Dental implants are now considered to be a predictable and successful treatment option for replacement of missing teeth.1–4 periimplantitis is particularly high. The incidence of periimplantitis reported in participants is 18% to 56% and in implants is 9% to 40%.5–7
Periimplantitis is an inflammatory reaction that occurs in periimplant tissues. The clinical symptoms are similar to those of periodontitis and include swelling, bleeding, suppuration of the periodontal tissue, and crestal bone resorption. Implants can eventually become destabilized if bone resorption progresses markedly.8
Regarding the management of periimplantitis , various therapeutic and preventive techniques are applied at different disease stages, including mechanical debridement, brushing, antibiotic therapy, and a surgical approach of eliminating infection and restoring bone defects and function.9–11 periimplantitis has been established. Because periimplantitis is a common biological complication in implant therapy and a main cause of implant failure today, a new strategy of preventing periimplantitis is therefore required to improve the success rate of implant therapy.
It is recognized that titanium dioxide (TiO2 ) promotes photocatalytic activity under ultraviolet (UV)-light irradiation.12 2 on the surface because of the oxidation from the atmosphere.13
Surface modification of implants with UV-light irradiation has been shown to generate hydrophilicity, decrease surface hydrocarbons, and improve electrostatus. Modifying the surfaces of the implant enhances the attachment, spread, proliferation, and differentiation of osteoblasts and mesenchymal stem cells in vitro ; furthermore, modifying the surfaces has been shown to promote protein adsorption and increase the bone-to-implant contact (BIC) from 55% to 98.2% in animal models.14–18
Clinical investigations have reported that UV-light treatment of titanium implants resulted in a high success rate based on the criteria put forward by Smith and Zarb and improved implant stability as evaluated by the implant stability quotient (ISQ) at implant placement and commencement of functional loading using Osstell ISQ (Integration Diagnostics, Savedalen, Sweden).19,20
Based on previously reported findings, implant surface modification with UV-light irradiation is believed to resist development of periimplantitis and improve osseointegration. However, the biological responses of UV-irradiated implants to periimplantitis have yet to be examined. Therefore, this study was conducted to examine the preventive effect of UV-light irradiated implant in periimplantitis . The null hypothesis tested was that the bone resorption around the implant surfaces with and without UV-light irradiation in a dog periimplantitis model did not differ.
Materials and Methods
Experimental Animals
Three female beagle dogs aged 12 months (weight, 9–10 kg) with clinically healthy periodontal tissue were used in this pilot study. The experimental protocol in animals and the surgical procedure followed the guidelines for Animal Care approved by Kanagawa Dental University. The dogs were anesthetized with pentobarbital (Sommnopentyl; Kyoritsu seiyaku, Tokyo, Japan) 25 mg intravenously before all surgical and experimental procedures.
Surgical Procedures
A total of 12 pure titanium screw implants (diameter 3.3 mm, length 8 mm, Standard Implant bone level type, SLA RN; Straumann, Basel, Switzerland) were used. UV-light irradiation was performed using a photodevice (TheraBeam Affiny; USHIO Inc., Tokyo, Japan) for 15 minutes before the placement of implants.
The UV-irradiated implants were placed on the right side (UV group), and non–UV-irradiated implants on the left side (non-UV group) of the mandible. Four implants, 2 each of which were UV and non–UV irradiated, were placed in each animal.
Schematic outline of the experimental procedure is shown in Figure 1 . Intraoral preventive treatment and scaling were performed 14 days before tooth extraction and implant placement. After full-thickness flaps were raised in an aseptic condition, mandibular premolars (P2, P3, and P4) were extracted bilaterally, and the alveolar crest was flattened. Titanium screw implants were immediately placed in the alveolar socket. The SLA surface of implant was placed by fitting them to the bone margin, and healing abutments were attached. The gingival flaps were then sutured by simple interrupted sutures (coated vicryl, 4.0 polyglactin 910; Ethicon, Johnson & Johnson, New Brunswick, NJ).
Fig. 1: Experimental procedure. Plaque control was performed during the 90-day healing period after implant placement using 0.12% chlorhexidine. A ligature was applied on the 90th day, and clinical and radiographic examinations were performed on the 90th and 180th days. Implants: n = 12.
Plaque formation was controlled using 0.12% chlorhexidine for the period of 90 days. After this period, periimplantitis was induced by applying dental floss around the neck of implants. Experimental periimplantitis was initiated after 180 days.
Dental Radiographic Analysis
Dental radiographs (70 kv, 15 mA, 0.25 seconds) were taken to evaluate the changes of alveolar bone. Radiography was standardized by attaching a silicone bite block prepared using a polyvinyl silicone impression material to the film holder.
The progression of bone resorption was monitored in both groups. Bone resorption variables were calculated objectively, as the mean of the mesial and distal values by image analyzing application (ImageJ; National Institutes of Health).
Bone resorption (%) = (length of BIC after 90 day − length of BIC after 180 days)/length of BIC after 90 × 100. BIC was defined as the contact area of bone and implant by visual examination.
Micro–Computed Tomography Scanning Analysis
Micro–computed tomography (micro-CT ) (MCT-CB100MF; Hitachi Medico Co., Ltd., Tokyo, Japan) was performed to examine the volume of bone resorption (enlargement rate × 3; voxel size, 43 μm; tube voltage, 70 kV; tube current, 100 μA; 201 slices in total, 8.6 mm). Mandibular images were obtained with 100 slices each to both the mesial and distal directions from the center of the implant. The mandible was extracted from the mesial and distal slices of micro-CT images, and using them as reference images, a masked image of the mandibular contour was prepared by interpolation. The actual contour of the mandible was traced, the bone defect was extracted, and its volume was calculated by extracting differences between the trace and masked image. Image processing and volume calculation were performed using image analyzing application (TRI/3D BON; Ratoc System Engineering, Tokyo, Japan).
Histological Preparation and Analysis
After 180 days, the common carotid arteries were cannulated, and Ringer solution containing 0.2% heparin was perfused until the jugular veins were cleared of blood. After perfusion, 2% glutaraldehyde phosphate buffer solution was injected into the carotid arteries for fixation. The mandible was removed after fixation, and these specimens were immersed in serial ethanol solutions adjusted to concentrations of 70% to 99% for dehydration and embedded in synthetic resin (Technovit 7200 VLC; Heraeus Kulzer, Hanau, Germany). Nondecalcified grind sections (thickness, 100 μm) of the mandible were created by hard tissue microtome (SP1600 saw microtome; Leica, Wetzlar, Germany). The grind sections were stained by methylene blue and examined under a light microscope.
Statistical Analysis
Data are expressed as the mean ± standard deviation. Student t test for unpaired was used to compare the UV and non-UV groups by SPSS software (IBM SPSS, Chicago, IL). P < 0.05 was considered statistically significant.
Results
Clinical Examination
Intraoral photographs of both the UV and non-UV groups are shown in Figure 2 . Both groups after 90 days demonstrated normal periodontal tissue (Fig. 2, A–C ). Both groups had developed experimental periimplantitis after 180 days, and signs of active inflammation including swelling and bleeding of periodontal tissue were observed around the implant (Fig. 2, B–D ).
Fig. 2: Intraoral photographs.A , Non-UV group after 90 days, (B ) UV group after 90 days, (C ) non-UV group after 180 days (90 days after dental floss application), and (D ) UV group after 180 days (90 days after dental floss application).
Dental Radiographic Findings
Figure 3 shows the standardized digital dental radiographs. White line in the photograph indicates the border of the SLA processed surface. In the non-UV group, bone–implant contact was observed along the whole implant surface 90 days after placement (Fig. 3, A ). At 180 days (90 days after dental floss application), remarkable bone resorption was noted around the neck of implant (Fig. 3, B ). In the UV group also, bone–implant contact was observed along the whole implant surface 90 days after placement (Fig. 3, C ). At 180 days (90 days after dental floss application), bone resorption was also seen (Fig. 3, D ).
Fig. 3: Radiographic findings.A , Non-UV group after 90 days. B , Non-UV group after 180 days (90 days after dental floss application). C , UV group after 90 days. D , UV group after 180 days (90 days after dental floss application). White lines are flat form of implant.
Figure 4 shows the change of bone level between 90 and 180 days in each implant of both groups. Figure 5 shows the amount of bone resorption in both groups.
Fig. 4: The change of bone level between 90 and 180 days in each implant of UV and non-UV groups.
Fig. 5: Comparison of the bone resorption around non-UV implant and UV implant after 180 days by radiographic findings. Data in mean ± SD. *Statistically significant difference between the 2 groups,P < 0.05.
Amount of bone resorption for UV groups was 2.0 ± 0.5 mm, whereas that for non-UV group was 2.7 ± 0.4 mm. It was significantly smaller for UV groups than for non-UV group as indicated in Figure 5 .
Micro-CT Scanning FindingsBone resorption at 180 days after implant placement was also examined by micro-CT . Bone resorption variables in buccolingual and mesiodistal sections are presented in Figure 6, A and B (non-UV group) and Figure 6, D and E (UV group).
Fig. 6: Micro-CT findings after 180 days.A , A buccolingual section of the non-UV groups. B , A mesiodistal section of the non-UV groups. C , A 3D image of the non-UV groups. D , A buccolingual section of the UV group. E , A mesiodistal section of the UV group. F , A 3D image of the UV group. The broken lines in (B ) and (E ) are the mesiodistal lines between the necks of adjacent teeth.
The broken lines are the mesiodistal lines between the necks of adjacent teeth as shown in Figure 6, B–E . Bone resorption seemed as a large defect in the non-UV group, whereas it was smaller in the UV group.
In 3D imaging, large ring-like bone resorption surrounding the implant neck was observed clearly in the non-UV group (Fig. 6, C ), whereas shallow plate-like bone resorption was observed, and less bone was resorbed in the UV group (Fig. 6, F ).
The mean volume of bone resorption was calculated from micro-CT images (Fig. 7 ). Bone resorption volume in UV group was 45.7 ± 9.6 mm3 , whereas that in non-UV group was 64.4 ± 10.6 mm3 . Bone resorption volume was significantly smaller for UV group than for the non-UV group.
Fig. 7: Comparison of the bone resorption for non-UV implant and UV implant after 180 days by micro-CT findings. Data are the mean ± SD. *Statistically significant difference between the 2 groups,P < 0.05.
Histological Findings
Representative histological images in both UV and non-UV groups are presented in Figure 8 .
Fig. 8: Light microscopic histological images (after 180 days). The grind samples were stained by methylene blue and examined under a light microscope.A , Cervical and middle areas of non–UV-irradiated implant at sagittal section. B , Cervical area of the non–UV-irradiated implant at horizontal section. C , Middle area of the non–UV-irradiated implant at horizontal section. D , Cervical and middle areas of UV-irradiated implant at sagittal section. E , Cervical area of the UV-irradiated implant at horizontal section. F , Middle area of the UV-irradiated implant at horizontal section.
Figure 8, A shows the sagittal grind sections of non-UV group. At the cervical area, remarkable bone resorption was observed, and the interface of bone and implant was destroyed. At the middle area, bone trabeculae could not be observed clearly on the implant surface except at the threads (arrow).
Figure 8, B and C show the horizontal section of non-UV group. Cortical bone resorption of the cervical area was also seen above the implant. Bone trabeculae in the middle area were not attached to the implant surface in some parts (arrow).
However, Figure 8, D shows the sagittal grind sections of UV group. At the cervical area, bone resorption was not observed, and interface of bone and implant was maintained. At the middle area, bone trabeculae were attached to the threads of the implant and most of the implant surface was seen facing continuous bone tissue (arrow).
Figure 8, E and F show the horizontal section of the UV group. In the cervical area, the whole implant surface was surrounded by cortical bone tissue. Thick bone trabeculae were clearly observed around the implant in the middle area.
Discussion
Plaque accumulation on periimplant tissue21–23 24,25 periimplantitis .
Clinical investigations have identified a correlation between inadequate oral hygiene leading to plaque accumulation and inflammation of periimplant tissue, which can result in periimplant marginal bone resorption.26–28 2,29–32 33,34 35–38 periimplantitis . Therefore, the experimental animal model used in this study used a ligature-induced periimplantitis without occlusal stress.
Spontaneous progression of ligature-induced periimplantitis in UV and non–UV-irradiated implants was compared in this study. The amount of bone resorption during the plaque accumulation period by the placement of dental floss was significantly smaller at the UV-irradiated implant than non–UV-irradiated implant as shown by both dental radiographic and micro-CT scanning findings. The histological finding also revealed partial disintegration of bone and implant surfaces in the non–UV-irradiated implant. One important question to come up during this study is why UV irradiation of the implant surface is effective in ligature-induced periimplantitis . One of the reasons could be the improved implant surface property.
Implant surface modification can be performed by various techniques such as hydroxyapatite or calcium-phosphate coating, sandblasting, oxidation, and acid-etching.39–43 40–42
UV irradiation was recently introduced as a new method of surface modification for titanium implants, and it is characterized by generation of hydrophilicity and hemophilicity, a unique mechanism and simple delivery.13,18 44 45 15 13
In a dog model, Hirakawa et al46 14
We speculated that UV light treatment increases integration between bone and implant surfaces without connective tissue and also enhances the sealing effect of the marginal area. Although BIC value was not indicated in our data, the radiographic and histological findings of UV-irradiated implant showed that integration between bone and implant surfaces was maintained in marginal and cortical bones.
At this point, the positive effects of UV-irradiated implant in ligature-induced periimplantitis are understood to be due to an increase of BIC and the vigorous osteogenesis that occurs at the implant interface in the marginal area. However, the generalization of the present findings in small number of beagle dogs to humans must be interpreted with caution. A larger sample size and further clinical investigations are necessary.
The results of this study of an animal model with ligature-induced periimplantitis demonstrate that the photocatalytic activity of titanium reduces bone resorption around UV-irradiated implants.
Conclusion
Although these results can only be considered preliminary, especially the small sample, less pronounced bone resorption was observed around UV-irradiated implants than around non–UV-irradiated implants as revealed by both dental radiographic and micro-CT observations. Histological observation of tissue sections also revealed no attachment and partial destruction at the bone–implant interface in non–UV-irradiated implants. As a result, UV-light irradiation of implants seems to suppress spontaneous progression of periimplantitis .
Disclosure
The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
Acknowledgments
This work was supported by JSPS KAKENHI, Grant Number 23592904. K. Ishii, M. Matsuo, and N. Hoshi have contributed equally.
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Approval
The experimental protocol in animals and the surgical procedure followed the guidelines for Animal Care approved by Kanagawa Dental University (No 144.145).
