Dental implants count as the most successful biomaterials implants in the human body. Failure rates due to infections are very low regardless of the fact that they function in a highly nonsterile environment in the oral cavity. This success can be attributed to the soft-tissue seal, protecting the osseo-integrated implant part against bacterial invasion. The seal consists of a layer of keratinocytes covering gingival fibroblasts, integrating the implant. In instances where the implant failure occurs, high patient discomfort and high costs of replacing an infected implant have been reported. Consequently, this calls for the development of improved, infection-resistant dental implant materials. Normally, new materials are often evaluated in mono-culture, examining bacterial adhesion or tissue interactions separately and neglecting the more real 3D-structure of the tissue seal. Unfortunately, during such endeavors, the 3D-tissue structure of the soft-tissue seal is mostly neglected and therewith the important physiological interplay between keratinocytes, fibroblasts, bacteria and implant surfaces. This is a severe shortcoming of current in vitro mono-culture models, especially in an era in which animal studies are regarded with more and more societal scrutiny and regulatory impediments are increasing.
On this account, there is need to advance the current in vitro models, including a better mimic of the 3D-model of the soft tissue seal, for the evaluation of new, infection-resistant dental implant materials under development. On this ground, a team of researchers from the University of Groningen and University Medical Center of Groningen in The Netherlands: Xiaoxiang Ren (PhD candidate), Professor Henny C. van der Mei, Professor Yijin Ren and Professor Henk J. Busscher developed a new 3D-tissue infection model of the soft-tissue seal around a dental implant using a transwell system in which bacterial challenges of the soft-tissue seal formed on different materials could be studied. Their work is currently published in the research journal, Acta Biomaterialia.
In the approach considered, the degree of physiological cell-to-cell contact was regulated by employing membrane filters with different pore sizes. The researchers set up a 3D-tissue model of the soft-tissue seal, in order to establish the roles of oral keratinocytes, gingival fibroblasts and materials surface properties in the protective seal. The model was developed using titanium oxide as an implant surface, and Streptococcus oralis and Staphylococcus aureus as post-operative challenging organisms. Finally, the sensitivity to different surface-chemistries of substratum materials in the model was demonstrated by using hydroxyapatite and silicone rubber, in addition to titanium dioxide surfaces.
The authors observed that in the absence of keratinocytes on the membrane, fibroblasts growing on the TiO2 surface could not withstand challenges by commensal streptococci or pathogenic staphylococci. In addition, keratinocytes growing on the membrane filters were observed to withstand bacterial challenges, but tight junctions widened to allow invasion of bacteria to the underlying fibroblast layer in lower numbers than in absence of keratinocytes.
In summary, through the newly presented 3D-tissue infection model, important physiological interactions between keratinocytes, gingival fibroblast, bacteria and materials surfaces were accounted for in the in vitro evaluation of new implant materials that had not been revealed in mono-culture models. Remarkably, it was seen that the protection offered by the soft-tissue seal appeared sensitive to surface properties of the implant material. In an interview with Advances in Engineering, Professor Henny C. van der Mei, the lead author further emphasized that the reported differential response to different surface-chemistries made the 3D-tissue infection model presented a useful screening tool in the development of new infection-resistant dental implant materials.
Xiaoxiang Ren, Henny C. van der Mei, Yijin Ren, Henk J. Busscher. Keratinocytes protect soft-tissue integration of dental implant materials against bacterial challenges in a 3D-tissue infection model. Acta Biomaterialia, volume 96 (2019) page 237–246.