Authors [Ref] (Year)
Mean time (years)
Sample size
Type of restorations
Veneer porcelain fracture (%)
Rinke et al. [5] (2013)
7
80
3–4 unit FPDs
28.8
Burke et al. [6] (2013)
5
33
3–4 unit FPDs
24.2
Monaco et al. [7] (2013)
5
1,132
Single crowns
Unknown
Vavřičková et al. [8] (2013)
3
102
Single crowns
Unknown
Rinke et al. [9] (2013)
3
52
Single crowns
5.8
Raiqrodski et al. [10] (2012)
5
23
3 unit FPDs
21
Ortorp et.al. [11] (2012)
5
143
Single crowns
3
Vigolo et.al. [12] (2012)
5
39
Single crowns
7.7
Schmitter et.al. [13] (2012)
5
30
4–7 unit FPDs
26.7
Schmitt et.al. [14] (2012)
5
25
3–4 unit FPDs
28
Kern et.al. [15] (2012)
5
20
3–4 unit FPDs
Unknown
Sorrentino et.al. [16] (2012)
5
48
3 unit FPDs
6.3
Sagirkaya et.al. [17] (2012)
4
107
Single crowns
0.9
Peláez et.al. [18] (2012)
4
20
3 unit FPDs
10
Salido et.al. [19] (2012)
4
17
4 unit FPDs
29.1
Ohlmann et.al. [20] (2012)
2
11
3–4 unit FPDs
18.2
Poggio et.al. [21] (2012)
1
102
Single crowns
2.0
Sax et.al. [22] (2011)
10
57
3–5 unit FPDs
28.0
Tartaglia et.al. [23] (2011)
3
463
Single or multiple-unit
Unknown
Roediger et.al. [24] (2010)
4
99
3–4 unit FPDs
13
Beuer et.al. [25] (2010)
3
68
Single or multiple-unit
7.4
Schmitt et.al. [26] (2010)
3
17
Single crowns
5.9
Tsumita et.al. [27] (2010)
2
21
3 unit FPDs
14.3
Clinical reporting on porcelain fracturing in zirconia all-ceramic restorations has shown a high rate of occurrence of short-term cohesive failure of porcelain, a rate of occurrence that is significantly higher than that in the case of porcelain fused to metal restorations. In one report, a chipping rate of 0–88.9 % in a one- to eight-year follow-up period has been mentioned [28]. Moreover, the most common form of clinical failure is porcelain chipping [29], and prevention of the fracturing of porcelain fired onto the zirconia frame has become a clinical challenge. In particular, the parameters of occlusion are a factor and the usage of a night guard or press ceramics has been proposed [28, 29].
27.3 Influence of Frame Thickness and Porcelain Firing Schedules
It is thought that owing to differences in the physical properties of metal frames, zirconia frames might more readily experience chipping from thermal factors during porcelain firing. The reason for this is that the thermal conductivity of zirconia is about 1/100 that of gold [30]. Porcelain firing is thus thought to proceed gradually inward starting not from the frame side but rather from the porcelain surface layer, which more readily conducts heat, and the fired porcelain interior is possibly more susceptible to partially incomplete firing or distortion. In addition, the heat capacity of the zirconia is about 3.5 times that of gold [30]. For this reason, a temperature gradient is believed to possibly occur in the process of cooling after firing, creating a shrinkage difference between the inner and outer surfaces of the sintered body and triggering cracking and other defects. The presence of the frame and thermal factors is thus expected to be a significant factor that triggers partially incomplete firing and defects. Modeling all-ceramic crowns mimicking clinical forms, Benetti et al. measured the temperature differences in porcelain interiors because of the differences between zirconia frames and metal frames and differences in cooling rates after firing. They noted that the specific heat, heat capacity, and thermal expansion rate of a material impacts early fracturing of all-ceramic crowns [31]. Nonetheless, though their research investigated temperature changes during sintering and cooling, there was no assessment of physical properties of porcelain caused by this. We have investigated how differences in the firing conditions of porcelain and frame material impact the fracture toughness of porcelain, in order to study how these factors impact the mechanical properties of porcelain. Our results showed that under conditions of faster heating rates, the fracture toughness decreases than that under manual conditions [32]. Regarding the thermal expansion coefficients of porcelain and zirconia, a zirconia frame has a slightly (about 10 %) greater thermal expansion coefficient than porcelain [33
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