CAD/CAM international magazine of digital dentistry No. 4, 2016

| study fracture resistance of restorations 14 CAD/CAM 4 2016 pin-retained amalgam guidelines.11 Titanium pins with a diameter of 0.6 mm were used (Stabilok; Fairfax Dental Inc.). Two pins were placed in each tooth at the appropriate line angles; pin 1 was placed on the mesial side whereas pin 2 was placed on the distal side of each molar tooth (Fig. 2). Pins were inserted to a 2 mm depth. The top 1mm was sheared off and smoothed.8 Pin length was slightly variable among the teeth. Radiographs were taken in a buccolingual and mesiodistal fashion to verify pin placement (Fig. 3). All tooth specimens were packaged and sealed in a moisture controlled con- tainerandshippedtoadentallab(DentUSA)forres- toration fabrication with e.max press (IPS e.max Press; Ivoclar Vivadent). Specimens were returned in the same manner along with the e.max onlay restorations (Figs. 4 & 5). Tooth specimens and res- torations were prepared and bonded (Fig. 6) using Multilink adhesive cementation system (Multilink Automix; Ivoclar Vivadent) following manufactur- ing recommendations.12 Cement flash was removed and the restorations were polished following standard Schulich Den- tistry protocols. The prepared tooth was fixed with ortho resin (Fig. 7) (acrylic resin, DENTSPLY Caulk) in the stabilization ring (Fig. 8). A universal loading machine (Instron laboratory testing unit: ITW) was utilized to apply an axial load to the tooth until the tooth fractured (Fig. 9). The machine applied pres- sureatamaximumcrossheadspeedof0.5mm/min. Tooth fracture was assessed visually and measured in Newtons for all the teeth in the control and test groups (Fig. 10). Results The force (Newtons) required to cause fracture of either the restoration or tooth, or a combination of the two, was extremely variable (Table I). The test group suggested greater variability among the values and the highest fracture resistance value. There was no significant difference in the fracture resistance between the non pin-retained e.max pressrestorationsandthepin-retainede.maxpress restorations(Fig.11).Anunpairedt-testresultusing P < .05 was P = .4443 in this assessment. Data were obtained by using an analysis of variance (ANOVA). Significant differences were set at a .05 level (Fig. 11). Discussion There was no statistical difference between the control group (non pin-retained restorations) and thetestgroup(pin-retainedrestorations)infracture resistance. The results indicated that the test group exhibitedgreatervariability.Thiscouldbeduetopin location, pin length, differences in pin angulations or variations in the width of the onlay preparation margin. The highest fracture resistance value was a pin-retained e.max onlay, which could be related to the increased surface area and subsequent bond strength.13 Pin-retained e.max onlays had a ten- dency to fracture in a very controlled manner, with much of the tooth-restoration complex remaining intact. Conversely, non pin-retained e.max onlays typicallyfracturedinsuchaviolentmannerthatthe tooth-restoration complex was destroyed. Due to the degree of variability, further laboratory testing would be warranted with a larger sample size. A clinical investigation, highlighting the pro- Fig. 8: Tooth sample secured in stabilization ring with Instron bearing. Fig. 9: Axial loading in Instron unit. Fig. 10: Tooth fracture/onlay failure. Table I: Fracture resistance values for samples (Newtons). Fig. 8 Fig. 9 Fig. 10 Control Group (N) Test Group (N) 3016 2679 2277 2436 2121 1605 3079 2606 2510 1716 2258 2927 3120 3060 2396 1575 2859 3118 2222 2385 Table I 42016 30162679 22772436 21211605 30792606 25101716 22582927 31203060 23961575 28593118 22222385

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