INTRODUCTION

Esthetic concerns have fostered the creation of ceramic dental materials used to manufacture metalfree crowns. Presently, due to its hardness and resistance, zirconium dioxide is used to manufacture copings for partial fixed prostheses. Zirconia can be used in CAD/CAM and pantographic systems¹ CAD/CAM systems operate by scanning a physical object transformed into a 3D image through a graphic design software. The design is sent to a robotic drilling device in order to obtain the final product.²

Manufacture of restorations by means of pantographic systems consists on the making of a replica that can be used as a guide; this device can be replicated in the desired material.³

As a rule, presintered zirconia is used in its porous initial state. Later sintering produces a 20-30%⁴ polymerization contraction, and it can be associated to defects in the resulting marginal precision.⁵ Zirconia is used as yttrium partially stabilized (Y-TZP) tetragonal zirconia, with excellent biocompatible, mechanical and esthetic properties.⁶ Resistance to fracture is associated to the zirconia hardness transformation phenomenon. This hardness was acquired during the change of tetragonal to monoclinic phase.⁷

In the manufacture of fixed prostheses, marginal precision is a factor of the utmost importance, since marginal fit defects produce long term failure of the rehabilitating treatment. Presence of space between coping and stump contributes to bio-film formation. As a consequence, gingival inflammation occurs as well as development of secondary caries.⁸ A study conducted on zirconia restorations after five year cementation determined the presence of secondary caries in 22% of cases.⁹ Marginal lack of precision increases bacterial prevalence in the mouth, incrementing thus periodontal disease incidence. Micro-leakage can produce endodontic problems.¹⁰ Cement dissolution and later de-cementation of the restoration are also possible.¹¹ Very thick cement layers increase tension strength in crown surfaces, producing porcelain wear.⁴ Acceptable space between coping and tooth must be under 120 μm.¹²

The aim of the present study was to compare marginal precision of zirconia copings with use of the following systems: CAD/CAM Cerec InLab (Sirona^®), CAD/CAM Zirkonzahn (Zirkonzahn^®) and Zirkograph 025 ECO pantographic system (Zirkonzahn^®). The project's hypothesis was that marginal fit of zirconia copings manufactured with the pantographic system Zirkograph 025 ECO (Zirkonzahn^®).

METHOD

An in vitro explicative, experimental and comparative study was conducted. A Cr-Co master model was fabricated, based on an ivorine preparation for zirconia crown of an upper premolar. Proximal shaping consisted on a 1 mm lingual and vestibular proximal reduction, and 1.5 mm in occlusal direction, with a rounded-shoulder gingival margin. This same carving was replicated in a Cr-Co master model, with aluminum based cubic design supporting structure (Figure 1). All samples were manufactured on that model.

Figure 1 Chrome cobalt master stump. 

Ten copings were manufactured for each system according to manufacturers’ instructions. Four groups were established, one for each of the following systems: CAD/CAM Cerec InLab (Sirona^®), CAD/CAM Zirkonzahn (Zirkonzahn^®), Zirkograph 025 ECO (Zirkonzahn^®) as well as one control group formed by metallic copings (Figure 2). Pre-sintered zirconia was used, which later completed sintering according to instructions for each system. Control group was formed by ten metallic copings manufactured following the lost wax system.

Figure 2 Copings of all analyzed systems. 

Copings were disinfected with 70% ethanol, this procedure was followed by an ultrasonic disinfection in water for 5 minutes.

Internal impressions of each coping were taken following replication technique. To this end light weight polyvinyl siloxane was used.

Elite HD+ Super Light Body Fast Set (Zhermack^®) was used as well as heavy weight polyvinyl siloxane elite HD+ Putty Soft Normal Set (Zhermack^®). A constant 50N force device was designed.^I This device was gauged at the Ecuador Institute of Normalization (Instituto Ecuatoriano de Normalización [INEN]), in order to control settling force of each coping on the master stump. This procedure was conducted under the supervision of Dr. Fernando Sandoval, at the Research Area of the School of Dentistry of the San Francisco de Quito University.

Stored samples were kept in a dry environment, 24 hours after taking the impression longitudinal sections were executed for observation under stereoscopic microscope Meiji Techno EMZ-13TR (Meiji Techno^®) at a 50x magnification.

One point per side in each coping was analyzed at marginal level. Marginal width (w) was measured in microns, and absolute marginal discrepancy was determined (z). To this effect, the method standardized by Holmes et al¹³ was used: this method measures horizontal (x) and vertical (y) marginal discrepancy. Measurements photographs were taken with Infinity 1 software (Lumera^®) (Figure 3). Said photographs were approved by Dr. Fernando Sandoval, Dean of the School of Dentistry, University San Francisco de Quito. Statistical analysis was conducted with SPSS IBM^® software. Obtained data and significance were executed with t test.

Figure 3 Determination of absolute (z), horizontal (x) vertical (y) marginal discrepancy and marginal width (w). Magnification of 50x, Meiji Techno EMZ-13TR stereomicroscope/(Meiji Techno®). 

RESULTS

Mean absolute marginal discrepancy and mean marginal thickness were 92.14 ± 38.59 and 78.62 ± 31.33 μm for CAD/Cam Cerec InLab (Sirona^®) system; 38.71 ± 12.62 and 36.91 ± 13.56 μm for CAD/CAM Zirkonzahn (Zirkonzahn^®); 77.92 ± 38.01 and 69.42 ± 33.23 μm for pantographic system Zirkograph 025 ECO (Zirkonzahn^®) and 44.11 ± 15.36 and 43.74 ± 15.70 μm for the metallic copings control group (Figure 4). The CAD/CAM Zirkonzahn group (Zirkonzahn^®) presented lower values of absolute mean marginal discrepancy and mean marginal thickness; it was followed by the control group of metallic copings, with an error in standard deviations lesser than 5% (3.17%). The group with greater mean marginal discrepancy (vertical, horizontal and absolute) and mean marginal thickness was Cerec InLab (Sirona^®) (Figure 5). With t test, statistically significant differences were recorded at the absolute marginal discrepancy (y) and marginal thickness (w) between groups Cerec InLab (Sirona^®) and Zirkograph 025 ECO (Zirkonzahn^®), in comparison with the control group of metallic copings (p = 0.0009) (p < 0.001). No statistically significant results were observed between CAD/CAM groups Zirkonzahn (Zirkonzahn^®) and control group (p > 0.001).

Color images can be found at: www.medigraphic.com/facultadodontologiaunam*Colors: purple Cerec InLab (Sirona^®) CAD/CAM system; red: Zirkonzahn (Zirkonzahn^®) system; orange: Zirkograph (Zirkonzahn^®) system; green: metallic copings.

Figure 4 Box plot representing distribution of horizontal (x), vertical (y) absolute (z) and marginal thickness (w) dimensions obtained in all analyzed systems. 

Figure 5 Graph showing mean values of absolute (z) marginal discrepancy and marginal width (w) of all systems used. 

DISCUSSION

In the present study, copings manufactured with CAD/CAM Zirkonzahn^® system proved to be the most precise. Copings manufactured with CAD/CAM Cerec InLab system (Sirona^®) exhibited greatest marginal discrepancy, with maximum values above clinically accepted values in literature. Therefore, the hypothesis offered for the present study was partially accepted, since copings manufactured with CAD/CAM system Zirkonzahn^® exhibited the greatest marginal precision.

Statistically significant differences between both assessed CAD/CAM groups could be due to factors related to the digitization of each system. Both systems are subject to limitations related to the scanning finite resolution. According to Reich et al,¹⁴ marginal precision errors are due to the formation or rounded angles or peaks formed due to the scanning optical system. Moreover, the use of opacifying spray for scanning might inhibit refl ection during the process, causing increase of the marginal thicknesses. Studies conducted by Beuer et al,¹⁵ Grenade et al¹⁶ and Kohorst et al¹⁷ also determined the fact that CAD/CAM elaboration processes (drilling and sintering) significantly affect marginal discrepancy. Influence of operator and dental clinican^14,15 establish the high sensitivity of the CAD/CAM technique.

Reich et al¹⁴ determined a 65 μm marginal discrepancy for zirconia copings of Cerec InLab system (Sirona^®). The study of Caparroso, Marín and Velásquez¹⁸ established a 47.34 μm marginal thickness, Bindl and Mormann¹⁹ established 43 μm, Beuer et al¹⁵ established 57 μm. When comparing the aforementioned values with those obtained in the present study, it can be established that the obtained mean marginal discrepancy value (78.62 μm) was greater than that reported in scientific literature.

Marginal discrepancy values of the Zirkograph 025 ECO system (Zirkonzahn^®) were above those obtained with the CAD/CAM Zirkonzahn system, but below to those achieved with the CAD/CAM Cerec InLab (Sirona^®) system. Santamaría, Aldana and Martín²⁰ assessed the systems CAD/CAM Cerec InLab (Sirona^®) and Zirkograph 025 ECO (Zirkonzahn^®) systems; in their pantographic study they found a greater marginal discrepancy of 121.3 μm than that obtained in the present study. Park et al²¹ established statistically significant differences between CAD/CAM and pantographic systems, and determined that the pantographic system exhibited better coping marginal precision. This concurs with findings of the present study which compared CAD/CAM Cerec InLab^® and Zirkograph 025 ECO (Zirkonzahn^®) groups.

Statistically significant differences were established between Zirkograph 025 ECO (Zirkonzahn^®) pantographic system and the metallic copings control group. In the present study as well as in other assessed studies found in scientific literature, mean marginal discrepancy was found to be below 120 μm, and therefore, was clinically accepted. The greater marginal discrepancy of pantographic systems is due to the greater influence exerted by applied manual procedures.

Caparroso, Marín and Velásquez¹⁸ determined that metallic copings exhibited better marginal adaptation than zirconia copings manufactured with the CAD/CAM Cerec InLab (Sirona^®) system. This result concurs with the result obtained among CAD/CAM Cerec InLab (Sirona^®) groups and the control group. Bindl et al¹⁹ stated there was no statistically significant difference in precision margin when comparing CAD/CAM systems and the metallic copings conventional technique. This concurs with results obtained in the present study of CAD/CAM Zirkonzahn groups (Zirkonzahn^®) and control group. Reich et al¹⁴ established that resulting marginal discrepancy of Cerec InLab (Sirona^®) system was similar to that obtained in metallic copings. This result does not concur with data of our study for the same CAD/CAM system, but it does concur for the CAD/CAM Zirkonzahn (Zirkonzahn^®) system.

Vodjani et al²² established that metal-porcelain crowns manufactured with conventional technique exhibited better marginal precision than CAD/CAM system crowns. This assertion in true among Cerec InLab (Sirona^®) systems and the control group, but is not applicable to CAD/CAM Zirkonzahn (Zirkonzahn^®) system and the control group.

Due to the contraction of the siloxane polyvinyl used in the replication technique, marginal discrepancy measures could indeed be lower than real measurements. In later research projects it is suggested to additionally conduct other marginal fit determination techniques such as cementation crosssectioned techniques.

CONCLUSIONS

CAD/CAM Zirkonzahn system (Zirkonzahn^®) exhibited the lowest absolute marginal discrepancy, it was followed by the metallic copings groups; no statistically significant differences were found between these two groups. The less precise system was CAD/CAM Cerec InLab (Sirona^®), which exhibited statistically significant differences in all studied groups. Marginal precision of CAD/CAM systems depended on the manufacturing processes of each separate system. In all groups mean values of absolute discrepancy and marginal thickness were lower than 120 μm, therefore, they were clinically acceptable.