In vitro precision of fit of computer-aided design and computer-aided manufacturing titanium and zirconium dioxide bars

Abstract

Objectives

Optical scanners combined with computer-aided design and computer-aided manufacturing (CAD/CAM) technology provide high accuracy in the fabrication of titanium (TIT) and zirconium dioxide (ZrO) bars. The aim of this study was to compare the precision of fit of CAD/CAM TIT bars produced with a photogrammetric and a laser scanner.

Methods

Twenty rigid CAD/CAM bars were fabricated on one single edentulous master cast with 6 implants in the positions of the second premolars, canines and central incisors. A photogrammetric scanner (P) provided digitized data for TIT-P ( n = 5) while a laser scanner (L) was used for TIT-L ( n = 5). The control groups consisted of soldered gold bars (gold, n = 5) and ZrO-P with similar bar design. Median vertical distance between implant and bar platforms from non-tightened implants (one-screw test) was calculated from mesial, buccal and distal scanning electron microscope measurements.

Results

Vertical microgaps were not significantly different between TIT-P (median 16 μm; 95% CI 10–27 μm) and TIT-L (25 μm; 13–32 μm). Gold (49 μm; 12–69 μm) had higher values than TIT-P ( p = 0.001) and TIT-L ( p = 0.008), while ZrO-P (35 μm; 17–55 μm) exhibited higher values than TIT-P ( p = 0.023). Misfit values increased in all groups from implant position 23 (3 units) to 15 (10 units), while in gold and TIT-P values decreased from implant 11 toward the most distal implant 15.

Significance

CAD/CAM titanium bars showed high precision of fit using photogrammetric and laser scanners. In comparison, the misfit of ZrO bars (CAM/CAM, photogrammetric scanner) and soldered gold bars was statistically higher but values were clinically acceptable.

Introduction

Computer-aided design and computer-aided manufacturing (CAD/CAM) allows for the fabrication of cement- and screw-retained prostheses made from a single, homogenous block of titanium (TIT) and yttria-stabilized tetragonal zirconia polycrystal (ZrO) . CAM uses industrial machines with Computerized Numerical Control (CNC) technology to mill the definitive framework based on the digital CAD information . Therefore, the intraoral three-dimensional (3D) position of the implant needs to be digitized . This can be done in two ways. One is to scan the implant position intraoral with specific scan abutments and an intraoral scanner . However, only few systems allow for this direct method for digital impression taking on abutment level. The second (traditional) way is to scan the implants in the master cast produced with a conventional impression , with laboratory scan bodies and a laboratory scanner. The introduction of new scanners with laser and photogrammetric technology and specialized milling centers allows for accurate and fast fabrication of CAD/CAM frameworks at a low price. Many companies provide a broad range of products including laboratory scanners and CAD software . While scanning may be performed by local laboratories, some production centers prefer to scan the model in-house for standardizing their procedures and minimizing external manual errors in the initial phase of the digital chain.

Precision of fit of CAD/CAM frameworks has been shown to be more accurate than conventional techniques . Although controversially discussed, misfit of a rigid framework may enhance the risk for technical and biological complications . To a certain degree, the gap between the components seems to be unavoidable and causes bacterial invasion into the implant-abutment interface . This may result in peri-implant infection with deep pockets and crestal bone loss .

While fixed implant-supported restorations can be cemented or screw-retained, the bar of implant-overdentures has to be screw-retained. Tightening of the occlusal screws in implants with divergent axes will increase the tension within the bar and transfer strains in the implant–bone complex, independent of the bar material .

However, no studies are available that compare CAD/CAM titanium bars using the latest scanners with different technologies. Thus, the aim of this study was to analyze the precision of fit of CAD/CAM TIT bars fabricated with the latest photogrammetric and laser scanners in comparison with ZrO bars (photogrammetry, CAD/CAM) and conventionally soldered gold bars. The null-hypothesis was that there is no difference in the mean vertical microgap at the implant-bar interface independent from the scanner, the CAD/CAM system and the material used.

Materials and methods

Master cast

A controlled laboratory study was performed as described by Katsoulis et al. . In short, an edentulous maxillary jaw model was made from polyester resin (EFCO Produkte GmbH, Germany) and stored for 7 days before implant analogon placement. A tooth set-up was produced with prefabricated teeth to fabricate a template to guide implant placement in this model. Six implant analogs with a diameter of 4.3 mm and a flat platform (Replace Select Tapered Regular Platform, Nobel Biocare, Gothenburg, Sweden) were placed in FDI positions 15, 13, 11, 21, 23, and 25. The axes of the four posterior implants were parallel and vertical, while the two implants in the central incisors position were angulated by 10° in the sagittal plane. The linear distance between the center points was 11 mm between the implants 25 and its closest implant 23, while the most distant implant (position 15) had 40 mm of distance. All the bars were fabricated with this model after storage for another 7 days. Table 1 summarizes the following production procedures.

Table 1
Details of the production procedure of the 4 study groups.
Group TIT-P TIT-L ZrO-P Gold
Production center Allshape AG NobelBiocare SA Allshape AG Private dental laboratory/Cendres&Métaux
Scanner Photogrammetry (Imetric) Laser (Procera) Photogrammetry (Imetric)
Scanner location Production center Private dental laboratory Production center
Spray Yes No Yes
CAD software Pro/ENGINEER NobelProcera™ Software 4.6.1 Pro/ENGINEER
CAD location Production center Private dental laboratory Production center
Number of axes CNC-machine (CAM) 5 axes 5 axes 5 axes
Bar material Titanium grade 5, block Titanium grade 2, block Zirconium dioxide, presintered block Prefabricated gold copings and rigid bar attachments, solder alloy
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Nov 25, 2017 | Posted by in Dental Materials | Comments Off on In vitro precision of fit of computer-aided design and computer-aided manufacturing titanium and zirconium dioxide bars
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