journal of dentistry 42 (2014) 677–683

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Impact of digital impression techniques on the adaption of ceramic partial crowns in vitro Oliver Schaefer a,*, Mike Decker a, Frank Wittstock a, Harald Kuepper a, Arndt Guentsch a,b a

Policlinic of Prosthetic Dentistry and Material Science, Centre for Dental Medicine, Jena University Hospital, Jena, Germany b School of Dentistry, Marquette University, Milwaukee, WI, USA

article info

abstract

Article history:

Objectives: To investigate the effects, digital impression procedures can have on the three-

Received 16 September 2013

dimensional fit of ceramic partial crowns in vitro.

Received in revised form

Methods: An acrylic model of a mandibular first molar was prepared to receive a partial

15 January 2014

coverage all-ceramic crown (mesio-occlusal-distal inlay preparation with reduction of all

Accepted 27 January 2014

cusps and rounded shoulder finish line of buccal wall). Digital impressions were taken using iTero (ITE), cara TRIOS (TRI), CEREC AC with Bluecam (CBC), and Lava COS (COS) systems, before restorations were designed and machined from lithium disilicate blanks. Both the

Keywords:

preparation and the restorations were digitised using an optical reference-scanner. Data

Digital impressions

were entered into quality inspection software, which superimposed the records (best-fit-

CAD/CAM

algorithm), calculated fit-discrepancies for every pixel, and colour-coded the results to aid

Lithium disilicate ceramics

visualisation. Furthermore, mean quadratic deviations (RMS) were computed and analysed

Marginal fit

statistically with a one-way ANOVA. Scheffe´’s procedure was applied for multiple compar-

Internal fit

isons (n = 5, a = 0.05). Results: Mean marginal (internal) discrepancies were: ITE 90 (92) mm, TRI 128 (106) mm, CBC 146 (84) mm, and COS 109 (93) mm. Differences among impression systems were statistically significant at p < 0.001 ( p = 0.039). Qualitatively, partial crowns were undersized especially around cusp tips or the occluso-approximal isthmus. By contrast, potential high-spots could be detected along the preparation finishline and at central occlusal boxes. Conclusions: Marginal and internal fit of milled lithium disilicate partial crowns depended on the employed digital impression technique. Clinical significance: The investigated digital impression procedures demonstrated significant fit discrepancies. However, all fabricated restorations showed acceptable marginal and internal gap sizes, when considering clinically relevant thresholds reported in the literature. # 2014 Elsevier Ltd. All rights reserved.

1.

Introduction

Currently two separate genres of digital impression systems exist. The first one uses scans of the oral cavity to instantly generate virtual working casts that allow clinicians to design

their restorations at the chair-side. This design is subsequently sent to an in-house milling-unit that manufactures customised restorations from prefabricated material blanks. Thereby, defective teeth may be restored within a single appointment.

* Corresponding author at: An der Alten Post 4, 07740 Jena, Germany. Tel.: +49 3641 934481; fax: +49 3641 934482. E-mail address: [email protected] (O. Schaefer). 0300-5712/$ – see front matter # 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jdent.2014.01.016

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journal of dentistry 42 (2014) 677–683

Systems that belong to the second genre rather concentrate on image acquisition and recognition to build virtual copies of the natural dentition. Data are transferred to specialised dental laboratories or dedicated production centres, where restorations are designed (computer-aided design, CAD), further processed, and finally manufactured (computer-aided manufacturing, CAM) most commonly by subtractive technologies like computer-numeric-controlled milling (CNCmilling).1 Some digital impression systems allow the practitioner to occasionally choose from one of the aforementioned treatment approaches. However, both genres offer considerable benefits over conventional impressions, such as an increased patient comfort, time- and therefore cost-efficient treatment procedures, or the opportunity to easily review and adjust preparations in real-time.2 Another advantage is the access to industrially prefabricated materials with refined composition and microstructure.3 Succeeding leucite-reinforced ceramics, lithium disilicate materials offer improved physical characteristics, while also remaining suitable for CAM in a pre-sintered state. Mechanical properties include a modulus of elasticity of 91.0 GPa, a hardness of 5.5 GPa, and a biaxial-flexure-strength of 375 MPa.4 Moreover, these glass ceramics were able to withstand mouth-motion cyclic loading and showed no failures up to load levels of 1100–1200 N, which exceeded physiologic chewing forces.5 Due to a favourable translucency and shade-variability, monolithic restorations can be fabricated and subsequently characterised both chair- and labside. Based on these properties, lithium disilicate ceramics may be used to replace missing anterior and posterior teeth up to a second premolar as a pontic in fixed dental prostheses.3 Adequate marginal and internal adaption is considered a decisive factor for the clinical longevity of these restorations.6 Although threshold values can vary, gap sizes larger than 150 mm were reported to promote discoloration, exposure of luting resin, dissolution of cement, microleakage, plaque retention, secondary decay, and gingival inflammation.7,8 Optical- or scanning electron microscopy is usually used to evaluate marginal and internal discrepancies in vitro. Common sample sizes range from 5 to 10 specimens per group, with 2–150 different measuring locations, selected in a systematic or random manner. In this context, Groten et al. suggested that a minimum of 50 measurement locations along the margin of a crown yielded clinically relevant information and a consistent estimate for gap size.9 Furthermore, dental ceramics cannot withstand elastic deformation to the same extent as tooth structures or resinous materials. Stress concentrations depend on the geometry of the specimen material, loading conditions, the presence of

intrinsic or extrinsic flaws, and marginal and internal fit. However, resin-based luting agents have been shown to reduce, yet not completely absorb, the resulting shearforces.10 Although digital impressions possess clear advantages compared to their conventional analogues, the fitting-accuracy of resulting restorations remains questionable.11 Therefore, the objective of this in vitro study was to estimate the marginal and internal fit of lithium disilicate partial crowns fabricated using different digital impression techniques. The tested null hypothesis was that different intraoral scanning principles would not affect the marginal or internal fit of partial coverage crowns fabricated from lithium disilicate glass ceramics.

2.

Materials and methods

2.1.

Tooth preparation

An acrylic model of a mandibular left first molar (AG-3 ZE 36, Frasaco GmbH, Tettnang, Germany) was prepared to receive a partial crown restoration using a standard set of diamond burs (Set 4562, Brasseler GmbH, Lemgo, Germany). The preparation featured a 1.5 mm occlusal height reduction, a 1 mm rounded shoulder finish line of the buccal wall, and a 1 mm deep occlusal box. The 3 mm deep proximal grooves were finished with oscillating diamond tips (SONICflex prep ceram, KaVo Dental GmbH, Biberach, Germany) to achieve 908 margins as well as rounded and soft internal line angles. Prior to preparation, an initial impression was taken with a sectional tray system (Multi Tray, Kettenbach GmbH, Eschenburg, Germany) and a vinyl polysiloxane material (Panasil Putty, Kettenbach GmbH). To ensure standardised reductions, the impression was removed from the tray, sectioned vertically, and regularly set back in place to verify the progress made towards a predetermined preparation depth. Finally, the prepared cavity was evaluated digitally by a preparation assistant system (PREPassistant, KaVo Dental GmbH), originally developed for preclinical dental training.

2.2.

Impression taking and virtual cast fabrication

The prepared tooth was mounted in a typodont (AG-3, Frasaco GmbH) and scanned 5 times with 4 different digital impression systems (Table 1). The iTero (ITE) scanner uses a parallel confocal imaging technology in combination with a red laser beam to capture teeth and surrounding soft tissues. Reflected light is split and led through a focal filter allowing only the image within the focal point of the lens to project onto the sensor. In this case, the distance between the lens and the

Table 1 – Selected characteristics of the digital impression systems used in the present study, as provided by the respective manufacturer. Token ITE TRI CBC COS

System

Manufacturer

Measurement principle

Measurement type

Measurement prerequisites

iTero cara TRIOS CEREC AC with Bluecam Lava Chairside Oral Scanner

Cadent Heraeus Kulzer Sirona Dental Systems 3M ESPE

Confocal imaging Ultrafast optical sectioning Stripe-light projection Active wavefront sampling

Still images Video Still images Video

– – Powdering Powdering

journal of dentistry 42 (2014) 677–683

scanned surface equals the focal length of the lens itself. In order to digitise an entire object, the lens is moved up and down from one focal plane to another. According to the manufacturer the system can capture up to 100,000 laser points at 300 focal depths, each spaced approximately 50 mm apart.12 Little information is disclosed on the technical principle of the cara TRIOS (TRI) system. It employs a technology called ultrafast optical sectioning that is said to combine confocal microscopy with the projection of structured light. A time- and location depending illumination pattern is projected onto the teeth, while the lens system is moving from one focal plane to another.13 The CEREC AC with Bluecam (CBC) projects a continuous series of light stripes onto the teeth, which are in turn reflected back to a sensing unit, built into the scanning head. Thereby, the introduction of blue LED-cells with a shorter wavelength has increased the overall accuracy and the achievable level of detail.14 The distance between a projected and reflected lightray is measured. Since projector and sensor are mounted fixed at a predefined angle, depth can be calculated through Pythagoras’ theorem.12 However, the CBC system still required a complete coating with titanium dioxide particles, to prevent reflections of glossy surfaces that could otherwise lead to high measurement uncertainties.15 The Lava Chairside Oral Scanner (COS) uses a technology called active wavefront sampling to build three-dimensional datasets. Therefore, hard- and soft-tissue structures are illuminated by 192 blue LED-cells. The reflected light is led through a system of 22 lenses and is finally captured by three separate CCD sensors. Proprietary image recognition algorithms use the corresponding in-focus and out-of-focus information of the lens system to generate surface patches.16 The scanner connects single patches by combining overlapping image parts, which are acquired once the scanning head is moved from one tooth to another. In order to properly detect overlapping areas, small ‘‘optical connectors’’ need to by applied onto the surfaces in question. This is achieved by lightly dusting the latter with a titanium dioxide powder.15 A single dentist proficient with any of the systems performed the scans following the instructions of the respective manufacturer. ITE impressions were taken with respect to the instructions of the acquisition software (so called guided scanning). The protocol featured buccally and orally tilted views of the dental arch and additional preparation scans from different angles. Calibration was not necessary. The cara TRIOS scanner (TRI) required a specially designed scanning tip to be attached to the camera after the system finished preheating. Scanning started from an occlusal direction before lingual and buccal aspects of the preparation were captured. Calibration was performed with a calibration tip when requested by the acquisition software. Since the CBC-system required preliminary powdering, the typodont was completely coated with an aerosol spray of titanium dioxide particles (Optispray, Sirona Dental Systems GmbH, Bensheim, Germany). Digital impressions were taken according to a scanning protocol recently described by Ender and co-workers. It included occlusal preparation views together with scans from an approximately 308 buccally and

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orally angulated direction.17 Additional calibration was not necessary. The applied powdering layer and any residues were removed under 2.7 magnification (StarVision SV 1, StarMed, Munich, Germany) using a soft brush. Impressions with the COS system required a light dusting with titanium dioxide powder (Lava Powder, 3M ESPE, Seefeld, Germany). Digital impressions were taken following a slow zig-zag path while moving the camera from one tooth to another. Calibration had to be performed both before and after each scan, to compensate for errors that might have occurred in between. One and the same tooth was used throughout the entire study. Since the application of a powdering-layer might negatively influence the digitalisation results, the following scan order was adapted:

Reference scanner ITE TRI Light dusting, COS, removal of powder with soft brush and air (e) Powdering, CBC, removal of powder with soft brush and air

(a) (b) (c) (d)

2.3.

Partial crown fabrication

Every digital impression was saved in a proprietary file format, compatible with dental design software (Dental Designer 2012, 3Shape, Copenhagen, Denmark). These files describe the surface geometry of an object by triangles in a threedimensional Cartesian coordinate system. Moreover, a unit normal is assigned to determine the orientation of the triangulated surface (the direction from which to object is looked at). The generation of virtual working casts can be performed instantly by inverting the normal vector, which means flipping the direction from which the impression is looked at but maintaining the exact spatial information as captured by the scanning devices. Preparation margins were selected and restorations were computed from a database of anatomical crown shapes. All calculations were performed with respect to the minimum material thicknesses required for veneer crowns and with respect to the respective parameters for lithium disilicate restorations as provided by the manufacturer (vertical and horizontal cement gap: 60 mm, cervical cement gap: 20 mm, distance to margin: 1.2 mm, margin thickness: 0.2 mm, offset angle: 658, drill radius: 0.605 mm). The designed datasets were then used to operate a five-axis milling-unit (Ultrasonic 20 linear, Sauer GmbH, Stipshausen, Germany) that machined ceramic partial crowns from lithium disilicate blanks (IPS e.max CAD, Ivoclar Vivadent AG, Schaan, Liechtenstein). The conventional rotation of the employed grinding tools was superimposed by an additional oscillation movement with a predefined vibration amplitude (so called kinematic oscillation). Thereby, low process forces could be realised, that enabled the fabrication of thin-walled structures and significantly reduced surface and subsurface damage accumulation.7 The milling unit featured a special mounting mechanism that was able to pick up and fix the metal mandrel bonded to the lithium disilicate blank before milling.

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2.4.

journal of dentistry 42 (2014) 677–683

Digitalisation and evaluation

The prepared reference tooth and the fabricated ceramic crowns were digitised using a self-calibrating structure-light scanner (Flex 3A, Otto Vision Technology GmbH, Jena, Germany). The instrument captures multiple images of the object in question and employs triangulation routines to calculate data points in a common three-dimensional coordinate system. Thereby, a homogenous point-to-point distance (point spacing) of 5 mm with a noise level 5 mm is achieved. STL datasets as the primary output format of the reference scanner were computed for each restoration and the reference tooth as well. Virtual crowns were superimposed upon the prepared tooth by computing all possible orientations (Qualify 12, Geomagic GmbH, Stuttgart, Germany) and selecting the one with the best object-to-object penetration.18 The mean quadratic deviation (root mean square, RMS) between the virtual preparation surface and the corresponding surface of the partial crowns was used as an estimate of the congruency of two superimposed records. Measurements were performed along the preparation finish line (marginal fit) and covering the entire prepared surface of the reference tooth (internal fit). Colour-coded difference images were used to analyse differences semi-quantitatively.

2.5.

Statistical analysis

All statistical computations were made using the IBM SPSS Statistics 19 software package (IBM SPSS Inc., Chicago, IL, USA). Means (RMS), standard deviations, and 95%-confidence intervals were calculated for marginal and internal discrepancies. Saphiro-Wilk’s test and Levene’s test were performed to verify departures from basic assumptions about variance and normality. A one-way analysis of variance (ANOVA) was conducted to assess the overall statistical significance of differences among digital impression techniques. Scheffe´’s multiple comparisons were used to test differences between groups. Furthermore, two separate contrast tests were employed to segregate the impression systems in accordance with their basic measurement type (video versus still image capturing) as well as possible measurement prerequisites (powder-based versus powder-free systems). Statistical significance was accepted at p < 0.05. A power analysis was conducted to estimate the required sample size. Assuming four test groups, an effect size of 1.25, and type I and type II error probabilities of 0.05 and 0.95, respectively, 20 specimens (5 per group) were required.

3.

Results

3.1.

Quantitative analysis

Means, standard deviations and 95%-confidence intervals of marginal and internal discrepancy measurements are listed in Table 2. The applied single-factor ANOVA showed statistically significant differences among the digital impression systems (marginal discrepancy: pmarg  0.001, internal discrepancy: pint = 0.039). With Shapiro–Wilk’s test ( pmarg = 0.319, pint = 0.124)

Table 2 – RMS-results of marginal and internal fit measurements. Means, standard deviations (SD), and 95% confidence intervals [95%-CI] from n = 5 are given in mm. Different superscript letters (a, b, c) within each column indicate statistically significant differences at p = 0.05. Marginal

ITE TRI CBC COS

Internal

Mean

(SD)

[95%-CI]

Mean

(SD)

[95%-CI]

90 a 128b,c 146 b 109a,c

(14) (9) (17) (11)

[71–108] [117–139] [125–167] [96–123]

92a,b 106 a 84 b 92a,b

(9) (7) (16) (10)

[81–104] [117–139] [63–104] [80–105]

and Levene’s test ( pmarg = 0.319, pint = 0.124) being non-significant, normality and equality assumptions were met. ITE and TRI impressions required no preliminary powdering or dusting and performed significantly better ( pmarg = 0.006, pint = 0.04) than COS and CBC scans, that needed such coating procedures. However, no statistically significant differences were detectable ( pmarg = 0.882, pint = 0.741) among systems that captured video frames (COS, TRI) and systems that recorded still images (ITE, CBC).

3.2.

Qualitative analysis

Colour-coded difference images (Fig. 1) allowed for semiqualitative information analyses. ITE impressions led to both under- and oversized reproductions of the original surface. Favourable outcomes (RMS range: 50 to 0 mm) can be seen as small green coloured portions of the buccal wall as well as the central occlusal box. The preparation margin showed yellow to light brown areas, which indicate a substantial over contouring (RMS range: 0 to +100 mm). Blue coloured inner aspects (RMS range: 100 to 50 mm) reflect undersized dimensions, especially at buccal and oral cusp tips. Restorations fabricated from TRI and CBC scans showed a comparably distributed deviation pattern. Over contoured dimensions were recorded at approximo-lingual margins, while buccal and central approximal parts of the preparation finishline were accurately reproduced. Internal surfaces were substantially undersized in general. Thereby, highest deviations were again detected at mesio- and distobuccal cusp tips (RMS range: 100 to 50 mm). LAV impressions resulted in oversized dimensions at lingual parts of the preparation finish line (RMS range: 0 to +50 mm). Though occasional high-spots were recorded, central parts of the occlusal box were accurately reproduced. By contrast, spot-shaped areas around the buccal cusp tips, the occluso-approximal isthmus and the lingual groove were highly under contoured (RMS range: 150 to 100 mm).

4.

Discussion

The present study aimed to measure and compare the fittingaccuracy of partial coverage restorations manufactured following four different digital impression procedures. The stated null hypothesis was rejected, as statistically significant

journal of dentistry 42 (2014) 677–683

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Fig. 1 – Colour-coded difference images for qualitative deviation analysis of internal (upper row) and marginal surfaces (lower row). Sample images that contribute best to the authors’ findings were selected for each impression technique. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.) differences in marginal and internal fit emerged among the examined systems. Brawek et al. evaluated the marginal and internal fit of zirconia crowns fabricated from COS and CBC scans. Marginal discrepancies were reported to range from 51 to 83 mm, depending on both digitalisation- and veneering-techniques. In accordance with the findings of this study, the application of the COS system resulted in significantly lower marginal misfit when compared to CBC scans. Considering internal fit, COS and CBC impressions performed comparably well, yielding no statistically significant differences.19 Seelbach et al. compared the accessible marginal and internal openings of single-unit restorations following CBC, COS and ITE impressions. Supporting the results of the present investigation, no statistically significant differences were detected between COS and ITE neither at marginal nor internal surfaces.20 However, the CBC system resulted in significantly higher internal misfit, which is contrary to the findings presented in this paper. One explanation might be the three-dimensional nature of the analysis protocol. Classical methods often focus on a small number of single internal measuring locations that are prone to variations. By contrast, restorations in this study were digitised with a mean number of approximately 1.7 million data points per specimen, thus permitting comprehensive and precise analyses. The employed triangulation routines resulted in inaccuracies during digital capturing and feature system-related variations of

Impact of digital impression techniques on the adaption of ceramic partial crowns in vitro.

To investigate the effects, digital impression procedures can have on the three-dimensional fit of ceramic partial crowns in vitro...
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