The Effect of Cementation Techniques on Retention of Full Veneer Crowns; an In-vitro Study

1. Tooth selection The procedure started by informing the oral surgery department to reserve mandibular premolar teeth that were being extracted each day, followed by taking patient consent for the purpose of research. The extracted teeth collected were kept in hydrogen peroxide for 5 minutes and then washed under tap water for any visible debris. The teeth were then stored in Formalin 10% for a period of 7-days, as per Centers for Disease Control and Prevention disinfection protocols. The teeth were then transported in Hanks Balanced Salt solution to maintain disinfection and tooth quality.
2. Preparation of tooth mounts The extracted teeth were then mounted in an acrylic block using self-cure acrylic resin (Shofu self cure acrylic resin, Japan). 0.5 mL of liquid was dispensed into a mixing cup, then 1 g of powder was added and quickly mixed using a spatula for 10 - 15 sec. Preformed plastic cylinders (Height; 55mm, Diameter; 20mm) were used, and inner walls painted with petroleum jelly for easy removal. The extracted teeth were then inserted in these cylinders filled with acrylic dough. Following manufacturer's instructions, it was ensured that the entire procedure was completed within 2 minutes after mixing was finished and at room temperature. The moulds were then left for an hour at room temperature to ensure complete chemical curing.
3. Crown preparation Following complete chemical cure the teeth were prepared using a high-speed handpiece with water irrigation with diamond bur (Head Diameter: 1.2mm, Head Length: 7.7mm, 06-125µmm). They were first reduced in occlusal anatomical planes and a chamfer finish line (approximately 0.75mm) was given all around. Total Occlusal Convergence was kept at 20◦ and 25◦ with a clinical height of 5mm. Preparation dimensions (Total Occlusal Convergence, height) of the teeth were checked using computer software (AutoCAD 2000) after taking pictures with digital single-lens reflex camera with 105mm lens at 100mm. The distance was reproduced using a standard metal scale. The tooth mounts were labelled and kept in Hank's Balanced Salt Solution to maintain tissue health. Any tooth that did not meet the preparation requirements was discarded according to the Centers for Disease Control and Prevention criteria.

The data collection was performed by the principal investigator. Dental photograph measurements were initially performed by the supervisor. To avoid the photographic error the photographs were calibrated by measuring the prepared tooth and scaling it on the AutoCAD software. The complete photography setup was stationary during the data collection process. Then, the data between the two operators were subjected to correlation analysis, and a strong correlation value was found (0.839). Furthermore, 20% of photographs were then re- assessed after a period of 2 weeks by the same operator. The data was analyzed later by formula to confirm intra-operator reliability through correlation statistics. As proven earlier through studies by dental morphometric measurements done by 2D image analysis can be performed by multiple operators with an excellent reliability. 4. Crown fabrication Wax-up for metal crowns was done with metal extension (25mm), to engage them in the Universal Testing Machine, using Dental Inlay casting wax (Inlay wax medium stick; GC-Dental Corp. Japan). The wax up was then invested using phosphate bonded gypsum material (GC-Super, Japan) and cast with Nickel-Chromium Ni-Cr metal alloy at the Prosthodontics laboratory of Fatima Jinnah Dental College. The cast crowns were then finished used metal finishing discs, making sure the metal extensions were not less than 20mm. The fitting surface of all crowns were sandblasted accordingly and were made ready for cementation process. The teeth were numbered between 1-120 for record purposes and random assignment to either of the study groups. 5. Cementation Tooth mounts were randomized using their serial numbers, to either of the three cement application techniques. This random allocation was done by generating a random sequence using Microsoft Excel by one of the department's lecturers, not directly related to the study. 40 teeth were allocated to each group. This sequence was passed on to an external operator (a general dentist), again not related to the primary research team, who was trained by the PI prior to study commencement on how to apply cement according to the operational definitions of the three cementation techniques under observation in this study. The cementation process started with mixing of permanent glass ionomer type-I luting cement (3M Ketac Cem Radiopaque Hand mix, USA) according to the manufacturer's recommendation by a single operator to ensure uniformity of mixing technique. The liquid component of Glass Ionomer Cement was measured using a 1cc syringe, while powder was measured using manufacturer provided scoop to ensure equal amount of cement was mixed. The mixing was done on a glossy paper pad provided by the manufacturer.

The external operator cemented the metal crowns on the tooth mounts according to their allocation of cementation category. They were then seated by applying 80Newton pressure using a hydraulic press for 10 minutes. After ensuring the cement had dried completely the cemented crowns (on mounts) were left for seven days, soaked in distilled water to ensure hydration and to replicate intra-oral moisture conditions. 6. Testing for retention On the eighth day, the PI undertook the experiment of retention testing. To help in minimizing bias while performing the retention testing, the PI and field assistant did not know which cement application technique had been used under mounted tooth being observed. The two components (Tooth mount with cemented crown) were then attached to a Universal Testing Machine (INSTRON-4301) and tensile forces were applied to check the retention of the crown. The amount of force required was monitored on the computer. The force of traction was increased slowly till the crown de-bonded and came off the prepared tooth. That value of force in Newtons was noted in the proforma against the tooth mount's serial number. The entire procedure was done under supervision of a field assistant from Pakistan Council of Scientific and Industrial Research laboratory. PI was unblinded after completion of data analysis. The serial number allocation to study groups was directly conveyed to the statistician by the external observer. All data were recorded in pre-formed proforma.