Rodent peri-implantitis models: a systematic review and meta-analysis of morphological changes

Peri-implantitis is the leading cause of implant failure, affecting 22% of patients worldwide [1]. It is defined as a biofilm-induced, pathological condition of the peri-implant tissues, characterized by inflammation within the supra-crestal connective tissue and progressive marginal bone loss [2]. Traditionally, its etiology and pathogenesis were thought to resemble periodontitis. As such, periodontal treatment protocols have been adapted to manage peri-implantitis [3]. However, these protocols are often deemed either ineffective or unable to predictably resolve peri-implantitis, regenerate lost tissues, and achieve re-osseointegration [4]. The observed inefficacy may be attributed to discrepancies in disease characteristics between peri-implantitis and periodontitis [5], especially microbiological and anatomical differences arising from the implant’s surface properties and its unique hard and soft tissue interface [6]. Clinically, peri-implantitis is further complicated by several risk factors at the patient, implant, and site levels that may influence disease initiation and progression [2]. This results in a complex disease that remains poorly understood and impossible to fully unravel with just clinical studies alone.

To address these issues, there is a need for mechanistic studies to shed light on the pathologic processes of peri-implantitis and the biological pathways through which risk factors exert their influence. For this purpose, in vivo research using animal models is indispensable. In the last decade, there have been increasing efforts to develop rodent intraoral experimental peri-implantitis models. Rodent models would enable researchers to conduct mechanistic studies more efficiently with reduced costs and a shorter healing time [7]. Furthermore, rodents possess several advantages, including the availability of many established protocols for reproducing systemic diseases, knockout and transgenic rodents, and the wide array of antibodies against rodent antigens for immunohistochemical analyses. These advantages provide a means to probe into the roles played by systemic disease, specific genes, and pathways in the disease process of peri-implantitis.

While rodents have been employed extensively for periodontal research, they are not well established for experimental peri-implantitis. In addition, there is a lack of consensus regarding these models, since existing systematic and narrative reviews were published before the development of these models and were focused on larger animals, particularly dogs and non-human primates [8]. The poor understanding of the histological, morphological, and biochemical characteristics of these models has thus limited their acceptance and utility. Therefore, this review sought to assess the construct validity of rodent peri-implantitis models by characterizing the morphological changes of rodent experimental peri-implantitis, in particular marginal bone loss (MBL), the key hallmark that distinguishes it from peri-implant mucositis.

To improve the management of peri-implantitis, additional insights into its etiology, disease processes, and systemic implications. While such mechanistic studies have been traditionally performed in larger animals, rodent models have emerged as a potential alternative in the last decade. However, the validity of these models remains controversial due to the lack of consensus regarding their histological, morphological, and biochemical characteristics. To address this issue, 23 studies were systemically identified and analyzed to characterize the morphological changes following experimental peri-implantitis in rodents.

The key histological hallmarks of peri-implantitis are inflammation of the supracrestal soft tissue and formation of peri-implant pockets with the underlying circumferential bone loss [2]. These histological features are clinically measured as increased probing depth (≥6 mm), bleeding or suppuration on probing, and radiographic bone loss (≥3 mm) [31]. Considering the rodent’s size, and the extent of MBL (0.25 mm in mice and 0.26 mm in rats), it would not be appropriate to directly apply these clinical diagnostic criteria to rodents. Instead, there is a need to identify the underlying histological and morphological characteristics that these clinical parameters represent. Based on the results of the meta-analysis, experimental peri-implantitis resulted in a significantly greater linear MBL than observed in healthy controls in both rats and mice, without species-specific differences. These radiographic findings were consistent with the histomorphometric measurements [15, 17, 19, 22]. The extent of bone loss was comparable for the buccal, mesial, distal and lingual surfaces of the implant, suggesting that MBL was circumferential [19, 22]. This was confirmed by a volumetric analysis, which further identified the presence of suprabony and infrabony components [13, 14, 23, 26, 30, 32, 33, 34]. This concurred with the findings reported in dog models and the defect configuration observed clinically [35]. In addition to MBL, the included studies have also reported the apical migration of an ulcerated pocket epithelium [19, 22] and expanding inflammatory cell infiltration within the supracrestal connective tissue [19, 24]. Moreover, these soft tissue changes also presented as visible signs of inflammation, such as edema and erythema [20, 22, 25, 28, 29], consistent with the clinical features of peri-implantitis. Thus, this review confirmed that the histological and morphological hallmarks of peri-implantitis were successfully recreated in rodent models of experimental peri-implantitis, supporting the construct validity of these models. However, construct validity is only one of the many considerations when choosing an animal model, especially since no model can replicate all aspects of peri-implantitis. Therefore, there is a need to discuss rodent models’ potential role in peri-implantitis research, while considering their advantages, limitations, and challenges.

Animal research in peri-implantitis can be broadly categorized into studies that focus on the disease processes and those focusing on therapy. The former involve research on the pathogenesis and wound healing of peri-implantitis, and its systemic interactions. Compared to their larger counterparts, rodents have several advantages that make them particularly suitable for such research. Longitudinal observations will be required to investigate the mechanisms involved in the initiation, progression, and resolution of peri-implantitis, translating to a considerable sample size, which is not feasible in larger animals considering the logistics, costs, and ethical concerns. This would be more achievable in rodent models due to their lower costs and shorter healing time [7]. Furthermore, the availability of transgenic and knockout rodents and a wide array of antibodies against rodent antigens would enable researchers to probe the roles played by specific cells, genes, and pathways in the disease process of peri-implantitis. For example, 2 studies utilized Toll-like receptors 2 and 4 knockout mice to explore the role of pattern recognition receptors in mediating the expression of proinflammatory cytokines and osteoclastogenesis that result in MBL [32]. This study shed light on the differences in molecular features between peri-implantitis and periodontitis, highlighting the rodents’ potential utility.

These advantages enable the use of rodents for research on various systemic diseases, giving rise to the availability of various protocols to reproduce these conditions in rodents. Thus, rodent models can be potentially used to explore the interactions between these diseases and peri-implantitis. As discussed in this review, rodent models have been used to determine the impact of diabetes mellitus [17, 21, 27] and xerostomia [18] on peri-implantitis. While an association between diabetes mellitus and peri-implantitis has been described, this relationship is poorly defined [2]. The rodent studies clarified this by highlighting the effects of hyperglycemia on inflammatory cytokine pathways, linking it to osteoclastogenesis and MBL [17, 21, 27]. In addition, the negative influence of xerostomia on the oral microbiota, mucosal inflammation, and MBL are novel findings with significant clinical implications [18]. These studies demonstrated rodent models’ potential to provide insight into disease processes and systemic interactions that will impact the clinical management of peri-implantitis.

Nonetheless, for studies that seek to develop or test novel therapeutics, larger animal models such as dogs may be a more suitable candidate. The small size of rodents precludes the use of clinical-grade dental implants and biomaterials and limits the possible routes of administering therapeutics to injections and topical applications [16, 34, 36]. These inherent limitations would affect the utility of rodent models in studies seeking to explore the underlying mechanism of action of these novel therapeutics. Furthermore, the healing potential and regenerative capacity of these models are poorly defined since the majority of the studies were concluded after successfully inducing peri-implantitis. While 2 studies reported that the MBL resulting from ligature-induced peri-implantitis was critical, to the point that spontaneous regeneration was not possible [20, 26], their responsiveness to treatment and regenerative capacity remains unexplored. In the current studies, therapeutic interventions were administered before or during the induction of peri-implantitis [12, 15, 16, 33, 34]. Thus, the significant reductions in MBL achieved would represent the intervention’s ability to prevent peri-implantitis, rather than their therapeutic or regenerative efficacy. Factoring in these limitations, even though rodents can serve as a screening platform, further translational research in larger animal models would still be required.

Nevertheless, for both types of research, rodent models have a fundamental limitation, which is the difficulty in ensuring successful osseointegration. In this review, failure rates of 11.6% and 11.3% were observed for mice and rats respectively. Considering rodents’ limited alveolar bone volume, difficulties in achieving primary implant stability may predispose implants to micromotion during healing, resulting in fibrous encapsulation instead of osseointegration [37]. To address these issues, future studies may adopt implants with cutting thread designs that help achieve primary stability or surface modifications that accelerate the process of osseointegration. Furthermore, to avoid confounders arising from the synergistic effects of the experimental peri-implantitis and post-insertion infection during the osseointegration phase [13], at least 28 days should be provided for the healing processes of osseointegration to be completed in both rats and mice [38, 39]. These issues will need to be factored in and addressed when planning future studies.

As with most research, this systematic review has its strengths and limitations. This is the first review to systematically appraise the validity of rodent models of experimental peri-implantitis. In addition, this review also summarizes the various experimental peri-implantitis protocols, providing a reference for future studies (Table 2). Four different methods for inducing peri-implantitis were identified, including the use of titanium particles, a topic of controversy. While the majority of the literature supports a microbial etiology for peri-implantitis [2], there is limited evidence that peri-implant bone loss could result from foreign body reactions provoked by titanium particles and their corrosion products [40]. Future research will be needed to clarify the individual roles played by and the interactions between biofilms, microbial products, and titanium particles in the disease processes of peri-implantitis. To facilitate this, an inclusive approach was adopted.

This systematic review was limited by the inherent shortcomings of the included studies. Among the included studies, heterogeneous definitions were used for both linear and volumetric measurements. Linear measurements of MBL have been defined as the distance from the bone crest to either the implant’s first thread, top, or apex. To account for these variations, the difference in MBL between healthy implants and those with experimental peri-implantitis was first calculated for each study. This difference was then used for meta-analysis. Unfortunately, a similar approach could not be adopted for the volumetric analysis because variations in the horizontal dimension in the VOI precluded a meta-analysis. Since an optimal VOI definition cannot be inferred from the available evidence, future studies will need to report a clear description of their respective VOI definitions to facilitate replication or comparison with other studies.

Another limitation of the available literature was the lack of measures to minimize biases. To improve the quality of pre-clinical evidence, future studies will need to incorporate measures such as clear sequence generation, allocation concealment, and blinding during the experiment and at the time of outcome assessment. In addition, to account for possible implant loss during osseointegration, this attrition should be considered during sample size calculation and changes in the sample size for each experimental group should be reported. Future studies may consider performing randomization only after confirming successful osseointegration, reducing the possible biases before peri-implantitis induction, and ensuring clearly reported and equal sample sizes for the different experimental groups.

In conclusion, the key features of peri-implantitis, including supracrestal soft tissue inflammation, peri-implant pocket formation, and circumferential MBL, have been successfully recreated in rodent models of experimental peri-implantitis. While the current systematic review revealed several limitations and challenges, these models are promising alternatives to larger animals for mechanistic studies pertaining to the disease processes and systemic interactions of peri-implantitis. We propose that future studies standardize their peri-implantitis protocols and improve the reporting of methodology and results, to enable replication and comparisons across the literature.