materials Article Effect of Cement Layer Thickness on the Immediate and Long-Term Bond Strength and Residual Stress between Lithium Disilicate Glass-Ceramic and Human Dentin João Paulo Mendes Tribst 1, * , Alison Flavio Campos dos Santos 1 , Giuliane da Cruz Santos 1 , Larissa Sandy da Silva Leite 1 , Julio Chávez Lozada 2 , Laís Regiane Silva-Concílio 1 , Kusai Baroudi 1 and Marina Amaral 1   Citation: Tribst, J.P.M.; dos Santos, A.F.C.; da Cruz Santos, G.; da Silva Leite, L.S.; Lozada, J.C.; Silva- Concílio, L.R.; Baroudi, K.; Amaral, M. Effect of Cement Layer Thickness on the Immediate and Long-Term Bond Strength and Residual Stress between Lithium Disilicate Glass-Ceramic and Human Dentin. Materials 2021, 14, 5153. https:// doi.org/10.3390/ma14185153 Academic Editors: Lavinia Cosmina Ardelean and Laura-Cristina Rusu Received: 9 August 2021 Accepted: 3 September 2021 Published: 8 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Dentistry, University of Taubaté (UNITAU), Taubaté 12020-340, Brazil; flaviosantosdr@outlook.com (A.F.C.d.S.); giucs90@gmail.com (G.d.C.S.); larissasandy.ls@gmail.com (L.S.d.S.L.); regiane1@yahoo.com (L.R.S.-C.); d_kusai@yahoo.co.uk (K.B.); marinamaral_85@yahoo.com.br (M.A.) 2 Department of Operative Dentistry, FO-National University of Córdoba, Córdoba 5016, Argentina; juliochavezlozada@gmail.com * Correspondence: joao.tribst@gmail.com Abstract: This study tested whether three different cement layer thicknesses (60, 120 and 180 μm) would provide the same bonding capacity between adhesively luted lithium disilicate and human dentin. Ceramic blocks were cut to 20 blocks with a low-speed diamond saw under cooling water and were then cemented to human flat dentin with an adhesive protocol. The assembly was sectioned into 1 mm 2 cross-section beams composed of ceramic/cement/dentin. Cement layer thickness was measured, and three groups were formed. Half of the samples were immediately tested to evaluate the short-term bond strength and the other half were submitted to an aging simulation. The microtensile test was performed in a universal testing machine, and the bond strength (MPa) was calculated. The fractured specimens were examined under stereomicroscopy. Applying the finite element method, the residual stress of polymerization shrinkage according to cement layer thickness was also calculated using first principal stress as analysis criteria. Kruskal–Wallis tests showed that the “cement layer thickness” factor significantly influenced the bond strength results for the aged samples (p = 0.028); however, no statistically significant difference was found between the immediately tested groups (p = 0.569). The higher the cement layer thickness, the higher the residual stress generated at the adhesive interface due to cement polymerization shrinkage. In conclusion, the cement layer thickness does not affect the immediate bond strength in lithium disilicate restorations; however, thinner cement layers are most stable in the short term, showing constant bond strength and lower residual stress. Keywords: dental bonding; polymerization; finite element analysis; dental materials 1. Introduction When performing a ceramic restoration, the most recommended protocol is the use of resin-based cements combined to adhesive protocols during the cementation proce- dure [1,2]. This recommendation aims to achieve a clinical long-lasting bond between ceramic/resin cement and between resin cement/dental tissues [3,4]. In addition, the resin cements are easily handled, have an adequate setting time, and have the potential for both mechanical and chemical bonding [2,5]. However, the vertical misfit, or cement thickness, between the restoration and tooth preparation is an important factor which affects the success and survival of ceramic restora- tions [6]. The literature recommends a cement layer thickness around 50–100 μm for resin cements in ceramic crowns [7]. Furthermore, the bonding properties have been shown to be significantly reduced for cement thickness of 450–500 μm due to the residual stress Materials 2021, 14, 5153. https://doi.org/10.3390/ma14185153 https://www.mdpi.com/journal/materials