Preserve Natural Teeth With Biomimetic Dentistry

The core tenets of biomimetic dentistry are preserving pulp vitality, restoring natural biomechanics and function, and extending the lifespan of the tooth. Greater life expectancy and retained natural teeth at older ages have increased the demand for minimally invasive procedures to preserve tooth structure.¹ Biomimetic dentistry helps meet this demand. Some basic principles include absolute isolation, selective caries removal, maximizing adhesion, and choosing biomechanically compatible materials.²

When dealing with larger defects, traditional techniques use full-coverage crowns. While long-term success rates are high, the aggressive nature of preparations and associated longevity with crowned teeth are concerning. On average about 67% to 75% of the coronal aspect of the tooth is removed, which is significantly higher than partial coverage preparations (32% to 47%) and clinical data indicate crowns have the fastest time to extraction when compared to minimally invasive restorations.^3,4 Other research demonstrates that the removal of marginal ridges significantly reduces fracture resistance.⁵ Crown preparation may also increase the incidence of pulpal necrosis.⁶

Adhesive approaches point toward the use of partial coverage restorations when handling more significant defects. A 16.9 year clinical study showed a 95% survival rate for partial-coverage ceramic restorations.¹ Another study found a significantly im­proved survival rate for partial coverage lithium disilicate (LDS) restorations when compared to crowns at 98.7% after 15.4 years.⁷ An in vitro study showed superior fatigue performance for computer-aided design/​​​computer-aided manufacturing LDS partial coverage restorations in comparison to single crowns.⁸ At this point, the longevity and benefits of partial coverage restorations are well established in the literature. Figures 1 through 6 provide an example of such a restoration.

Isolation

Maintaining strong isolation is critical in preventing contamination of adhesive restorations. While rubber dam isolation is not a new technique, many clinicians do not use it in daily practice. Some of the many benefits of rubber dam isolation include soft tissue retraction and protection, improved visual field, reduced stress for operator and patient, aseptic isolation, and a reduction in contamination from aerosol generating exposure by 70% to 98.9%.⁹ A recent clinical in-situ study concluded that enamel bond strengths were significantly higher with rubber dams, independent of bonding agents studied.¹⁰

Caries Removal

Selective caries removal in deep lesions aims to preserve vitality by preventing pulpal exposure. This involves complete removal of carious tissue 2 to 3 mm circumferentially around the caries lesion and selective removal (removing infected dentin and leaving affected dentin) over areas that risk exposure.^2,11 It has long been postulated that selective caries removal paired with a strong adhesive seal can decrease bacterial load, allowing the dentin to remineralize, thereby allowing vitality preservation without compromising the overall bond strength of the restoration.^11,12 Several clinical studies have highlighted the benefits of this technique.¹³

While significant clinical data support selective caries removal in deep carious lesions, there is certainly valid criticism.¹⁴ Clinical studies often adopt success criteria to be absence of pain, swelling, and fistula. These criteria are not equivalent to the absence of pulpal pathology, considering pulpal necrosis can occur in the absence of symptoms in 14% to 60% of cases.¹⁵ The follow-up periods on the studies are insufficient with most studies having a 1- to 3-year follow-up.^14,16 These studies are often not long enough to allow accurate assessment of a chronic process that can develop into pulpal pathology. Only one such study has been done, with a 10-year follow-up.¹⁷ However, no study has consistently investigated pulpal response and subsequent histological success.¹⁶

The case for complete caries removal can be made with respect to biological pulp tissues, especially with the advances in vital pulp therapy and its success rates of 96.7% at 5 years (histologic)¹⁶ and 91.6% at 10 years (clinical and radiographic).¹⁸

When deciding on excavation modality, the patient must be considered. Systemic immunocompetence, caries risk, and the patient’s desired outcomes must not be overlooked. Clinician skill and knowledge as always are determining factors as well.

Preparation Guidelines

Preparation techniques should be defect driven and dictated by the removal of caries. Undermined cusps (< 1.5 mm thickness at base or if occlusal margin involves cusp tip) should be removed especially if it involves a fracture or crack.¹⁹ While some studies indicate cuspal coverage can protect against crack propagation and increase extracoronal strengthening,¹⁹ adhesion alone can have this affect regardless of cusp reduction.²⁰

A recent in vitro/​​​in silico study examining preparation design on the fracture resistance of compromised molars restored with LDS inlay/​​​overlay preparations concluded that inlay preparations showed reduced fracture resistance when compared to onlays.²¹ However, overlay preparations showed more catastrophic failures. This is likely due to the supraphysiologic forces applied in this study. These destructive failures of ceramic restorations were not seen in clinical studies that included onlays.²² The authors concluded that the undermined inlay preparation is a viable option to restore compromised molars.²¹

The overall design should be rounded, avoiding sharp corners or preparing additional retention features. This results in the most favorable biomechanical loading of the restored tooth.²³ Lastly, a 45° bevel was found to have the greatest fracture resistance when compared to butt-joint and chamfer finish lines.²⁴ While certain guidelines can improve performance, there is no one size fits all solution to partial coverage preparations.

Immediate Dentin Sealing

Maximizing bond strength is accomplished via immediate dentin sealing (IDS) with gold standard bonding agents. IDS has several published benefits including, but not limited to, decreased post-operative sensitivity, improved bond strength, decreased bacterial leakage, and improved fracture strength.^25,26

The current literature continues to affirm these benefits. A recent systematic review concluded that IDS improves bond strength of indirect restorations.²⁷ Another recent systematic review concluded IDS significantly increases the success and survival rate of indirect restorations while decreasing the intensity and incidence of post-operative sensitivity compared with delayed dentin sealing.²⁸

The technique is best applied on freshly cut dentin with the use of either a three-step total etch filled adhesive or a two-step self-etch adhesive system.²⁷ Air particle abrasion with 25 micron aluminum oxide significantly improves surface preparation and bond strength when using a self-etch adhesive.²⁹ An additional application of a thin 0.2 to 0.5 mm layer of a highly filled flowable resin (resin coat) is required to overcome the oxygen inhibited layer (OIL) when using an unfilled adhesive.²⁷ The resin coat aims to protect the hybrid layer and allow a higher degree of conversion. It additionally aids the blocking of undercuts and controls the mode of failure by placing them above the resin layer.^2,30

The OIL must be removed prior to impression to prevent inference of impression material setting or adherence to temporary restorative material. This is accomplished by a final light cure through glycerin gel applied to the surface or scrubbing with an alcohol-soaked cotton pellet.²⁷

Restorative Foundation

The additional placement of a dentin replacement composite (with shrinkage rates less than 3% and near 20 GPa elastic modulus) further aids in blocking undercuts and creating an ideal adhesive interface.^2,19

The use of fiber-reinforced composites (FRC) as a dentin replacement can further enhance the performance of indirect restorations. They are excellent dentin replacements due to their structural and mechanical resemblance. The short fibers resemble the collagen matrix and mechanically, the modulus of elasticity, tensile strength, and fracture toughness are nearly uniform to natural dentin.^30,31

FRC have been shown to act as a stress breaker that can help reduce catastrophic failure.³² This in vitro study investigated severely damaged endodontically treated teeth (ETT) and nonETT and found that the groups restored with a bilayered approach (core with FRC veneered with direct composite) had a significantly more favorable fracture pattern. The vital and ETT groups that were restored with FRC had 75% and 66.7% restorable fractures respectively, while the nonFRC groups (regardless of ETT or nonETT) only had 25% of fractures that were favorable.³²

The application of a restorative foundation with FRC can help control the mode of failure, improve fracture toughness, and aid a minimally invasive preparation by filling voids and undercuts from the result of caries excavation.

Resin Cement vs Preheated Composite

Etched and silane-treated LDS partial coverage restorations can be luted with either resin cement or with preheated restorative composite. The emergence of thermodynamic luting has some benefits based on laboratory studies. Luting with preheated composite offers improved physical properties, higher bond strengths and improved fracture toughness.^33-35 However, not all composites heat equally nor reach adequate film thickness.³⁶ The evidence in regard to adaptation of the restoration is mixed. Magne et al³⁷ reported an improved adaption using preheated composite compared to resin cement when luting various 3-mm thick partial coverage prosthetics. A recent systematic review reported that luting with preheated composite may negatively influence adaption, although the results were limited due to heterogeneity of existing data.³⁸ Again, clinician skill and knowledge may best dictate the luting protocol.

Conclusion

Biomimetic dentistry offers a valuable, minimally invasive approach that aligns with modern patient demands for tooth preservation and extended longevity. By emphasizing selective caries removal, partial coverage restorations, and advanced adhesive techniques, it reduces the risks associated with more aggressive procedures like full-coverage crowns. As clinical evidence continues to support the long-term success of these methods, biomimetic principles present a compelling option for achieving optimal outcomes in restorative dentistry.

References

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From Decisions in Dentistry. October/November 2024;10(6):16-21.