Clinical effectiveness of contemporary adhesives: A systematic review of current clinical trials
Introduction
The major shortcoming of today's adhesive restoratives is their limited durability in the mouth [1]. Adhesive restorations only remain in optimum condition for 3–5 years [2]. The most cited reasons for failure of adhesive restorations placed with earlier adhesives are loss of retention and deficient marginal adaptation [3], [4]. An improved retention of adhesive restorations was recorded with the introduction of the ‘total-etch’ (now referred to as ‘etch-and-rinse’) technique in the early 1990s, by which phosphoric acid is applied simultaneously to enamel and dentin [1], [5], [6], [7]. Nowadays, there is an obvious trend in the development of adhesives with a simplified and thus more user-friendly application procedure.
The basic mechanism of bonding to enamel and dentin is essentially an exchange process involving replacement of minerals removed from the hard dental tissue by resin monomers that upon setting become micro-mechanically interlocked in the created porosities. Based upon the underlying adhesion strategy, three mechanisms of adhesion are currently in use with modern adhesives [8], [9].
Following an ‘etch-and-rinse’ approach, the tooth is first etched (mostly 30–40% phosphoric acid) and rinsed off. This conditioning step is followed by a priming step and application of the adhesive resin, resulting in a conventional three-step application procedure. This etch-and-rinse technique is definitely still the most effective approach to achieve efficient and stable bonding to enamel. Tags are formed through in situ polymerization of resin within the created etch pits, thereby enveloping individually exposed hydroxyapatite crystals. At dentin, the primary bonding mechanism of etch-and-rinse adhesives is primarily diffusion-based and depends upon hybridization or micro-mechanical interlocking of resin within the exposed collagen fibril scaffold. Simplified two-step etch-and-rinse adhesives combine the primer and the adhesive into one application (often referred to as ‘one-bottle’ adhesives).
‘Self-etch’ adhesives use non-rinse acidic monomers that simultaneously condition and prime dentin. The tooth is no longer rinsed, which not only lessens the clinical application time, but also significantly reduces technique-sensitivity. In addition to a two- and one-step application procedure-depending upon the use of a separate solvent-free bonding agent or not-, a further distinction should be made between ‘mild’ and ‘strong’ self-etch adhesives. ‘Strong‘ self-etch adhesives have a rather low pH (<1) and have been documented with a bonding mechanism and interfacial ultra-morphology resembling that produced by etch-and-rinse adhesives. Consequently, the underlying bonding mechanism of ‘strong’ self-etch adhesives is primarily diffusion-based, similar to the etch-and-rinse approach. ‘Mild’ self-etch adhesives (pH ±2) only partially dissolve the dentin surface, so that a substantial amount of hydroxyapatite remains available within a submicron hybrid layer. Adhesion is consequently obtained micro-mechanically through shallow hybridization and by additional chemical interaction of specific carboxyl/phosphate groups of functional monomers with residual hydroxyapatite [10].
Glass–ionomers are still considered the only materials that are self-adhering to tooth tissue [11]. Nevertheless, a short polyalkenoic acid pre-treatment is recommended, resulting in a two-step approach. The polyalkenoic acid conditioner cleans the tooth surface; it removes the smear layer and exposes collagen fibrils up to about 0.5–1 μm depth [12]; herein, glass–ionomer components interdiffuse, establishing a micro-mechanical bond following the principle of hybridization [8], [13]. Chemical bonding is additionally obtained by ionic interaction of the carboxyl groups of the polyalkenoic acid with calcium of hydroxyapatite that remains attached to the collagen fibrils [11].
Because adhesives have evolved so rapidly during the last few years, the time is right to prepare a status report on the clinical effectiveness of contemporary adhesives. Although laboratory testing of contemporary adhesives bonded to sound tooth substrate under optimal laboratory conditions has been shown to predict clinical effectiveness [9], [14], the ultimate test method to assess bonding effectiveness remains a clinical trial. When investigating clinical effectiveness of adhesives, only studies involving non-carious class-V adhesive restorations should be considered, for which there are many reasons [1]: (1) cervical lesions do not provide any macro-mechanical retention; (2) they require for at least 50% bonding to dentin; (3) when restored, they result in an enamel as well as dentin margin; (4) they are widely available; (5) they are usually found in anterior teeth or premolars with good access; (6) preparation and restoration of class-V lesions is minimal and relatively easy, reducing somewhat practitioner variability; (7) despite varying cavity-configuration factors of class-V lesions [15], [16], and thus resultant interfacial stress, the mechanical properties of the composite used are relatively unimportant [17], [18], [19]; and (8) ineffective bonding commonly results in restoration loss, which is the most objective evaluation parameter.
The purpose of this paper was to review current literature on the clinical effectiveness of contemporary adhesives when used to restore cervical non-carious class-V lesions. Restoration retention as a function of time was recorded in order to find out if adhesives with a simplified application procedure are as clinically effective as conventional three-step adhesives.
Section snippets
Materials and methods
Literature published from January 1998 up to May 2004 was reviewed for university-centred clinical trials that tested the clinical effectiveness of adhesives in non-carious class-V lesions. Clinical trials of which the data of successive recalls were reported in more than one paper/abstract were combined and counted as one study. Restoration–retention rates per adhesive reported in peer-reviewed papers as well as IADR–AADR abstracts and ConsEuro abstracts were included (Table 1) and depicted as
Results
A list of published class-V clinical trials selected from the literature (January 1998–May 2004) is shown in Table 2. The adhesives tested in the selected class-V clinical trials are listed per adhesive class in Table 3. The retention rates (in %) of all adhesives are schematically presented as a function of time and per adhesive class in Figure 1, Figure 2, Figure 3, Figure 4, Figure 5. The average annual failure rates (in %) are schematically presented for each adhesive category in Fig. 6.
In
Discussion
We reviewed current literature published between January 1998 and May 2004 with regard to class-V clinical trials in order to draw up the current status of clinical effectiveness of contemporary adhesives. Until now, this has never been done before. In total, 85 clinical trials have been published in peer-reviewed journals, IADR–AADR abstracts and ConsEuro abstracts during the last 6.5 years. Unfortunately, almost 60% of the clinical trials were published in abstracts. This must be ascribed in
Conclusions
Comparison of retention of class-V adhesive restorations as a measure to determine clinical bonding effectiveness of adhesives revealed that glass–ionomers bond most effectively and durably to tooth tissue. Three-step etch-and-rinse adhesives and two-step self-etch adhesives showed a clinically reliable and predictably good clinical performance. The clinical effectiveness of two-step etch-and-rinse adhesives was less favourable, while an inefficient clinical performance was noted for the
Acknowledgements
This study was supported in part by the Toshio Nakao Chair with B. Van Meerbeek and P. Lambrechts awarded as chairholders.
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