journal of dentistry 42 (2014) 1151–1155

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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

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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


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/




Results: After the three-year follow-up, none of the zirconia-based restoration was lost


(‘‘apparent’’ survival rate 100%) otherwise, the chipping rate of the veneering material


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.



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). 0300-5712/# 2014 Elsevier Ltd. All rights reserved.


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.



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




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).


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


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


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


– – – – – – – 0.907 – – – 0.860 – – – – – – – – – – – – – 0.698

– – – – – – – 0.031 – – – 0.037 – – – – – – – – – – – – – 0.050


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.


Fig. 1 – Survival curve of the 86 placed crowns at 36 months of follow-up.

No. of cumulative events


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


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.


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A 3 years retrospective study of survival for zirconia-based single crowns fabricated from intraoral digital impressions.

To evaluate the clinical performance of glass-ceramic/zirconia crowns fabricated using intraoral digital impressions - a retrospective study with a th...
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