Introduction

Regardless of the place of margins, proper marginal fit of dental fillings and crowns is essential to prevent periodontal diseases and recurrent caries (1-3). Several researchers have announced that clinical methods (visual inspection and/or an explorer) can be used to evaluate the gingival margin of filling and crown. They have also considered it to be a difficult diagnostic task, particularly when the restoration margin is interproximal and subgingival (1,2). Some other researchers have taken the use of explorer into question, as it tends to stick, whether or not the tissue adjacent, to the filling is carious; thus, radiographic methods are suggested to be used for diagnosis of the lesions adjacent to restorations (1,4). Despite the fact that all defective restorations do not necessarily bring about disease, marginal misfits are crucial to be appropriately diagnosed so that the tooth and the surrounding tissues can be maintained as much as possible; so it should be included as a part of the overall evaluation of the quality of the restoration. The margin of a single unit extracoronal restoration is the most critical item in long-term prognosis and the most susceptible part to
distort (5). Therefore, success and failure of the fixed restoration are completely related to adaptation and location of the crown margin (5). Some other items that affect the marginal adaptation are marginal beveling, venting, type and thickness of the cement, type of the impression material (dimensional accuracy), and the design of the margin preparation (6,7).

The best place for the crown margin is where the best access is provided both for the dentist to make impression, and for the patient to clean the
restoration (5). The fourdeterminingfactors in choosing the marginal location are periodontal consideration, esthetic consideration, adequate retention, and finally extending the preparation to the sound tooth structure (2,8).

The teeth restored with full-crowntend to exhibit more gingival inflammation and pocket depth than nonrestored ones (9). The extent of inflammation around the restored teeth depends on particular elements such as emergence profile, adequate access to polish the subgingival margins and biologic width invasion (3). Biological width is defined as the distance between the alveolar crest and the junctional epithelium and is estimated to be approximately 2.04mm (10). Migration of the junctional epithelium and bone resorptionare the consequences of marginal invasion of the crown to biologic width; which occurs to create the former biological distance (10).

Evaluation of the marginal adaptation can be performed either qualitatively or quantitatively. Qualitative assessment is done by direct visualization and sense of touch(by use of an explorer) (11), by using impression materials, or through radiological assessment (12). For quantitative assessment, employing a microscope in high magnification would be the best choice to measure the gap space. Graded explorers and parallel radiography can also be used for this purpose (13,14). Radiography is routinely used for quantitative evaluation in Nance and Hixon-Old father’s methods which are specialized for space measurement in mixed dentition (15).

In the case of rehabilitation with dental implants, most implant systems consist of two components – the implant screw and a connecting transmucosal structure, the abutment. The prosthetic crown can be either connected to the abutment or be an independent element (16), consequently, a gap can exist between the implant and the abutment.Also between the crown and the abutment, there might be a gap or an overextension of the luting agent. Since the presence of excess cement may result in peri-implant inflammation, radiographic evaluation has been proposed by some studies to ensure the appropriate seating and debridement of subgingival restorations (17, 18). Marginal misfit in cement-retained implant single crowns can also be accompanied by changes in crestal bone (17). Researches on radiographic assessment have reported the radiopacity of the restorative material and the technique to have effects on the assessment of marginal misfits (1). A number of in-vitro studies have used conventional and digital radiography to evaluate the diagnosis of gaps, and adopted marginal discrepancies from 0.01 to 0.5 mm between the restoration and the tooth (12, 19). Also Bjorn used radiography to measure the size of overhang and the marginal gap, as well as the distance between the crown margin and the bone crest (20). Radiography is particularly essential to estimate and calculate the bone dimensions before dental implant surgery(21). Hence, various radiographic techniques such as digital radiography, periapical, and computed tomography have been devised to evaluate the proximal surfaces; however, within the literature there exists little consensus on their individual use. Thus, in an attempt to search for a rationale on the use of parallel radiography and to suggest the best protocol, the current study was enrolled to evaluate the marginal adaptation and location of the crowns (the most critical item in fixed restoration) at the delivery point by employing radiographic assessment in Shiraz Dental School, Shiraz, Iran.

Materials and Method

In this descriptive study, 200 fabricated single-unit crowns were evaluated for the location and adaptation of margins by parallel radiography using Kodak E speed dental x-ray film (Eastman Kodak, Rochester NY, USA). The study was performed in the Department of Fixed Prosthodontics and OMF Radiology of Shiraz Dental School, Shiraz, Iran in 2012. The research protocol was submitted for consideration, comment, guidance and was approval by the research ethics committee of Shiraz School of Dentistry (ID: EC-2013-166). Considering the Declaration of Helsinki as the ethical principles, allpatients were informed in details about the nature of the trial, and individual voluntary informed consent was signed.

All the crowns were related to the posterior teeth and all of them were porcelain fused to metal (PFM). The type of margin preparation for all of them was shoulder bevel, and all of them were cemented by zinc phosphate cement.

All the radiographs were taken by parallel technique in the same condition (kvp:60,mA:8,exposure time:320ms). Then they were scanned and digital images were saved in computer. All the linear measurements were done using digital radiological software (Dr. Suni, China). To calculate the radiographic magnification, a metallic sphere (4mm in diameter) was placed on the occlusal surface and was fixed with a piece of sticky wax along the crown margin. The coefficients of magnification were calculated by measuring the diameter of the radiological image of the sphere divided by the actual diameter of it. Thereafter, the vertical and horizontalmarginal discrepancies (Fig. 1) (the vertical and horizontal distances between the crown margin and the preparation margin in either vertical or horizontal axis respectively), and the distance between the crown margin and the bone crest were also measured using the previously mentioned software.

Figure 1. Method of measuring horizontal and vertical marginal discrepancy

The final values were recorded regarding the coefficient of magnification. The values for marginal discrepancies were evaluated by descriptive statistical analysis of the registered data.

Generally the biologic width around different teeth is different and can just be measured by histological evaluation. In the current study, however, the lowest biological width (2mm) was considered as standard and the extents less than thatwere considered as invasion to the biological width. Since a space of at least 0.5mm should be available between the preparation finishing line and the junctional epithelium crest (10), the spaces less than 2500µm between the crown margin and the bone crest were considered as biologic width violation;besides, the open margin was attributed to the marginal gap of more than 50µm (5).

Results

The largest recorded value around the tooth is considered as the marginal discrepancy value; however, the buccal and lingual surface of the teeth could not be detected in radiographic evaluation. Hence, only mesial and distal surfaces were evaluatedin this survey. The acceptable marginal gap in this study was 50µm. Accordingly, the crowns with horizontal and/or vertical gap of more than 50µm in mesial or distal surfaces were considered as having open margin. As represented in table 1, 75.5% of the crowns had vertical and 60% of the crowns had horizontal discrepancy. Likewise, 85% of all crowns had a marginal gap more than the acceptable extent in either horizontal or vertical dimensions.As demonstrated in table 2, 77% and 70%of the crowns invaded the biological width in the mesial and distal surfaces, respectively.

Out of 200 evaluated teeth in this study, 70 teeth were detected to have some remaining cement around the teeth in the gingival sulcus, indicating that they had not been completely cleaned.

The margins of 15 crowns (7.5%) were on amalgam restoration with overhang. A total of 8 (4%) did not have the effective emergence profile, although they did not have marginal gap either. In 40 cases restored by a castable post, 8 core and 10 crowns had space between the post and Gutta-percha in the root canal and in 3 cases, the prepared post was deviated from the root canal direction.

Table1. Number and percentage of crowns with or without horizontal or vertical gap

Table 2. Number and percentage of crowns have more or less space than 2500µ between the crown margin and the crestal bone

Discussion

The current study was carried out to find a rationale on using radiography as an adjunct method for diagnosing the misfit in dental prostheses and restorations. When a filling or a crown is placed, the restoration surface should be aligned with the tooth margin (4, 22). Misfit is usually defined as the lack of adaptation between the restoration and the prepared tooth. They must be prevented because they are prone to cause accumulation of biofilm and consequently lead to development of carious lesions (1,2,23).It might be disadvantageous to the gingival and marginal bone tissues, as well (23,24). Likewise, healthy periodontal conditions are maintained through providing proper fitting of abutments on implants (17,25). There are a number of studies in which Radiographic examination has been recommended for the assessment of dental prosthesis or restorations (19,26) as well as the abutment adaptation (18). Two recent publications have suggested the presence of digital radiographic artifacts beside the metal restorations, which could impede the interpretation of such images (27,28). Another study compared the digital and conventional radiographs to assess the diagnostic accuracy of metal restoration misfit, and found no significant difference between conventional and digital original images (29). Based on the controversial results of this study, the presence of vertical marginal gap in 75.5% and horizontal gap in 60% of crowns can endorse the fact that evaluation of the marginal adaptation by using only explorer (30), rouge and chloroform, or even by an impression material would not be adequate. The results from clinical and radiographic examination were brought to comparison; the study that had assessed marginal gaps adjoining to implant components hadreported higher accuracy from the radiographic recording (31). In the case of assessing Class II restorations, the number of false positive diagnosis was reduced due to making use of both clinical and radiograph evaluation together (32). On the other hand, the number of correct cases diagnosed by the two methods were found to have no difference in an in-vivo study (33), whereas another study reported radiographs to have helped diagnosing more correct cases (34).Some studies were conducted concerning the impact of x-ray beam angulation (35,36) and they all agreed that the most accurate angulation for marginal misfit diagnosis in dental restorations is the orthogonal projection. Regarding image acquisition system, conventional film radiography was used in majority of the studies, while some others had used digital radiography, both CCD-based sensors and PSP systems (12,21). However, the use of computed tomography in diagnosing the misfits was assessed by none of them.The vertical marginal gap could probably be due to incomplete seating of the crowns (37), and incomplete seating by itself can be caused by tight proximal contact or presence of pressure spot.Vertical marginal gap can also be the result of short ditching in the laboratory by the technician and inaccuracy in making the impression of marginal preparation by the clinician (Fig. 2) (37). Practically, the subgingival margins, especially in the posterior regions, cannot be registered accurately by the impression material due to the poor access and the presence of blood (37). Some authors have proposed that radiography can assess the restored proximal surfaces better (1,2). In a study the proximal margins of Class II amalgam restorations were evaluated by employing radiographic examination solely and in association with clinical method. The resultrevealedbetter quality of diagnosis with the combined method (32). The horizontal flaws can be in two forms of ledge or overhang. Ledge is the result of wrong preparation (37); for instance, the chamfer margin should never be prepared wider than half of the thickness of a diamond bur tip, otherwise an unsupported lip of enamel may create marginal ledge (Fig. 3a) (37).

Presence of positive horizontal space (an overhang) can be allied to the technician or the operative dentist. In some instances, the technician may ditchthe die beyond the finishing line of the preparation and subsequently this flaw would lead to an overhang margin that can invade the gingival tissue and develop inflammation (Fig. 3b) (23). Sometimes also the undercut which is created in the preparation of the crown by the operative dentist can cause the wax pattern distortion during removal from the die and would produce marginal gap and overhang (5). Invasion of the biologic width can reversely be associated with the operative errors of the dentist(23). It must be concerned that the preparation should not be extended more than 0.5mm into the gingival sulcus (Fig. 4) (23).

Despite the fact that the cement residue was cleaned out by the explorer and dental floss in the present study, radiographic evaluation revealed that it had not been completely cleaned in 70 cases, which could cause gingival irritation and inflammation as a consequent (Fig. 5)(38). Inappropriate emergence profile exhibited in 8 cases could be related either to the wrong ditching of the technician or to the inadequate preparation of the dentist. Both instances can cause gingival inflammation and periodontal problems due to inadequate access for proper oral hygiene (23). The current study only made use of conventional radiography, so further researches are recommended to compare the accuracy of various types of digital radiographies in assessing marginal adaptation of indirect restorations. Digital radiography methods are preferred because of their lower irradiation dosage andimage processing which are the advantages of this type of radiography compared to older ones.

Figure 2. Radiographic image showing vertical marginal gap

Figure 3.a. Radiographic projection illustrating horizontal marginal discrepancy in the form of ledge formation.

b. Radiographic picture showing horizontal marginal discrepancy in the form of overhang

Figure 4. Radiographic picture showing invasion of the restoration margin to the biologic width (arrow)

Figure 5. Radiographic projection displaying the Figure 3a. Radiographic projection illustrating horizontal marginal discrepancy in the form of ledge formation3b Radiographic picture showing horizontal marginal discrepancy in the form of overhange residual cement that has not been completely cleaned