Three-dimensional analysis of the interchangeability of a semiadjustable articulator system in service over time.

STATEMENT OF PROBLEM: Some contemporary articulator systems claim to be highly precise in their interchangeability, with tolerances below 10 µm in vertical error; however, the claims have not been independently verified. PURPOSE: The purpose of this study was to investigate the interchangeability of calibrated semiadjustable articulators in service over time. MATERIAL AND METHODS: A calibrated mounting articulator served as the master articulator, while the test groups were used articulators with a minimum of 1-year use by predoctoral dental students (n=10); used articulators with a minimum of 1-year use by prosthodontic residents (n=10); and new articulators (n=10). One set of mounted maxillary and mandibular master models was positioned in the master and test articulators. High-precision reference markers on the master models were used to determine interarch 3D distance distortions (dRR, dRC, and dRL), interocclusal 3D distance distortion (dRM), interocclusal 2D distance distortions (dxM, dyM, and dzM), and interocclusal angular distortion (dθM) relative to the master articulator. All measurements were conducted three times using a coordinate measuring machine and then averaged to derive the final data set. RESULTS: For interarch 3D distance distortion, the mean dRR ranged from 4.6 ±21.6 µm for new articulators to 56.3 ±47.6 µm for articulators used by prosthodontic residents; mean dRC ranged from 65 ±48.6 µm for new articulators to 119.0 ±58.8 µm for articulators used by prosthodontic residents; and mean dRL ranged from 12.7 ±39.7 µm for articulators used by prosthodontic residents to 62.8 ±75.2 µm for new articulators. For interocclusal 3D distance distortion, the mean dRM ranged from 21.5 ±49.8 µm for new articulators to 68.6 ±64.9 µm for articulators used by predoctoral dental students. For the 2D distance distortions, the mean dxM ranged from -17.9 ±43.4 µm for articulators used by predoctoral dental students to -61.9 ±48.3 µm for articulators used by prosthodontic residents; mean dyM ranged from 18.1 ±59.4 µm for new articulators to 69.3 ±115.1 µm for articulators used by prosthodontic residents; and mean dzM ranged from 29.5 ±20.2 µm for new articulators to 70.1 ±37.8 µm for articulators used by prosthodontic residents. Mean dθM ranged from -0.018 ±0.289 degree for new articulators to 0.141 ±0.267 degree for articulators used by prosthodontic residents. One-way ANOVA by articulator type revealed statistically significant differences among the test groups for dRR (P=.007) and dzM (P=.011) only, where articulators used by prosthodontic residents fared significantly poorer than the other test groups. CONCLUSIONS: The new and used articulators tested did not fulfill the manufacturer's claim of accuracy of up to 10 µm in the vertical dimension. Up to 1 year of time in service, none of the investigated test groups fulfilled the criterion for articulator interchangeability, even if the more lenient threshold of 166 µm were accepted.

Rotational Load Fatigue Performance of Titanium vs Titanium-Zirconium Implant-Abutment Connections.

PURPOSE: Titanium-zirconium (Ti-Zr) alloy has been developed to strengthen the implant body, but clinically relevant information is still limited. The aim of this in vitro study was to compare the rotational load fatigue performance of implant-abutment connections in narrow-diameter (3.3-mm) and regular-diameter (4.1-mm) implants made with commercially pure grade 4 titanium alloy (CPTi-G4) and Ti-Zr. MATERIALS AND METHODS: Narrow-diameter (N) and regular-diameter (R) implants with CPTi-G4 (Ti) or Ti-Zr (Tz) materials were tested. This resulted in four test groups: NTi, NTz, RTi and RTz. Five specimens were made for each group (n = 5). Abutments used were milled from titanium-aluminum-niobium alloy abutment blanks. A rotational load fatigue machine applied a sinusoidally varying load at an angle of 45 degrees to produce an effective bending moment of 35 Ncm at a frequency of 14 Hz in air at 20°C. The number of cycles to failure was recorded. The upper limit was set as 5 million cycles. Results were evaluated using analysis of variance (ANOVA) and Tukey post hoc tests. Failure locations and patterns were evaluated with scanning electron microscope (SEM). RESULTS: All regular-diameter test groups reached the upper limit of 5 million cycles without failure. All narrow-diameter test groups failed within the range of 402,530 cycles to 3,374,353 cycles. It could be observed that NTz showed a higher mean cycle count as compared to NTi. NTi test group recorded two implants damaged, one implant fracture, five abutment fractures, and four screw fractures. NTz test group showed only abutment fractures at the level of implant platform, with no damage to the implant bodies. Significant difference was found between implants of different diameters. There was no significant difference between implants of different materials. CONCLUSION: Regular-diameter implants performed significantly better than narrow-diameter implants, regardless of material, while no significant difference in cyclic load to failure was found between groups of different alloys. All NTz failures were at the abutment only, without damage to the implant. This failure pattern can potentially be clinically advantageous in terms of retrieval and subsequent replacement of a failed prosthesis.

Crown preparations by undergraduate dental students: A comparison of conventional versus digital assessment via an intraoral scanner.

PURPOSE: This study aims to assess the effectiveness of the 3Shape TRIOS intraoral scanner (IOS) in student crown preparation evaluation. DESIGN: Students were tasked to perform a full metal crown preparation on the upper left first molar on a patient simulator within 45 minutes. Marginal Width, Occlusal Reduction, Presence of Undercuts, Taper, Planes of Reduction, Line Angles, Conformity to Gingival Contour, and Smoothness were evaluated via 3 assessment modes: (a) Conventional assessment by 2 supervisors; (b) Conventional assessment by students; (c) Digital assessment by students. Agreement between assessment modes was investigated using Kappa (κ), with a threshold set at κ > 0.4. Effectiveness of IOS for objective parameters was determined via physical verification, while that for subjective parameters was defined by agreement with the stricter supervisor grade. RESULTS: Substantial agreement (κ = 0.631) was found between IOS measurement of Marginal Width and physical verification. Two of 5 subjective parameters met κ > 0.4 for agreement between IOS and the stricter supervisor grade. Agreement between supervisors ranged from slight (κ = 0.103) for Occlusal Reduction to Fair (κ = 0.399) for Marginal Width. Agreement between conventional assessments of supervisors and students ranged from less than chance (κ = -0.142) for Occlusal Reduction to moderate (κ = 0.577) for Line Angles. Agreement between conventional assessments of supervisors and digital assessments of students ranged from slight (κ = 0.130) for Planes of Reduction to moderate (κ = 0.538) for Line Angles. CONCLUSIONS: IOS may be used to overcome limitations in conventional assessment of objective parameters and some subjective parameters. Digital assessment of crown preparations cannot completely replace conventional assessment.