2.1 Study participants
This study included 40 patients with 54 full-crown prostheses seen at the Department of Prosthodontics, Peking University School and Hospital of Stomatology, China, between August and December 2021. We included patients aged ≥ 18 years with at least one premolar or molar full-crown restoration with a supragingival or flush gingival margin implanted at least 6 months prior to the study. Included patients were also without systemic disease or active periodontitis, had full behavioral autonomy, had the ability to express themselves, and exhibited good compliance. Patients were excluded if they had poor oral hygiene, acute or chronic disease in teeth adjacent to the full-crown restoration, orthodontic bracket attachments on the tooth surface, or other characteristics that may affect photography of fixed restorations.
All subjects provided written consent prior to participation. The study was performed in accordance with the Declaration of Helsinki and approved by the Biomedical Ethics Committee of Peking University Hospital of Stomatology, Beijing, China (No. PKUSSIRB-202165097).
2.2 Black and white calibration sheet
As a black and white calibration sheet, a 1.5-mm-diameter semicircle, which was half black (R:0, G:0, B:0) and half white (R:255, G:255, B:255), was printed on a piece of self-adhesive paper and calibrated. A photograph of this card, along with one of a standard black and white card, was imported into Adobe Photoshop CC 2018 (Adobe Inc., Mountain View, CA, USA) to ensure that the black and white calibration sheet was consistent with the standard color card.
2.3 Examiners and calibration
To ensure reliability of the clinical examinations (the reference standard), the prosthodontist was trained on how to assess the marginal adaptation of full-crown restorations according to FDI World Dental Federation [27] criteria, and the periodontist was trained in the use of the Modified Gingival Index (MGI) to assess the gingival status of full-crown restorations [28]. The prosthodontic and periodontal specialists who performed the clinical examinations were experts with more than 10 years of experience.
To ensure accurate evaluation of the full-crown restorations based on photographs obtained using an intraoral camera (Zhimei YF200B; Baden Co., Ltd., Beijing, China), three evaluators were trained. During training, the three evaluators assessed photographs similar to those evaluated in the actual study, to ensure that they understood the evaluation methods and criteria. The study photographs were assessed once the total agreement score for the three evaluators was ≥ 85% [27, 29]. The evaluators were general dentists with less than 3 years of clinical experience.
Table 1
FDI [27] and MGI [28] levels
| FDI | | MGI |
1 | Harmonious outline, no gaps, no white or discolored lines. | 0 | Absence of inflammation. |
2 | Marginal gap (< 150 µm), white lines; small marginal fracture removable by polishing; slight ditching, slight step/flashes, or minor irregularities. | 1 | Mild inflammation or with slight changes in color and texture, but not in all portions of marginal or papillary gingiva. |
3 | Slight ditching, slight step/flashes, minor irregularities; non-removable gap < 250 µm; several small marginal fractures; major irregularities, ditching or step/flashes. | 2 | Mild inflammation, such as the preceding criteria, in all portions of marginal or papillary gingiva. |
4 | Gap > 250 µm or dentine/base exposed; severe ditching or marginal fractures; larger irregularities or steps (repair necessary). | 3 | Moderate, bright surface inflammation, erythema, edema and/or hypertrophy of marginal or papillary gingiva. |
5 | Restoration (complete or partial) is loose but in situ; generalized major gaps or irregularities. | 4 | Severe inflammation, erythema, edema and/or marginal gingival hypertrophy of the unit or spontaneous bleeding, papillary congestion, or ulceration. |
2.4 Clinical examination
Clinical examinations were performed by trained senior clinicians (prosthodontal and periodontal experts) with more than 10 years of clinical experience. The examinations were performed on the same day, using the same dental equipment and light conditions. The instruments used for the examinations included disposable mouth mirrors and triple syringes. Prior to the examination, the teeth were cleaned with sterile gauze to remove food deposits. Teeth were examined in a wet state, but excess saliva was removed using a triple syringe if necessary [24, 30, 31]. The average time spent on the examination was almost 1 min per patient. The prosthodontist assessed the margins of the full-crown restorations according to FDI [27] criteria, and the marginal adaptation of the full-crown restorations was rated as clinically acceptable (levels 1–3; 0) or unacceptable (levels 4 and 5; 1). The periodontist assessed the gingival status of the full-crown restorations according to the MGI [28], and the gingival status was rated as clinically acceptable (levels 0–2; 0) or unacceptable (levels 3 and 4; 1).
2.5 Intraoral photographs
After the clinical examination, intraoral photographs were taken under standardized conditions by a dentist trained in the use of photographic equipment [30]. The intraoral camera (Zhimei YF200B; Baden Co., Ltd.) had a 6-LED light source and provided images with a fixed resolution of 1,600 × 1,200. The patients were positioned on a dental chair, with the Frankfort plane at an angle of 45° relative to the floor. No external light source was used. The teeth were wet when photographed, but excess saliva was removed using a sterile gauze or triple syringe. The position of the full-crown restorations was determined and a black and white calibration sheet was taped over one third of the buccal side. After covering the intraoral camera with a disposable protective sheet, it was inserted into the mouth. The camera was positioned at a 45° angle relative to the buccal surfaces of the full-crown restoration, so that the full-crown restoration was located in the center of the image [23]. After conventional intraoral photographs were obtained, two linear polarizers (Edmund Industrial Optics, Barrington, IL, USA) were perpendicularly placed in front of the light source and prism to enable cross-polarization. Owing to the cross-polarization, the horizontally polarized reflected light was blocked by the perpendicularly polarizing filter, such that the color distortion caused by specular reflection on the tooth surface was eliminated [26]. Photographs taken using the intraoral camera under these conditions were categorized as polarized. The photographs were evaluated for acceptability and quality; more photographs were obtained as needed.
The photographs were saved on a personal computer (Inspiron 5408; Dell Inc., Round Rock, TX, USA) as .JPG files and assigned a numerical code to protect patients’ identities [24, 30]. The photographs from the conventional intraoral photographs group were imported into Photoshop CC 2018 (Adobe) for editing and processing (of the black and white calibration). These photographs were classified as calibration photographs and stored on the personal computer as .JPG files. Sample photographs are displayed in Fig. 1.
2.6 Photographic evaluation
Three trained dentists who did not participate in the data collection process evaluated photographs showing the full-crown restoration margins and gingival status based on the FDI [27] criteria. Photographs from the conventional, calibration, and polarization groups were randomly displayed on a 14-inch high-definition display. Each evaluator independently assessed the photographs for 30 s and rated the full-crown restoration margins and gingival status as clinically acceptable (0) or unacceptable (1). Cases where at least two of three assessors were in agreement were analyzed [23].
2.7 Statistical analysis
The data were analyzed using SPSS software (version 26.0; IBM Corp., Armonk, NY, USA). The marginal adaptation and gingival status of full-crown restorations were compared among the conventional, calibration, and polarization groups. The χ2 test was used to compare the assessment results among the three groups; p < 0.05 was considered to indicate a statistically significant difference. The sensitivity and specificity of the camera-based assessments were calculated for each group and compared to the results of clinical examinations of the oral cavity.
Cohen’s kappa statistic was used to assess the agreement between the intraoral camera-based assessments of the marginal adaptation and gingival status of full-crown restorations and the clinical examinations. Kappa values were classified as poor (≤ 0.20), fair (0.21–0.40), moderate (0.41–0.60), good (0.61–0.80), or very good (0.81–1.00) [32].