of the supporting bone observable radiographically. Peri-implant mucositis is a reversible inflammatory reaction around the implant [17]. We diagnosed peri-implantitis when the probing pocket depth (PPD) around the implant was 6 mm or more, suppuration and bleeding were observed at the time of probing, and bone resorption was present radiographically for 25% or more of the implant length [18].