Bonding Surface Designs in Fixed Orthodontic Attachments

Fixed orthodontic attachments/appliances work as a medium to transfer the force applied to the teeth. In bonded types, several factors affect the attachment bond strength and their clinical success. The primary approach for increasing the bond strength focused on altering the time and concentration of acid etching; however, the results showed that these changes might increase susceptibility to enamel decalcification. The bonding mechanism of orthodontic attachments may be chemical, mechanical, or a combination of both. Most attachment bonding surfaces (ABSs) have no chemical bond to resin composites. Hence, mechanical retention plays a major role. Developing more bonding surfaces by increasing the macroscopic size of the attachments has esthetic and hygienic limitations, so the ABS design plays a more important role in maintaining and improving the bond strength. In this research, different ABS designs are reviewed and categorized according to manufacturing methods and their features.


Introduction
Fixed orthodontic treatments need attachments to transfer the force applied to the target teeth [1]. In bonded types, bond strength is crucial for clinical success, and its failure delays the treatment progress and exerts an economic impact [2]. Several variables infuencing the bond strength are resin-related, enamel-related, attachment-related, and bonding condition-related factors [3].
Primary attempts to improve bond strength include increasing ABS macroscopic area and better ABS adaptation on the tooth surface [1,7,9,[12][13][14]. Round or triangular ABSs have less area than quadrangular ones. Trapezoid shapes have more area than rectangular, rhomboid, and square ones [4]. Extension of ABS in occluso-gingival direction in attachments with gingival ofset feature is another efort to increase the bonding surface area. When the area is even, the ABS shape has no signifcant efect on the bond strength [15].
Aesthetic and hygienic considerations have led to smaller fxed attachments and consequently reduced macroscopic ABSs [8]. Te reduced macroscopic ABS area increases the importance of its design and its microscopic area for maintaining optimum bond strength [16,17].
Te bonding mechanism of attachments may be chemical, mechanical, or a combination of both [18]. Since most ABSs have no chemical bond to resins, mechanical retention, due to ABS design, has a major role in the bonding success [5,8,19,20].
Mechanical retention is achieved by producing surface roughness to increase the surface area and to develop effective undercuts for the resin to penetrate and interlock after polymerization [21].
In this paper, diferent ABS designs are reviewed and categorized according to manufacturing methods and their features.

Materials and Methods
Electronic databases, including PubMed, Scopus, Web of Science, and Google Scholar, were searched for relevant articles up to Jun 20, 2022. Searched MeSH terms included orthodontic fxed appliances, orthodontic brackets, and shear strength and to complete the search strategy we used other relevant terms including base design, mesh design, orthodontic attachments, pad design, bond strength, and a combination of them. Inclusion criteria were English language articles, full-text availability, and discussion on ABS-related issues. Tere was no time limitation. Finally, 46 articles were included in this review.

Results
Te database search identifed 162 articles. Of these, 14 were duplicates. Of the remaining 148, screening the titles and abstracts discarded 93. Of the remaining 55 articles, for which the full text was examined, we excluded 9. Te remaining 46 articles were included in this review. Te main exclusion criteria were not mentioning the type of bonding surface of the brackets used in the research and the lack of comparison between bonding surfaces as the main variable.
Diferent ABS designs have been introduced to improve mechanical retention. Tey can be categorized into three groups according to the method of increasing contact area and creating efective undercuts: (1) Mesh-based ABSs: Metal net/mesh is connected to an ABS. Mesh is woven fne metal strings with diferent sizes and holes, [20] lacing patterns, and layer numbers ( Figure 1).  [23]. Te mesh gauge is determined by counting wires per linear inch in the enlarged photographs. Te sizes available for single mesh designs included 40, 50, 60, 80, and 100 [24]. Te size of the mesh spaces (apertures), including space area and space volume, plays a key role in the bond strength of meshbased ABSs. [20]. Te aperture diameter in the fnest mesh used, i.e., 100 gauge, is about 150 microns [20,25]. Te attachments with larger apertures show better air displacement from the surface and better resin penetration into the spaces, resulting in higher bond strengths [14,20]. Although several studies have reported that fne mesh gauges due to larger apertures demonstrate higher bond strength, some studies have indicated no signifcant diferences between various mesh gauges [8,13,16,24,[26][27][28][29]. Knox et al. revealed that certain combinations of bonding composite resin and mesh gauge could result in maximum bond strength, so determining an exact trend seemed difcult. It is related to the fact that more viscous composite resins could not penetrate into the fne spaces of some meshes [30].

Mesh Design.
Te mesh string direction could be vertical or diagonal relative to the crown long axis. Te diagonal pattern showed higher bond strength than the vertical one [31].

Attachment-Mesh Connection.
Te mesh may be connected to the ABS by welding or brazing. Te weld spots can reduce the bond strength by reducing the available retentive surface area and obliterating the mesh strings. Locating these welding points on the margins of ABS may result in voids beneath the weld spots, subsequent leakage, and enamel decalcifcation. Te weld spots may act as stress concentration points, initiating the resin fracture at the attachment-resin interface [16,24,32].
To eliminate the weld spot-related problems, brazing of the mesh to the attachment was suggested. Tis involves joining metal parts by melting the intermediate metal fllers. Te melting point of fllers should be below the solidus temperature of the metal parts and also above 840 F (450 C ) [33]. Te brazed mesh bases have been found to show higher bond strength than the welded ones [24,34].

Mesh Layers (Single or Double).
Te ABSs may consist of single or double mesh layers. In double-mesh attachments, the superfcial one is coarser. Tese diferent types can infuence the stress distribution in ABS during the debonding time, thereby causing the bracket base fexibility. In this regard, in double-mesh attachments, the stress is lower in the superfcial mesh layer than in the deeper layer [20]. Tis reduced stress yields less stress in the cementbracket interface due to decreased physical deformation [35]. Some studies have reported that double-mesh attachments have higher bond strength than single-mesh ones [14,20]. And one study showed no signifcant diference between them [36].

Non-mesh Subtractive ABSs
3.2.1. Perforated. One of the earliest ABS designs was perforated, which entailed 12 to 16 holes per ABS ( Figure 2). Te bonding resin would seep through the perforations and interlock after polymerization to provide mechanical retention. Te resin penetrating the oral environment through the perforation would increase the risk of hygienic and aesthetic problems [11,16,33]. Tese designs, especially those with peripheral perforations, showed lower bond strength than the mesh-based ones, which might be due to the smaller number of retentive components [24,34].

Retention Grooves.
Retention grooves are horizontal depressions on ABS which open at the mesial/distal ends ( Figure 3). Te V-shape grooves are also located vertically on ridges [19]. Tese vertical grooves are designed to provide an escape way for composite resins and avoid air entrapment in combination with the horizontal ones [37]. Tese cross-cuts increase the ABS-resin contact area, too [11]. Several studies have evaluated the bond strength of ABSs with retention grooves. Inconsistent results have been found due to different bonding agent viscosities, ABS treatment, and fabrication processes [19,27,30,[37][38][39][40]. Controlled studies are needed to evaluate the efect of these retentive grooves on the bond strength compared with others.

Spherical Concavities.
Te shallow spherical concavities were made on ABS to provide mechanical retention ( Figure 4). Tey are also named photo-etched, microlock, concave circular, and microetched ABSs. Tey have lower bond strength than mesh-based and retentive grooves [16,24,34,37]. It might be due to less efcient undercuts for the resin to lock.

Laser Structured.
Te laser-structured ABS was introduced by Olivier Sorel in 2002 on stainless steel brackets (Discovery ® , Dentaurum, Germany). In the fabrication process, the Nd: YAG laser used to produce various sizes of hole-shaped undercuts caused by selectively melted metal provided micro-and macromechanical retention ( Figure 5). Tis type of ABS had superior bond strength to enamel and porcelain compared with the mesh-based ones [7,[41][42][43][44]. In this type of ABS, bond failure was reported at the resinenamel interface due to maximum contact in ABS-resin interface [7].
Furthermore it seems that laser etching of the bonding surface can improve the bond strength of other ABS designs [13].

Anchor Pylon.
It consisted of miniature pylons which act as anchors penetrating and embedding in the bonding agent to provide mechanical retention ( Figure 6). Tree studies demonstrated better bond strength with the anchor pylon design rather than the mesh-based types [2,45,46].

Concentric Grooves.
Concentric grooves onto ABS were introduced with two diferent widths (100 μm and 150 μm) (Figure 7). Te torsional stresses in this design may transfer more uniformly to the substrate due to the hydrodynamic analogy as a physical principle and produce a higher bond strength than the mesh-based design. Te wider grooves (150 μm) had higher bond strength [47].

Triangular and Quadrilateral Mesh.
Attachments with triangular and quadrilateral mesh baseplates were compared by variable mesh lengths and mesh spacing diameters (mesh lengths 100, 200, and 400 μm, and mesh spacing diameters 100, 150, and 200 μm). Te triangular mesh baseplate with a 400 μm mesh length and 200 μm mesh spacing had the highest bond strength, so increased aperture area has led to increased bond strength [6].   (Figure 8) showed higher bond strength than the mesh-based design, whereas there were no signifcant diferences between rail-shaped and mesh-based designs [17,48]. Sandblasting, microetching, and laser etching of ABSs can improve bond strength due to increased microscopic contact area [20] although the efect of laser etching is more [13,49].

Nonmesh Additive ABSs.
Adding metal or ceramic particles in spherical or irregular shapes to ABSs can make roughness and efcient undercuts to provide mechanical retention (Figure 9) [11]. Te metal coating processes involve sintering stainless steel or cobalt-chromium beads in various sizes onto the ABSs. Ceramic beads may fuse to the stainless steel ABSs with sintering or chemical bonding agents.
It has been revealed that porous metal-coated ABSs have higher bond strength than mesh-based ones [50,51]. Smith and Maijer showed the sintered porous metal-coated group had the highest bond strengths and bond failure was in the resin layer, whereas the porous ceramic-coating group had the lowest result, and bond failure was in the ceramiccoating layer. As the metal particles were coarser, the bond strength was higher in this study [50].

Discussion
Te clinical success of treatments done with bonded orthodontic attachments is dependent on their bond strength [26]. Tis bond is afected by proper enamel preparation, resin viscosity, curing and strength, and ABS morphology [3].
Numerous studies have attempted to increase the bond strength by modifying the acid etching technique, the adhesive material, and the ABS design. Although initial studies focused on altering the time and concentration of acid etching, the results showed that these changes might cause enamel loss and increase susceptibility to decalcifcation. Tus, a more conservative approach to increasing the bond strength is developing stronger adhesives and more retentive ABSs [23].
Te bonding agent has a signifcant efect on the bond strength in such a way that a lower percentage of fller and a smaller particle size can lead to more penetration of the resin [32]. Te variations in the thickness of adhesive material layer may afect the bond strength. Each product has its own critical thickness, in which the bond strength will be maximum [1]. However, the resin viscosity reduction to compensate for the narrow and small apertures reduces the mechanical properties of the resin and thus increases the risk of bond failure. Terefore, designs with very efective undercuts might not show the desired strength properties due to the use of resins with incompatible viscosity [30,39].
Te most important factor in ABS design which can afect the bond strength is efective contact area between resin and ABS. Debonding forces are usually parallel to ABS. Terefore, areas of ABS that are perpendicular to these forces are more efective than areas parallel to them if resin can    contact them due to good penetration into undercuts. Resin penetration into undercuts is dependent on resin viscosity, dimensions of undercuts, and pressing force onto the attachment toward the tooth. Te air trapped between ABS and resin inhibits resin penetration. Te presence of lateral spaces from undercuts makes air escape possible, improves resin penetration, and provides an efective ABS-resin contact area [30,39].
Several studies have investigated the infuence of ABS design. Despite the common clinical perception, a smaller bracket base does not lead to insufcient bond strength if there is sufcient mechanical retention. Terefore, as the macroscopic retentive surface area of the bracket bases are reduced for aesthetic and hygienic reasons, the ABS design and morphology have a greater infuence on bond strength [23].
Tis review reports diferent ABS designs and features and shows patterns with more efective undercuts, such as laser structured and anchor pylon, which have better bond strength [42,43,45,46]. Also, the attachment fabrication method can infuence the efectiveness of undercuts. For example, cast molded attachments have inadequate retentive grooves because the detachment of these brackets from the mold requires the absence of deep undercuts, but threedimensional printed methods can create deeper undercuts because they are printed directly onto the base of the brackets [41].
Additionally bonding condition factors can infuence bond strength, such as saliva or blood contamination. According to the previous studies these factors can cause bond failure; therefore, proper bonding technique and precise isolation during the bonding process is necessary [52,53].
Stress distribution is another important factor afecting bond strength. Te more uniform it is, the better the bond strength will be. ABSs with concentric grooves and double mesh have this advantage [30,35,47].
Tree-dimensional printing is an emerging technology with the potential to streamline bracket production for personalized and precision orthodontics. Metal printing can be performed with both stainless steel and titanium alloys, materials commonly used in orthodontic brackets because of their biocompatibility and resistance to intraoral corrosion. Tree-dimensional metal printing permits retentive features with undercuts to be printed directly onto the base of a bracket. Tis makes it possible to create innovative features which cannot be produced by traditional methods [54].
Te limitations of the present review were in the following aspects: Tere is a need for further evidence-basedhigh-quality studies/randomized clinical trials (RCTs) to evaluate the efect of attachment bonding surface on bond strength and in connection with bonding condition-related factors.
3D printing technology develops possibility of designing various attachment bases and direct printing of the samples without the need for milling or casting processes. Application of this method in future studies seems to provide a new horizon.

Conclusion
As for proper enamel preparation, bonding conditions, and suitable viscosity of the bonding agent, the most important factor infuencing the bond strength is the attachment bonding surface (ABS) design.
Tree-dimensional designing and printing can provide efective undercuts, more air escape ways, more resin penetration, a more efective ABS-resin contact area, better stress distribution, and fnally higher clinical success.

Data Availability
Due to the review nature of our article, our data for this article are actually studies found after a wide search in databases, and upon journal request the authors can send the endnote fle of all found articles separately.