journal of dentistry 42 (2014) 1151–1155
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/jden
A 3 years retrospective study of survival for zirconia-based single crowns fabricated from intraoral digital impressions Enrico Gherlone a, Federico Mandelli a,*, Paolo Cappare` a, Giuseppe Pantaleo c,a, Tonino Traini a,b, Francesco Ferrini a a
Department of Dentistry, Vita Salute University, San Raffaele Hospital, Milan, Italy Department of Medical, Oral and Biotechnological Sciences, University of Chieti-Pescara, Chieti, Italy c Faculty of Psychology, San Raffaele University of Milan, Italy b
article info
abstract
Article history:
Objective: To evaluate the clinical performance of glass-ceramic/zirconia crowns fabricated
Received 20 February 2014
using intraoral digital impressions – a retrospective study with a three-year follow-up.
Received in revised form
Methods: 70 consecutive patients with a total of 86 glass-ceramic/zirconia crowns were
4 June 2014
treated by a single clinician using standardized clinical and laboratory protocols. A complete
Accepted 5 June 2014
digital workflow was adopted for the purpose except for the veneering procedure for the glass-ceramic crowns. Occlusal adjustments were made before the ceramic glazing procedure. Before cementation, all abutments where carefully cleaned with a 70% alcoholic
Keywords:
solution and air dried. Cementation was performed using dual-curing, self-adhesive resin
Zirconia crown
cement. Patients were re-examined after 12, 24 and 36 months, to assess crown chipping/
CAD/CAM
fractures.
Chipping
Results: After the three-year follow-up, none of the zirconia-based restoration was lost
Fracture
(‘‘apparent’’ survival rate 100%) otherwise, the chipping rate of the veneering material
Veneer
increased from 9.3% after 12 months, to 14% after 24 months to 30.2% after 36 months. As a consequence, the ‘‘real’’ success rate after 3 years was 69.8%. Conclusions: After 3 years the success rate of zirconia-based crowns was 69.8%, while the incidence of the chipping was 30.2%. Assuming an exponential increase in chipping rate between 12 and 36 months it can be argued that, among others, the fatigue-mechanism could be advocated as the main factor for the failure of glass-ceramic veneered zirconia especially after 24 months. # 2014 Elsevier Ltd. All rights reserved.
1.
Introduction
To achieve better aesthetic results and biological integration with soft tissues, clinicians have been evaluating the feasibility of all ceramic crowns since 19651; at the beginning
the major problem was a low mechanical stability. Years later, better results were achieved by introducing new materials, such as glass-infiltered alumina2 but despite the improvements achieved, failure rates remained high when these materials were used for posterior crowns. More recently, the introduction of zirconia seemed to offer a valid answer to the
* Corresponding author at: Department of Dentistry, via Olgettina, 58, 20132 Milan, Italy. Tel.: +39 02 26432970. E-mail address: [email protected] (F. Mandelli). http://dx.doi.org/10.1016/j.jdent.2014.06.002 0300-5712/# 2014 Elsevier Ltd. All rights reserved.
1152
journal of dentistry 42 (2014) 1151–1155
Table 1 – Distribution of the crowns divided by teeth group.
mechanical needs of the posterior regions. It has been documented3 that the mechanical properties of zirconia are the highest ever reported for any dental ceramic with fracture strength values of 1240 MPa; this material was also used to fabricate a solid fixed dental prosthesis framework, with very high aesthetic results.4 Tetragonal zirconia polycrystals (3YTZP) is the one mainly adopted by manufacturers (Cercon1 – Dentsply International, LavaTM – 3MTM ESPETM, Procera1 zirconia – Nobel BiocareTM, YZ cubes for Cerec InLab1 – VidentTM and IPS e.max1 ZirCAD – Ivoclar Vivadent3). The two biggest clinical concerns using glass-ceramic on zirconia framework are the chipping of glass-ceramic veneering material, and the degradation (ageing) of zirconia exposed to the oral environment that could lead to long-term failures.5–9 There is evidence that designing frameworks with an anatomical profile decreases the failure incidence of the crowns.10 Manufacturing zirconia-core crowns implies a computeraided design/computer-aided manufacturing (CAD/CAM) process: the workflow starts with a conventional oral impression from which a stone cast is produced and then digitalized to a virtual 3D model. After these steps the CAD/CAM process proceeds till the planned framework/crown is ready to be shipped back to the dental office. Accuracy of the final product depends on the sum of the possible errors from the start to the end of the workflow. Conventional impressions can deliver high quality results in terms of stability and precision9 but problems such as undefined reproduction of the preparation margins, tearing of the impression material, presence of debris, lack of material within key areas and undefined margins on the casts could be present.11,12 Clinical results in daily practice need further improvements.13 Digital oral impression was introduced in order to eliminate the aforementioned sources of error: digital impression should permit a total digitization of the workflow from the mouth of the patient to the final restoration. Studies on the accuracy have been published14,15 but data is still inconsistent. Furthermore, this relatively new technology is evolving rapidly and, at the time of writing, third generation oral scanners are already available on the market, with improved clinical performances. The LAVATM Chairside Oral Scanner, (3M ESPE, Seefeld, Germany) is one of the various models present on the market and it is based on the principle of active wave front sampling.16 Three sensors capture images from different perspectives; 3D surface patches are generated in real time by means of proprietary image processing algorithms using in-focus and out-of-focus information. Published articles have confirmed its reliability both in vitro17,18 and in vivo.19 The aims of this longitudinal retrospective clinical study were to evaluate the success and the failure rates of zirconiabased crowns fabricated by a complete digital workflow with a three-year follow up.
treated with 86 crowns; one patient received three crowns, fourteen received two and all the rest one. The position of the crowns is summarized in Table 1. Tooth abutments were prepared by FF with knife-edge finish line20 and temporarily restored with composite-resin interim crowns during the same session. After eight weeks of tissue conditioning, the patient came for the definitive impression. Two displacement cords (Ultrapack; Ultradent Products) of different sizes, chosen individually for each patient, were gently placed into the gingival sulcus; the cord of narrower diameter was placed apically with respect to the wider diameter. Mouth was rinsed with water and air-dried. A dedicated powder for optical scanning (LavaTM Powder for Chair-side Oral Scanner, 3M ESPE) was used to apply a thin layer of dust on the teeth as recommended by the manufacturer. The wider cord was removed and the scan performed immediately afterwards. The quality of the digital impression, magnified compared to the natural size, was then evaluated on the computer screen. Abutment had to be clearly visible beyond gingival margin, without powder excesses and possible elements of distortion like bubbles or undefined areas; adjacent teeth were evaluated as well. If any problem was found, the scanning process was repeated. The workflow required a second scan for the opposite arch and a third scan for bite registration (less time-consuming). After the digital scan was accepted, all the frameworks were designed and milled by the same centre, with the same technology (LavaTM, 3M ESPE). The veneering process was made using the same glass-ceramic material (Willi Geller/Creation ZI-F) and performed by the same dental technician. The occlusal adjustments were made by means of a diamond bur before the ceramic glazing procedure. Before cementation, all abutments where carefully cleaned with a 70% alcoholic solution and air-dried. Cementation was performed using dual-polymerizing, self-adhesive resin cement (RelyxTM Unicem – 3M ESPE). A specific data-form was used to record: gender, age and position of the crowns as well as the day of the restoration delivery. Patients were re-evaluated after 12, 24 and 36 months, adding data regarding crown chipping/fractures (yes/no). Along with fractures, in this study chipping is also considered a failure because it means the crown has lost its integrity.
2.
3.
Materials and methods
Seventy patients, 50 males and 36 females with a mean age of 45.9 years (SD = 11.6) in need of restorations for single crowns, were treated by the same experienced operator (FF) using the same clinical and laboratory protocol. The patients were
Region Incisor Canine Premolar Molar Total
Maxilla
Mandible
Total
9 4 19 24 56
0 0 8 22 30
9 4 27 46 86
Statistical analysis
The time-dependent survival analysis of the crowns was calculated with the Kaplan–Meier model21 using a statistical package software IBM SPSS Statistics for Windows, Version 21.0. (IBM Corp. Armonk, NY; USA).
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journal of dentistry 42 (2014) 1151–1155
Table 2 – Survival table of the 86 placed crowns. Time (months)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
4.
12 12 12 12 12 12 12 12 24 24 24 24 36 36 36 36 36 36 36 36 36 36 36 36 36 36
Status
Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured
Cumulative proportion surviving at the time Estimate
SE
– – – – – – – 0.907 – – – 0.860 – – – – – – – – – – – – – 0.698
– – – – – – – 0.031 – – – 0.037 – – – – – – – – – – – – – 0.050
Results
After the three-year follow up, 60 crowns were free of complication, corresponding to a 69.8% success rate (Table 2).
No. of remaining cases
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
85 84 83 82 81 80 79 78 77 76 75 74 73 72 71 70 69 68 67 66 65 64 63 62 61 60
The chipping rate was 9.3% after 12 months 14% after 24 months and 30.2% after 36 months (Fig. 1) with an exponential increase between 24 and 36 months (Fig. 2). No correlation between crown fractures and patient gender or tooth position was found.
5.
Fig. 1 – Survival curve of the 86 placed crowns at 36 months of follow-up.
No. of cumulative events
Discussion
Stabilized zirconia is one of the latest restoration materials introduced in clinical practice. Its key feature is a flexural strength higher of twice compared to other ceramics such as alumina. This strength derives from a stress-induced transformation from the metastable tetragonal form to the stable monoclinic form (t ! m). This transformation was associated with an increase in volume of the grain, thereby closing the crack tip and preventing further crack propagation.22 Notwithstanding the aforementioned mechanical properties in clinical practice, the most common complication observed in zirconia-based restorations is the chipping and fractures of the veneering ceramic with or without exposing the underlying Y-TZP framework.22 To our knowledge, this is the first report evaluating the survival rate of zirconia-based crowns manufactured with a complete digital workflow. The results of the present study, over a 3-year follow-up period, demonstrated a success rate of 68.6%. The chipping rate was 9.3% after 12 months 14% after 24 months and 30.2% after 36 months, with an exponential increase between 24 and
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journal of dentistry 42 (2014) 1151–1155
Fig. 2 – Cumulative chipping rate of the crowns. Note the ‘‘exponential-like’’ aspect of the red trend line. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)
36 months (Fig. 2). By contrast in a 3-year follow-up study Ortorp23 reported a chipping/fractures rate of 2% for single crowns made using conventional impressions; nevertheless, after two further years of follow-up, the same author19 reported an increase in the chipping/fractures rate of up to 10.2%.24 The lack of criteria used to define the chipping events can be advocated to explain the differences with the present results.25 Another study on single zirconia-based crowns reported one framework fracture and no chipping after a 2year follow-up.26 Nevertheless, the power of the study was very low due to limited number of crowns studied (n = 15). AlAmleh27 in a systematic review on zirconia-based fixed partial dentures (FPD) reported a chipping or fracture rate of the veneering material ranging from 0% to 51%. Heintze28 in a systematic review comparing zirconia FPD and metal supported FPD reported a chipping rate of 24% for zirconia prostheses. Recently, Raigrodski7 underlined that chipping was one of the most frequent complications observed but could not perform a statistical comparison between the included articles because, among them, ‘‘There was no consensual guideline for quantifying chipping of the veneering porcelain’’. Glass ceramics are brittle materials because of atomic bonds that do not allow the atomic planes to slide apart when subjected to load. Thus, ceramics cannot withstand deformation of >0.1% without fracturing. On the other hand, due to the monolithic structure (single phase) the glass ceramics have very low fracture toughness with a tendency to coalesce onto both the surface and subsurface micro cracks. Because they do not yield, monolithic ceramics behave as brittle materials and a propagating crack need only to overcome the surface energy of the material. As result of the occlusal function, the formation of microscopic flaws and micro cracks can lead to the failure of dental ceramics by means of accumulating damages. However, in glass ceramics, toughness and initial strength are not the only factors that determine long-term survival:
subcritical crack growth (SCG) significantly decreases survival time of glass ceramics.29 SCG is defined as a crack propagation at stress intensity factor (KI) levels lower than the critical stress intensity factor, or fracture toughness (KC).30–32 Occlusal load could be considered a long-term or repetitive low-level loading that may cause pre-existing subcritical crack flaws to grow until failure occurs. Moreover, the SCG is affected by several other factors which act as additional stressors.33 Embrittlement effects due to mechanical finishing and grinding procedures have been reported.34,35 Nonetheless, the presences of Griffith defects in glass ceramics are critical for SCG.36 A less documented factor influencing the SCG is the misfit of coefficient of thermal expansion between zirconia core and glass ceramic veneer that leads to failure due to a residual stress.37 The amount of the residual stress in the surface of the glass ceramic veneering appears to be increased by cooling rate.38 The mechanisms responsible for loss of strength in dental glass ceramics appear to be governed by combined factors. Meanwhile, the stress corrosion by water molecules at the crack tip and mechanical degradation of the material permits crack propagation at stress levels in some cases to less than 50% of the initial material strength.39 Analyzing the data here reported we assume a fatigue-mechanism fracture for chipping of glass veneered zirconia that appeared to be exacerbated in time reaching a critical point at 24 months. Further research is necessary to clarify the relationship between the ageing process and clinical failure.
Conflict of interest The authors declare no conflicts of interest.
references
1. Pjetursson BE, Sailer I, Zwahlen M, Hammerle CH. A systematic review of the survival and complication rates of all-ceramic and metal-ceramic reconstructions after an observation period of at least 3 years. Part I: Single crowns. Clinical Oral Implants Research 2007;18:73–85. 2. Magne P, Belser U. Esthetic improvements and in vitro testing of In-Ceram Alumina and Spinell ceramic. International Journal of Prosthodontics 1997;10:459–66. 3. Denry I, Kelly JR. State of the art of zirconia for dental applications. Dental Materials 2008;24:299–307. 4. Hannink RHJ, Kelly PM, Muddle BC. Transformation toughening in zirconia-containing ceramics. Journal of the American Ceramic Society 2000;83:461–87. 5. Taskonak B, Borges GA, Mecholsky Jr JJ, Anusavice KJ, Moore BK, Yan J. The effects of viscoelastic parameters on residual stress development in a zirconia/glass bilayer dental ceramic. Dental Materials 2008;24:1149–55. 6. Swain MV. Unstable cracking (chipping) of veneering porcelain on all-ceramic dental crowns and fixed partial dentures. Acta Biomaterialia 2009;5:1668–77. 7. Raigrodski AJ, Hillstead MB, Meng GK, Chung KH. Survival and complications of zirconia-based fixed dental
journal of dentistry 42 (2014) 1151–1155
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prostheses: a systematic review. Journal of Prosthetic Dentistry 2012;107:170–7. Preis V, Behr M, Hahnel S, Handel G, Rosentritt M. In vitro failure and fracture resistance of veneered and fullcontour zirconia restorations. Journal of Dentistry 2012;40:921–8. Tang X, Nakamura T, Usami H, Wakabayashi K, Yatani H. Effects of multiple firings on the mechanical properties and microstructure of veneering ceramics for zirconia frameworks. Journal of Dentistry 2012;40:372–80. Rosentritt M, Steiger D, Behr M, Handel G, Kolbeck C. Influence of substructure design and spacer settings on the in vitro performance of molar zirconia crowns. Journal of Dentistry 2009;37:978–83. Christensen GJ. The state of fixed prosthodontic impressions: room for improvement. Journal of the American Dental Association 2005;136:343–6. Christensen GJ. Laboratories want better impressions. Journal of the American Dental Association 2007;138:527–9. Samet N, Shohat M, Livny A, Weiss EI. A clinical evaluation of fixed partial denture impressions. Journal of Prosthetic Dentistry 2005;94:112–7. Mehl A, Ender A, Mormann W, Attin T. Accuracy testing of a new intraoral 3D camera. International Journal of Computerized Dentistry 2009;12:11–28. Ender A, Mehl A. Full arch scans: conventional versus digital impressions – an in-vitro study. International Journal of Computerized Dentistry 2011;14:11–21. Rohaly J, Hart DP, Brukilacchio TJ. Three-channel camera systems with non-collinear apertures. Google Patents 2008. Seelbach P, Brueckel C, Wo¨stmann B. Accuracy of digital and conventional impression techniques and workflow. Clinical Oral Investigations 2013;17:1759–64. http://dx.doi.org/ 10.1007/s00784-012-0864-4. Almeida e Silva JS, Erdelt K, Edelhoff D, Araujo E, Stimmelmayr M, Vieira LC, et al. Marginal and internal fit of four-unit zirconia fixed dental prostheses based on digital and conventional impression techniques. Clinical Oral Investigations 2014;18:515–23. Brawek PK, Wolfart S, Endres L, Kirsten A, Reich S. The clinical accuracy of single crowns exclusively fabricated by digital workflow–the comparison of two systems. Clinical Oral Investigations 2013;17:2119–25. http://dx.doi.org/10.1007/ s00784-013-0923-5. Loi I, Di Felice A. Biologically oriented preparation technique (BOPT): a new approach for prosthetic restoration of periodontically healthy teeth. European Journal of Esthetic Dentistry 2013;8:10–23. Hannigan A, Lynch CD. Statistical methodology in oral and dental research: pitfalls and recommendations. Journal of Dentistry 2013;41:385–92. Denry IL, Holloway JA. Microstructural and crystallographic surface changes after grinding zirconia-based dental ceramics. Journal of Biomedical Materials Research Part B: Applied Biomaterials 2006;76:440–8.
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23. Ortorp A, Kihl ML, Carlsson GE. A 3-year retrospective and clinical follow-up study of zirconia single crowns performed in a private practice. Journal of Dentistry 2009;37:731–6. 24. Ortorp A, Kihl ML, Carlsson GE. A 5-year retrospective study of survival of zirconia single crowns fitted in a private clinical setting. Journal of Dentistry 2012;40:527–30. 25. Pantaleo G. Guidelines for planning and conducting scientific research. Italian Oral Surgery 2012;11:47–58. 26. Cehreli MC, Kokat AM, Akca K. CAD/CAM Zirconia vs. slipcast glass-infiltrated Alumina/Zirconia all-ceramic crowns: 2-year results of a randomized controlled clinical trial. Journal of Applied Oral Science 2009;17:49–55. 27. Al-Amleh B, Lyons K, Swain M. Clinical trials in zirconia: a systematic review. Journal of Oral Rehabilitation 2010;37:641–52. 28. Heintze SD, Rousson V. Survival of zirconia- and metalsupported fixed dental prostheses: a systematic review. International Journal of Prosthodontics 2010;23:493–502. 29. Zhu Q, de With G, Dortmans LJ, Feenstra F. Subcritical crack growth behavior of Al2O3-glass dental composites. Journal of Biomedical Materials Research Part B: Applied Biomaterials 2003;65:233–8. 30. Lawn BR. Fracture of brittle solids. 2nd ed. Cambridge: Cambridge University Press; 1993. 31. Weiderhorn S. Subcritical crack growth in ceramics. In: Bradt R, Hasselman D, Lange F, editors. Fracture mechanics of ceramics. New York, NY: Plenum Press; 1974. p. 613–46. 32. Michalske T. Fractography of slow fracture in glass. In: Mecholsky Jr JJ, Powell Jr SR, editors. Fractography of ceramic and metal failures. Washington, DC: American Society for Testing and Materials; 1984. p. 121–36. 33. Glasstone S, Laidler KJ, Eyring H. The theory of rate processes. New York, NY: McGraw-Hill; 1941. 34. Mecholsky Jr JJ, Freiman S, Rice RW. Effect of grinding on flaw geometry and fracture of glass. Journal of the American Ceramic Society 1977;60:114–7. 35. Traini T, Gherlone E, Parabita SF, Caputi S, Piattelli A. Fracture toughness and hardness of a Y-TZP dental ceramic after mechanical surface treatments. Clinical Oral Investigations 2014;18:707–14. 36. Griffith AA. The phenomena of rupture and flow in solids. Philosophical transactions of the royal society of london Series A Containing Papers of a Mathematical or Physical Character 1921;221:163–98. 37. Fischer J, Stawarczyk B, Tomic M, Strub JR, Hammerle CH. Effect of thermal misfit between different veneering ceramics and zirconia frameworks on in vitro fracture load of single crowns. Dental Materials Journal 2007;26:766–72. 38. Mainjot AK, Schajer GS, Vanheusden AJ, Sadoun MJ. Influence of cooling rate on residual stress profile in veneering ceramic: measurement by hole-drilling. Dental Materials 2011;27:906–14. 39. Salazar Marocho SM, Studart AR, Bottino MA, Bona AD. Mechanical strength and subcritical crack growth under wet cyclic loading of glass-infiltrated dental ceramics. Dental Materials 2010;26:483–90.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/jden
A 3 years retrospective study of survival for zirconia-based single crowns fabricated from intraoral digital impressions Enrico Gherlone a, Federico Mandelli a,*, Paolo Cappare` a, Giuseppe Pantaleo c,a, Tonino Traini a,b, Francesco Ferrini a a
Department of Dentistry, Vita Salute University, San Raffaele Hospital, Milan, Italy Department of Medical, Oral and Biotechnological Sciences, University of Chieti-Pescara, Chieti, Italy c Faculty of Psychology, San Raffaele University of Milan, Italy b
article info
abstract
Article history:
Objective: To evaluate the clinical performance of glass-ceramic/zirconia crowns fabricated
Received 20 February 2014
using intraoral digital impressions – a retrospective study with a three-year follow-up.
Received in revised form
Methods: 70 consecutive patients with a total of 86 glass-ceramic/zirconia crowns were
4 June 2014
treated by a single clinician using standardized clinical and laboratory protocols. A complete
Accepted 5 June 2014
digital workflow was adopted for the purpose except for the veneering procedure for the glass-ceramic crowns. Occlusal adjustments were made before the ceramic glazing procedure. Before cementation, all abutments where carefully cleaned with a 70% alcoholic
Keywords:
solution and air dried. Cementation was performed using dual-curing, self-adhesive resin
Zirconia crown
cement. Patients were re-examined after 12, 24 and 36 months, to assess crown chipping/
CAD/CAM
fractures.
Chipping
Results: After the three-year follow-up, none of the zirconia-based restoration was lost
Fracture
(‘‘apparent’’ survival rate 100%) otherwise, the chipping rate of the veneering material
Veneer
increased from 9.3% after 12 months, to 14% after 24 months to 30.2% after 36 months. As a consequence, the ‘‘real’’ success rate after 3 years was 69.8%. Conclusions: After 3 years the success rate of zirconia-based crowns was 69.8%, while the incidence of the chipping was 30.2%. Assuming an exponential increase in chipping rate between 12 and 36 months it can be argued that, among others, the fatigue-mechanism could be advocated as the main factor for the failure of glass-ceramic veneered zirconia especially after 24 months. # 2014 Elsevier Ltd. All rights reserved.
1.
Introduction
To achieve better aesthetic results and biological integration with soft tissues, clinicians have been evaluating the feasibility of all ceramic crowns since 19651; at the beginning
the major problem was a low mechanical stability. Years later, better results were achieved by introducing new materials, such as glass-infiltered alumina2 but despite the improvements achieved, failure rates remained high when these materials were used for posterior crowns. More recently, the introduction of zirconia seemed to offer a valid answer to the
* Corresponding author at: Department of Dentistry, via Olgettina, 58, 20132 Milan, Italy. Tel.: +39 02 26432970. E-mail address: [email protected] (F. Mandelli). http://dx.doi.org/10.1016/j.jdent.2014.06.002 0300-5712/# 2014 Elsevier Ltd. All rights reserved.
1152
journal of dentistry 42 (2014) 1151–1155
Table 1 – Distribution of the crowns divided by teeth group.
mechanical needs of the posterior regions. It has been documented3 that the mechanical properties of zirconia are the highest ever reported for any dental ceramic with fracture strength values of 1240 MPa; this material was also used to fabricate a solid fixed dental prosthesis framework, with very high aesthetic results.4 Tetragonal zirconia polycrystals (3YTZP) is the one mainly adopted by manufacturers (Cercon1 – Dentsply International, LavaTM – 3MTM ESPETM, Procera1 zirconia – Nobel BiocareTM, YZ cubes for Cerec InLab1 – VidentTM and IPS e.max1 ZirCAD – Ivoclar Vivadent3). The two biggest clinical concerns using glass-ceramic on zirconia framework are the chipping of glass-ceramic veneering material, and the degradation (ageing) of zirconia exposed to the oral environment that could lead to long-term failures.5–9 There is evidence that designing frameworks with an anatomical profile decreases the failure incidence of the crowns.10 Manufacturing zirconia-core crowns implies a computeraided design/computer-aided manufacturing (CAD/CAM) process: the workflow starts with a conventional oral impression from which a stone cast is produced and then digitalized to a virtual 3D model. After these steps the CAD/CAM process proceeds till the planned framework/crown is ready to be shipped back to the dental office. Accuracy of the final product depends on the sum of the possible errors from the start to the end of the workflow. Conventional impressions can deliver high quality results in terms of stability and precision9 but problems such as undefined reproduction of the preparation margins, tearing of the impression material, presence of debris, lack of material within key areas and undefined margins on the casts could be present.11,12 Clinical results in daily practice need further improvements.13 Digital oral impression was introduced in order to eliminate the aforementioned sources of error: digital impression should permit a total digitization of the workflow from the mouth of the patient to the final restoration. Studies on the accuracy have been published14,15 but data is still inconsistent. Furthermore, this relatively new technology is evolving rapidly and, at the time of writing, third generation oral scanners are already available on the market, with improved clinical performances. The LAVATM Chairside Oral Scanner, (3M ESPE, Seefeld, Germany) is one of the various models present on the market and it is based on the principle of active wave front sampling.16 Three sensors capture images from different perspectives; 3D surface patches are generated in real time by means of proprietary image processing algorithms using in-focus and out-of-focus information. Published articles have confirmed its reliability both in vitro17,18 and in vivo.19 The aims of this longitudinal retrospective clinical study were to evaluate the success and the failure rates of zirconiabased crowns fabricated by a complete digital workflow with a three-year follow up.
treated with 86 crowns; one patient received three crowns, fourteen received two and all the rest one. The position of the crowns is summarized in Table 1. Tooth abutments were prepared by FF with knife-edge finish line20 and temporarily restored with composite-resin interim crowns during the same session. After eight weeks of tissue conditioning, the patient came for the definitive impression. Two displacement cords (Ultrapack; Ultradent Products) of different sizes, chosen individually for each patient, were gently placed into the gingival sulcus; the cord of narrower diameter was placed apically with respect to the wider diameter. Mouth was rinsed with water and air-dried. A dedicated powder for optical scanning (LavaTM Powder for Chair-side Oral Scanner, 3M ESPE) was used to apply a thin layer of dust on the teeth as recommended by the manufacturer. The wider cord was removed and the scan performed immediately afterwards. The quality of the digital impression, magnified compared to the natural size, was then evaluated on the computer screen. Abutment had to be clearly visible beyond gingival margin, without powder excesses and possible elements of distortion like bubbles or undefined areas; adjacent teeth were evaluated as well. If any problem was found, the scanning process was repeated. The workflow required a second scan for the opposite arch and a third scan for bite registration (less time-consuming). After the digital scan was accepted, all the frameworks were designed and milled by the same centre, with the same technology (LavaTM, 3M ESPE). The veneering process was made using the same glass-ceramic material (Willi Geller/Creation ZI-F) and performed by the same dental technician. The occlusal adjustments were made by means of a diamond bur before the ceramic glazing procedure. Before cementation, all abutments where carefully cleaned with a 70% alcoholic solution and air-dried. Cementation was performed using dual-polymerizing, self-adhesive resin cement (RelyxTM Unicem – 3M ESPE). A specific data-form was used to record: gender, age and position of the crowns as well as the day of the restoration delivery. Patients were re-evaluated after 12, 24 and 36 months, adding data regarding crown chipping/fractures (yes/no). Along with fractures, in this study chipping is also considered a failure because it means the crown has lost its integrity.
2.
3.
Materials and methods
Seventy patients, 50 males and 36 females with a mean age of 45.9 years (SD = 11.6) in need of restorations for single crowns, were treated by the same experienced operator (FF) using the same clinical and laboratory protocol. The patients were
Region Incisor Canine Premolar Molar Total
Maxilla
Mandible
Total
9 4 19 24 56
0 0 8 22 30
9 4 27 46 86
Statistical analysis
The time-dependent survival analysis of the crowns was calculated with the Kaplan–Meier model21 using a statistical package software IBM SPSS Statistics for Windows, Version 21.0. (IBM Corp. Armonk, NY; USA).
1153
journal of dentistry 42 (2014) 1151–1155
Table 2 – Survival table of the 86 placed crowns. Time (months)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
4.
12 12 12 12 12 12 12 12 24 24 24 24 36 36 36 36 36 36 36 36 36 36 36 36 36 36
Status
Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured Fractured
Cumulative proportion surviving at the time Estimate
SE
– – – – – – – 0.907 – – – 0.860 – – – – – – – – – – – – – 0.698
– – – – – – – 0.031 – – – 0.037 – – – – – – – – – – – – – 0.050
Results
After the three-year follow up, 60 crowns were free of complication, corresponding to a 69.8% success rate (Table 2).
No. of remaining cases
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
85 84 83 82 81 80 79 78 77 76 75 74 73 72 71 70 69 68 67 66 65 64 63 62 61 60
The chipping rate was 9.3% after 12 months 14% after 24 months and 30.2% after 36 months (Fig. 1) with an exponential increase between 24 and 36 months (Fig. 2). No correlation between crown fractures and patient gender or tooth position was found.
5.
Fig. 1 – Survival curve of the 86 placed crowns at 36 months of follow-up.
No. of cumulative events
Discussion
Stabilized zirconia is one of the latest restoration materials introduced in clinical practice. Its key feature is a flexural strength higher of twice compared to other ceramics such as alumina. This strength derives from a stress-induced transformation from the metastable tetragonal form to the stable monoclinic form (t ! m). This transformation was associated with an increase in volume of the grain, thereby closing the crack tip and preventing further crack propagation.22 Notwithstanding the aforementioned mechanical properties in clinical practice, the most common complication observed in zirconia-based restorations is the chipping and fractures of the veneering ceramic with or without exposing the underlying Y-TZP framework.22 To our knowledge, this is the first report evaluating the survival rate of zirconia-based crowns manufactured with a complete digital workflow. The results of the present study, over a 3-year follow-up period, demonstrated a success rate of 68.6%. The chipping rate was 9.3% after 12 months 14% after 24 months and 30.2% after 36 months, with an exponential increase between 24 and
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Fig. 2 – Cumulative chipping rate of the crowns. Note the ‘‘exponential-like’’ aspect of the red trend line. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)
36 months (Fig. 2). By contrast in a 3-year follow-up study Ortorp23 reported a chipping/fractures rate of 2% for single crowns made using conventional impressions; nevertheless, after two further years of follow-up, the same author19 reported an increase in the chipping/fractures rate of up to 10.2%.24 The lack of criteria used to define the chipping events can be advocated to explain the differences with the present results.25 Another study on single zirconia-based crowns reported one framework fracture and no chipping after a 2year follow-up.26 Nevertheless, the power of the study was very low due to limited number of crowns studied (n = 15). AlAmleh27 in a systematic review on zirconia-based fixed partial dentures (FPD) reported a chipping or fracture rate of the veneering material ranging from 0% to 51%. Heintze28 in a systematic review comparing zirconia FPD and metal supported FPD reported a chipping rate of 24% for zirconia prostheses. Recently, Raigrodski7 underlined that chipping was one of the most frequent complications observed but could not perform a statistical comparison between the included articles because, among them, ‘‘There was no consensual guideline for quantifying chipping of the veneering porcelain’’. Glass ceramics are brittle materials because of atomic bonds that do not allow the atomic planes to slide apart when subjected to load. Thus, ceramics cannot withstand deformation of >0.1% without fracturing. On the other hand, due to the monolithic structure (single phase) the glass ceramics have very low fracture toughness with a tendency to coalesce onto both the surface and subsurface micro cracks. Because they do not yield, monolithic ceramics behave as brittle materials and a propagating crack need only to overcome the surface energy of the material. As result of the occlusal function, the formation of microscopic flaws and micro cracks can lead to the failure of dental ceramics by means of accumulating damages. However, in glass ceramics, toughness and initial strength are not the only factors that determine long-term survival:
subcritical crack growth (SCG) significantly decreases survival time of glass ceramics.29 SCG is defined as a crack propagation at stress intensity factor (KI) levels lower than the critical stress intensity factor, or fracture toughness (KC).30–32 Occlusal load could be considered a long-term or repetitive low-level loading that may cause pre-existing subcritical crack flaws to grow until failure occurs. Moreover, the SCG is affected by several other factors which act as additional stressors.33 Embrittlement effects due to mechanical finishing and grinding procedures have been reported.34,35 Nonetheless, the presences of Griffith defects in glass ceramics are critical for SCG.36 A less documented factor influencing the SCG is the misfit of coefficient of thermal expansion between zirconia core and glass ceramic veneer that leads to failure due to a residual stress.37 The amount of the residual stress in the surface of the glass ceramic veneering appears to be increased by cooling rate.38 The mechanisms responsible for loss of strength in dental glass ceramics appear to be governed by combined factors. Meanwhile, the stress corrosion by water molecules at the crack tip and mechanical degradation of the material permits crack propagation at stress levels in some cases to less than 50% of the initial material strength.39 Analyzing the data here reported we assume a fatigue-mechanism fracture for chipping of glass veneered zirconia that appeared to be exacerbated in time reaching a critical point at 24 months. Further research is necessary to clarify the relationship between the ageing process and clinical failure.
Conflict of interest The authors declare no conflicts of interest.
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