DROGODEPENDENCIA Y SEXUALIDAD

7.1.1 Objectives

Previous studies have investigated the failure o f resin/dentine interfaces by a variety o f methods including tensile and shear bond tests, and by examining the surfaces o f fractured samples with conventional light or scanning electron microscopy. These were reported in the literature review (Chapter 1.7). None have recorded the propagation o f the crack along the interface in real time. The objectives o f this study were to gain further information on interfacial failure using fluorescence confocal microscopy. This would be achieved by:

• examining and recording the interface under load in real time, until the point o f failure

• examining the interface o f the reassembled fractured sample.

7.1.2 Method

Lower third molar teeth were included in auto-polymerising denture base acrylic (Simplex Rapid, Associated Dental products Ltd., Kemdent Works, Swindon UK) in cylindrical moulds (internal dimensions: 30 mm diameter, 25 mm height. Model 813- 019 Leco, USA). The samples were sectioned with a slow speed, water cooled diamond saw to expose an approximal dentine surface. Samples were stored in water.

even once the teeth had been included in acrylic, to ensure that the dentine remained hydrated. In order to produce a very small crack or deficiency within the interface, thin PTFE tape (plumber’s jointing tape) was draped and adapted over the most coronal 2mm o f the sectioned approximal dentine. A matrix for a resin composite bobbin was made from a section o f a drinking straw (external diameter 4mm). This was positioned so that the margin o f the PTFE tape produced a cord o f a circle across the edge o f the drinking straw and secured to the dentine surface with modelling putty (Figure 7.1). The dentine within the matrix was then prepared with the SBMP system according to the manufacturer’s instructions (Table 2.2), but with a fluorescent label in the adhesive, either rhodamine B or auramine O. A 2 mm deep ZlOO restoration was then packed in the matrix and cured. The teeth were sectioned longitudinally through the restoration as indicated in Figure 7.1 to expose an interface on a flat surface which could be examined using a TSM. The remainder o f the acrylic sample was sectioned to produce a block o f suitable size (8 mm wide x 23 mm long x 15 mm high) for mounting in a jig on a straining stage (Figure 7.2).

Figure 7.1 Line diagram illustrating the positioning of the straw matrix, PTFE tape and

composite bobbin on the tooth sample in the resin block, (b) buccal, (1) lingual aspect. The line x-x

illustates the final section through the block and composite bobbin.

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Figure 7.3 Photograph of a sectioned tooth sample included in an acrylic resin block, with

composite bobbin bonded to the approximal dentine. The coronal aspect of the bobbin was trimmed

with a razor blade to make a flat surface for the application of the load. The load was applied in the

direction indicated by the arrow.

Figure 7.4 An image of a composite bobbin and the dentine/adhesive interface prior to

loading. The defect in the resin composite (C) / dentine (D) interface formed by the PTFE tape (*) is

illustrated. The adhesive (A) is labelled with fluorescein. 450/520 nm. x20/0.8NA 01. Scale bar

100 pm.

Figure 7.5 Illustrates the disadvantages of the PTFE tape in some samples, with the polishing

debris lodged within the defect site. 450/520 nm. x20/0.8NA OI. Scale bar 100 pm.

Figure 7.6 Approximated fracture surfaces following failure of the dentine/restorative

interface, illustrating failure between the hybrid zone (H), and the adhesive (A), a cohesive failure of

the composite (C), and a crack within the resin composite (*). The dotted line indicates the original

relationship of the fractured surfaces. The adhesive was labelled with fluorescein 450/520 nm.

X20/0.8NA 01. Scale bar 100 pm.

Figure 7.7 Approximated fractured surface following failure of the dentine/restorative

interface, illustrating failure between the adhesive (A) and the composite (C). The primer (P) is

labelled with rhodamine B. 546/600 nm. x20/0.8NA 01. Scale bar 100 pm.

Figure 7.8 Approximated fractured surfaces following failure of the dentine/restorative

interface, illustrating failure within the resin composite (C), the adhesive (A), and the dentine (D).

Cracks within the resin composite are also visible (*). The dotted line indicates the original

relationship of the fractured surfaces. The adhesive is labelled with fluorescein. 450/520 nm.

Tlie coronal aspect o f the sectioned ; r Loration v/as triinmcd using a razor blade in order to produce a flat surface for loading. This produced a slight step in the dentine surface at th e . maigin o f the restoration giving an excellent reference point . for relocating the composite bobbin on the dentine following fracture (Figure 7.3). The samples were mounted in a miniature straining device which could be positioned on the TSM stage (Watson 1994). A servo motor driven pusher (or blade 0.7 mm wide) was used to load the resin composite button, llie pusher was aligned to run parallel to the interface less than 300 pm the dentine surface (Figure 7.2). This jig was placed on the microscope stage and the position o f the sample adjusted to lie parallel with the focal plane o f the objective as the stage travelled in the x and y plane. The pusher was also adjusted to travel in this plane. This enabled the whole length o f the interface between dentine adhesive and resin composite to be examined and avoided damage to the objective during loading of the sample. The interface was examined through a glass C O vers lip using a x20/0.8 NA oil immersion objective. The rate o f application o f the load to the samples was varied so that once a crack was seen, the rate was reduced to zero. This allowed cracks to be generated and then to propagate with their own energy before the blade was advanced again. The interface under load and during failure was recorded on video with a CCD camera and on 35 mm film after the fracture event.

7.1.3 Results

Figure 7.4 shows a TSM image o f an interface prior to loading, illustrating the resin composite restoration (C) and the deficiency in the restoration interface formed by the PTFE tape (*). Some problems arose in the application o f the PTFE tape and it was difficult to form a consistent defect. Some samples de-bonded during or following the preparation prior to applying the load, and it was also noted that the adhesive overlying the PTFE had a gel like consistency. The appearance o f the samples was marred by the polishing debris which became lodged within the defect site (Figure 7.5).

Development of the crack could be followed at video rate as the bond between the adhesive and the hybrid zone became 'unzipped' in a slip/stick failuie pattern with teais extending into the adhesive. Propagation o f the failure could later be examined frame by frame.