The marginal discrepancy of lithium disilicate onlays: Computer-aided design versus press.

AIM: The aim of the study was to evaluate and compare the vertical marginal discrepancy of computer-aided design (CAD)/computer-aided manufacturing (CAM) and pressable lithium disilicate onlays.
MATERIALS AND METHODS: A maxillary first premolar typodont tooth was prepared to receive lithium disilicate onlay. Mesio-occluso-distal cavity was prepared with palatal cusp reduction and collar preparation. In the proximal box, gingival seat was placed 1 mm coronal to the cementoenamel junction and mesiodistal width of the seat was kept to 1 mm. Thirty stone models were prepared from thirty rubber base impressions and divided into two groups, based on the technique of fabrication of onlays: (1) Group CL (CAD/CAM lithium disilicate) and (2) Group PL (Pressable lithium disilicate). Fifteen onlays per each group were fabricated by following the manufacturer instructions. Marginal fit of all the samples were analyzed by using stereomicroscope with Image Analysis software. Statistical analysis was done by t-test.
RESULTS: Statistical significant difference was found between both the groups. The lowest marginal discrepancy (41.46 μm) was measured for Group CL (CAD/CAM lithium disilicate) specimens, and the highest (55.95 μm) discrepancy was observed on the Group PL (Pressable lithium disilicate) specimens.
CONCLUSION: Although there was a statistically significant difference between the two groups, marginal gap of both the groups were in clinically acceptable levels. Copyright:

INTRODUCTION

The final goal of restorative dentistry is to fabricate the restorations similar to natural teeth. Ceramics are the best materials when esthetic restorations are demanded.[1] Esthetic and conservative treatment options for teeth weakened by caries or fractures are tooth-colored partial-coverage indirect restorations.[2] In particular, all-ceramic restorations have acquired popularity because of patient demands for highly esthetic restorations for severely compromised posterior teeth. As all-ceramic partial coverage restorations can be luted with adhesive-luting cement, these can be an alternative to the conventional traditional full-coverage crown in restoring weakened or missing tooth structure.[3] Exceptional esthetics and excellent biocompatibility are further advantages.[4] Especially, lithium disilicate has got utmost popularity in dental practice, presenting undebatable benefits.[5]

Lithium disilicate is a glassy ceramic that composed of quartz, lithium dioxide, oxides of phosphor and potassium, alumina, and other components. IPS e.max lithium disilicate is one of the all-ceramic materials where lithium disilicate crystals (SiO2–LiO2) are embedded into a glass matrix to minimize microcrack propagation, thereby enhancing mechanical stability. It was introduced in 2005 by Ivoclar Vivadent (AG, Schaan, Liechtenstein).[6]

Marginal fit is one of the key determinants in the success of restorations. Inferior marginal fit might lead to cement dissolution, marginal discoloration or staining, microleakage, and secondary caries. Therefore, it is important to maintain proper marginal fit to decrease the occurrence of associated complications.[7] Recent evolution in dental ceramic materials and fabrication techniques has led to increased strength and improved marginal fit.[1]

IPS e.max lithium disilicate restorations can be fabricated by two different techniques: lost-wax hot-pressing technique (IPS e.max Press) or computer-aided designed/computer-aided manufacturing (CAD/CAM) milling procedures (IPS e.max CAD).[8]

As the literature is usually confined to the comparison between different CAD/CAM systems, it is still controversial if the restorations fabricated by the CAD/CAM systems exhibit comparable marginal fit to the restorations made by the conventional dental laboratory techniques.[910] As there were very few studies evaluating marginal discrepancy of lithium disilicate onlays fabricated by different techniques, the aim of this in vitro study was to evaluate and compare the marginal fit of CAD/CAM lithium disilicate onlays with those using pressing technique.

MATERIALS AND METHODS

A typodont maxillary first premolar tooth was prepared to receive lithium disilicate ceramic onlay following the protocol: Mesio-occluso-distal cavity prepared by using FG271 and FG169 L carbide burs (SS White, Lake wood, USA), cavity width of one-third the intercuspal distance, pulpal floor depth of 1.5 mm at the central groove area, an overall preparation angle of 6° toward the occlusal aspect, gingival seat width of 1 mm in proximal box which was placed 1 mm coronal-to-cementoenamel junction, buccal cuspal reduction of 1.5 mm, and palatal cuspal reduction of 2 mm with a collar preparation of 1 mm [Figure 1].

Figure 1

Representative images of cavity design. (a) cavity design on typodont tooth. (b) schematic representation of cavity design. (A) 1.5-mm reduction of the buccal cusp. (B) 2-mm reduction of the palatal cusp. (C) Occlusal cavity depth (1.5 mm). (D) Width of gingival seat (1 mm). (E) Level of gingival seat (1 mm above CEJ). (F) Width of the collar on the palatal surface (1 mm)

A primary impression was made using alginate impression material (Aroma fine; GC Corp., Tokyo, Japan) and a plaster (Samwoo plaster; Samwoo Corp., Seoul, Korea) die was fabricated. Two sheets of baseplate wax (Modeling wax; Pemaco Inc., St. Louis, MO, USA) applied on the plaster die as spacer for the custom-made tray. Thirty custom-made trays were made using acrylic resin (Quicky; Nissan Dental Products Inc., Kyoto, Japan). Thirty final impressions were made with polyvinyl siloxane (Examixfine; GCAmerica Inc., Alsip, IL, USA) using thirty custom trays and thirty master Type IV die stone models were fabricated (Rhombrock; Mitsubishi, Tokyo, Japan). The thirty stone models were divided into two groups, 15 models per each of the following groups: (1) Group CL (CAD/CAM lithium disilicate) and (2) Group PL (Pressable lithium disilicate). Fifteen onlays were fabricated per each group by using stone models.

Fabrication of onlays

All onlay restorations were made by single technician using the same protocol following manufacturer's instructions. Onlays from Group CL (CAD/CAM lithium disilicate) were designed and milled with a CAD/CAM system (CerecinLab 3D Software v3.01, Sirona, Bensheim, Germany) using presintered lithium disilicate blocks (IPS e.max CAD, Ivoclar Vivadent, Schaan, Liechtenstein). After milling, final sintering was done following manufacturer's instruction. Glazing (IPS e.max Ceram Glaze Paste, Ivoclar Vivadent, Schaan, Liechtenstein) was done as final treatment. Onlay restorations from Group PL (Pressable lithium disilicate) were made using Pressable lithium disilicate ingots (IPS e.max Press, Ivoclar Vivadent, Schaan, Liechtenstein). IPS PressVEST Speed (Ivoclar Vivadent, Schaan, Liechtenstein) investment material was used for investment of final wax patterns. Pressing was done in a press furnace EP600 (Ivoclar Vivadent, Schaan, Liechtenstein) by following manufacturer's recommendations of firing parameters. Then, the onlay restorations were devested by keeping pressed parts in an aqueous mixture of 1.7% sulfuric acid and 0.6% hydrofluoric acid. Two glaze (IPS e.max Ceram Glaze Liquid, Ivoclar Vivadent, Schaan, Liechtenstein) firings were performed in a Programat P200 furnace (Ivoclar Vivadent, Schaan, Liechtenstein).

The marginal fit was estimated by measuring the space between the edge of the restoration and the die cavosurface margin by stereomicroscope (Labomed CZM6, Labo America Inc., USA) at ×5 magnification. Measuring was done at randomly selected 60 points for each specimen [Figure 2]. The marginal gap of one onlay was described as a mean value of these 60 measurements. The marginal gap was measured without cementation on the stone dies. A small quantity of Variolink II Try-In Paste was used to avoid displacement of restoration from the die. ProgRes C14 plus microscope camera (Jenoptik, Germany) was used to capture the required points. The required points were captured by ProgRes C14 plus microscope camera (Jenoptik, Germany). ProgRes CapturePro software (Jenoptik, Germany) was used for image analysis. Marginal gaps were evaluated on the computer screen with a magnification of ×200. All measurements were performed by one operator.

Figure 2

(a) Lithium disilicate onlay on stone die. (b) Measuring the marginal gap under stereomicroscope with image analysis

The means and standard deviations were calculated for both the groups. T-test was used for statistical analysis at 0.05 level of significance.

RESULTS

Table 1 shows the means and the standard deviations of both the groups. The mean value of marginal gap was 41.46 μm for Group CL (CAD/CAM lithium disilicate) onlays and 55.95 μm for Group PL (Pressable lithium disilicate) onlays. T-test was performed to analyze statistical significance between both the groups. The results showed that there was a statistically significant difference in the marginal fit of Group CL and Group PL. In the present study, P < 0.01 was considered as significant. Our study showed highly significant results and the P value was 0.001.

Table 1

The mean and standard deviation of marginal gap of test groups

	N	Mean	Std. Deviation
Group CL	15	41.4630	15.94282
Group CL	15	55.9540	26.68181

DISCUSSION

Systematic review on all-ceramic crowns by Contrepois et al.[11] stated that the three main factors influencing the success of an all-ceramic restoration, namely, esthetics, fracture resistance, and the marginal fit. As the fabrication technique also one of the factors that affecting the marginal gap,[12] in this study, we compared CAD/CAM, the evolving fabrication technique of lithium disilicate restorations with that of the conventional lost-wax pressing method.

The ideal die should be dimensionally stable, hard enough to resist the fabrication process, and accurate. Various die materials such as Type IV, Type V die stone, epoxy resin, and metal have been used for evaluation of the marginal fit.[13] In this study, Type IV stone dies were used to mimic regular clinical and laboratory scenarios.

Some investigators[1415] found a significant increase in the marginal gap after cementation. These results, however, varied based on the luting agent used.[13] Therefore, the marginal gap measured without cementation for a better standardization in this study.

Direct viewing is a nondestructive and a comparatively simple method.[13] Direct viewing to measure the marginal gap can be done by the use of stereomicroscope with image analysis software,[13] optical microscope with image analyzing software.[7] The other nondestructive methods reported in the literature are profile projector and laser videography.[13] In this study, stereomicroscope was used for direct viewing of the specimens. The visibility of the data obtained was greatly enhanced by the image analysis software.[7]

According to Groten et al.,[16]50 or at least 20–25 measurements (either selected or random) are needed for each specimen on all sides to acquire clinically relevant information about the marginal fit. Sixty points were selected randomly in this study.

In this study, the mean marginal discrepancy in Group CL (CAD/CAM lithium disilicate) (41.4 μm) was smaller than in the Group PL (Pressable lithium disilicate) (55.9 μm). The results of this in vitro study exhibited statistically significant difference between the marginal discrepancy of lithium disilicate onlays fabricated by CAD/CAM and pressing techniques similar to the study done by Ng et al.[17] who have observed poor marginal fit in pressed crowns than lithium disilicate CAD/CAM (LAVA COS scanning unit).

The findings of the present in vitro study could be explained by the following: e. max press restorations require an accurate negative replica of the dentition with an elastomeric impression material. While transporting the impression to the dental laboratory could result in dimensional changes due to variations in temperature.[18] In addition, the time elapsing between impression making and fabrication of the stone cast, as well as disinfection may cause further more distortion.[19] The subsequent steps such as application of a die hardener and die spacer, the preparation of a wax pattern of the restoration, and investing the pattern and pressing process could also be the cause of errors.[20]

Conversely, some previous studies[821] found poor marginal fit in lithium disilicate CAD/CAM restorations in comparison to pressed lithium disilicate restorations. However, Guess et al.[3] observed no significant difference between the marginal gaps of lithium disilicate pressed onlays and CAD/CAM (CEREC 3D InLab) onlays. These discrepancies might be due to the difference in CAD/CAM units, methods of measurements, types of microscope, magnification level, and location and number of measurements.[17]

McLean and von Fraunhofer[22] stated that a restoration would be successful if marginal gaps and cement thickness is <120 μm. In the literature, it has been reported that the all-ceramic crowns exhibited a mean marginal gap ranging from 19 to 160 μm.[23] In the present study, though there was a statistical significant difference between two groups, marginal gap of both the groups are in clinically acceptable levels.[22]

However, there are few limitations in this study. In vitro study environment varies noticeably from everyday clinical practice. The tooth selected for onlay preparation was intact artificial tooth which cannot be attributed to clinical scenario. Additional clinical follow-up studies are required to evaluate the marginal fit in clinical conditions.

CONCLUSION

Differences in marginal fit were recorded for lithium disilicate onlays fabricated using different manufacturing techniques (CAD/CAM and Pressing). However, both the study groups exhibited acceptable marginal discrepancies. Long-term follow-up clinical studies are required to evaluate the clinical outcome of different fabrication techniques.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.