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journal of dentistry 37 (2009) 627–637

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All-ceramic partial coverage restorations—Midterm results of a 5-year prospective clinical splitmouth study Petra C. Guess a,b,*, Joerg R. Strub a, Niclas Steinhart a, Martin Wolkewitz a,c, Christian F.J. Stappert a,b,d a

Department of Prosthodontics, Albert-Ludwigs-University, Freiburg, Germany Department of Biomaterials and Biomimetics, New York University College of Dentistry, New York, USA c Institute of Medical Biometry and Medical Informatics, Albert-Ludwigs-University, Freiburg, Germany d Department of Periodontology and Implant Dentistry, New York University College of Dentistry, New York, USA b

article info

abstract

Article history:

Objectives: Midterm-evaluation of a 5-year prospective clinical splitmouth-investigation on

Received 8 January 2009

survival rate and long-term behavior of all-ceramic partial coverage restorations (PCRs) on

Received in revised form

molars. Pressed ceramic and CAD/CAM fabricated PCRs were compared.

12 April 2009

Methods: 80 vital molars of 25 patients were restored with all-ceramic PCRs (40 IPS e.max

Accepted 19 April 2009

Press*[IP] and 40 ProCAD*[PC]). IP-PCRs were heat pressed following the lost-wax method. PC-PCRs were fabricated with Cerec 3** and Cerec InLab** CAD/CAM system (**Sirona Dental Systems, Bensheim, Germany). All PCRs were adhesively luted

Keywords:

with a light-polymerizing composite (Syntac*/Tetric*) (*Ivoclar Vivadent, Schaan,

Ceramic

Liechtenstein). Clinical reevaluations were performed at baseline and 13, 25, and 36

Press ceramic

months after insertion of the PCRs according to the modified United States Public Health

CAD/CAM

Services (USPHS) criteria. Absolute failures were demonstrated by Kaplan–Meier survival

Partial coverage restoration

rate. Results: After an observation time up to 3 years, survival rate of IP-PCRs was 100% and 97% for PC-PCRs due to one severe fracture. The PC-PCR had to be replaced after 9 months. Secondary caries and endodontic complications did not occur. Increased clinical service time resulted in significant decrease of marginal adaptation ( p = 0.031) and enhanced marginal discoloration ( p < 0.0001). Both PCR ceramic materials demonstrated significant deteriorations in color match ( p < 0.0001) and surface roughness ( p < 0.0001), IP-PCRs were significantly more affected ( p  0.005). Regarding anatomic form IP-PCRs performed significantly better ( p = 0.0012). Conclusion: Pressed ceramic and CAD/CAM fabricated partial coverage restorations exhibited a reliable treatment option to restore larger defects in posterior teeth. Marginal degradation of the resin cement and deterioration of the all-ceramic materials during clinical function determine the clinical long-term performance of partial coverage restorations. # 2009 Elsevier Ltd. All rights reserved.

* Corresponding author at: Department of Biomaterials and Biomimetics, New York University College of Dentistry, Arnold and Marie Schwartz Hall of Dental Sciences, 345 East 24th Street (Rm 804S), New York, 10010 NY, USA. Tel.: +1 212 998 9986; fax: +1 212 995 4244. E-mail address: petra.guess@uniklinik-freiburg.de (P.C. Guess). 0300-5712/$ – see front matter # 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2009.04.006


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Introduction

The concept of bonded all-ceramic inlays and onlays has been introduced to the dental community in the early 1980s.1 When treating posterior teeth with small sized defects such as Classes I and II cavities with all-ceramic restorations, pressed ceramics,2,3 feldspathic ceramics1,4 and CAD/CAM systems5–9 have been widely used and scientifically well documented. Long-term clinical data covering observation periods of 17–18 years with feldspathic ceramics and CAD/ CAM systems are reported.5,8 Treating extended multisurfaced defects in posterior dentition often demands more than covering the intra-coronal part of the residual tooth structure. Extensive removal of compromised tooth structure increases the width of the intra-coronal defect and can result in a lack of dentin support of the buccal and oral cusps. Partial or complete coverage of the occlusal surface within the preparation is a preventive measure to avoid cusp fracture.10– 12 Full ceramic crown restoration is a widely utilized treatment choice for extended tooth structure defects.13 Metal–ceramic crowns have historically proven to be reliable for these indications since technicians and clinicians have substantial experience with this technique.14 Comparable results have also been reported for all-ceramic densely sintered alumina and reinforced glass–ceramic crowns.14 However, since full crown preparation requires removal of extensive tooth structure, defect oriented restoration designs have been developed.15 Early experiences with all-ceramic partial crowns and castable glass ceramic systems showed major clinical shortcomings and revealed high fracture rates of 25% after an observation period of 6–84 months.16 In subsequent years, progress in adhesive bonding techniques and improved luting composites as well as the development of ceramic materials with improved mechanical properties lead to a broader range of partial coverage restorations in posterior dentition. Conventional feldspathic ceramic materials have been strengthened by increasing the leucite crystal content.17 Pressable ceramics using the lost-wax technique as well as industrially prefabricated machinable ceramics for lab- and chairside CAD/CAM systems have been developed as modification of the conventional powder slurry fabrication technique to obtain predictable long-term performance.18 High fracture resistance as well as satisfying marginal conditions of the presently investigated press and CAD/ CAM ceramic system for partial coverage restorations application could be observed in preceding in vitro studies simulating 5-year clinical service.19,20 Present clinical data often lack of detailed information about extension of partial coverage restorations, preparation designs and all-ceramic materials used.5,12,16,21 Few studies exist on the clinical longterm behavior of all-ceramic partial coverage restorations involving the entire occlusal tooth surfaces.1,22,23 Preliminary results of all-ceramic partial coverage restoration in short and medium-term clinical observation are promising.24–31 However, conclusive evidence-based statements on clinical longevity of extended partial coverage restorations cannot be made yet. The present prospective study investigated the longevity and clinical performance of pressable and CAD/CAM fabricated all-ceramic partial restorations in a splitmouth design.

As a null hypothesis no difference between the two allceramic materials and fabrication techniques on clinical longterm success of all-ceramic partial restorations was estimated. In the present evaluation the 3-year results are reported.

2.

Materials and methods

2.1.

Patient selection and indication

The study comprised 25 patients who required at least two partial coverage restorations (PCRs). All patients were recruited from the patient contingent of the Department of Prosthetic Dentistry of the University hospital of Freiburg. Eighty extended Class II molar cavities were restored with 40 adhesively luted pressed PCRs (IPS e.max Press [IP]) and 40 CAD/CAM fabricated PCRs (ProCAD [PC]; Cerec 3 (4 restorations)/Cerec InLab (36 restorations)). According to the splitmouth study design, at least two restorations were placed in each patient (one IP and one PCPCR), limited to a maximum of four restorations per patient. For each patient, the material for the first qualified tooth (according to the FDI system) was chosen randomly. The following tooth received the alternate material, respectively. Molars in antagonist position could not be restored within the study. Molars had to fulfill the following requirements to be restored with all-ceramic PCRs: 1. Absence of pain; 2. Absence of pulpal disease or pathological changes in the periapical region; 3. Absence of active periodontitis; 4. Ability to apply rubber dam during PCR placement; 5. Presence of large occlusal restorations or defects extending to more than one outer surface (making the tooth unsuitable for an intra-coronal restoration); 6. Molars were in occlusion and had at least one proximal contact. The exclusion criteria for teeth of study related partial coverage restorations were: 1. Consistently poor oral hygiene; 2. Parafunctions (distinct bruxism) or pronounced malocclusion (e.g. cross bite); 3. Non-vitality or existing root canal treatments. All patients were treated in the Department of Prosthetic Dentistry of the University hospital of Freiburg by clinicians experienced with placing all-ceramic PCRs. The study was conducted according to the Declaration of Helsinki and was inspected and approved by an ethics committee (International ethics committee Freiburg, Germany, Registration Number: 277/01). All patients were required to give written informed consent and agreed to the 5-year recall program.

2.2.

Preparation

The molars were prepared for PCRs with a butt joint margin of the non-bearing cusps and a rounded shoulder finishing line


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(1.2 mm) of the bearing cusps. The preparation design was based on a MOD inlay cavity with a 2 mm deep occlusal box and an isthmus width of 2 mm. The cavities were slightly divergent with an overall preparation angle of 68 towards the occlusal aspect. The occlusal tooth surface was reduced by 2 mm following the occlusal anatomy. The proximal boxes ended above the cemento-enamel junction. Preparation margins were not beveled. All inner line angles were rounded. Preparations were performed using 80 mm grit diamond burs (837KR.314.012, 847KR.314.016) and finished with 30–40 mm finishing diamonds (8837KR.314.012, 8847KR.314.016, 8390.204.016) (Gebr. Brasseler, Lemgo, Germany). Manufacturer’s restrictions regarding minimum material thickness (occlusally 1.5 mm and axially at least 0.8 mm), were consistently maintained during preparation. A chemical curing composite filling material (Clearfil, Kuraray Medical Inc., Okayama, Japan) was used to block out defect related undercuts to maintain a standardized preparation protocol. Fillings were applied under rubber dam. Full-arch impressions were taken using a polyether impression material (Permadyne, 3M ESPE, St. Paul, MN, USA). Master models were made with type 4 dental stone (GC FujiRock EP, GC Europe, Leuven, Belgium). Provisional restorations were fabricated chair-side with a silicone key and self-curing provisional composite material (Protemp 3 Garant, 3M ESPE, St. Paul, MN, USA) and were cemented with eugenol-free temporary cement (Freegenol, GC Europe, Leuven, Belgium). Shade selection was carried out with an universal shade guide (Chromascop, Ivoclar Vivadent, Schaan, Liechtenstein). The ProCAD PCRs were fabricated by means of the Cerec 3 and the Cerec InLab system (Sirona, Bensheim, Germany). The restorations were milled out of leucite-reinforced glass– ceramic ProCAD blanks (Ivoclar Vivadent, Schaan, Liechtenstein). The press ceramic IPS e.max Press PCRs (Ivoclar Vivadent, Schaan, Liechtenstein) were processed according to the manufacturer’s instructions. The staining technique according to the manufacturer’s recommendation was used to individualize the IPS e.max Press and ProCAD PCRs.19,25 Minor corrections of the glazed restorations during try-in were made chair-side. Changes to the occlusal and labial surfaces were re-polished to a high-gloss using ceramic silicone polishers (Dialite Polishing Set Ceramic, Gebr. Brasseler, Lemgo, Germany). PCRs that required major corrections or needed complete revisions were sent to the dental laboratory and received an entirely new coating of the ceramic glazing. All PCRs were adhesively luted under rubber dam. The prepared teeth were thoroughly cleaned with a polishing brush and a fluoride-free cleaning paste (Pell-ex Hawe Neos Dental, Bioggio, Switzerland). Enamel (40 s) and dentin (15 s) were etched with 37% phosphoric acid (Total-Etch, Ivoclar Vivadent, Schaan, Liechtenstein), rinsed with water for 15 s, and dried. Syntac Primer was applied to the etched surface with a brush for 15 s and dried after a reaction time of 10 s. Then Syntac Adhesive (Ivoclar Vivadent, Schaan, Liechtenstein) was applied for a reaction time of 10 s. Heliobond (Ivoclar-Vivadent AG, Schaan, Liechtenstein) was applied to enamel and dentin and blown to a thin layer. To avoid inaccuracies of fit, Heliobond was not light-polymerized

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before restoration placement. At the same time, each PCR was cleaned with 99% isopropanol, and the inner surfaces were etched with 4.9% hydrofluoric acid (IPS Ceramic Etching Gel, Ivoclar Vivadent, Schaan, Liechtenstein), ProCAD PCRs for 60 s and IPS e.max Press PCRs for 20 s, respectively. The etched ceramic surfaces were rinsed for 60 s, dried, and then silanized (Monobond-S, Ivoclar Vivadent, Schaan, Liechtenstein). After a reaction time of 60 s the silanized ceramic surfaces were dried with air. The light curing fine particle hybrid composite material Tetric (Ivoclar Vivadent, Schaan, Liechtenstein) was used for adhesive luting and was applied to the prepared tooth structure and to the inner surfaces of the restoration. To decrease the viscosity, the resin material Tetric was preheated to 54 8C.32 Restorations were inserted with slowly increasing pressure (ca. 5–10 N). Excess cement in all marginal areas was removed with foam pellets and Superfloss (Oral-B, London, UK) immediately. Prior to polymerization, luting composite at the preparation margin was covered with glycerin gel (LiquidStrip, Ivoclar Vivadent, Schaan, Liechtenstein) to prevent an oxygen inhibition layer. Polymerization of the luting agent was performed by light curing from different directions (oral, vestibular, occlusal and proximal surfaces for 40 s, respectively) with a light intensity of at least 650 mW/cm2 (Elipar Free Light 2; 3 M ESPE, St. Paul, MN, USA) for 200 s. Residual excess cement was removed with a 15c scalpel (#371716, Bard-Parker; Becton-Dickinson, Dr. Franklin Lakes, NJ, USA). Epigingivally excess cement was removed with a finishing diamond (#852EF.314.014; Komet Dental) after 24 h and coronal excess cement with flexible polishing discs (Soft-Lex Pop-On, 3 M ESPE, St. Paul, USA). Restoration margins were polished with silicone polishers (#9418.204.030, #9419.204.030, #9547.204.030; Komet Dental) and inter-proximal polishing strips (Soft-Lex Finishing Strips, 3M ESPE, St. Paul, USA). Centric and eccentric occlusal contacts were adjusted using diamond finishing burs.

2.3.

Clinical evaluation

The PCRs were classified by two independent calibrated investigators according to the modified United States Public Health Service (USPHS) criteria33,34 (Table 1) at baseline and after 13, 25, and 36 months post-insertionem (Table 2). The restorations were visually inspected with dental mirror and probe, and clinically examined with wax-free dental floss. Deviations in color match, and anatomic form were recorded and photographed. Each restoration was examined for cracks, fractures, and debonding. Pulp vitality was verified with CO2test. The patients were questioned about possible postoperative complaints. For additional documentation, representative restorations were investigated by scanning electron microscopy (SEM). After taking impressions (Permagum Putty Soft, Dimension GrantL, 3M Espe, Seefeld Germany), epoxy resin replicas (Epofix Resin, Struers, Ballerup, Denmark) were fabricated, gold sputtered and viewed under SEM (Model 3500S, Hitachi, Osaka, Japan).

2.4.

Statistical evaluation

For statistical evaluation, a Kaplan–Meier survival rate accounting for absolute failures was calculated and graphically


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depicted.35 Absolute failure was defined by clinical unacceptable fracture and crack development which required a replacement of the entire restoration, and/or secondary caries as well as endodontic complications. The beginning of the observation interval started with incorporation of the restoration and the end of the interval was defined by incidence of an absolute failure. The modified USPHS criteria were tabulated using the statistics program R Development Core Team 2004 (R Foundation for Statistical Computing, Vienna, Austria). For each outcome (marginal adaptation, marginal discoloration, surface roughness, color match and anatomic form), a random intercept logistic regression model using the SAS procedure PROC GLIMMIX was fitted. Following effects were considered: material, recall and the interaction between material and recall.

3.

Results

Forty PC and 40 IP partial coverage restorations (PCR) were inserted in 11 male (44%) and 14 female (56%) patients (overall mean age 43.5 years, range 25–60 years). Thirty percent (n = 12) of PC-PCRs and 37.5% (n = 15) of IP-PCRs were placed in maxillary molars and 70% (n = 28) of PC-PCRs and 62.5% (n = 25) of IP-PCRs in mandibular molars. Of an initial group of 25 patients, 22 patients were summoned for a follow-up examination after a mean observation period of 13 months. For the second recall appointment after a mean follow-up period of 25 months, 22 patients were examined. Fifteen patients attended the third recall examination (mean follow-up period of 36 months) (Table 2). Presently, the drop-out rate regarding restorations was 10% (8 restorations), corresponding to a patient drop-out of 8% (2 patients). One patient was unable to participate in the

clinical examination due to work-related problems and one patient relocated. Two further patients (each with 4 restorations) could not attend the first and second recall examination after 13 and 25 months due to pregnancy (Table 2). Since the partial coverage restorations were inserted over a period of 1.8 years some of the restorations did not reach the third recall appointment yet, resulting in a reduced number of examined restorations. The data will be accomplished in the ongoing evaluation. No secondary caries, endodontic complications or postoperative complaints were observed in the given follow-up periods. The carbon dioxide vitality test indicated that all restored teeth, which were diagnosed as vital at the time of treatment maintained vital at the time of examination. Significant decreasing Alpha rankings over time (baseline to third recall) were observed for the following criteria: marginal adaptation ( p = 0.031), marginal discoloration ( p < 0.0001), surface roughness ( p < 0.0001), color match ( p < 0.0001) and anatomic form ( p = 0.0012). Development of Bravo ratings for marginal adaptation over time was comparable for both materials ( p = 0.297). Results for marginal discoloration showed a similar trend over time for both materials. Significantly increased surface roughness (predominately Bravo and few Charlie ratings) were observed for both restoration materials over time and was more pronounced with IP restorations ( p < 0.0001). Decreasing color match was noticed after the first recall and became more dominant in both restoration materials with time, particularly with IP-PCRs ( p = 0.005) (Table 2). Increasing surface roughness as well as decreased color match was mainly related to wear of the surface staining glazing material in occlusal contact point areas (Fig. 1). From the baseline evaluation onwards CAD/CAM fabricated PC-PCRs exhibited significantly

Table 1 – USPHS criteria for classification of partial coverage restorations. Characteristics

Rating

Secondary caries

Alpha Bravo

No evidence of caries contiguous with the margin of the restoration Caries evident contiguous with the margin of the restoration

Marginal adaptation

Alpha Bravo Charlie

No visible evidence of crevice along margin; no catch or penetration of explorer Visible evidence of crevice and/or catch of explorer; no penetration of explorer Visible evidence of crevice; penetration of explorer

Marginal discoloration

Alpha Bravo Charlie

No discoloration on the margin between the restoration and the tooth structure Superficial discoloration on the margin between the restoration and the tooth structure; does not penetrate in pulpal direction Discoloration has penetrated along the margin of the restorative material in pulpal direction

Surface roughness

Alpha Bravo Charlie

Visual fine polished glossy surface, no palpable roughness Slight, visible and palpable roughness Coarse, visible and palpable roughness, unglazed surface

Color match

Alpha Bravo

No mismatch in color, shade and/or translucency between restoration and adjacent tooth Mismatch between restoration and tooth structure within the normal range of color, shade and/or translucency (<1 shade off; Vita shade guide) Mismatch between restoration and tooth structure outside the normal range of color, shade and/or translucency (>1 shade off; Vita shade guide)

Charlie

Anatomic form

Alpha Bravo Charlie

Criteria

The restoration is continuous with tooth anatomy The restoration is not continuous with tooth anatomy. The restoration is slightly under- or over-contoured The restoration is not continuous with tooth anatomy. Restoration material is missing; a surface concavity is ascertainable


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Table 2 – Modified USPHS criteria evaluation of the partial coverage restorations at baseline and after a mean follow-up period of 13, 25, and 36 months (number of patients and restorations, USPHS criteria in percent). Baseline

1. Recall after 13 months

2. Recall after 25 months

0 25 80

13 (8–18) 22 67

25 (20–28) 22 67

Mean recall time (min–max) Patients Restorations

USPHS criteria in %

Baseline PC (n = 40)

Secondary caries Alpha Bravo Marginal adaptation Alpha Bravo Charlie

100 –

92.5 7.5 –

1. Recall after 13 months

IP (n = 40) 100 –

PC (n = 33) 100 –

IP (n = 34) 100 –

36 (31–41) 15 47

2. Recall after 25 months PC (n = 33) 100 –

3. Recall after 36 months

IP (n = 34) 100 –

3. Recall after 36 months PC (n = 23) 100 –

IP (n = 24) 100 –

95 5 –

84.9 15.1 –

88.2 11.8 –

81.8 18.2 –

85.3 14.7 –

65.2 34.8 –

83.3 16.7 –

Marginal discoloration Alpha Bravo Charlie

100 – –

100 – –

84.9 15.1 –

76.5 23.5 –

57.6 42.4 –

55.8 41.2 3

47.8 52.2 –

62.5 37.5 –

Surface roughness Alpha Bravo Charlie

100 – –

100 – –

48.5 51.5 –

11.7 79.4 8.9

30.3 63.6 6.1

3.0 88.2 8.8

17.4 74.0 8.6

– 79.2 20.8

Color match Alpha Bravo Charlie

95 5 –

92.5 7.5 –

42.4 57.6 –

20.5 79.5 –

39.4 60.6 –

11.8 88.2 –

39.1 60.9 –

8.3 91.7 –

Anatomic form Alpha Bravo Charlie

77.5 22.5 –

97.5 2.5 –

78.8 21.2 –

94.1 5.9 –

69.7 30.3 –

85.3 14.7 –

56.5 43.5 –

79.2 20.8 –

more Bravo ratings for criterion anatomic form than pressed IP-PCRs ( p = 0.001) (Table 2 and Fig. 2). Bravo ratings for the criterion anatomic form increased significantly during the present recall period. This trend was comparable for both materials ( p = 0.321).

Fig. 1 – SEM view of occlusal contact point and surrounding surface roughness (arrow) on a replica of a IPS e.max Press restoration after 25 months.

One mandibular PC-PCR demonstrated an absolute failure by a clinically unacceptable marginal ceramic fracture of the lingual-occlusal cusp area. The fracture occurred after a service time of 9 months and was limited to the ceramic

Fig. 2 – Clinical splitmouth comparison of CAD/CAM fabricated and pressed PCRs after 36 months; Bravo rating for PC-PCRs and Alpha rating for IP-PCRs for anatomic form criterion.


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extension of the cohesive fractures, the restorations could remain in situ. The fractures were observed in the subsequent follow-ups, and showed no further changes. During the reported observation period of up to 41 months macroscopically visible cracks could not be notified in neither of the restoration materials. None of the partial coverage restorations debonded.

4.

Fig. 3 – Kaplan–Meier success probability (%) according to material (IPS e.max Press and ProCAD) and time interval of duty (months) (graph). Estimated survival probability as well as number of patients and restorations is presented in relation to time period in months (table).

restoration material; the underlying tooth structure was not affected. The PC restoration had to be replaced by a conventional cast gold PCR and was not listed in any of the subsequent investigations. The Kaplan–Meier survival probability of the PC restorations was thus 97%. All IP restorations remained in situ (survival probability of 100%) (Fig. 3). Minimal ceramic cohesive fractures (chippings) were found in three additional patients, effecting one PC and two IP restorations. The clinically acceptable chipping fracture of a maxillary molar PC restoration was located at the margin in relation to the occlusal surface (buccal part) and occurred after 12 months. Two mandibular IP restorations showed ceramic chippings at the restoration margins at the lingual part of the occlusal surface (Fig. 4). The minimal fractures were observed after 25 and 38 months, respectively. Owing to the limited

Discussion

During an observation period of 3 years, IPS e.max Press and ProCAD ceramic partial coverage restorations showed promising survival rates of 100% and 97%, respectively. All IPS e.max Press restorations remained in situ in good function. One ProCAD restoration demonstrated an absolute failure after an observation period of 9 months. Valid comparisons to similar studies are limited, since similar designed prospective longitudinal clinical investigations on partial coverage restorations comparing different all-ceramic systems are not available in the literature, yet. To best of the authors’ knowledge no study has investigated the clinical performance of all-ceramic partial coverage restorations using an identical standardized preparation configuration in a prospective splitmouth study design. Some reports can be found in the literature when considering clinical behavior of pressed and CAD/CAM fabricated all-ceramic partial coverage restorations separately. Limitations due to differences in material selections, clinical procedures and study designs have to be considered. However it is possible to ascertain some tendencies by comparing the clinical performance of available studies. The survival probability results of IPS e.max Press partial coverage restorations are comparable to29 or better than26 the reports on IPS Empress partial coverage restorations in the literature. IPS Empress partial coverage restorations achieved survival rates of 92.7–100% in midterm (2–4 years)26,29 and 81– 93.4% in long-term evaluation (7 years).22,36 The present survival rate obtained for the Cerec ProCAD partial coverage restorations can only be related to a 93.1% 3year survival rate of Vita Mark II partial crowns31 and to a 90– 90.4% survival rate for Vita Mark I inlay and onlay restorations after 9–10 years.37,38 The survival probability of onlays made of

Fig. 4 – Clinical (a) and SEM (b) replica evaluation of a IPS e.max Press partial coverage restoration 25 months postinsertionem. Wear of the luting cement and ceramic cohesive fracture (chipping) at the margin of the IPS e.max Press ceramic is marked with an arrow.


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feldspathic ceramics was distinctively lower with 56–60.7% after an observation period of 6–7 years.16,23 Different clinical reports in the literature on all-ceramic onlay and partial coverage restorations reveal that bulk fractures of the allceramic material are still the most frequent cause for clinical failure.22,26,36,38 Unacceptable fractures were observed in 2.7%36 to 4.8%22 to 6.1%26 of IPS Empress partial coverage restorations. Vita Mark II partial crowns revealed a fracture rate of 3.6%.31 In the present study none of the pressed IPS e.max Press partial coverage restorations exhibited unacceptable fractures. Fracture resistance of CAD/CAM fabricated restorations was also rated positively. Only one mandibular ProCAD restoration (2.3%) showed an absolute failure in form of a ceramic bulk fracture at the lingual cusp area. Similar fracture patterns resulting in occlusal partial material loss were described in 5.8% and 6% of Cerec/Vita Mark II partial coverage restorations.12,24 One of the major life limiting failure mechanisms of dental all-ceramic restorations is contact damage accumulation during oral function. Strengths of dental ceramics are significantly lower after multi-cyclic loading.39 The presence of internal surface flaws in particular as well as surface flaws arising from occlusal wear and adjustments act as localized stress concentrators and can initiate the formation of catastrophic clinical failure.40,41 Sub-critical crack growth in ceramics is attributed to corrosion assisted stress at the crack tip or at any pre-existing defect in the ceramic.42,43 Residual stresses in the ceramic and/or in the bonding system also promotes micro-crack formation and propagation.44 Polymerization shrinkage of the luting composite creates stress concentrations at the adhesive interface and at the ceramic subsurface and can be considered as an additional reason for crack formation.45 Failure of the adhesive bond was also made responsible for the ceramic fracture in the present study, since the fractured ceramic fragment detached entirely from tooth substance without cohesive fracture, leaving the underlying composite build up material intact.46 Clinically acceptable cohesive fractures (chippings) are reported to occur mainly at the marginal area of a restoration, involving small or severe material loss and leaving an irregular oblique fracture plane.47 In the present study two of 40 IPS e.max Press partial coverage restorations (5%) demonstrated minimal ceramic cohesive fractures (chipping). Comparable chipping fracture rates of 1–2.7% after 4–5 years26,36 were observed for IPS Empress onlays. In long-term studies a significant increase of chipping fractures has been recorded over time (26% of the IPS Empress inlay and onlay restorations after 8 years).3 In the present study one Cerec ProCAD restoration (2.5%) showed a cohesive fracture at the occlusal-buccal marginal area. Slightly higher chipping fracture rates for Cerec Vita Mark II partial coverage restorations were obtained by Federlin et al.30 Two Vita Mark II restorations (6.9%) showed occlusal chipping without need of replacement. One further restoration (3.6%) exhibited severe chipping at the marginal ridge and proximal contact after 2 years and had to be replaced at the 3year follow-up interval.31 In the present study, no correlation of ceramic cohesive fractures to previous rotary instrumentation during occlusal

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adjustment or to occlusal contact areas or wear was observed (Fig. 4), like it was reported in the literature.3 The satisfying clinical outcome of the presently investigated materials may be related to the enhanced mechanical properties of the all-ceramic systems. As a result of the lithium disilicate microstructure, the pressable glass–ceramic IPS e.max Press material reveals a flexural strength of up to 400 MPa (manufacturer’s information). The leucite-reinforced ProCAD blocks developed for CAD/CAM technology exhibits a lower flexural strength of 140 MPa (manufacturer’s information) but is reported to possess a high structural homogeneity due to industrially prefabrication.19,48,49 In the current study, none of the partial coverage restorations detached after an observation period of 36 months. Van Dijken et al.36 reported a loss of retention of four IPS Empress restorations (2.1%) and Federlin et al.30 of one Vita Mark II partial coverage restoration (1.7%) after 4.9 and 2 years, respectively. Secondary caries did not occur at either pressed or CAD/ CAM fabricated partial coverage restorations; this coincides with other studies on IPS Empress3,29 and Vita Mark II restorations.31 Secondary caries has only rarely been mentioned in the literature as a cause for the loss of all-ceramic partial coverage restorations.12,22,26,36 Biological failures in terms of endodontic failures did not occur. Pulp vitality could be preserved during the entire investigation period of 36 months. No patient reported pain, postoperative hypersensitivity or was in need of endodontic treatment. Similar results were found in other clinical studies on partial coverage restorations over a 3–7 years observation period.22,24 The used modified UPSHS criteria proved to be reliable and suitable for the evaluation of tooth colored restorations as previously reported in several studies.3,8,12,22,24,29,31,36,50 Each criteria will be discussed in particular. Regarding the criterion marginal adaptation, a distinct deterioration was found over time, independently of the fabrication techniques and all-ceramic materials used. Alpha ratings decreased from 95% to 83.3% for IPS e.max Press and from 92.5% to 65.2% for ProCAD restorations. In accordance to the present investigation, a decrease in marginal adaptation of ceramic restorations over time is reported in the current literature. IPS Empress restorations revealed 28.6%, 40–62.1% Bravo ratings after 4, 5 and 7 years.22,26,36 CAD/CAM fabricated Vita Mark II partial coverage restorations showed 21.4–43.1% Bravo ratings12,31 or slight submargination (69% beta rating) after 3 years.24 Local margin imperfections such as reduced continuous margin and slight palpable ditching resulted in increased Bravo ratings. Marginal deterioration can be attributed to the degradation and wear of the cementation composite. The restoration-composite-tooth structure bond is susceptible to chemical, thermal and mechanical fatigue in the oral cavity.20,51 Aging of the adhesive luting components as a result of primary polymerization shrinkage, different coefficients of thermal expansion of the bonding materials, composite degradation and increased elution of fillers due to water uptake52 is accepted as a known inherent general drawback in adhesive dentistry.16,45,49


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Although degradation of the luting resin over time has not been associated with absolute failure of all-ceramic inlay and onlay restorations and marginal deterioration was within the limits of clinical acceptability, it has been shown to result in micro-cracks and chipping of the all-ceramic material at the margins.53 The high wear rate of the luting cement can also be associated with a high modulus of elasticity of the ceramic restoration materials which during chewing forces transmits stress to the margins and to the cement whose modulus is lower.29,45,54 The SEM evaluation revealed distinct changes with time in the luting gap, namely substance loss of the luting composite in the contact free marginal areas (Fig. 4). Surface breakdown and wear of the luting composite might compromise the support of the brittle ceramic materials potentially causing marginal chipping.51,55 Crack formation in enamel at the marginal areas as reported by Kra¨mer et al.3,40 was not observed in the present study. The criterion marginal adaptation of partial coverage restorations is closely correlated with the occurrence of marginal discoloration.45 In the present study, marginal discoloration revealed to be the most significant change over time for both ceramic materials, showing a statistically significant difference after 1–3 years when compared to baseline data. 37.5% of the IPS e.max Press and 52.2% of the ProCAD restorations showed Bravo ratings at the 3-year recall. A significant number of clinical investigations have shown that the initial good marginal adaptation does not remain stable over time. Marginal discolorations have been reported to be 13%, 16.4%, and 31% for IPS Empress restorations after an observation period of 4, 4.9 and 7 years, respectively,22,26,36 and 14.3% and 34.5% for CAD/CAM fabricated Vita Mark II restorations after 3 years.12,31 Exogenous deposits and plaque,49 the width of the luting space and the capacity for staining of the exposed luting composite have been discussed as factors for marginal discolorations.31 In preceding laboratory studies on the investigated allceramic systems and preparation design, marginal accuracies of 56 mm for IPS e.max Press and 71 mm for the ProCAD partial coverage restorations were obtained after 5-year artificial aging and met the clinical requirements.20,54 Higher average composite joint width of 207 mm to was 308 mm after 1.4–4.6 years were found in a clinical follow-up investigation on CAD/ CAM fabricated partial coverage restoration.24 However, incidence of secondary caries was only reported in the present literature in correlation with extensive marginal gaps (gap widths > 250 or 400 mm).47 In the majority of studies dual curing composite resins have been used to cement all-ceramic inlays and onlays.8,21,22,41 The solely light curing one phase composite resin Tetric was used in the present study due to its user friendly application, increased working time and high radio opacity. Tetric has also been applied for adhesive cementation of pressed56,57 and CAD/CAM generated all-ceramic restorations by other author groups.12,24 The use of light curing composite agents has been questioned.36 However Kra¨mer et al.40,57 could not reveal any difference regarding clinical performance and luting gap degradation between dual and light curing composites. The application of a light curing

composite in the present study did not result in increased failure rates when compared to restorations cemented with dual-curing resin cements reported in the literature.22,29,31 After 3 years of observation the surface texture of IPS e.max Press and ProCAD partial coverage restorations became significantly rougher. In the present study the alterations in surface roughness were additionally imaged in the SEM using replica models (Fig. 4).47,58 The initial smooth surface texture changed to localized rough areas mainly at the occlusal contact areas. 79.2% of the IPS e. max Press and 74% of the ProCAD restorations showed Bravo ratings after 36 months. In the literature, shortcomings in surface texture were also reported for IPS Empress (54.4–70% Bravo ratings after 4–5 years26,36) and for Vita Mark II (50–59% Bravo ratings after 4.2 years24) partial coverage restorations. In the present study, vast numbers of partial coverage restorations irrespective of the all-ceramic system used showed minor color deviation during the given midterm observation. The color mismatch (Bravo rating) increased from 5% for ProCAD and 7.5% for IPS e.max Press partial coverage restorations at baseline to 60.9% for ProCAD and 91.7% for IPS e.max Press at the 3-year examination. Decreasing color stability has also been reported for IPS Empress (24% Bravo ratings after 4 years26) and Cerec/Vita Mark II restorations (90% Bravo ratings after 4.2 years).24 Color changes and deterioration of the surface could be the result of occlusal contact wear, extrinsic mechanical wear and chemical degradation of the glazing material.3,49,59 However the increase of surface roughness as well as the color deviation and alterations over time were not deemed to be a significant clinical problem both in this investigation nor in any earlier studies and were only evaluated by the examiners. Due to the minor extent, replacement of any of the restorations was not required. To overcome these esthetic shortcomings regarding initial and long-term color match and to improve the chameleon effect of full anatomic surface stained partial coverage restorations, high translucent ingots have been recently developed to increase translucency of pressable ceramics. At the time when the study was initiated, a major drawback of CAD/CAM fabricated restorations consisted in the uniform color of monolayered all-ceramic blocks. Application of surface staining was necessary to obtain individual characterization. Meanwhile multilayered all-ceramic blocks with multiple translucencies and a balanced blend of chroma have been designed to feature a natural appearance in shade, translucency and fluorescence. Anatomic form was the only criteria that revealed significance between the two investigated fabrication techniques from the baseline evaluation onwards. Whereas generally well contoured occlusal and marginal surface morphology was found in pressed partial coverage restoration, significantly lower numbers of CAD/CAM fabricated restorations were scored Alpha with regard to anatomic form (Fig. 2). Both all-ceramic systems showed distinctive wear and loss of contour with increasing Bravo rating scores within the evaluation period of 3 years. Similar results were described for IPS Empress3,36 and Cerec/Vita Mark II8,12,31 partial coverage restorations. It must be emphasized that these results were observed with an earlier version of the Cerec system’s


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hardware and software protocol. Further advances in CAD/ CAM technology have been reported. Technically improved hardware and software will overcome the shortcomings in marginal fit and restoration design faced at the time when the study was started. New models of automated CAD such as biogeneric tooth reconstruction will be able to rebuild the occlusal morphology of teeth in specific clinical situations and to improve the esthetic outcome.8 Preparation guidelines and standards for all-ceramic partial coverage restorations have not been described yet and the importance of preparation design and ideal extension of all-ceramic intra- and extra-coronal restorations is still discussed controversially in present literature.36 Whereas some clinical studies could not observe any difference in longterm behavior of inlay compared to onlay restorations,3,8,36,37,56,60 a tendency towards better performance of onlay restorations has also been reported.41 In the present study, the preparation design using a butt joint reduction of the occlusal surface and a rounded shoulder of the bearing aspect followed guidelines mentioned in the literature46,61 and was developed for extended defects in posterior teeth. Previous in vitro studies revealed favorable fracture resistance for the investigated partial coverage configuration.19,54,62 Although improved aesthetics and color match may result form gradual transition from ceramic to natural tooth structure with buccal knife edge and 458 bevel finish lines, the ceramic margins are particularly vulnerable to crack formation and fracture.28 Partial coverage restorations with butt joint margins can be aesthetically challenging when labial aspects are involved as the transition between tooth and ceramic can be apparent and difficult to mask.46 However the preparation form provides a maximum thickness of the ceramic material which is reported to minimize the risk of crack formation and fractures and to contribute to an increased crown stiffness in severely damaged posterior teeth.63 In finite element analysis, partial coverage restorations that included the complete occlusal surface and revealed a rounded shoulder margin at the bearing cups were characterized by favorable stress pattern as a result of increased adhesive surface and uniform load distribution.63–65 Occlusal function at the margin of the restoration could be avoided due to the defect size determined extension to the buccal and lingual aspect including the complete occlusal surface.61 All-ceramic pressed and CAD/CAM fabricated partial coverage restorations for restoring larger cavities in posterior teeth present a satisfactory tissue saving alternative to full crown treatment. However, because of the time dependent degeneration of the ceramic material, possible crack propagation and marginal deterioration, it is important to further investigate the presently used all-ceramic systems and materials in the oral environment to predict the long-term outcome of resin retained ceramic partial coverage restorations. The accomplishment of the 5-year data within this prospective clinical study will allow further evaluation of the described treatment option. The susceptibility of the luting material towards degradation in clinical function will be a major aim of further improvement.

5.

635

Conclusions

Based on the midterm results of this prospective clinical splitmouth study covering an observation period of up to 36 months, pressed and CAD/CAM fabricated ceramic partial crowns can be considered as a reliable treatment option for the restoration of larger defects in the posterior dentition. The long-term success of adhesively cemented partial coverage restoration is determined by the durability of the adhesive bond and cementation composite as well as by the mechanical reliability of the all-ceramic material used.

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a DepartmentofProsthodontics,Albert-Ludwigs-University,Freiburg,Germany b DepartmentofBiomaterialsandBiomimetics,NewYorkUniversityCollegeofD...