# A Literature Review of Adhesive Systems in Density: Key Components and Their Clinical Applications
## 4 Effect of Dentin Structure on Resin-Dentin Bonding
### Role of the Smear Layer in Dentin Bonding  
The production of the smear layer occurs while preparing the dental cavity. It manifests on the dentinal surface after the application of the instruments and is made up of varying debris and an altered morphology [75]. The resulting porous layer, with a thickness ranging between 0.5 and 7 \(\upmu\)m, is comprised of saliva, water, HAp, bacteria, blood, and modified collagen [76]. The quality and thickness of the smear layer depend on the instrumentation used, the mode of use, the dentin site, and whether a splash of water is applied. A rougher surface is produced through diamond burs rather than carbide burs, and stronger friction is created by dry cutting than by wet cutting [75, 77].  
As soon as the layer binds to the dentinal surface, the entrance of the dentin tubules is plugged to a depth between 1 and 10 \(\upmu\)m, effectively blocking any direct contact of materials with the dentinal substrate. The final layer is referred to as \"Smear Plugs\", which reduce the permeability of the dentin by 86% [78, 79].  
Addressing the issue created by the smear layer requires a certain level of etching prior to chemical adhesion to the dentinal substrate. There are two possible effective options:  
* The full elimination of the smear layer with H\({}_{3}\)PO\({}_{4}\) before bonding in an ER approach (smear layer-removal approach) [74].
* The application of bonding agents that can enter beyond the smear layer (various acidic primers are used), while integrating it following a SE approach (smear layer-modified approach) [80].  
The bond to dentin depends on micro-mechanical interlocking in both processes [9, 17].  
In early generations, adhesives were bonded directly to the surface which is covered by the smear layer [81, 82]. The BS of these adhesives in an in vitro setting is lower than 5 MPa, which might lead to clinical debonding [83]. The specimens appearing after debondingare classified mainly as a cohesive failure inside the smear layer due to the adhesive resin being hydrophobic which prevents it from entering through the debris zone [84].  
In the initial phases of adhesive research, the most frequently used abrasive paper for generating a standard smear layer across dentin surfaces in research laboratories was 600-grit silicon carbide papers (SiC) (average particle size: 29 \(\upmu\)m) [85; 86; 87]. Also, they are regularly employed by manufacturers before introducing a new adhesive system. Consequently, clinicians extrapolate these outcomes to clinical substrates, which are markedly different. For example, the smear layer generated by regular-grit diamond burs (average particle size: 100 \(\upmu\)m) is denser, thicker, and rougher compared to that produced by 600-grit SiC [88]. Consequently, it can weaken the dentin bond, especially when SE adhesives are used [89; 90], impacting material selection and, eventually, the quality of bonded resin restoration. Previous studies addressing such issues have recommended using coarser SiC, such as 180-grit (average particle size: 63 \(\upmu\)m) [31; 91; 92; 93], or 120-grit (approximately 125 \(\upmu\)m in thickness) and 400-grit SiC (about 35 \(\upmu\)m in thickness) for dentin preparation [94]. The aforementioned factors that contribute to the characteristics of the smear layer, and the smear layer's influence on the bond performance of adhesive systems should always be studied and highlighted.