Marginal adaptation vs esthetics for various dental ceramic restorations

 The aim of dental clinicians, prosthodontists, and manufacturers throughout the history of dentistry has been to achieve esthetically and functionally ideal restorations. Ceramic crowns have an impressive natural appearance and biocompatibility, and their clinical application has now been expanded to the complete arch (1). The success of a dental restoration is determined by 3 main

factors: esthetic value, resistance to fracture, and marginal adaptation (2). Inadequate fit results in plaque accumulation, which increases the risk of carious lesions and can cause microleakage and inflammation of the endodontics. Plaque accumulation, especially with subgingival margins, can also cause periodontal diseases. Finally, misfit may lead to cement
dissolution, particularly when using zinc phosphate cements (3). 

Until the late 1980s , researchers who measured marginal fit did not always measure the same distances. In 1989 Holmes et al proposed a simple terminology. In general, marginal fit is assessed by measuring the marginal gap or absolute marginal discrepance. In Specification No. 8, the American Dental Association specifies that the film thickness of zinc phosphate cement does not exceed 25 mm for Type 1 cements and 40 mm for Type 2 cements. However, experiments that analyzed marginal adaptation have shown that this goal was rarely achieved.(4) Gardner3 found that it was difficult to achieve this degree of accuracy even under ideal conditions. 

After a 5-year analysis of more than 1000 restorations, McLean and von Fraunhaufer concluded that 120 mm was the highest tolerable marginal opening. Ceramic crown manufacturing may require such different methods as
conventional slip casting, hot pressing, or computer-aided design / computer-aided manufacturing (CAD / CAM). The most esthetic restoration was considered to be porcelain jacket crown. However, the inherent weaknesses and brittleness of dental porcelain along with the high sintering shrinkage were noted as serious drawbacks to its universal acceptance as a complete veneer restoration.

Despite the many available techniques, the growing desire for better esthetics and improved biocompatibility has resulted in the development of a large number of all-ceramic restorative systems. Some use a single layer glass-ceramic material such as Dicor and IPS Empress, while others use a dual layer design such as In-Ceram and Procera method. With the advent of computer-aided design and milling (CAD / CAM) systems (5) further advances were made in highstrength all-ceramic technology.

 

In order to allow the restoration to survive in the oral cavity, the margins should be closely adapted to the preparation's cavosurface finish line. The type of reparation finish line has the main role in dictating the shape and bulk of restorative material in the restoration margin. It also has a main effect on the marginal adaptation and the degree of seating of the restoration and hence the clinical longevity of orcelain jacket crowns (6).

Types of marginal finish lines and their connection to ceramic restoration

There are several finish line configurations. They are given according to the requirement and tooth structure. Ideally, all finish lines should be placed, whenever possible, supragingivally. Due to esthetic and carious considerations, but sometimes margin is placed subgingivally for aesthetics purpose and in these situations, they should preserve the integrity of epithelial attachment and should not exceed more than half the depth of the gingival sulcus (7). It was obvious that there is no one type of finish line can be used for all crown preparations.
The chamfer finish line is an obtuse-angled finish line that exhibits the least stress concentration and most conservation (Fig. 1). It is indicated for a complete metal crown or the lingual aspect of metal-ceramic restoration.

the obtuse angle produces shearing forces which are not well tolerated by porcelain (8).

Shoulder finish is a right-angled finish line that can resist compressive occlusal forces but it requires more preparation, so it is not conservative. A shoulder finish line is recommended for allceramic as it minimizes the stresses and preventing the possibility of porcelain fracture (Fig. 2) (8). However, shoulder or chamfer finish lines can be selected for all-ceramic crowns that are bonded to prepared teeth. In a laboratory study, when all-ceramic crowns were internally etched

and cemented on natural teeth with a resinous cement, there was no significant strength reduction or in a longitudinal retrospective clinical evaluation of all-ceramic crowns (9).

 

A bevel can be made on the external edge of the shoulder finish line to reduce the marginal discrepancy of the restoration as it can be burnished (Fig. 3). It also protects the edge of the finish line preventing chipping. Shoulder with bevel is primarily indicated for the facial finish line of metal-ceramic restorations to hide the supragingival facial metal margin of the restorations. It is also used as the gingival finish line on the proximal box of inlays and onlays, and as occlusal finish line for onlays and partial veneer crowns (8).

On the other hand, the knife-edge is the more conservative finishing line, but it is difficult to wax and cast and susceptible to distortion and may results in an over-contoured restoration (8). In lower anterior teeth, periodontically treated teeth, very convex axial surfaces, and tilted teeth where preparation is minimal, the knife-edge finish line appears to be the treatment of choice (7,8). Similar to knife-edge finish line, Chisel edge and feather edge finish lines, and they are rarely used (8).

Deep chamfer finish line (also known as a heavy chamfer) is similar to chamfer but prepared with a diamond of greater. It is indicated for all-ceramic crowns and provides better support for a ceramic crown than does a conventional chamfer, but it is not as good as a shoulder. A deep chamfer can produce an unsupported lip of enamel that is susceptible to fractured during or after cementation of the restoration (Fig. 4) (6).

The radial shoulder is a modified form of shoulder finish line with a rounded internal line angle that is rounded using an end-cutting diamond and finished with a bin-angle chisel. There is less stress concentration in the tooth structure than with a classic shoulder, so it provides good support for ceramic restoration walls. However, the amount of tooth structure destruction required for this configuration is not significantly less than that required for a classic shoulder (6).

The effect of different margins on the final esthetics of ceramic restorations (10) In restoring extensively damaged or previously restored teeth, the contemporary dentist has a wide range of options to utilize. All-ceramic crowns are also used to restore both esthetics and function. One of the essential to success with either option is proper tooth preparation, which
includes proper selection and preparation of the preparation cervical margin (10).Regardless of the margin geometry, it is imperative to properly place the prepared gingival margin in relation to the free gingival margin, the epithelial attachment, and the alveolar housing. Wherever the
esthetic demands allow, there should be margins in a supra-gingival location. 

Margins must be positioned in an intracrevicular position in many patients in order to cover those margins with healthy gingival tissue and thus have an appropriate esthetic result. Clinicians need to realize that placing a cervical margin in an intracrevicular position is a precision exercise. If the margin is placed within a short distance of the free gingival margin, a small amount of gingival recession can result in margin exposure and esthetic failure. A margin placed too deep in the sulcus risks violation of biological width and concomitant chronic inflammation of the gingiva
There are two potentially useful landmarks to guide the clinician in precisely placing margins. A generally useful guideline is to place 0.5 mm apical cervical margins on the healthy free gingival margin. A more accurate method is to sound through the attachment to probe the alveolar bone crest and place at least 4 mm of coronal cervical margins on that alveolar crest. With either landmark, it is critical that the prepared cervical margin follow the alveolar bone scalloped anatomy, the attachment, and the gingival tissue. Clinicians make the most common error of flattening the cervical margin in the inter-proximal regions, violating biological width and eliciting a chronic inflammatory response. The clinician has three options in circumstances where esthetics is important: The first is to use an all-ceramic crown. While in recent years , the overall lifetime of most all-ceramic options has increased, metal-ceramic crowns can provide longer service. However, several all-ceramic systems provide excellent longevity-enhancing esthetics and can be used on anterior teeth with relative confidence. All-ceramic alternatives on posterior teeth should be avoided. Though many all-ceramic systems are available, the design of the cervical margin is identical for all the systems. 

A shoulder margin with a rounded inner angle should be ready to end at about 90 to the outer angle of the labial or buccal surface. A slight slant of no more than 5 is acceptable. The margin should be as smooth as possible, and should be between 1.2 and 1.5 mm in depth to provide maximum esthetics and strength (Fig.7). This design provides an adequate volume for esthetics and strength, and places the cervical margin during function in compression. It also allows glazing of the restore without rounding the ceramic margin of the terminal. Tensile forces occur in function when a more pronounced slant is produced, which can result in half-moon fractures in the cervical region. 

The marginal fit of different recently introduced dental ceramics to the dental
field 

 One of the important factors used in clinically assessing their long term performance is the optimal fit of all-ceramic restorations. In a restauration the presence of marginal discrepancies exposes the luting agent to the oral environment. The marginal opening allows for more plaque accumulation that can cause inflammatory gingival reactions and may result in degradation of the soft tissue due to periodontal disease. It also causes recurrent caries and bone loss. Many studies have assessed the marginal fit of various all-ceramic crowns.(11) The results show great variations within a single crown system. The evaluation of the marginal discrepancy of the crowns depends on several factors: the measurement of cemented or noncemented crowns, the type of abutment used for the measurements, the type of microscope, the factor of enlargement used for the measurement location and the quantity of single measurements. Variations exist on what is a clinically acceptable margin. McLean and von Fraunhofer30 suggested a restore if marginal gaps and cement thicknesses of < 120 mm can be achieved. Tuntiprawon and Wilson31 found that ceramic crowns at the axial wall with smaller gap dimensions and marginal opening showed the best compressive strengths when loaded onto the dies. In fixed prosthodontics marginal accuracy is an important quality criterion. Many studies examined the marginal fit of crowns,(12)and in many of those studies the authors used steel or resin dies to measure the marginal precision. Natural teeth show great variations due to their age, individual structures and post-extraction storage time, thus creating difficulties in obtaining standardized abutments. The benefit of this approach is the potential for all crown systems to achieve a consistent planning. The master steel die remained  clear and damage-free, which was an added benefit. The cementation of crowns before determining marginal fit is an important factor in the literature. 

Most authors studied cemented crowns.(13) The marginal distance usually decreases after luting, which is clinically important. Moreover, due to cementation, additional systemic and statistical faults are introduced.4 For this reason, cementing procedures have been avoided in this review. To avoid an increase in statistical error, all measurements were carried out by the same investigator. Marginal fit measurements are often done by sectioning a crown and measuring it with a light microscope or a scanning electron microscope. This method however has the potential for artifact distortion, provides only limited numbers and measurement positions, and is destructive. In the present case, a direct measuring technique with image analysis software under a microscope allowed for non-destructive quantification and multiple measurements.

 Nonetheless this research has had many limitations. Several researchers measured the internal fit of the crowns, but in this study it was difficult to calculate the internal fit of the crowns, since measuring the internal fit of the crowns involves cementing the crowns and sectioning the specimens. Another limitation of this study was that the marginal gap was perpendicularly and orthoradially oriented on the computer monitor, prior to the microscopic measurement. Nevertheless, precise placement of a specimen was difficult, since the researcher could only visually monitor the measurements. In addition , this study did not completely simulate clinical conditions by examining the marginal fit without cementation of crowns. Important factors are sample size and number of measurements per specimen. For each group some authors selected six24 or eight specimens. Many authors conceived studies using 10 samples per test. Just 4-2 positions at the margin of each crown specimen / abutment were measured in most earlier studies. (14) Some authors, however, made measurements at multiple  eference points, each with small distances, resulting in 150 measurements for a single specimen / abutment. Other authors randomly selected points along the crown's margin without precise positions, and determined the mean of all measurements for the crown 's marginal fit. Groten et al suggested that ideally, it would require 50 or at least measurements to obtain clinically relevant information on the size of the gap. 

 

In this analysis, 20 crowns were prepared for each group, and 80 measurements per crown were made to improve the accuracy of the statistics. Only if internal relief is sufficient to accommodate the luting agent can an improvement in the seating of a crown. The internal relief must accommodate the coating of cement and any defects on the surface of the tooth and the inner crown. In this analysis, die spacers were applied as per the instructions of the manufacturer. Yeo et al6 reported that the marginal openings of the In Ceram and IPS Empress 2 crowns were respectively 112 and 46 mm, which is consistent with the results of this study.

 

 The marginal distance of II Ceram crowns was measured by Sulaiman et al8 and found to be 160,66 mm. Those results are also consistent with this study. However for In Ceram crowns, Rinke et al2 registered a marginal gap of 33.5 mm. The differences in the methodology used by various investigators researching marginal accuracy may be an explanation for the lack of consensus. Sulaiman et al8 suggested the use of different measurement instruments may be the cause. An important consideration is also the skill of a dental technician who performed the restorations. The explanation the In-Ceram crowns demonstrated the largest marginal gap in this study may have been correlated with two factors. First, the glass mixture appears to settle during glass infiltration firing, which produces an unnecessary bulk at the edge of the coping after the shooting is finished, and must be trimmed using a rotary tool. Careless removal of excess material at the margin may theoretically lead to increased marginal difference. The second explanation could be the need for  a second additional impression from gypsum dies to prepare a slip-cast. Because of these procedures a dimensional change could have occurred.