INTRODUCTION:

Prosthodontics is branch of dentistry with marked innovation. As a result, the desire to retain their natural teeth has markedly increased for the past decades and therefore the expectations for esthetically pleasing restorations have increased¹. This has been reflected with the increased demand for ceramics as ideal tooth colored restoration for fixed prosthodontics².

The increased use of ceramic crowns luted with resin composite cements and the introduction of CAD/CAM as a manufacturing technique had intensified the debate about the internal and marginal fit of dental restorations³.

In an attempt to control quality and improve speed, dental laboratories are undergoing mechanization with some outsourcing certain stages to other labs. The use of CAD/CAM and copy milling, particularly with regard to core material is increasing⁴.

There is also rise in the use of pressable ceramic rather than conventional hand layering techniques.In attempts to increase the fracture toughness and strength of dental prosthesis, several new ceramic materials and techniques have been used during the last decade. Improvements in ceramic technology have taken the form of novel processing technique. This, along with patients demands for superior esthetics has led to a new interest in all-ceramic dentistry⁵. All ceramic materials can be used for making all kinds of dental restoration.⁶ Optimal esthetics and mechanical properties like high wear resistance and low thermal conductivity make all-ceramic materials ideal for processing of fixed dental prosthesis⁷.

The fabrication of all ceramic dental prosthesis can meet a wide variety of error while processing in the machine during sintering or heat pressing. Elimination of investment material and proceeding for bonding by modifying the surface with sandblasting may cause defect in the surface geometry⁸. As a result, lot of defects in the material, which will bring down the strength and variability. In CAD-CAM fabricated restorations where spacing can be digitized into interior and margins to permit accurate fit.

Heat pressing technique has become an alternative technique to produce a veneering layer of all-ceramic restorations. Anecdotal evidences and marketing information from the dental companies had reported that dental technicians prefer the pressing technique because of its speed, accuracy, and stability⁹. The system uses the lost wax technique and processing cycles are short. It can also be used as a conventional porcelain furnace⁵. It was therefore of interest to study whether the different manufacturing processes and ceramics influence the fit of the restorations. Therefore, the purpose of this study was to evaluate and compare the internal fit of zirconia copings manufactured by a CAD/CAM system and pressable ceramic copings manufactured by conventional casting procedure.

MATERIALS AND METHOD:

Tooth preparation:

A typodont mandibular left first molar was prepared about 1.5mm occlusal reduction and with shoulder finish line of 1.0mm width around the entire circumference and convergence angle of 6 degrees. All the line angles were rounded. Before starting the preparation, silicone putty index of the unprepared tooth to be made to facilitate the accurate reduction of the tooth. (Figure.1).

Figure 1: Occlusal view of prepared tooth

Fabrication of die:

The prepared tooth was duplicated using silicon putty impression material. (Aquasil; Dentsply, India). An auto-polymerizing PMMA resin (cold cure resin, DPI, India) was poured into the impression to form a pattern which was invested with investment material (Bego, Germany) and casting was done after burnout at 950°C temperature with a metal alloy. (Bellasun, Bego, Germany) The metallic die with sprue button was used which aid in easy removal of the die from the impressions. Then wash impression was made with putty and light body silicone impression material. (Aquasil; Dentsply, India). The metal die was removed in one stroke slowly to avoid tear in the impressions made. After that, two gypsum die per impression was poured with a low expansion die stone. Thereafter, corresponding gypsum models were formed with type IV die stone (Elite Rock, Germany). The recommended ratio of 20 ml of distilled water to 100g of powder was vacuum mixed (BEGO Motova 100, Germany), then slowly vibrated (laboratory vibrator, serial no. 106513, Korea) into the impressions and allowed to set for 45 minutes as recommended by the manufacturer.

Distribution of samples:

All the 20 dies were separated into two main groups. Group.1 comprised of 10 dies on which CAD/CAM milled zirconia copings were fabricated and Group.2 comprised of 10 dies on which pressable ceramic copings were fabricated using lost wax and heat pressing technique.

Fabrication of Zirconia copings:

Under Group 1, 10 dies were there for the fabrication of YTZPi- zirconia copings (Ceramill Zi, Amann Girbach, Austria). The internal laser scanner unit of the CAD/CAM system scanned each die. After scanning, the copings were designed on the computer (Ceramill map300, Amann Girbach, Germany). Die spacer covering the coronal 80% of the die with a virtual layer of 0.05mm and a coping thickness of 0.9mm. The die was scanned and the enlarged version of the virtually manufactured coping was sent to the dry milling machine (Ceramill motion, Amann Girbach, Germany). The enlarged copings were milled from the YTZPi- zirconia blanks of 12mm thickness (Ceramill Zi/Lot no. 1111889, Austria). Finally the substructures were sintered in a special furnace (Ceratherm, Amann Girbach, Germany) for 12 hours at 1400°C temperature for the optimum shrinkage to occur. All copings were distributed to their corresponding die and evaluated for marginal discrepancy by visual method. (Figure.2)

Figure 2: Zirconia Coping

Fabrication of heat pressable lithium disilicate ceramics:

Under Group 2, 10 dies were there for the fabrication of heat pressed lithium disilicate copings (IPS, e max press, Ivoclarvivadent, Liechenstein). Each die was scanned by using the internal laser scanner unit of the CAD/CAM system (Ceramill map300, Amann Girbach, Germany). The virtually manufactured coping on the data scanned was same as the settings made previously for the zirconia copings i.e. a cement space of 0.05mm and a coping thickness of 0.9mm. The data was sent to the dry milling maching (Ceramill motion, Amann Girbach, Germany), wax blocks of 20mm thickness (Polywax, Bilkim co. ltd,Turkey) were used for the production of wax patterns.

The wax patterns were attached with sprues and invested with gypsum bonded investment with the ratio of 40 ml to 160g of investment material (Bellavest SH, Wirobond, Bioponto star, Germany) was vacuum mixed (Whipmix, Kentucky, USA) then slowly vibrated (Laboratory vibrator, serial no.106513, Korea) into the ring containing the wax patterns, which were applied with debublizer to avoid any void during pressing. The investment was allowed to set for approximately half an hour and then placed in the heat furnace (Sirio muffle furnace, Italy) from 15°C to 950°C temperature for the burnout procedure. After reaching to the desired temperature in about 1- 1.5 hours, the all-ceramic ingot (IPS, e max press, Ivoclarvivadent, Liechenstein) was placed in the vent present in the crucible former. Then the whole unit was placed in the pneumatic ceramic furnace (EP 600, Ivoclarvivadent, Switzerland) for heat pressing. At the temperature of approximately 950°C, the melted ingot was pressed by the pneumatic arm to fill in the area of the wax pattern, which was lost during the burnout procedure.

Internal fit evaluation:

The crowns were fixed to the respective dies with cyanoacrylate cement using finger pressure. The crown die complexes were then embedded in a rectangular auto-polymerizing acrylic block.The rectangular block was made with the help of a custom-made metal mould where the sample die was kept in the center and auto-polymerizing resin was poured all around the housing made for the embedding. The samples were then sectioned facio-lingually with a precision saw following the guide marks on the dies (Bain UT cutting machine, Chennai Metco Pvt Ltd). There were 2 interfaces for each point of sectioning so measurements can be done at 3 points around the crown. The three zones were 2mm above the marginal finish line in both buccal and lingual side and one in the center of the occlusal surface. Mean vertical and horizontal internal adaptation discrepancies were calculated, from the values obtained and were compared. After sectioning of the coping die complexes, examined under Scanning electron microscope. (TESCAN VEGA 3)(Figure.3)

Figure 3: SEM analysis of samples

Statistical Analysis:

The collected datas were statistically analyzed using SPSS windows 17 ^th version (IBM; New York, United State). The descriptive statistical methods like 25th percentile, 75th percentile, mean, median and standard deviation were calculated. The mean values were compared using the non-parametric inferential method namely Mann-Whitney test. The level of significance was fixed as 5 %.

RESULTS:

The mean internal gap width for the group was 280.3 μm with the standard deviation of 6.1 μm. The mean internal gap width for the pressed group was 238.33 μm with the standard deviation of 9.49 μm. The Mann-Whitney statistical test was applied to compare the two mean values. The significant p-value indicates that internal gap width in the heat pressed ceramic copings treated group was lesser than CAD –CAM milled zirconia treated group. (Table.1)

Table 1: Mean and standard deviation of internal gap width at Mid-Occlusal by groupwise

Group	Mean (μm)	Standard deviation	Mann-Whitney test value	P-value
CAD-CAM Milled Zirconia Copings (Group 1)	280.32	6.10	-3.780	<0.001
Heat Pressed Ceramic Copings (Group 2)	238.33	9.49

The mean internal gap width in μm at mid-buccal for the CAD-CAM milled Zirconia copings group was 137.3μm with the standard deviation of 14.05μm. The mean internal gap width at mid-buccal for the heat pressed ceramic copings group was 170.5μm with the standard deviation of 8.96μm. The non-parametric Mann-Whitney test was been applied to compare these two mean values. The significant p-value indicates that the internal gap width was less in the CAD-CAM milled group compared to Heat Pressed ceramic copings group.(Table.2)

Table 2: Mean and standard deviation of internal gap width at Mid-Buccal by groupwise

Group	Mean (μm)	Standard deviation	Mann-Whitney test value	P-value
CAD-CAM Milled Zirconia Copings (Group 1)	137.310	14.0580	3.553	<0.001
Heat Pressed Ceramic Copings (Group 2)	170.530	8.9630

The mean internal gap width at Mid-Lingual for the CAD-CAM Milled Zirconia Copings group was 145.18 μm with the standard deviation of 20.9 μm. The mean internal gap width at Mid-Lingual for the Heat pressed ceramic copings was 172.18 μm with the standard deviation of 5.30 μm. The significant Mann-Whitney test result confirms that the internal gap at Mid-Lingual of Heat pressed ceramic copings was higher than CAD-CAM Milled Zirconia copings. (Table.3)

Table 3: Mean and Standard deviation of internal gap width at Mid-Lingual by groupwise

Group	Mean (μm)	Standard deviation	Mann-Whitney test value	P-value
CAD-CAM Milled Zirconia Copings (Group 1)	145.180	20.9908	-3.025	.002
Heat Pressed Ceramic Copings (Group 2)	172.180	5.3072

DISCUSSION:

All ceramic restorations are preferred by most of the patients because of aesthetics. Hence, the dental practitioners should have knowledge on recent advancement in restorative materials and explain the details to the patients about the benefit of the recent advancement in esthetic restoration¹⁰. Zirconia ceramics are superior in bending strength and fracture toughness compared to other ceramic, and yttrium partially stabilized zirconia (Y-TZP) was used as reinforced zirconia.The metal like nature of ceramic leads to use of zirconia in making dental prosthesis¹¹. There many techniques used for all ceramic systems are the heat-press and computer-aided design and manufacturing (CAD/CAM)system¹². CAD/CAM system means that a major part in the working sequence is carried out by industrial machine¹³.

The milling involves fabrication of a substructure with zirconia or the entire unit (s) could be machined. The machinable ceramics have more advantage than conventional prosthesis because it eliminates the clinical steps of impression making and laboratory steps like cast pouring,articulation, die sectioning, casting, and subsequent layering which will save enormous time but the supremacy of machinable system over the conventional layered ceramics, with effect to marginal discrepancy, is not clearly established in the literature¹⁴. Screw retained zirconia showed better marginal adaptation than cement retained crown¹⁵. The marginal fit of CAD/CAM milled Zirconia coping is more accurate than Ni-Cr alloy coping fabricated in conventional manner¹⁶.

In accordance with the results observed in the current study, it is related to the clinical investigation that widest gap dimensions were found in occlusal adaptation. Literature showed that cement space thickness of 80μm have been reported to be more favorable for the mechanical stability of zirconia-based restorations.Laboratory milling of semi-sintered zirconia exhibited similar accuracy as centralized milled zirconia substructures¹⁷. It was estimated in another study that the mean gap at the cusp tip and central fossa of the occlusal surface were about 120μm which was less than the results obtained in the present study. It was suggested that more dilution of the special liquid with water was necessary to reduce the expansion at the occlusal surface.

In another study significant differences found in internal gap between the Procera system and Cerec 3D system, where the Procera system had better fit than the Cerec 3D system. Internal gap of the conventional all-ceramic crowns was within the range of 123 to 154μm as in the previous studies conducted. For the single layer system fabricated directly from the milling block, relatively large internal gaps existed because of bur size and the accuracy of milling and scanning¹⁸. This was not in conjunction with the current study where the values of internal fit were more than the previous studies,i.e within the range of 220-290μm. Even so, it had been reviewed earlier in studies that the range was clinically acceptable.

The adaptation of most zirconia based dental prosthesis manufacturing with CAD-CAM method was within the acceptable range for meeting clinical requirements and was explained by many authors who had concluded that rounded shoulder or chamfer preparations were recommended for the finish line designs of zirconia-based restorations where in the current study the finish line design may also have aided in the fit of the restoration. Increasing the convergence angles of the tooth abutments reportedly improved the internal and marginal adaptation of zirconia-based crown in a study^19,20.

It was reported in another study that complex fabrication process and variability of manual procedures for CAM only system, such as definitive die preparation with spacer, and stated that waxing and wax pattern removal from the die might cause differences in adaptation¹⁷.

Several studies had showed that milling of pre-sintered zirconia material yielded results superior to those for milling of fully sintered zirconia and green stage zirconia²¹. Using CAD software, found that computer fixed cement spaces might have an effect on adaptation of zirconia-based crowns²². In another study the post-machining sintering effect on marginal and internal fit of CAD-CAM fabricated zirconia framework were assessed and found that the mean value of thickness of luting space at the occlusal surface was larger than the set value which was in conjunction with the present study. This may be due to the anisotropic shrinkage of the zirconia blanks subjected to post-machining sintering. As a result, sintering shrinkage in tooth axis was smaller than that in the horizontal axis²³. Also, the literature showed that There was a marked difference between marginal fit all-ceramic crowns of zirconia with layering and monolithic crowns with the influence of firing cycles²⁴.

In the present study, the internal gap of 230 to 290 μm at the occlusal surface was more than the axial surface (120 to 190 μm). These results were the same as another CAD/CAM system was reviewed in a study²⁴. In the literature, values of 49 to 136 μm had been reported for the internal fit of all- ceramic restorations. The data supported that there was not much difference in the marginal and internal gap between the 2-fabrication method and the amount of internal and marginal discrepancy was in the clinically acceptable range.

The limitations of the present study where the gap dimensions were measured using the cross-section technique. As a result, the precision was measured at only 3 defined areas per coping, which may not be the representation of accurate fit. Sectioning of the sample may deteriorate the fitness of the sample. Also, all retainers were produced and tested under ideal conditions, which may reflect the precision in daily clinical use. Only one set of standard settings of CAD/CAM of die spacer thickness had been experimented. Different settings might influence the accuracy and also, cyanoacrylate was used to position the copings in place with no definitive loading force to ensure the complete fit. The anisotropic shrinkage of zirconia blanks subjected to post-machining sintering. Due to this shrinkage property, either uniform zirconia blanks need to be prepared for post-machining sintering or that dimensional changes of frameworks need to be corrected/adjusted in view of the anisotropic shrinkage of blanks.

Clinical Significance:

There is evidence of influence of excessive cement space on failure of the prosthesis.The viability of a computer-aided and three-dimensional approach for assessing marginal and internal fit of indirect restorations was minimal, thereby, reducing the microleakage of the restoration which will increase the shelf life of the fixed dental prosthesis.

CONCLUSION:

Within the limitations of the present study, the authors concluded that fit of the single crown coping was acceptable except for the wider gap at the occlusal surface than in the axial surface. There was significant difference in the internal fit comparison between the two fabricated copings. To guarantee good fit of CAD-CAM fabricated zirconia frameworks, it was mandatory to perform delicate dimensional adjustment during the CAD process as well as implement good framework design guidelines to prevent the distortion of frameworks. It is necessary for the dental practitioners to decide the material of choice for the restoration, due to the increased variability that can occur due to its properties while fabrication.

CONFLICT OF INTEREST:

The authors have no conflict of interest regarding this investigation.

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Received on 04.09.2021 Modified on 09.05.2022

Research J. Pharm. and Tech 2023; 16(10):4591-4596.

DOI: 10.52711/0974-360X.2023.00747