## 1 Introduction  
Dental implants are widely used in dentistry, with a continuous increase in the number of patients treated and a parallel continuous developing of new techniques and technologies available for clinicians. A study by [PERSON] et al. estimated that there will be a prevalence of dental implants from 5.7% to 23% in the American population in 2026, so it is clear how important this treatment option is [1]. Despite this large distribution of dental implants, there are still some problems that can occur during treatment--lack of osseointegration, peri-implantitis, occlusal overloading, fixture fractures, and abutment screw loosening [2, 3]. In particular, peri-implantitis is surely one of the major problems that can occur. In 2017, a new classification was proposed by the American Academy of Periodontology and the European Federation of Periodontology--a presence of bleeding or suppuration with gentle pressure on probing and some variations from previous examinations, such as increased probing depth and increased bone loss. In the case of absence of previous examinations, peri-implantitis is defined as presence of bleeding and/or suppuration on gentle probing, probing depths \(\geq\) 6 mm, and bone levels \(\geq\) 3 mm apical to the most coronal portion of the intraosseous part of the implant [4]. Moreover, it is very important to consider that different classifications, with restrictive diagnostic criteria, are associated with significantly reduced presence of peri-implantitis [5]. In the etiology of peri-implantitis, in order of severity, we can consider:  
1. Microbial contamination (periodontal disease) [6];
2. Bone quality (>PI in regenerated bone: caution is required in the event of poorly vascularised bone) [7];
3. Surgical damage. Thermal injury to bone [8];
4. Soft Tissues morphology (i.e., the absence of keratinized mucosa around dental endosseous implants has been associated with a higher susceptibility to plaque-induced tissue destruction [9], but still there is no consensus on this argument [6];
5. Implant surface [10];
6. Abutment connection [11];
7. Prosthetic damage (design, overloading, and no passive bridges) [12];
8. Cementation [13]. Based on the surface roughness, it has been proposed to categorize implants as ([PERSON] and [PERSON] 2004) [14]:  
- smooth S\({}_{\text{a}}\)! 0.5 \(\mu\)m;  
- minimally rough S\({}_{\text{a}}\): 0.5-1.0 \(\mu\)m;  
- moderately rough S\({}_{\text{a}}\): 1.1-2.0 \(\mu\)m;  
- rough S\({}_{\text{a}}\)? 2.0 \(\mu\)m.  
S\({}_{\text{a}}\) values have to be interpreted with care, because there are other parameters that characterize implant surfaces. Literature reveals that rough surface implants (i.e., titanium plasma sprayed; TPS), once exposed to the oral cavity, have a higher tendency to develop peri-implantitis than minimally rough implants. Moreover, some studies assess that rough implant surfaces are more susceptible to peri-implantitis establishment and progression [15]. However, several authors have indicated that frequent patients recalls are involved in peri-implantitis prevention, and a close recall scheme is essential [2; 4; 5; 6]. Once peri-implantitis is established, laser-assisted treatment seems favored in literature [16]; various wavelengths are used: diodes (from 450 nm to 980 nm), Nd:Yag (1064 nm), Nd:Yap (1340 nm), and Er:Yag (2940 nm). Lasers could be applied alone or in combination with a photosensitizer; photodynamic therapy (PDT) is the combination of light with a chromophore and oxygenated tissues. The goal of PDT is to deliver singlet oxygen to all tissues affected by pathogenic bacteria. If the wavelength used is absorbed in the chromophore used, laser ray cannot penetrate more in deeper, and if the power density is too low (usually PDT protocols use LLLT (low level laser therapy) energy, in order to avoid any thermal damage), the decontaminating effects are not enough.