Dental Asia Mar/Apr 2019

Clinical Feature DENTAL ASIA MARCH / APRIL 2019 44 O ver the past five decades, the evolution of adhesive techniques has transformed the scope of dental practice. The vast majority of direct and indirect restorations are bonded to natural tooth structure rather than cemented or mechanically retained. Ex t ens i ve resear ch and produc t development have improved adhesives, concurrently with patient demands for improved oral appearance. The widespread demand and use of dental adhesives has fueled an intensive development of better and easier dental adhesives in rapid succession; inundating dentists with successive “generations” of adhesive materials. While the term “generation” has no scientific basis in the realm of dental adhesives and is to a great extent arbitrary, it has served a useful purpose in the organisation of the myriad of commercially available materials into more comprehensible categories. The “generational” definitions assist in identifying the chemistries involved, the strengths of the dentinal bond, and the ease of clinical use. Ultimately, this type of classification benefits both dentist and patient by simplifying the clinician’s chairside choices. The beginning of the generational adhesive systems The 1 st generation adhesives in the late 1970s were rather unsuccessful. While their bond strength to enamel was high, their dentinal adhesion was pitifully low at typically less than 2 MPa. Generally, all the adhesive generations bond well to the microcrystalline structure of enamel; it is their bond strength to the semi-organic dentin that is the greater concern. “Bonding” was achieved through chelation of the bonding agent to the calcium component of the dentin. While tubular penetration did occur, it contributed little to the retention of the restoration. It was common to see debonding at the dentinal interface within several months. 1 These bonding agents were recommended for small, retentive Class III and Class V cavities. 2 Post-operative sensitivity was common when these bonding agents were used for posterior occlusal restorations. 3 In the early 1980s, a distinct 2 nd generation of adhesives was developed, attempting to use the smear layer as a bonding substrate. 4 This layer bonded to the underlying dentin at a negligible level of 2-3 MPa. The weak 2-8 MPa dentinal bonding strength of 2 nd generation adhesives meant that mechanical retention form was still required. Restorations with margins in dentin saw extensive microleakage, and posterior occlusal restorations were likely to exhibit significant post-operative sensitivity. The long-term stability of 2 nd generation adhesives was problematic; one-year retention rates were as low as 70%. 5,6 In the late 1980s, revolutionary two- component primary/adhesive systems we r e i n t r oduc ed . The i r ma r k ed clinical improvement warranted their classification as 3 rd generation adhesives. Significant increases in dentin bonding strength (8-15 MPa) diminished the need for cavity retention form. Erosion, abrasion, and abfraction lesions were treatable with minimal tooth preparation, hence the beginning of ultraconservative dentistry. A noticeable decrease in postoperative sensitivity with posterior occlusal restorations was very welcome. 3 rd generation adhesives were the first “generation” that bonded not only to tooth structure, but (weakly) to dental metals and ceramics as well. The downside of 3 rd generation bonding agents was their lack of longevity; intraoral adhesive retention started to decrease after three years. Despite significant levels of post- operative sensitivity, patient demands for tooth-coloured restorations convinced some dentists to begin providing posterior composite fillings routinely. 7,8,9 The transformation and popular generations of theadhesive systems In the early 1990s, 4 th generation bonding agents transformed dentistry. Their high bond strength to dentin (17-25 MPa) and limited post-operative sensitivity in posterior occlusal restorations encouraged many dentists to switch from amalgam to direct posterior composite fillings. 4 th generation is characterised by the process of hybridisation at the dentin-composite interface. Hybridisation is the replacement of the hydroxyapatite and the water in the surface dentin by resin. This resin, in combination with the remaining collagen fibres, constitutes the hybrid layer. Hybridisation involves both the dentinal 7 TH GENERATION ADHESIVE SYSTEMS By George Freedman DDS, FAACD, FIADFE, Dipl.ABAD