الفهرس 
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Abstract
Crown retention is a very important factor in the success of cemented implant restorations. The necessity for retention is related to the abutment geometry (height and width), surface roughness, and cement type. Surface roughness, grooves and luting agents are factors that can be controlled by the clinician. Surface roughness increases retention because of the resulting microretentive ridges, while groove patterns create abutment cement keys.
The aim of this study was to introduce the use of circumferential and axial grooves on implant abutments and to evaluate their effect on the retention of provisionally cemented implant restorations.
Twenty straight abutments (4,5mm diameter and 8.0 mm height) with abutment screws and corresponding stainless steel laboratory internal implant analogs (4.5 mm diameter) were used in this study. Abutments were divided into 4 groups (n=5): group I (control group): abutments were left without grooves group II: abutments with 1 axial groove group III: abutments with 2 circumferential grooves group IV: abutments with 3 circumferential grooves.
Each implant abutment/analog specimen was embedded and stabilized in a prefabricated split copper mold filled with self-polymerizing acrylic resin to form acrylic bases to facilitate the testing procedure. Ni Cr crown copings were fabricated from waxing sleeves, attached to it a wax loop with a flat wax platform to provide a flat surface to receive the cyclic loads. Two cotton pellets were compacted on top of the abutment screw of each abutment and screw access channels were sealed flush with softened and compacted modeling wax. The intaglio surfaces of the crown copings were sandblasted in
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the lab then filled with a RelyX Temp NE mixed according to the manufacturer’s instructions. Each crown coping was seated on the corresponding abutment and held in place with finger for 10 seconds, and then a static load of 6-kg weight was applied for 10 minutes to provide complete seating of the crown. After cementation, specimens were placed in an incubator at 100% humidity at 37°C for 24 hours, then subjected to 500 cycles in a thermocycling machine between 5ºC (±2ºC) and 55ºC (±2ºC) with a 20 seconds dwell time in each bath, and 5-10 seconds interlude between water baths to simulate the thermal stresses in the oral environment. Then, the specimens were subjected to 5000 cycles of compressive load to simulate 6 months of average human masticatory function. The specimens were mounted on a universal testing machine and a vertical tensile force was applied at a crosshead speed of 2.5 mm per minute, to dislodge the copings from the abutments. The peak load to dislodgment was recorded in Newton and used to indicate the retentive values.
After the retention test, the copings and abutments were evaluated for failure mode according to the location of the residual cement with Stereoscopic Microscope. Full thickness residues on the abutment or coping were denoted as adhesive failure. Cohesive failure was denoted when the failure was within the cement and partial thickness residues were seen on the abutment and the opposing surface of the coping. A combination of adhesive and cohesive failure was considered a mixed failure. Collected data were analyzed by SPSS using one-way analysis of variance (ANOVA) and Tukey’s HSD tests.
The retentive values of copings cemented on abutments with three circumferential grooves had the highest mean tensile bond strength (157.52 N), followed by abutments with two circumferential grooves (136.94 N), then
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abutments with one axial groove (112.16 N), then abutments without grooves (111.86 N). Tukey HSD test revealed the mean tensile strength of the three circumferential grooves was significantly higher than the other groups (P<0.0001) but there was no significant difference between the axial groove and control group (P=0.999)
The cement failure mode seen in this investigation was generally adhesive in nature, although some adhesive/cohesive failures were observed. Copings cemented on group I abutments and group II exhibited adhesive type failure which occurred at abutment-cement interface with cement residues attached to the coping. Full thickness cement remnants were found primarily (more than 50%) over the inner-surface of the copings while small full thickness cement residues were found on the abutment shoulder.
Copings cemented on group III and group IV exhibited mixed (adhesive/cohesive) type failures. In group III, less than 50% of partial thickness cement residues were found on abutment surface within and around its circumferential grooves as result of cohesive failure mode. While greater than 50% of full thickness remnants were found on the copings intaglio denoting adhesive failure mode occurring at abutment/cement interface. While in group IV, greater than 50% of partial thickness cement residues were found on abutment surface within and around its circumferential grooves as result of cohesive failure mode. While less than 50% of full thickness remnants were found on the copings intaglio denoting adhesive failure mode occurring at abutment/cement interface.