Authors
Department of Fixed Prosthodontics, School of Dentistry, Shiraz University of Medical Sciences, Shiraz, Iran
Abstract
Keywords
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.
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
Table1. Number and percentage of crowns with or without horizontal or vertical gap
Total |
Either horizontal or vertical |
Horizontal dimension |
Vertical dimension |
|
|||
Without gap |
With gap |
Without gap |
With gap |
Without gap |
With gap |
||
200 |
30 |
170 |
80 |
120 |
49 |
151 |
Number |
100 |
15% |
85% |
40% |
60% |
24.5% |
75.5% |
Percentage |
Table 2. Number and percentage of crowns have more or less space than 2500µ between the crown margin and the crestal bone
Percentage |
Number |
Marginal space to the crestal bone(µm) |
|
23% |
46 |
More than 2500 |
Mesial |
77% |
154 |
Less than 2500 |
|
24% |
48 |
More than 2500 |
Distal |
70% |
140 |
Less than 2500 |
Figure 2. Radiographic image showing vertical marginal gap
a |
b |
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