Effect of screw access hole preparation on fracture load of implant-supported zirconia-based crowns: an in vitro study

Abutment connection structural changes in dual-retained and screw-retained metal-ceramic implant-supported restorations: an in vitro study, part I

BACKGROUND

This study sought to compare the rotational freedom and structural changes of abutment connections in dual-retained and screw-retained metal-ceramic implant restorations at different fabrication stages and following thermomechanical loading.

METHODS

Twenty metal-ceramic restorations were equally divided into two groups. Group 1 (G1) consisted of dual-retained restorations on prefabricated titanium abutments, while Group 2 (G2) comprised screw-retained restorations on UCLA chromium-cobalt overcast abutments. Specimens underwent 500 cycles of thermocycling and 500,000 cycles of mechanical loading. Changes in connection dimensions and rotational freedom were compared within and between the groups before- and post-loading. Statistical analyses were conducted using a generalized linear model (GLM). The significance level was set at α = 0.05.

RESULTS

Initially, no significant differences in connection dimensions were observed between the groups (P >.05). After loading, G2 exhibited significantly smaller hexagon side lengths and diagonal measurements, along with increased hexagonal angle deformation and concentricity (P <.001) compared to G1. Rotational freedom was significantly greater in G2 compared to G1 both before and after thermomechanical loading (P <.001). G2 experienced significant dimensional changes before and after loading (P <.001), whereas G1 showed no significant changes in connection dimensions pre- and post-loading (P >.05).

CONCLUSIONS

Dual-retained restorations outperformed screw-retained ones with minimal connection alteration and higher rotational stability.

Effect of Vent Hole and Cement Type on Fracture Resistance of CAD-CAM Monolithic Zirconia Crowns

PURPOSE

To assess the fracture resistance of monolithic zirconia crowns cemented with different types of cement on cement-retained implant abutments.

MATERIALS AND METHODS

Forty implant analogues were positioned in acrylic resin blocks, and cement-retained straight implant abutments were fastened to the analogues. Crowns were designed with/without occlusal vent holes and produced from monolithic zirconia blocks by the CAD-CAM technique. The two crown types were divided into two groups and cemented with resin and zinc-polycarboxylate cement under 5 kg weight. A universal testing machine applied compressive forces to the crowns until fracture. Fracture resistance values were analysed using two-way ANOVA and the independent samples t-test with statistical significance set at p < 0.05.

RESULTS

According to the two-way ANOVA results, although the crown design did not have a significant effect on fracture resistance (1417.65 ±337.39 N, 1565.16 ±517.12 N; crowns with and without vent holes, respectively), the main effect of the cement variable on the fracture resistance was significant. Zinc-polycarboxylate cement (1680.1 ±375.23 N) showed higher fracture resistance than resin cement (1302.71 ±420.64 N) in the crowns designed with vent holes (p< 0.005).

CONCLUSION

The use of cement-retained implant-supported monolithic zirconia crowns with an occlusal vent hole is safe, and zinc-polycarboxylate cement use may be an appropriate choice for cementation of these crowns. This article is protected by copyright. All rights reserved.

The effect of aging on the fracture resistance of different types of screw-cement-retained implant-supported zirconia-based restorations

Structural durability of screw-cement-retained implant-supported zirconia-based restorations is an important factor in choosing the best type of restoration for clinical use. This study aimed to evaluate the effects of thermocycling on the fracture resistance of different types of screw-cement-retained implant-supported zirconia-based restoration. Two experimental groups (monolithic zirconia and porcelain-veneered zirconia) and a control group of porcelain-fused-to-metal restorations were fabricated via CAD-CAM (n = 14 per group). Half of the specimens of each group (n = 7) were subjected to 10000 thermal cycles. The compressive force was applied and the force leading to fracture was measured by using a Universal Testing Machine. The fractured modes were classified under a scanning electron microscope. The data were analyzed through two-way ANOVA, one-way ANOVA, and independent samples t-test (α = 0.05). Among the non-thermocycled subgroups, the monolithic zirconia specimens were significantly more fracture-resistant than the porcelain-veneered zirconia and porcelain-fused-to-metal groups (P<0.05); but it was not the same with aging (P>0.05). Thermocycling decreased the fracture resistance of all groups; however, the difference was not statistically significant (P<0.05). The monolithic zirconia presented higher fracture resistance than the bilayered restorations for screw-cement retained implant-supported restorations. Thermocycling decreased the fracture resistance of all types of restorations insignificantly which can be clinically important.

Clinical and Laboratory Outcomes of Angled Screw Channel Implant Prostheses: A Systematic Review

The purpose of this systematic review was to evaluate the clinical and laboratory outcomes of angled screw channel (ASC) restorations and to summarize the influencing factors. An electronic search of the English language literature was performed in four databases and enriched by manual searches. Retrieved studies were screened against the predefined exclusion and inclusion criteria. Eight clinical and seven laboratory studies were eligible for the analysis. The risk of bias for included observational studies was performed using the Newcastle–Ottawa quality assessment scale. Laboratory studies quality assessment method was adapted from previous published systematic reviews. Two clinical studies focused on technical outcomes and the rest reported the biological outcomes of the ASC restorations. Out of the seven laboratory studies, two studies investigated the fracture resistance of ASC restorations, four studies evaluated the reverse torque value of the nonaxially tightened screws, and one study evaluated both variables. The present review revealed that while the performance of ASC restorations is promising in short-term clinical studies, the evidence of their long-term reliability is still lacking. The laboratory studies indicated comparable fracture resistance results of the ASC restorations with the straight screw channel restorations. In addition, factors, such as initial torque value, configuration of the screw driver, screw design, abutment system, and the angulation of screw channel, were shown to influence the screw resistance to loosening.

Effect of screw access hole design on the fracture resistance of implant-supported zirconia-based restorations

OBJECTIVE

To evaluate the effect of screw access hole design on the fracture resistance of cement-retained implant-supported zirconia-based restorations with screw access holes.

MATERIALS AND METHODS

Thirty cement-retained implant-supported zirconia-based molar crown specimens were fabricated. The specimens were divided into 3 groups of 10: without access holes (C), with screw access holes (S), and with screw access holes accompanied with a surrounding zirconia wall (W). The veneering ceramic thickness was 0.8 mm in all groups. Implants were vertically mounted in a metal block. Abutments were screwed on the implants and the specimens were cemented to the abutments. A universal testing machine applied compressive forces to the specimens until fracture. Fracture resistance values of the specimens were measured. One-way analysis of variance (ANOVA) and Tukey HSD tests were used to analyze data (P < .05).

RESULTS

The mean fracture resistance values were 5794.85 N for C, 2691.48 N for S, and 3878.06 N for W. The screw access hole design significantly affected the fracture resistance (P < .0001). Significant differences were found between C and S (P < .001), C and W (P < .001), and S and W (P = .026).

CONCLUSIONS

The screw access hole decreased the fracture resistance. A surrounding zirconia wall for the screw access hole increased the fracture resistance.

CLINICAL SIGNIFICANCE

Screw access hole design may affect the fracture resistance of cement-retained implant-supported zirconia-based restorations with screw access holes. This study introduced a screw access hole design to improve the fracture resistance of these restorations.

Effects of the Screw-Access Hole Diameter on the Biomechanical Behaviors of 4 Types of Cement-Retained Implant Prosthodontic Systems and Their Surrounding Cortical Bones: A 3D Finite Element Analysis

PURPOSE

To investigate the effect(s) of screw-access hole (SAH) in different diameters on the cement-retained implant prosthodontic systems and surrounding cortical bones.

MATERIALS AND METHODS

Twenty finite element models were divided into 4 groups: 2 types of full-contour (FC) crowns (Y-TZP, gold alloy) and 2 types of porcelain-fused-to-metal crowns (based on Co-Cr, Au-Pd alloy). For each group, 5 crowns were simulated by varying the diameter of SAH (0, 1, 2, 3, and 4 mm). A vertical load of 200 N and an oblique load of 100 N (45°s) were applied. All models were analyzed with finite element analysis software.

RESULTS

The stress on the occlusal surface of crowns was almost unchanged when the SAH was within 0 to 3 mm, whereas it showed an obvious increase when it reached 4 mm. The stress concentration was also suddenly changed from the loading area to the hole margin under vertical loading. As for the screw, a lower stress level was observed in vertical loading when an FC crown with an SAH within 0 to 1 mm was applied. The stress concentration was constantly located at the beginning of the first thread. Stresses of other components remained almost unchanged.

CONCLUSIONS

From the aspect of biomechanics, an FC crown with a 1-mm access hole is recommended when a combined cement- and screw-retained crown was used in the posterior region.

The color masking ability of a zirconia ceramic on the substrates with different values

Background. The color masking ability of a restoration plays a significant role in coveringa discolored substructure; however, this optical property of zirconia ceramics has not been clearly determined yet. The aim of this in vitro study was to evaluate the color masking ability of a zirconia ceramic on substrates with different values.

Methods. Ten zirconia disk specimens,0.5 mm in thickness and 10 mm in diameter, were fabricated by a CAD/CAM system. Four substrates with different values were prepared, including: white (control), light grey, dark grey, and black. The disk specimens were placed over the substratesfor spectrophotometric measurements. A spectrophotometer measured the L^*, a^*, and b^* color attributes of the specimens. Additionally, ΔE values were calculated to determine the color differences between each group and the control,and were then compared with the perceptional threshold of ΔE=2.6. Repeated-measures ANOVA, Bonferroni, and one-sample t-test were used to analyze data. All the tests were carried out at 0.05 level of significance.

Results. The means and standard deviations of ΔE values for the three groups of light grey, dark grey and black were 9.94±2.11, 10.40±2.09, and 13.34±1.77 units, respectively.Significant differences were detected between the groups in the ΔE values (P<0.0001).The ΔE values in all the groups were more than the predetermined perceptional threshold(ΔE>2.6) (P<0.0001).

Conclusion. Within the limitations of this study, it was concluded that the tested zirconia ceramic did not exhibit sufficient color masking ability to hide the grey and black substrates.