Evaluation of the dislodgement force of splinted restorations with engaging conical abutments over multiple nonparallel implants (in-vitro study).

BACKGROUND: Splinted multiunit cement-retained restorations with screw access channels over engaging abutments are viable implant prosthetic options. However, information regarding the maximum degree of divergence between multiple implants is lacking. The purpose of this in vitro study was to determine the maximum degree of divergence between 2 adjacent implants with conical connections that allows insertion and removal of splinted restorations with engaging preparable abutments or titanium base abutments. METHODS: Two implants were aligned in a stone base, one straight and the other at an angle ranging from 0 to 20 degrees. The implants represented an implant system that had an internal conical connection and a hexed abutment engaging the base of the connection. Two straight preparable engaging cement retained abutments were screwed onto the implants and splinted together using acrylic resin. A total of 11 angles were tested, with 7 specimens for every angle. Evaluation of dislodging force was performed by pulling out the splinted abutments after unscrewing them. This was performed subjectively by 3 blinded investigators who applied a tactile pulling force. A scale of 0-10 was used to estimate the pulling force. Objectively the dislodging force was measured in Newtons using a universal testing machine. A statistical correlation was made between the subjective and objective dislodging force values using Spearman's rank correlation coefficient. RESULTS: The mean subjective values increased gradually from 0 to 16 degrees. A sudden rise was noticed at 18 degrees (9.71 ± 0.23) and, at 20 degrees, the investigators were not able to remove the splinted abutments from the implants. The mean objective dislodgement force values increased gradually from 0 to16 degrees and abruptly from 16 degrees (13.57 ± 0.45 N) to 18 degrees (25.40 ± 0.66 N) and 20 degrees (35.22 ± 0.64). The correlation between the subjective and the objective evaluations assessed using the Spearman's rank correlation coefficient was 0.98 indicating a statistically significant correlation (P < .001). As the objective dislodging force increased, the subjective dislodgement difficulty increased. CONCLUSIONS: Splinting cement retained restorations with screw access channels on engaging abutments is possible when multiple implants with conical connections having an internal flare angle of 8 degrees are used, with implant divergence of up to 16 degrees.

Effect of pattern materials and fabrication techniques on the color of a pressed lithium disilicate ceramic: An in vitro study.

STATEMENT OF PROBLEM: Pattern materials and their fabrication techniques have been reported to affect the marginal accuracy and internal fit of pressed lithium disilicate ceramic restorations. However, information regarding their effect on the color of such restorations is lacking. PURPOSE: The purpose of this in vitro study was to evaluate the color of a pressed lithium disilicate glass-ceramic fabricated from wax or resin patterns by using conventional and milling techniques. MATERIAL AND METHODS: Fifty-six Ø14×1.5-mm disk pattern specimens were divided into 4 groups based on the pattern material and fabrication technique: conventional wax pattern group (CW), conventional resin pattern group (CR), milled wax pattern group (MW), and milled resin pattern group (MR). Lithium disilicate specimens were fabricated by using the lost wax technique. The CIELab values were measured with a spectrophotometer (VITA Easyshade Advance). The ΔE00 values were calculated and compared with perceptibility (ΔE00=0.8) and acceptability (ΔE00=1.8) thresholds to interpret the color differences. The 2-way ANOVA and Tukey post hoc tests were used to analyze data (α=.05). RESULTS: Two-way ANOVA revealed that ΔE00 was significantly affected by the pattern material (P<.001), fabrication technique (P=.034), and their interaction (P<.001). The ΔE00 of the CW group (0.76 ±0.04) was clinically imperceptible. No significant difference in the ΔE00 values was found between the reference and the MW (1.48 ±0.13) and MR (1.51 ±0.15) groups (P=.926), which were both clinically perceptible and acceptable. The highest ΔE00 was that of the CR group (2.06 ±0.22). CONCLUSIONS: The color of the pressed lithium disilicate ceramic was influenced by the wax and resin pattern materials fabricated by using conventional or computer-aided design and computer-aided manufacturing (CAD-CAM) techniques. Only the CW pattern group showed color differences below the clinically perceptible threshold (ΔE00=0.8). The CR pattern group showed the highest color differences, which were not clinically acceptable.

Effect of screw access channel on the fracture rate of lithium disilicate cement-retained implant-supported posterior crowns.

STATEMENT OF PROBLEM: Whether the presence of a screw access channel on the occlusal surface of a lithium disilicate cement-retained single tooth implant restoration reduces its fracture rate is unclear. PURPOSE: The purpose of this randomized controlled clinical trial was to compare the fracture rate of lithium disilicate cement-retained posterior single tooth implant restorations with or without screw access channels. The study also evaluated whether the crown dimensions and location of the occlusal screw access channel would affect the fracture rate. MATERIAL AND METHODS: A split-mouth study was conducted where 40 participants having 2 implants placed to restore single missing posterior teeth with crowns were enrolled. Implants were randomly allocated into a conventional crown group or a screw access group, receiving cement-retained restorations fabricated from lithium disilicate glass-ceramic with and without an occlusal screw access channel. Crown dimensions were recorded in millimeters from the designing software program. Follow-up evaluations were made at 3, 6, and 12 months by observing visible cracks or fractures of the crowns. The Kolmogorov-Smirnov test of normality was used. Comparisons were carried out by using the Student and Welch t tests (α=.05). RESULTS: After a 12-month follow-up period, no crowns fractured or revealed visible cracks, resulting in a 100% success rate. No statistically significant differences were found between the conventional crown group and screw access channel group in all crown dimensions (P>.05). CONCLUSIONS: The presence of an occlusal screw access channel did not affect the short-term fracture rate of lithium disilicate cement-retained single implant-supported posterior crowns. The amount of axial cantilever, occlusal ceramic thickness, and location of the screw access channel on the occlusal surface of the restoration have no effect on its fracture rate.