Assessment of linear measurements of bone for implant sites in the presence of metallic artefacts using cone beam computed tomography and multislice computed tomography

Abstract

The objective was to evaluate the influence of dental metallic artefacts on implant sites using multislice and cone-beam computed tomography techniques. Ten dried human mandibles were scanned twice by each technique, with and without dental metallic artefacts. Metallic restorations were placed at the top of the alveolar ridge adjacent to the mental foramen region for the second scanning. Linear measurements (thickness and height) for each cross-section were performed by a single examiner using computer software. All mandibles were analysed at both the right and the left mental foramen regions. For the multislice technique, dental metallic artefact produced an increase of 5% in bone thickness and a reduction of 6% in bone height; no significant differences ( p > 0.05) were detected when comparing measurements performed with and without metallic artefacts. With respect to the cone-beam technique, dental metallic artefact produced an increase of 6% in bone thickness and a reduction of 0.68% in bone height. No significant differences ( p > 0.05) were observed when comparing measurements performed with and without metallic artefacts. The presence of dental metallic artefacts did not alter the linear measurements obtained with both techniques, although its presence made the location of the alveolar bone crest more difficult.

Computed tomography (CT) has become one of the most widely used imaging modalities in implant dentistry. The quality, accuracy, and precision of the images increase the safety of implant placement, enhancing the success rates of implant-supported prostheses . The use of CT is important to avoid injuries to the inferior alveolar nerve and the maxillary sinus . Two techniques have been commonly employed in dental practice: the multislice computed tomography (MSCT) and cone-beam computed tomography (CBCT).

MSCT is considered to be the latest spiral CT technology. It is a reproductive and accurate technique. MSCT has been used in medical and dental practice . It produces detailed, undistorted, and precise images of the maxillofacial complex . Its high costs and doses of radiation limit its use in daily dental practice .

CBCT is a recent technology that, despite having a lower resolution than MSCT , also provides precise and accurate measurements. When compared with MSCT, CBCT is associated with lower costs and produces lower doses of radiation, so practitioners have adopted its use for implant placement .

There is evidence that both techniques are precise and accurate, but dental metallic artefacts seem to interfere with the analysis of their Images . When scanned by CT, the dental metallic restorations attenuate the X-ray beam more than soft tissue and bone. The photons are not detected by the CT appliance, primarily because of the high atomic number of contrast agents or metal implants that jeopardize the location of anatomic structures. The increased fraction of photoelectric interactions causing photopenic holes on the projection data are displayed on CT images as sunburst streaks , which emanate radially from the site of the metal object. The severity of the ‘sunburst’ artefact is related to the physical size of the fixation hardware and its composition . In the reconstructed images, dental metallic artefacts may be observed as hypodense areas surrounded by hyperdense areas (beam hardening) with over 1000 Hounsfield units (density grey scale of CT) that can be observed near any type of metal, including titanium implants and metallic restorations .

The presence of artefacts can make it impossible to plan the implant location or to determine the location of important structures, such as the mental foramen . The aim of the present study was to evaluate the influence of dental metallic artefacts on linear measurements (height and thickness) taken on para-sagittal cross-sections of the alveolar ridge using two different CT techniques: MSCT and CBCT.

Materials and methods

10 dried mandibles were selected for this study. The exclusion criteria were the presence of an extremely resorbed alveolar ridge (height < 4 mm) and intra-bony lesions.

Each mandible was scanned twice using MSCT and CBCT, with and without a dental metallic artefact. Immediately before the second scanning, metallic restorations were placed at the top of the alveolar ridge coronal adjacent to the mental foramen region. The mandibles were scanned in the presence of such artefacts. When a tooth was present at the region adjacent to the mental foramen, a nickel-chromium onlay restoration was made. In cases of edentulous areas, a nickel-chromium metallic restoration was positioned with wax over the alveolar ridge adjacent to the mental foramen region ( Fig. 1 ). The distance between the onlay restorations, which were positioned with wax onto the alveolar crest, and the mental foramen region was approximately the same distance established between the onlay restorations prepared on tooth crowns. Subsequently, cross-sectional images were acquired using each technique.

Fig. 1
Panoramic reconstruction showing a metallic restoration positioned with wax, adjacent to the mental foramen. The arrow indicates the mental foramen. The hyperdense image over the alveolar crest represents the metallic restoration.

CBCT images were taken using the iCAT machine (Imaging Sciences International, Hatfield, PA, USA) with a 0.25 voxel size every 40 s with a field of view of 8 cm, 90 Kv, and 7 mA. The same specimens were also submitted to scanning with the MSCT 64 canals (Aquilion, Toshiba Medical, Tustin, CA, USA) with the following parameters: 0.5 mm slice thickness, 0.3 mm of interval of reconstruction, obtained every 0.4 s, with a matrix of 1024 × 1024, at 120 kVp and 300 mA.

The same software was used for image visualization, manipulation, and image analysis (Imaging Studio ® software, Anne Solutions, São Paulo, Brazil). Following the acquisition of para-sagittal cross-sections, each mandible was analysed at both the right and the left mental foramen regions. The measurements of height and thickness were selected because they are important for planning implant placement. The mental foramen region was selected for image analysis because it is an important anatomic landmark. Dental implants must be positioned at a safe distance from the mental nerve to avoid nerve injury .

Linear measurements of each section were made by a single examiner. Thickness was measured as the distance from the external buccal cortical to the external lingual cortical at the region immediately above the coronal cortex of the mental foramen (horizontal linear measurement) ( Fig. 2 ). Height was measured as the distance from the top of the alveolar ridge (external cortical) to the coronal cortex of the mental foramen (vertical linear measurement) ( Fig. 2 ).

Fig. 2
Parasagittal cross-section images illustrating the measurements performed in the present study. The height measurements are indicated by the arrows and the thickness measurements are indicated by the squares.

The mental foramen images of CBCT and MSCT were analysed without dental metallic artefacts. The same analysis was performed using the images for CBCT and MSCT with the presence of dental metallic artefacts. Mean values for height and thickness were calculated for each hemi-mandible using all of the measurements obtained from the cross-sectional images at the region of the mental foramen. The calibration of the observer was confirmed by the intra-class correlation coefficient ( Table 1 ), obtained by the repetition of measures on 20% of the hemi-mandibles, which were randomly selected. For statistical purposes, the means of the measurements performed using images with or without artefacts were compared in the CBCT and the MSCT techniques. Given that the same mandible was measured twice (with and without artefacts), comparisons were conducted using the paired t -test ( α = 0.05).

Table 1
Intra-examiner calibration. Intra-class correlation coefficient (ICC) for measurements 1 and 2.
ICC (IC 95%) P
Measurement 1 0.976 (0.869–0.995) <0.001
Measurement 2 0.985 (0.916–0.997) <0.001

Results

Two hemi-mandibles were excluded due to the presence of high bone resorption, so 18 hemi-mandibles were included in the study. For both techniques, the anatomic structures were more easily defined, primarily at the top of the bone crest and cortical bone ( Figs. 3A and 4A ), on the images without metallic restoration and without dental metallic artefacts. Images with dental metallic artefacts showed the presence of streaks and beam hardening at the top of the alveolar bone crest, which impaired its examination, location, and measurement ( Figs. 3B and 4B ). In some cases ( Fig. 4 B), the alveolar bone crest was not clear and showed discontinuity.

Fig. 3
Parasagittal cross-section images obtained by CBCT technique. (A) An image obtained without restoration placement and, consequently, without dental metallic artefacts. The top of the alveolar crest is evident. (B) An image obtained with restoration placement adjacent to the mental foramen region and, consequently, with dental metallic artefacts. Observe the presence of beam hardening near the alveolar crest (arrows). The presence of beam hardening may impair the identification of the top the alveolar crest.

Fig. 4
Parasagittal cross-section images obtained by MSCT technique. (A) An image obtained without restoration placement and, consequently, without dental metallic artefacts. The top of the alveolar crest is evident. (B) An image obtained with restoration placement adjacent to the mental foramen region and, consequently, with dental metallic artefacts. Note the presence of beam hardening near the alveolar crest (arrows). The top of alveolar crest is less evident due to the presence of beam hardening.

With regard to the MSCT technique, the presence of dental metallic artefacts produced an increase of 5% in bone thickness and a reduction of 6% in bone height. No significant differences ( p > 0.05) were detected when comparing the measurements taken with and without dental metallic artefacts for both parameters ( Table 2 ).

Feb 5, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Assessment of linear measurements of bone for implant sites in the presence of metallic artefacts using cone beam computed tomography and multislice computed tomography
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