Frauke Müller (FM): I have been passionate about research since my early days as an assistant at the Prosthodontic Department of the University of Bonn in Germany. Since 2003, I have held the Chair for the Division of Gerodontology and Removable Prosthodontics at the University of Geneva, Switzerland. At the same time, I am responsible for several satellite dental clinics at the Department of Rehabilitation and Geriatrics of the University Hospitals of Geneva. This is a unique opportunity to treat geriatric and special in-patients who present various special challenges in regard to their oral health and dental treatments. Our research team includes faculty staff, under- and postgraduate students. We also collaborate very closely with Professor Martin Schimmel, now Chair of the Department of Reconstructive Dentistry and Gerodontology, School of Dental Medicine, University of Bern, and Professor Murali Srinivasan, who was appointed Chair of the Clinic of General-, Special Care- and Geriatric Dentistry, Center of Dental Medicine, University of Zurich in 2019. Both trained in Geneva, and they are somehow still members of the “extended family”. And then of course we have several international collaborations in the area of gerodontology. It was Murali who ran the experiments for this RCT and is first author of the corresponding publication. 

FM: The research question for this project arose from the clinical need to treat edentulous patients with complex functional and anatomical conditions. The conversion of a mandibular complete denture to an implant-overdenture is one of the most effective interventions in dentistry. The shortcomings of conventional mandibular dentures are well documented in the literature, and are witnessed by our patients in daily clinical practice. The functional improvements associated with implant placement concern the chewing efficiency and bite force, which allow the patient to adopt a diet with a broader range of “healthy” food stuffs. Structural benefits comprise improved chewing muscle bulk, and reduced bone atrophy in the peri-implant area. But most importantly, there are numerous psycho-social benefits for edentulous patients, who are able to enjoy a more active social life, enhanced self-confidence and better quality of life. So the benefits of implant-overdentures go way beyond the mouth! The novel treatment concept introduced in this ITI-funded RCT investigated the option of providing posterior occlusal support without increasing the number of implants. In younger edentulous patients – yes, in our discipline an average age of 72.5 years is considered young – the increased chewing force with implant-overdentures may accelerate posterior bone loss, where the dentures are merely mucosa-born. Therefore, 4 implants are recommended for this patient group, providing an extended support area to protect the posterior bony structures. However, this approach can only be adopted when the anatomical conditions are favorable, and if the increased invasiveness and cost are acceptable to the patient. Our study showed that extended bars on 2 implants provide posterior support while invasiveness and cost remain reasonable, and proved not inferior to 2 implants with the classical retentive anchors (ball attachments).

FM: The ITI is the largest and leading non-governmental research funder in the field of implant dentistry and has been supporting research projects since 1988. Clearly, we applied for the research grant with the ITI not only for the funding, but also for the prestige and reputation associated with such a grant. Applications are meticulously reviewed by highly competent experts on the ITI Research Committee, providing valuable input and a “quality label” to the project. Having myself been a member of the ITI Board of Directors and Chair of the Leadership Committee for two terms, I also have emotional bonds to the ITI, which provided numerous opportunities for my professional development and network. In short, the ITI seemed the obvious option to support this research project. 

Murali Srinivasan (MS): The study was designed as a randomized controlled trial, where edentulous patients with 2 interforaminal mandibular implants were randomized into either having their superstructure converted to a long-extension bar or 2 retentive anchors in the form of ball attachments. Outcome parameters such as implant survival, marginal bone loss, chewing efficiency, maximum bite force, distribution of occlusal contacts as well as patient-reported outcome parameters like denture satisfaction and oral health-related quality of life were investigated at baseline, 2 weeks, 6 and 12 months and every year thereafter. The results revealed an implant survival rate of 100% and confirmed the non-inferiority of the novel treatment modality over the first 12 months. Both the experimental and the control group experienced functional improvements, but within the observation period, they occurred earlier in the experimental group, which may be related to a facilitated adaptation process in dentures which are mechanically more stable. However, in a clinical context, one year is a very short observation period, as the implant-dentures are meant to be in service for many more years. Therefore, long-term observations are ongoing to reveal potential differences. Given that one treatment concept presents a rotational axis, and hence the sinking in of the posterior denture saddle, whereas the other has an extended support area, long-term differences might appear with regard to posterior bone resorption and/or the need for relines and occlusal adjustments to prevent a combination syndrome. 

MS: The novel treatment modality is promising and provides the general practitioner with an additional option to treat edentulous patients with mandibular implant-overdentures. Treatment planning for edentulous patients is complex, given the large spectrum of anatomical conditions, functional requirements and last but not least psycho-social suffering from tooth loss. The potential functional and structural advantages of a long extension bar might be particularly interesting for younger edentulous patients with high functional demands, but limited financial means. However, the 15-mm extensions we used are longer than recommended by the manufacturer, and the treatment was performed under the strictly controlled conditions of a clinical study. Before recommending the treatment concepts for general dental practice, a longer observation period should be documented, as implant loss and prosthodontic complications like fractures of the distal extensions may occur later on.

FM: As mentioned above, a longer observation period is needed before the novel treatment concept can be recommended for daily general practice, so we are continuing to follow the patient cohort and the 3-, 5- and hopefully 10-year observations will be duly published. Future research will also be even more minimally invasive and look into the possibilities of single-implant mandibular overdentures. A joint pilot project with the Prof. Claudio Leles from the University of Goias in Brazil, and Prof. Srinivasan from the University of Zurich will compare the use of one single implant in the canine area of the chewing side with the traditional 2-implant overdenture design. This approach aims to provide occlusal support where it is needed, namely close to the denture’s chewing center, rather than in the mandibular midline’s suture. Of interest is also the possibility to add a second implant later on, if additional budget is available and the patient desires further improvement. Once the pilot project is successful, guess where we will apply for a research grant to conduct the full-scale clinical trial?