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Researchers develop novel approach to dental restoration using 3D printing

The new technique is both cost-effective and time-saving for clinicians who still want to offer superior quality restorations. (Image and video: Keiichi Hosaka)

Digital workflows are constantly being improved with innovations in technique, software and materials and tailored to dental diseases and patient cases. Exploiting technological advancements, a team of international researchers has developed a more cost-effective, biomimetic, non-invasive and—most importantly—quicker treatment technique for restoring a missing mandibular lateral incisor utilising two flowable resin composites injected into two 3D-printed indices. Dental Tribune International spoke with lead researcher Prof. Keiichi Hosaka of Tokushima University in Japan about the new direct restoration technique.

Prof. Keiichi Hosaka. (Image: Keiichi Hosaka)

Prof. Hosaka, the study mentions the use of two specialised 3D-printed indices for the injection moulding process. Could you elaborate on the design process for these indices, and how they contribute to the overall success of the restoration?
The design process for our 3D-printed indices begins with the restoration of an ideal direct bridge as the first step. This is followed by an approximately 1 mm cutback on the labial side only and precise modelling of the internal anatomical structures at the incisal edges, namely the dental mamelons. In the third step, the two indices are designed as negative forms and 3D-printed using a flexible 3D-printing resin. Also, a 3D-printed stabilisation holder (made from a rigid 3D-printing resin) improves the adaptation of the flexible indices. This method allows the use of two different resin composites, one for the dentine and one for the enamel, with different mechanical properties or shades to control polymerisation shrinkage and achieve more biomimetic restorations with a shorter chair time.

What were some of the main challenges encountered in attempting to adapt the dual-resin composite injection moulding technique to the specific case of replacing a missing mandibular lateral incisor, and how were they overcome?
In this situation, three properties are needed for the 3D-printed index: flexibility to allow for undercuts; tearability or breakability for removal after light curing of the resin composite; and transparency for light curing and visibility. In this context, we had to choose a flexible 3D-printing material that is approved for intra-oral use in Japan. The material we used is coloured, but we have since developed a more transparent one.

Also, the 3D-printing process, including washing and post-curing, must be carefully calibrated. Inadequate washing may cause the index to bond to the resin composite, requiring a separator to be applied inside the index. Over-polymerisation may make the index stiffer than desired.

Considering the digital workflow integral to this technique, what are the implications for dental practices in terms of required equipment and training and overall feasibility of adopting this approach in a typical clinical setting?
Ideally, this would require that the dental clinic has an intra-oral scanner, CAD software and a 3D printer, but that may not be realistic for now. Instead, the clinic could rely on an index fabrication service. The dentist would simply send the scanned data to the fabricating company and the index would be returned to the dental clinic. This approach is supported as a form of teledentistry.  We also started following this approach in Japan last year with our university start-up, Amidex, inspired by advanced minimal intervention dentistry with index and digital transformation (DX).

Given the promising initial results, what specific long-term outcomes and potential complications do you anticipate in patients treated with this technique, and how do you plan to address them in future studies?
Although early results are promising, monitoring for potential complications such as debonding and composite fractures is essential. These problems are easily repaired intra-orally, demonstrating a practical advantage of direct composite restoration before considering more invasive or costly treatments.

The unique aspect of this simplified flowable resin composite bridge without reinforcing fibres provides a straightforward alternative for dentists. Preheated paste-type resin composites could serve as another option.

From a clinical standpoint, I believe that the standardised restorative process through laboratory and operative procedures will improve the quality of the restoration and extend longevity. Future clinical studies will focus on collecting longitudinal data on wear resistance, shade stability and patient satisfaction in order to continually refine the technique.

Editorial note:

The study, titled “A dual composite resin injection molding technique with 3D-printed flexible indices for biomimetic replacement of a missing mandibular lateral incisor”, was published online on 13 March 2024 in Journal of Prosthodontic Research, ahead of inclusion in an issue.

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