Monomer release from direct and indirect adhesive restorations: A comparative in vitro study



Indirect dental restorations produced by computer-aided design and computer-aided manufacturing (CAD/CAM) are relatively new in daily dental practice. The aim of the present study was to compare the monomer release between direct composite restorations and indirect CAD/CAM produced restorations (composite, ceramic and hybrid).


Identical crown restorations were prepared from three indirect materials (Cerasmart, Vitablocs Mark II and Vita Enamic) and one composite material (Clearfil AP-X). For each restoration, eight crown restorations were luted onto tooth samples and immersed into 2.5 mL of an aqueous extraction solvent. Additionally, three nonluted crowns of each restoration type were also immersed in the extraction solvent, and served as controls. Every week, the extraction solvent was collected and refreshed, during a period of 8 weeks. The released monomers were quantified using ultra-performance liquid chromatography-tandem mass spectrometry.


Indirect restorations release significantly lower quantities of residual monomers than direct restorations, and the monomers released by the luted indirect restorations are mainly derived from the composite material used for cementation. The quantity of monomers released by direct restorations greatly depended on the time of light polymerization.


In terms of monomer release, indirect restorations are a good alternative to direct restorations to limit patient exposure to residual monomers. It is important to ideally design the fit of indirect restoration so that the cement layer is as thin as possible and the monomer release from this cement layer remains as low as possible.


Composites are currently the material of choice for tooth restorations . They have many advantages including rapid polymerization, good adhesion to tooth tissue, easy handling, easy to repair, good mechanical properties, the possibility of tissue-saving cavity preparation and the fact that they are relatively cheap compared to indirect restorations. In practice, patients often prefer composites because of their pleasing esthetics. Despite their many advantages, there are also some disadvantages such as biological effects and biocompatibility.

In the clinical situation, composites will never completely polymerize, which results in the release of residual monomers in the oral cavity. There is a possibility that these eluted ingredients are systemically absorbed by the host via the mucosa or the pulp tissue, or through ingestion . Moreover, released monomers have the potential to elicit allergic reactions, most often due to intraoral T-cell mediated delayed hypersensitivity (type IV) and more rarely due to IgE-mediated immediate hypersensitivity (type I) .

In spite of their introduction in the mid-1980s, chairside indirect restorations produced by computer-aided design and computer-aided manufacturing (CAD/CAM) have only recently become a viable alternative to restore teeth with extensive tooth loss. Compared to large composite restorations, they have the advantage of more easy anatomical reconstruction and better mechanical properties . Compared to conventional crowns, the success of chairside restorations lies in the shorter workflow, and thus reduced patient-time and costs. Obtaining optimal adhesive bonding to both tooth and the CAD/CAM material is, however, the prerequisite for success. Typically, a composite cement is used for luting, but also restorative composites can be used as they exhibit better mechanical properties .

Both ceramic and indirect composite are used for CAD/CAM restorations . Ceramics are harder than composite materials and subsequently more wear-resistant. However, this hardness might also lead to increased wear of the opposing dentition and restorations. On the other hand, like glass, ceramics are brittle and more prone to fracture compared to composites . Recently, hybrid ceramics have been brought onto the market as fundamentally new combinations of ceramic and polymer, in an effort to combine the esthetics and wear resistance of ceramics with the advantages of composite resins. The ceramic ingredients provide for the mechanical stability, the polymer part for the elasticity.

Although conventional direct composite restorations may exhibit good esthetic and mechanical properties, indirect resin-composite blocks may represent a good alternative to direct composite restorations, especially in large posterior restorations. Not only do these industrially pre-polymerized indirect composite restorations have a higher degree of conversion (DC), they also redress the negative effect of polymerization shrinkage, consequently improving the overall mechanical and physical properties . Moreover, indirect composites exhibit better interproximal contacts, higher wear resistance, and simplified creation of natural and anatomical shapes in large defects .

The aim of this study was to investigate whether indirect adhesive restorations produced by chairside CAD–CAM technology can provide a more biocompatible way to restore teeth. For this purpose, the release of monomers from identically shaped crown restorations either made from direct (conventional) composite or CAD-CAM blocks (indirect composite blocks, feldspar ceramic blocks and polymer-infiltrated-feldspathic (hybrid) ceramic blocks) was quantified using ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The null hypothesis of this study was that the indirect restorations result in lower quantities of released monomers.

Materials and methods

Sample preparation

Preparation of tooth samples

Extracted third molars were collected in the university clinic of Leuven (UZ Leuven). Ethics approval for the collection of extracted third molars was obtained by the Commission for Medical Ethics of the university hospitals KU Leuven (ML8189, approved May 3, 2012). The teeth were stored in 0.5% chloramine until further use. First, the occlusal two third of the crown was removed with a diamond saw (Isomet 1000, Buehler, Lake Bluff, IL, USA). Since all restorations should have the same dimensions, the remaining part of the teeth was shaped into identical cylindrical specimens with the microspecimen former (University of Iowa, Iowa City, IA, USA).

The tooth samples ( n = 32) were thoroughly cleaned with Tubulicid (Dental Therapeutics AB, Saltsjo-Boo, Sweden) on a microbrush. Enamel, when present, was etched for 15 s with Scotchbond Universal Etchant (3M ESPE, Seefeld, Germany), followed by rinsing and drying with a mild air flow. Clearfil SE Bond (Kuraray Dental, Hattersheim am Main, Germany) was used as adhesive system following the manufacturer’s instructions. The primer was applied for 20 s, followed by drying with a mild airflow. After applying the bonding, airflow was gently applied to evenly distribute the bonding and the bonding was subsequently cured for 10 s with the Demi Ultra curing light (Kerr Dental, Orange, CA, USA) with an output exceeding 1200 mW/cm 2 .

Preparation of indirect restoration specimens

Three different indirect materials were tested: (1) composite resin blocks Cerasmart (GC Europe, Leuven, Belgium), (2) feldspar ceramic block Vitablocs Mark II (VITA Zahnfabrik, Bad Saeckingen, Germany) and (3) polymer-infiltrated-feldspathic (hybrid) ceramic blocks Vita Enamic (VITA Zahnfabrik). More details on these materials are summarized in Table 1 . The tooth preparation was digitally scanned using CEREC (Densply Sirona, Wals bei Salzburg, Austria) and a suitable restoration was digitally designed. Identical crowns of indirect materials were milled with the CEREC MC X milling unit (Densply Sirona). For each material, 11 specimens were prepared.

Table 1
Information and composition of the materials tested.
Material Classification Composition Shade Manufacturer
Filler content Mass% Monomers size
Clearfil AP-X Conventional composite resin Silanated barium glass filler, silanated silica filler, silanated colloidal silica 78 BisGMA TEGDMA CQ A3- Kuraray Dental, Tokyo, Japan
Cerasmart Resin nano-ceramic Silica (20 nm), barium glass (300 nm) 71 UDMA, DMA, Bis-MEPP A2 LT 14 GC, Tokyo, Japan
Vitablocs Mark II Fine-particle feldspar ceramic block Feldspathic crystalline particles
in glassy matrix
2M1C I10 Vita Zahnfabrik H. Rauter, Bad Säckingen, Germany
Vita Enamic Polymer-infiltrated-feldspathic (hybrid) ceramic block Silicon oxide
Aluminum oxide
Sodium oxide
Potassium oxide
Boron trioxide
Calcium oxide
86 UDMA, TEGDMA 3M2 EM-14 Vita Zahnfabrik H. Rauter, Bad Säckingen, Germany
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Jan 10, 2021 | Posted by in Dental Materials | Comments Off on Monomer release from direct and indirect adhesive restorations: A comparative in vitro study
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