The properties of abutment materials can influence the quality of mucosal attachment formation [2]. Zr abutments were introduced to facilitate the esthetic implantation treatment of the maxillary anterior teeth, particularly in cases of patients with thin mucosa [33]. However, there are only limited data on the soft tissue response to Zr, particularly in comparison with the response to Ti under peri-implantitis conditions. This self-control study provides valid data support to compare the expression of proinflammatory cytokines in PICF and the inflammatory infiltration of soft tissues around implant abutments fabricated from Zr and Ti with and without ligation. No evidence of significant differences between these two types of biomaterials was found.
The results of this study showed that peri-implant mucosal inflammation occurred around the ZrL and TiL healing abutments and soft tissues around these abutments were red and swollen. GI was significant higher of specimens with the ZrL and TiL abutments than those with the ZrN and TiN abutments on day 28. However, no significant differences were found in PD, indicating that peri-implant mucositis without bone loss was the type of inflammation. This result is consistent with that of a previous study in dogs conducted by Albouy et al., the findings of which indicated that peri-implant bone resorption occurred at 12 weeks after ligation [34].
Investigations of the biochemical parameters in the gingival sulcus or PICF have become increasingly popular because it is possible to monitor the health status of gingiva and peri-implant mucosa [19, 35]. These biochemical methods can provide with an early diagnosis and potential application in disease prevention. TNF-α and IL-1β are primarily secreted by monocytes and macrophages and are potent multifunctional cytokines in abundant signal transduction processes during inflammation by acting as proinflammatory proteins. Therefore, the levels of TNF-α and IL-1β in PICF were analyzed, in addition to the clinical parameters.
The volumes of PICF were significantly increased in the tissues around the ZrL and TiL abutments than in those around the ZrN and TiN abutments on day 28, which was related to the occurrence of peri-implant mucositis. These results are in good accordance with previous studies, which demonstrated a significant increase in the volume of PICF after plaque accumulation [19, 35]. However, after oral hygiene behaviors were resumed, the volume decreased. These findings demonstrated that oral hygiene reduces peri-implant mucosal inflammation [19]. These data suggested that an increased volume of PICF could be a useful marker of the early inflammation of peri-implant soft tissues.
In this study, the levels of TNF-α and IL-1β in PICF were increased in the tissues around the ZrL and TiL abutments on day 28. Elevated levels of TNF-α and IL-1β in PICF have been found to be associated with peri-implantitis and peri-implant mucositis [19, 20]. Nevertheless, no significant differences in the levels of TNF-α and IL-1β were observed in this study, which may be related to the short observation period. The findings of TNF-α and IL-1β in PICF obtained in the present study were in good agreement with the clinical findings, which indicated that TNF-α and IL-1β could be useful markers for assessing the peri-implant health status.
According to morphological criteria on the amount of inflammation that reflects soft tissue health status, no differences were observed on the soft tissue inflammatory infiltration around the Zr and Ti healing abutments. However, fewer inflammatory cells were observed around the Zr healing abutments than around the Ti healing abutments. Similar results were also observed in a recent study conducted by Brakel et al., which reported no difference in the inflammation grading scale score in peri-implant mucosa adjacent to the Zr and Ti abutments [24]. A canine study conducted by Welander et al. reported lesser inflammatory infiltration in the epithelium of peri-implant mucosa around the Zr abutments than in that around the Ti implants [1]. In a human histological study, Degidi et al. reported significant elevations in the proinflammatory infiltrates (lymphocytes, plasma cells, and histiocytes) as well as an increased expression of vascular endothelial growth factor and nitric oxide synthase isoforms 1 and 3 in the tissues adjacent to the Ti healing abutments than in those around the Zr healing abutments after a 6-month healing phase [18].
The immunohistochemistry assays in this study revealed the expression of inflammatory cells at the basement membrane zone, in the soft tissues adjacent to healing abutments, and in the small endothelial cells of vessels in the vicinity. These inflammatory cells may be related to the Langerhans cells in the basal layer and vascular endothelial cells. Langerhans cells are lymphocyte antigen-presenting cells that play an important role during the early immune response of periodontitis or gingivitis [36]. Vascular endothelial cells are involved in the inflammatory process via the release of proinflammatory cytokines.
Whether this observation was attributable to the favorable attachment properties for the surrounding connective tissues and the epithelium was not conclusively established. A reduction in the inflammatory reactions may not merely be an expression of better insulation through the soft tissue but may also due to the proven lesser accumulation of bacteria on ceramic surfaces [16, 17].