Comparison of loupes versus microscope-enhanced CAD-CAM crown preparations: A microcomputed tomography analysis of marginal gaps

STATEMENT OF PROBLEM

Long-term restoration success depends on a precision marginal fit to prevent marginal leakage and caries. The successful fit of a computer-aided design and computer-aided manufactured (CAD-CAM) crown may be affected by different workflow variables, including preparation, scanning, crown design, milling, sintering, and cementation. Discrepancies in any of these steps may result in poor marginal and internal fit. Evidence suggests that tooth preparation may be the most important step in the workflow for a successful outcome. Compared with the traditional means of crown preparation using the naked eye or loupes, the dental operating microscope provides higher magnification and more direct illumination. However, the impact of high magnification during preparation on the marginal quality of CAD-CAM crowns is unclear.

PURPOSE

The purpose of this in vitro study was to compare marginal fits of CAD-CAM crowns fabricated after initial preparation with loupes and subsequent preparation refinement with either loupes or a microscope. The null hypothesis was that no significant difference would be found in the marginal gap between the preparations with loupes and those with a microscope.

MATERIAL AND METHODS

Mounted extracted molars (N=18) received initial crown preparations with a coarse grit, rounded shoulder, diamond rotary instrument with loupes of ×3.0 magnification. The teeth were then randomly divided into 2 groups and refined for an additional 2 minutes with fine grit, rounded shoulder, diamond rotary instruments with either loupes (LOUP) or a microscope up to ×10.0 magnification (DOM). The prepared teeth were scanned with an intraoral scanner to fabricate zirconia-reinforced lithium silicate crowns manufactured with a 4-axis milling machine, sintered in a dental furnace in accordance with the manufacturer's instructions, and cemented with self-adhesive resin cement. All teeth with crowns were mounted and scanned with a microcomputed tomography (μCT) system at 21-μm nominal voxel size. The resulting Digital Imaging and Communications in Medicine (DICOM) images were imported into a semiautomatic segmentation software program. Marginal and absolute gaps were measured at 24 consistent circumferential points per specimen. Absolute gaps were labeled, and the total volume was calculated. Paired and unpaired t tests were used for statistical analysis (α=.05).

RESULTS

The mean marginal gap was 145.0 ±259.6 μm for LOUP and 35.6 ±110.6 μm for DOM, with a statistically significant difference (P<.001). The mean gap volume for LOUP was 0.975 ±0.811 mm3, and 0.250 ±0.477 mm3 for DOM, also statistically significantly different (P=.023). A significant difference was found between the absolute and marginal gaps for LOUP (P=.007), but for DOM, the difference was not significant (P=.063).

CONCLUSIONS

This study demonstrated that the higher magnification used during tooth preparation played a significant role in the size of marginal gaps present around CAD-CAM crowns. Crown preparations finished by using fine grit diamond rotary instruments with a microscope at higher magnification than loupes resulted in a more precise marginal fit with smaller gaps.

Staged esthetic crown lengthening: Classification and guidelines for periodontal-restorative therapy

OBJECTIVE

This article presents technical guidelines for perio-restorative esthetic crown lengthening, along with a discussion of the biologic rationale. A classification system is proposed to assist in treatment planning and sequencing the surgical and restorative phases.

CLINICAL CONSIDERATIONS

When esthetic crown lengthening is performed as an adjunct to restorative therapy, the surgical approach must be determined by the anticipated position of the restorative margins. The removal of sufficient bone to achieve the desired clinical crown length and preserve the supracrestal gingival tissue dimensions is facilitated by the use of a surgical guide fabricated according to the design of the restorations. A staged approach allows sequencing the provisional restoration to minimize unesthetic sequelae during the healing period. Inadequate bone resection and/or alteration of the soft tissue dimensions results in delayed healing, leading to coronal gingival rebound and biologic width impingement.

CONCLUSION

The identification and preservation of appropriate restorative and biologic landmarks is essential for success in pre-prosthetic esthetic crown lengthening treatment. A staged approach improves the esthetic management during the postsurgical healing and maturation period.

CLINICAL SIGNIFICANCE

A restorative driven classification system for sequencing and staging adjunctive esthetic crown lengthening procedures is presented. Technical guidelines to enhance gingival margin predictability are suggested, accompanied by relevant evidence. In addition, wound healing timelines following gingival and osseous resection are provided.

Accuracy of Dental and Industrial 3D Printers

PURPOSE

This in vitro study evaluated the dimensional accuracy of three 3D printers and one milling machine with their respective polymeric materials using a simplified geometrical model.

MATERIALS AND METHODS

A simplified computer-aided design (CAD) model was created. The test samples were fabricated with three 3D printers: a dental desktop stereolithography (SLA) printer, an industrial SLA printer, and an industrial fused deposition modeling (FDM) printer, as well as a 5-axis milling machine. One polymer material was used per industrial printer and milling machine while two materials were used with the dental printer for a total of five study groups. Test specimens were then digitized using a laboratory scanner. The virtual outer caliper method was used to measure the linear dimensions of the digitized 3D printed and milled specimens in x-, y-, and z-axes, and compare them to the known values of the CAD model. Data were analyzed with Kruskal-Wallis one-way ANOVA on Ranks followed by the Tukey's test.

RESULTS

Milled specimens were not significantly different from the CAD model in any dimension (p > 0.05). All 3D printed specimens were significantly different from the CAD model in all dimensions (p = 0.01), except the dental SLA 3D printer with one of the polymers tested (Bis-GMA) which was not significantly different in two (x and z) dimensions (p = 0.4 and p = 0.12).

CONCLUSIONS

The milling technology tested provided greater dimensional accuracy than the selected 3D printing. Printer, printing technology, and material selection affected the accuracy of the printed model.

Ceramic Printing - Comparative Study of the Flexural Strength of 3D Printed and Milled Zirconia

PURPOSE

To determine and compare the mechanical properties of 3D-printed yttriastabilized zirconia to milled isostatic pressed yttria-stabilized zirconia, with the following hypotheses: (1) The flexural strength of 3D-printed yttria-stabilized zirconia is comparable to milled yttria-stabilized isostatic pressed zirconia; and (2) thermocycling and chewing simulation do not affect the flexural strength of 3D-printed yttria-stabilized zirconia.

MATERIALS AND METHODS

A total of 30 bars of an experimental 3D-printed 3 mol% yttriastabilized zirconia (LithaCon 3Y 230, Lithoz) and 10 bars of milled isostatic pressed zirconia (Prettau Zirconia, Zirkonzahn) were utilized. The printed zirconia bars were divided into three groups (n = 10 bars per group): (1) untreated (control); (2) thermocycled; and (3) tested after chewing simulation. A flexural strength test was performed on all samples using a three-point bend test in an Instron Universal testing machine. One-way analysis of variance on ranks was used to compare milled to printed zirconia. The effects of thermocycling and load cycling on 3D-printed zirconia were also determined.

RESULTS

The flexural strength values for milled and printed zirconia were 936.3 ± 255.0 MPa and 855.4 ± 112.6 MPa, respectively. There was no statistically significant difference in flexural strength between the milled and printed zirconia (P = .178). No statistically significant differences were observed between the control 3D-printed zirconia group and the thermocycled (888.4 ± 59.3 MPa) or load-cycled printed zirconia (789.6 ± 133.8 MPa; P = .119).

CONCLUSION

Printed 3 mol% yttria-stabilized zirconia has comparable flexural strength to milled yttria-stabilized isostatic pressed zirconia. The thermocycling and chewing simulation used in this study did not significantly alter the flexural strength of the printed 3 mol% yttria-stabilized zirconia. These results indicate a promising role for 3D printing in the fabrication of zirconia restorations. Additional studies are needed to explore the full potential of this technology.

A novel digital approach for fixed full-mouth implant-supported rehabilitations: A case report

Successful rehabilitation of a patient’s entire dentition with implant-supported fixed prostheses requires restoration of function, esthetics and comfort. To achieve this goal, the clinician and laboratory technician must work in concert with one another to navigate the multiple steps from the patient’s initial evaluation to delivery of the final prostheses. Key to this is the ability of the clinician to provide the technician with detailed information regarding the patient’s extra- and intraoral characteristics in a manner that can be easily and accurately transferred to the lab bench where it then serves as the foundation for reconstruction of the dentition. In recent years, the impressive evolution of digital technology in dentistry has dramatically facilitated this complex process. The aim of this case report is to illustrate how digital profiles of a patient’s facial and intraoral features can be merged with one another and used to generate artificial teeth and gingival tissue of a full mouth implant supported rehabilitation via computer-aided design and computer-aided manufacturing (CAD/CAM) technology to successfully rehabilitate a patient that initially presented with a terminal dentition.

Key words:Facial scan, Zirconia, Implant-supported rehabilitation, Implant-supported prosthesis, Fixed prosthesis, Oral rehabilitation.

Sr[Li2Al2O2N2]:Eu2+-A high performance red phosphor to brighten the future

Innovative materials for phosphor converted white light-emitting diodes are in high demand owing to the huge potential of the light-emitting diode technology to reduce energy consumption worldwide. As the primary blue diode is already highly optimized, the conversion phosphors are of crucial importance for any further improvements. We report on the discovery of the high performance red phosphor Sr[Li2Al2O2N2]:Eu2+ meeting all requirements for a phosphor's optical properties. It combines the optimal spectral position for a red phosphor, as defined in the 2016 Research & Development-plan of the United States government, with an exceptionally small spectral full width at half maximum and excellent thermal stability. A white mid-power phosphor-converted light-emitting diode prototype utilising Sr[Li2Al2O2N2]:Eu2+ shows an increase of 16% in luminous efficacy compared to currently available commercial high colour-rendering phosphor-converted light-emitting diodes, while retaining excellent high colour rendition. This phosphor enables a big leap in energy efficiency of white emitting phosphor-converted light-emitting-diodes.

Bond strength of different veneering ceramics to zirconia and their susceptibility to thermocycling

PURPOSE

To evaluate the bond strength of recently developed feldspathic veneering ceramics to zirconium-oxide ceramic core material (Y-TZP). In addition, the effect of thermocycling on the bond strength was investigated.

METHODS

90 square Lava framework Y-TZP specimens were randomly assigned to three groups (n = 30) and veneered with one of the three zirconia veneering ceramics Cerabien ZR (CZR), GC Initial (GCI), or Lava Ceram (LC). 30 additional metal-ceramic specimens served as the control (VO). 15 specimens per group were stored in a saline solution for 3 days. To test the durability of the bond, 15 specimens per group were thermocycled for 10,000 cycles between 5 degrees to 60 degrees C with a 15-second dwell time. Shear strength was tested with an Instron universal testing system. A two-way ANOVA model and Tukey's HSD were applied for statistical analyses (alpha = 0.05).

RESULTS

The different material combinations revealed significantly different shear bond strength values (CZR > GCI > LC > VO). All veneering ceramic/zirconia combinations showed significantly higher bond strength than the metal-ceramic control VO. Bond strength values were not affected by thermocycling, except for CZR, which had significantly higher bond strength after thermocycling. Microscopic evaluation of failure modes revealed 100% cohesive failures in the veneering ceramic for all specimens.

CAD/CAM and telescopic technology: design options for implant-supported overdentures

There are many options with respect to materials, construction methods, and design concepts for the technical implementation of implant-supported dental prostheses. Different methods of anchorage can be used to attach removable superstructures to implants. Telescopic crowns make it possible to fabricate inexpensive superstructures with precise and passive fit. Computer-aided design/computer-assisted manufacture (CAD/CAM) technology allows copings to be fabricated from materials such as zirconia or titanium. Moreover, CAD/CAM crown copings can serve as a base for fabricating customized ceramic replacement teeth. Different veneering techniques, such as pressed-on ceramics for zirconia cores, ensure a fast and economic work process. With the use of electroforming it is possible to manufacture highly precise secondary structures that ensure passive seating of the prosthesis in a stable position. This article demonstrates a restorative treatment option using current techniques with the aim of rehabilitation with an esthetic and functional implant-supported removable denture.

Influence of impression material and time on the 3-dimensional accuracy of implant impressions

OBJECTIVE

To investigate, in vitro, the effect of time on the 3-dimensional accuracy of open-tray implant impression copings made of polyvinyl siloxane and polyether impression materials.

METHOD AND MATERIALS

Reference models with 4 internal connection implants were fabricated. Four sets of 71 direct impressions were made with square impression copings using an open-tray technique. One set of impressions was made with polyether material (material A) and each of 3 polyvinyl siloxane materials (materials B, C, and D). Three-dimensional changes in spatial orientation of the implant analogs were measured over 48 hours using an optical measurement device. Deviation values were summed over 4 points and averaged per impression. Global differences were tested with a nonparametric Kruskal-Wallis test followed by pairwise comparisons using the Mann-Whitney U test.

RESULTS

The mean dimensional accuracy of implant impressions significantly differed over time (P < .001). Materials A and B were found to produce comparable results, For materials C and D there was a significant increase in deviations, with a maximum increase at 2 hours. Deviations for material C were comparable to materials A and B at 48 hours. Baseline values were not maintained by any material.

CONCLUSIONS

Within the limits of this in vitro study, it can be concluded that time cannot be neglected as a factor affecting the accuracy of implant master casts.

Application of interim fixed reference points to maintain the occlusal vertical dimension: A technique for immediate loading protocols in the edentulous mandible

This article describes a technique for the clinical insertion and use of specially designed screws as fixed intraoral reference and attachment points in implant dentistry. After insertion, these screws serve to stabilize radiographic and surgical templates in the completely edentulous patient. This approach provides the advantage of guiding maintenance of the correct occlusal vertical dimension and ensuring accuracy and stability during subsequent phases of treatment.

Implant-supported prosthetic treatment in cases with hard- and soft-tissue defects

Tooth loss due to trauma or periodontal disease is often associated with extensive deficiencies of the supporting hard and soft tissues and atrophy of the alveolar ridges. The functional and esthetic predictability of an implant-supported prosthetic restoration in patients with extensive tissue augmentation is guarded, even when lost tissues were restored at an early stage by local ridge augmentation. The prosthetic reconstruction of major tissue defects is especially difficult, as lost structures of the alveolar ridge must be replaced by the restoration, which is expected to provide optimal function and esthetics, yet allow for optimal oral hygiene. This article presents prosthetic solutions for replacing missing hard and soft tissues with implant-supported fixed and removable restorations in cases of severe tissue deficiencies.