1

Introduction

Resin-based composite (‘composite’) restorations have a finite lifetime, depending on a range of factors . The traditional approach to a failed composite restoration is removal and replacement, however each time this is done, the cavity becomes larger, the tooth weaker, more complex restorations may result and pulpal symptoms may ensue . It has been estimated that half a general practitioner’s time is spent on ‘replacement’ dentistry, with the consequent high cost in time and money .

Consistent with the philosophy of ‘minimum intervention’ operative dentistry, defective restorations should be first evaluated for the possibility of repair, rather than being routinely replaced . There is now accumulating evidence that repair of composite can be a viable, long-term, clinical procedure . However, a recent questionnaire to 20 composite manufacturers revealed that all recommended repair of their materials, but more than half (65%) did not supply any repair instructions. The remainder (35%) gave guidance on repair using the same type and brand of composite as the original, i.e., that manufacturer’s own brand. The dilemma for the clinician, however, is that the type and brand of original composite may not be known, and now that composite monomers other than dimethacrylates are being used which require matching bonding agents (e.g., ‘acid’ resins in the polyacid-modified composites; siloranes), the repair task becomes less straightforward. It is also apparent that different repair techniques are being taught in different dental schools around the world .

Adhesive technology in dentistry is a complex area , and since 2003, several reports have been published on the repair bond strength of composite materials . The main area of interest in such papers was in the surface treatment of the original composite, including mechanical roughening, acid ‘etching’, silanating and applying various resin-based adhesives, and recently excellent reviews have been published . Most authors ‘repaired’ the original composite with the same composite, and all composites used were dimethacrylate-based. However, it should be noted that the bond strengths do not necessarily correlate with microleakage, at least in the laboratory . Regarding bond strength and clinical data, one recent review showed that in non-carious cervical lesions, ‘aged’ bond strength values and medium-term retention rates show a reasonable correlation. However, no data appear to exist on the correlation between bond strength of repaired resin composites and clinical performance.

The aim of this study was to evaluate the ‘repair’ strength of various combinations of composite, including a silorane-based material, using four manufacturers’ adhesive systems, and to compare the bond strengths with the cohesive strength of the original, unrepaired products. The null hypotheses were (a) the type of adhesive used did not influence the repair strength, (b) the chemical nature of the resin matrix did not influence the repair strength.

2

Materials and methods

Twelve disc-shaped specimens, 12 mm diameter × 3 mm thick, were prepared for each composite material ( Table 1 ) in a plastics mold. The mold was positioned on a glass microscope slide, the composite was injected, a second microscope slide pressed firmly onto the composite to remove excess, the composite photocured for 40 s (XL3000; 3M, St Paul, MN; minimum intensity 600 mW cm ⁻² ) and stored in water at 60 °C.

After 1 month the specimens of each material were removed from the water, the upper face ground unidirectionally with 80-grit carborundum paper and divided randomly into four groups of three specimens, one group being assigned to each of the repair adhesives ( Table 2 ). For each group of three, one specimen was randomly assigned to each of the repair composites ( Table 1 ).

The specimen of each sub-group was placed in the base of a 6-mm deep × 12-mm internal diameter plastics mold with the 80-grit abraded surface uppermost, the abraded surface treated with one of the adhesives ( Table 2 ) according to the manufacturer’s instructions ( Table 3 ) and the remaining 3-mm deep cavity in the mold filled with one of the repair composites ( Table 1 ). The composite was photocured as above as for the original composite.

The specimens were placed in tap water at 37 °C for 24 h, removed and fixed by the end to a 12-mm diameter acrylic stub for mounting in a diamond disc cutting device (Isomet; Beuhler; Lake Bluff, IL). Four arcs of the cylindrical specimen were removed to leave a square configuration of face approximately 10 mm × 10 mm, followed by a cut half way across the face to full specimen depth, followed by seven to eight evenly spaced cuts to full depth at right angles to the initial cut. The resulting 14–16 ‘sticks’ of material, approximately 6 mm × 0.8 mm × 4 mm, were fractured away from the mounting base. An approximately 1-mm wide semicircular ‘neck’ was created on both sides of the stick at the level of the original and repair composite interface using a 1.2-mm diameter high speed diamond bur with water cooling. The thickness of the specimens was measured to 0.01 mm at the level of the neck, the hour-glass specimens attached to a sliding jig ( Figs. 1 and 2 ) with cyanoacrylate adhesive and tested in tension at a cross head speed of 1 mm/min. After fracture, the failure load (N) was recorded, the mode of failure assessed at between 30 and 45 magnification under a binocular light microscope (ML9300, Meiji, Japan) and the width of the fracture site measured to 0.01 mm. Specimens that failed prior to testing were assigned a nominal failure load of 1 N.

Composite specimen fixed to the two halves of the tensile jig using cyanoacrylate adhesive. Note that the specimen is long enough to be placed across the recess in the jig, which was the case only for the unrepaired composite specimens.

Repaired specimen fixed as in Fig. 1 . Note that specimen in short and thus the recessed jig design cannot be used. The ‘join’ between old and new composites is positioned approximately between the two halves of the jig.

One disc-shaped specimen, 12 mm diameter × 6 mm thick, was prepared for each composite material in a plastic mold using the procedure described above, photocured from both sides for 40 s and stored in water at 60 °C for 24 h. 14–16 ‘sticks’ of each material, with a semicircular ‘neck’ at the middle, were prepared as above for microtensile cohesive strength testing.

The data were entered into a statistical package (SPSS for Windows; SPSS Inc, Chicago, IL) and the fracture surface area (width at fracture × thickness at fracture, sq mm) and failure stress (failure load/fracture surface area, N/sq mm = MPa) calculated for each specimen. For each group of specimens, the mean failure stress was calculated. The 36 groups were divided into three groups of 12 depending on the original composite (Durafill, P90 or Z250), and the various means within each group of 12 compared using 1-way analysis of variance and LSD t -tests.

2

Materials and methods

Twelve disc-shaped specimens, 12 mm diameter × 3 mm thick, were prepared for each composite material ( Table 1 ) in a plastics mold. The mold was positioned on a glass microscope slide, the composite was injected, a second microscope slide pressed firmly onto the composite to remove excess, the composite photocured for 40 s (XL3000; 3M, St Paul, MN; minimum intensity 600 mW cm ⁻² ) and stored in water at 60 °C.

After 1 month the specimens of each material were removed from the water, the upper face ground unidirectionally with 80-grit carborundum paper and divided randomly into four groups of three specimens, one group being assigned to each of the repair adhesives ( Table 2 ). For each group of three, one specimen was randomly assigned to each of the repair composites ( Table 1 ).

Table 2

Repair adhesives.

Name Batch number	Manufacturer	Composition
Adper Single Bond 2	3M ESPE, St Paul, MN
Scotchbond Etch 4CL		Phosphoric acid 35%
Adhesive N104096		Ethyl alcohol 25–35%, bisphenol a diglycidyl ether dimethacrylate 10–20%, silane treated silica (nanofiller) 10–20%, 2-hydroxyethyl methacrylate 5–15%, glycerol 1,3-dimethacrylate 5–10%, copolymer of acrylic and itaconic acids 5–10%, diurethane dimethacrylates 1–5%
Clearfil Repair	Kuraray Medical Inc, Okayama, Japan
K-Etchant Gel 00450B		Phosphoric acid 40–60%, colloidal silica, water
SE Bond Primer 00887A		2-Hydroxyethyl methacrylate 20–30%, 10-methacryloyloxydecyl dihydrogen phosphate, hydrophilic aliphatic dimethacrylates, dl -camphorquinone, water
Porcelain Bond Activator 00234A		3-Trimethoxysilylpropyl methacrylate 40–60%, hydrophobic aromatic dimethacrylates
SE Bond Bond 01306A		2-Hydroxyethyl methacrylate 25–35%, bisphenol A diglycidylmethacrylate, 10-methacryloyloxydecyl dihydrogen phosphate, hydrophobic aliphatic dimethacrylates, colloidal silica, dl -camphorquinone
Ecusit Composite Repair 628845	DMG GmbH, Hamburg, Germany	Aliphatic, aromatic dimethacrylate, carboxylic methacrylate
P90 System Adhesive Self-etch Primer: N109581 Bond: N098774	3M ESPE AG, Seefeld, Germany	2-Hydroxyethyl methacrylate (HEMA) 15–25%, bisphenol A diglycidyl ether dimethacrylate (BISGMA) 15–25%, water 10–15, ethanol 10–15%, phosphoric acid-methacryloxy-hexyl esters mixture 5–15%, silane treated silica 8–12%, 1,6-hexanediol dimethacrylates 5–10%, copolymer of acrylic and itaconic acids < 5, (dimethylamino) ethyl methacrylate <5%, dl -camphorquinone <3%, phosphine oxide <3%

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