CAD/CAM Ceramics in Dentistry: An Inside View

Headnote

Abstract

In the last 2 decades, exciting new developments in dental materials and computer technology have led to the success of contemporary dental computer-aided design/computer aided manufacturing. Several highly sophisticated chair side and laboratory CAD/CAM systems have facilitated the use of superior dental ceramics. CAD/CAM technology has led to improved ceramic processing with increased micro structural uniformity high density, less porosity and overall has increased clinical predictability of restoration. This article provides an overview of the various CAD/CAM systems and describes their components.

Keywords: Zirconia, Cercon, Lava, CAD/CAM.

Introduction

During the past 20 years, several all-ceramic crown systems have been introduced to dentistry. The use of high-strength materials like alumina and zirconia-based ceramics which can be shaped only by CAD/CAM systems, have increased the life and demand for CAD/ CAM restorations. It was a long way from the first attempt with all ceramic restorations to today's modern systems that led to different material and processing techniques. The idea of using CAD/CAM techniques for the fabrication of tooth restoration originated with Duret in the 1970.1 10 years later Mormann developed the CEREC system, the first chair side restorations. Since then, technology has evolved in 2 directions; first is intra-operatory application for one appointment restoration using the pre-fabricated ceramic monoblocks. In parallel, CAD/CAM systems for commercial production centers and dental laboratory emerged, expanding the range of materials.2 This article, presents an overview of current CAD/CAM systems describing components of CAD/CAM system.

The all-ceramic CAD/CAM restorations have long been the center of the profession's ardent quest for the consummate esthetic restoration. Over the years the optimal esthetics of the all ceramic crowns have been acknowledged, but its tendency to fracture over time rendered it an alternative restoration for specifically selected cases when compared to work-shoe of porcelain-fused to metal restorations. Simply using all-ceramic will not ensure predictable esthetic success. Precise attention to details regarding tooth preparation, cervical margin design and location, softtissue management are crucial for the success of the restoration.

Clinical guidelines

Appropriate measures should be taken while selecting the patient for all-ceramic restoration, in addition to detailed oral-examination as part of diagnosis. To facilitate patient selection for all-ceramic FPD's, there must be adequate prospective interocclusal space needed for the framework and veneering material. A4mm of clinical crown measured with periodontal probe from the interproximal papilla to the marginal ridge of the abutment indicates adequate connector height.

The concentration of heavy stresses in the connector area increases the risk of fracture. Therefore, it is mandatory to evaluate the abutment periodontal health with an emphasis on mobility. Abutments with increased mobility should not be selected for all-ceramic crowns. Finally, heavy bruxers who exhibit parafunctional activity should not receive all ceramic crowns.

Contraindications:

i) As cantilever pontic.

ii) In Class II div2 malocclusion patients, due to deep bite when there is insufficient space for labio-lingual connector.

iii) Mesial-tilting of abutment tooth with supraerupted teeth, which cannot be corrected with minimal enameloplasty

iv) Very short clinical crown that does not permit height forthe connector.3

CAD/CAM Components

All CAD/CAM systems have 3 functional components: data capture or scanning to capture and record data about the oral environment (tooth preparation, adjacent teeth and occluding tooth geometry); CAD to design the restoration to fit the preparation and to perform according to conventional dental requirements; and CAM to fabricate the restoration.

Scanner is a data collection tool that measures 3-dimensional jaw and tooth structures and transforms them in to digital data. Basically there are 2 different scanning possibilities:4

1. Optical scanners: The basis of this type of scanner is a collection of 3 -dimensional structures in a so-called triang-ulation procedure. Triangulation based scanners project a laser beam on an object at a given angle and the reflected beam then travels at a different angle through a lens. Through this angle the computer can calculate a 3-dimensional data from the image on the receptor unit. Unfortunately having separate pathways seriously limits measurements of steep angulations or deep cavities due to beam obscuration; therefore triangulation may miss important information from the walls and bottom surface of an object. Triangulation scanners can scan up to 40°-60°.5 Examples: Lava, Everest.

2. Mechanical scanners: In this scanner, the master cast is read mechanically line by line by means of a ruby ball and the 3-dimensional structure is measured. The scanner has a high scanning accuracy, where the diameter of the ruby is set to the smallest grinder in the milling system. The drawbacks of this system are long processing time and high expenditure compared to optical scanners. Example: Proc-era.

Processing devices: The construction data produced with CAD software are converted into milling strips for the CAM processing and loaded on milling device. Processing devices are distinguished by means of number of milling axis:4

3 axis: The milling machine has degree of movement in 3 spatial directions X, Y and Z axes. A 3-axis will allow the stock material or the cutter to move in the three axes. The stock material is held in a fixture that moves leftand right (first axis) and front to back (second axis), with a spindle that moves up and down (third axis). A three-axis mill essentially cuts from the top and bottom of the restoration, parallel to the path of insertion. It is not capable of milling undercuts. The advantages of these milling devices are short milling time and less cost. Examples: Inlab (Sirona), Lava (3M ESPE), Cercon (DeguDent).

4 axis: In this, in addition to 3 axis, stock material can also be turned infinitely variably. Example: Zeno.

5 axis: In 5-axis configuration, the milling is capable of constantly re-orienting the stock material or the spindle, offthe path of insertion while simultaneously cutting and moving in the other 3 axes. Because 5-axis milling can cut from the side, it can mill undercuts and parts can be tilted offthe path of insertion to allow the use of shorter stock material. Example: Everest (KaVo).

Processing can take place in different density stages. There are 2 types of zirconia blocks available for CAD/CAM applications. There is fully sintered or Hot isostatic pressing (HIP). HIP type of sintering utilizes high temperature and pressure to increase density of the material. Examples are Everest and DC Zirkon. The advantage of this is superior fit as there is no shrinkage, but has disadvantage of inferior machinability associated with wear of the tool. HIP zirconia has less strength as surface defects are introduced during grinding.

The second is partially sintered zirconia. Blanks of this material are manufactured by spraying dried zirconia powder that is then isostatically pressed and incompletely sintered. After milling, dried zirconia is then sintered completely in a furnace at 1,350°-1,500°C to achieve its final shape.6

It has advantage of easy machinability without wear on the tools or chipping of the materials. However, because of the extensive sintering shrinkage during post-sintering, shrinkage of 20-25% has to be compensated by increasing framework size to attain good marginal fit. The various types of commercially available zirconia, their advantages and milling procedures for each type are mentioned in Table 1.

CAM uses computer generated paths to shape a part

A diverse set of technologies has been used to create dental restorations. Early systems relied almost exclusively on cutting the restoration from pre-fabricated block with the use of burs, diamonds or diamond disks. This approach in which material is removed to create the desired shape is termed as subtractive method. This method creates complete shapes effectively but at the expense of material being wasted. As an alternative additive CAM approaches like those used in rapid prototyping technology are beginning to be used in dental restorations. Selective laser sintering is one of the technologies, in which the system sinters material along the path, building a part from bath of ceramic or metal powder and adding material until the complex is complete. One of the systems uses stereolithography, another additive process to produce 3D components from acrylics and occlusal splints.2

Cercon smart ceramic system: This system utilizes conventional waxing method for designing infrastructure for crowns and bridges with specific thickness. A special laser scanner scans the wax pattern and the data are transferred to the computer aided manufacturing unit. This data is then utilized for milling the framework from partially sintered Y-TZP blanks. Because of the strength and reliability of the core material, zirconia, coping with thickness of 0.3mm is sufficient. Connector sizes of 9-11mm2 are sufficient. The laboratory duplicates the preparation and the technician designs the restoration in wax. The wax model is placed in the cercon brain scanning. Non-contact scanning of the wax model is done and the generation of the corresponding data record occurs, following which the pattern is machined out of cercon base blank of enlarged size. The enlargement factor compensates for the sintering shrinkage and is contained in a bar code on each blank. The cercon base will bridge a lot of gaps. 6 or 7 units in the anterior or posterior region should pose no problems even with extensive dental arches. The blanks are available in 12, 30, 38 and 47mm in anatomical length.8 The shades of cercon framework available are neutral white and a natural shade.

In a comparative in vitro study, the marginal fitness of TZP all-ceramic crowns and fixed partial dentures (FPDs) were examined. The marginal discrepancies of the FPDs (29.3 urn) were not significantly different from those of the crowns (31.3um). The results of this study suggest that: 1) The marginal fitness of all-ceramic restorations fabricated by the Cercon system was satisfactory for clinical use and 2) The dimensional stability of the Cercon substructure was maintained during firing of the porcelain and successive glazing.9

Procera: To capitalize on the strength of alumina, the early 1900 saw the introduction of pure, densely sintered alumina core called Procera. A Procera coping is a suitable substructure for both anterior and posterior prostheses. The system uses an innovative concept for generating alumina and zirconia copings. 2 types of scanners are available-Procera Piccolo for single-unit restorations and Procera Forte for single and multiple-unit restorations. First, the scanning stylus is used to create 3D images of the master dies, which are sent to the processing center. The processing center then generates enlarged dies designed to compensate for the shrinkage of the ceramic material.10 Copings are manufactured by dry pressing high-purity alumina powder (>99.9%) against the enlarged dies. These densely packed copings are milled to the desired thickness. Subsequently, the copings are sintered at 2,000°C to impart maximum density and strength to the milled copings.

The system offers a choice of coping thickness (0.25, 0.4, 0.6mm), opacity (white or standard) and material (alumina or zirconia). A major advantage of a Procera coping is its ability to mask underlying tooth discoloration. Procera is classified as a high strength ceramic with a flexural strength 700MPa, sufficient to support the weaker veneering porcelain. Alumina and zirconia restorations may be cemented with either conventional methods or adhesive bonding techniques. Conventional conditioning required by leucite ceramics (eg., hydrofluoric acid etch) is not needed. Microetching with aluminum oxide particles on cementation surfaces removes contamination and promotes retention for pure aluminum oxide ceramic." In-vitro studies recommended that a resin composite containing an adhesive phosphate monomer in combination with a silane coupling/bonding agent can achieve superior long-term shear bond strength to the intaglio surface of Procera AllCeram and Procera AllZirkon restorations.12,13 According to recent data, the average marginal gap for Procera ranges from 54-64µm.14 A 5 year clinical success rate of 93.5% has been reported for the Procera crown and the major complications include fracture of veneering and coping.15

LAVA: A die is scanned by contact free optical scanner. The CAD software designs an enlarged framework that is milled from softer pre-sintered blanks. The oversized framework is then fully sintered by post-sintering. The flexural strength of lava is >100MPa and fracture toughness of 5-10MPa enable a lower framework wall thickness than other all-ceramic. Generally the coping thickness is not less than 0.5mm. There are following exceptions:

- Anterior copings >0.3mm, but not in cases of bruxism.

- Abutment tooth coping to the cantilever bridge unit for posteriorteeth>0.7mm.

The connector height is 7-12mm. The 7 shades in lava are obtained by immersing the pre-sintered blanks in shading liquid containing different coloring ion. At pre-sintered stage when it is still porous, the ions diffuses in to the zirconia material and gets incorporated in the structure during the final sintering step.

Hertlein et al., investigated the marginal fit of the Lava All Ceramic System for anterior and posterior teeth with a chamfered preparation margin under a stereomicroscope and the marginal adaptation was reported to vary between 40umand70um.16

Conclusion

CAD/CAM systems have dramatically enhanced dentistry by providing high quality restorations. The evolution of current systems and the introduction of new systems demonstrate increasing user friendliness, expanded capabilities, and improved quality and range in complexity and application. New material also are more esthetic, wear more nearly like enamel, and are strong enough for full crown and bridges. Dental CAD/CAM technology is successful todaybecauseofthevisionofmanygreatpioneers.

References

References

1. Duret F, Blouin JL, Duret B. CAD-CAM in dentistry. J AM Dent Assoc 1988;117;715-202.

2. Strub JR, Rekow ED, Witkowski S. Computer-aided design and fabrication of dental restorations: Current systems and future possibilities. JAmDentAssoc 2006;137:1289-96

3. Pilathadka S, Vahalova D. The Zirconia: A new dental ceramic: An overview. Prague Med Report 2007;108:5-12.

4. Beur F, Schweiger J, EdelhoffD. Digital dentistry: an overview of recent development for CAD/CAM generated restorations. Br Dent J 2008;208:505-11.

5. Geiselhoringer H, Hoist S. The new nobel procera system for clinical success: the next level of CAD/CAM dentistry. Cosmetic dentistry 2009;2:26-32.

6. Parker RM. Use of zirconia as restorative material. Dent Today 2007;26:114-17.

7. Cavalcanti AN, Foxton RM, Watson TF, Oliveira MT, Giannini M, Marchi GM. Y-TZP ceramics: key concepts in clinical application. Oper Dent 2009;34:344-51.

8. Trushkowsky R. A transformation-toughened polycryst- a lline ceramic posterior inlay/onlay fixed partial denture. Inside Dentistry 2008;5:1-5.

9. Liu RP A panorama of dental CAD/CAM restorative systems. Compend Contin Edu Dent 2005 ;26:507-12.

10. Andersson M, Oden A. A new all-ceramic crown: a dense-sintered, high purity alumina coping with porcelain. Acta OdontScand 1993;51:59-64.

11. Awliya W, Oden A, Yaman P, Dennison IB, Razzoog ME. Shear bond strength of a resin cement to densely sintered high-purity alumina with various surface conditions. Acta OdontolScandl998;56(l):9-13.

12. Blatz MB, Sadan A, Blatz U. The effect of silica coating on the resin bond to the intaglio surface of Procera All Ceramic restorations. Quintessence Int2003;34 (7):542-47.

13. Blatz MB,SadanA,MartinI,LangB.Invitro evaluation of shear bond strengths of resin to densely-sintered high-purity zirconium-oxide ceramic after long-term storage and thermal cycling. I Prosthet Dent 2004;91(4):356-62

14. May KB, Russell MM, Razzoog ME, Lang BR. Precision fit: The procera all ceramic crown. I Prosthet Dent 1998; 80:394-04.

15. Odman P, Anderson B. Procera all ceram crowns followed for 5-10.5years:Aprospectiveclinical study, hit I Prosthodont 2001;14:504-09.

16. Hertlein G, Hoscheler S, Frank S, Suttor D. Marginal fit of CAD/CAM manufactured all ceramic prosthesis. I Dent Res 2001;80:42-44