The in-vitro accuracy of fiducial marker-based versus markerless registration of an intraoral scan with a cone-beam computed tomography scan in the presence of restoration artifact.

OBJECTIVES: To determine the effect of restoration artifact ('metal artifact') on registration accuracy of an intraoral scan and cone-beam computed tomography (CBCT) scan, comparing fiducial marker-based registration with markerless registration. MATERIALS AND METHODS: A maxillary model was fitted with multiple configurations of zirconia crowns to simulate various states of oral rehabilitation. Intraoral scans and CBCT scans (half and full rotation) were acquired. Registration was performed using markerless (point-based registration with surface-based refinement) and fiducial marker-based registration. Each experimental condition was repeated 10 times (n = 320). The absolute deviation was measured at the canines and first molars, and the average and maximum values were analysed using multiple linear regression. RESULTS: R2 was 0.874 for average error and 0.858 for maximum error. For markerless registration, there were 0.041 mm (p < .001) and 0.045 mm (p < .001) increases in average and maximum error per crown, respectively. For fiducial marker-based registration, the effect of additional crowns was not statistically significant for average (p = .067) or maximum (p = .438) error. For a full arch of crowns, the regression model predicted average and maximum errors of 0.581 and 0.697 mm for the markerless technique, and 0.185 and 0.210 mm for the fiducial marker-based technique. Overall, the fiducial marker-based technique was more accurate for four or more crowns. The half rotation scan increased average error by 0.021 mm (p = .001) and maximum error by 0.029 mm (p < .001). CONCLUSIONS: Under the present study's experimental conditions, the fiducial marker-based technique should be considered if four or more full-coverage highly radiopaque restorations are present.

The influence of metal artifact reduction on the trueness of registration of a cone-beam computed tomography scan with an intraoral scan in the presence of severe restoration artifact.

PURPOSE: When planning guided implant surgery, highly radiopaque materials such as metals or zirconia produce streaking artifacts ('metal artifact') on cone-beam computed tomography scans, which can impair registration of the intraoral scan. This study aimed to determine the effect of metal artifact reduction on the trueness of registration in the presence of multiple full-coverage zirconia crowns. MATERIALS AND METHODS: A 3D-printed maxillary study model was restored with 12 full-coverage zirconia crowns and scanned with an intraoral scanner. Cone-beam computed tomography scans of the study model were acquired, with and without activation of the metal artifact reduction algorithm. Registration of the optical scans was performed using initial point-based registration with surface-based refinement, and the deviation was measured at four pre-defined dental landmarks. Welch's t-test was used to compare the registration error for the metal artifact reduction group with the control group. RESULTS: The average registration error was 0.519 mm (95% CI 0.507 to 0.531) with metal artifact reduction deactivated, compared to 0.478 mm (95% CI 0.460 to 0.496) without metal artifact reduction. Therefore, activation of the metal artifact reduction algorithm was associated with a 0.041 mm (95% CI 0.020 to 0.061, p < 0.001) increase in average registration error. CONCLUSIONS: The use of the metal artifact reduction algorithm slightly reduced trueness in this in vitro study. Clinicians are advised not to rely on a metal artifact reduction (MAR) algorithm for registration of a cone-beam computed tomography scan with an intraoral scan when planning guided implant surgery in the presence of restoration artifacts.

Risk factors for small pharyngeal airway dimensions in preorthodontic children: A three-dimensional study.

OBJECTIVE: To analyze which parameters, gathered from standard orthodontic diagnostic material, were most relevant for identifying small pharyngeal airway dimensions in preorthodontic children. MATERIALS AND METHODS: The sample was composed of 105 cone beam computed tomography scans of healthy preorthodontic children (44 boys, 61 girls; mean age, 10.7 ± 2.4 years). Airway volume and minimal cross-sectional area were three-dimensionally assessed. Cephalometric features and skeletal maturity were assessed on generated two-dimensional cephalograms. Associations were analyzed and adjusted for age, gender, and skeletal maturity by multiple regression analyses. RESULTS: Airway volume and minimal cross-sectional area were significantly smaller in prepubertal children (P < .001, P < .05, respectively) and positively associated with age (P < .001, P < .01, respectively). After adjustment of age, skeletal maturity and gender significant associations were found between pharyngeal airway dimensions and craniofacial morphology. Airway volume was positively associated with maxillary and mandibular width (P < .01; P < .001, respectively) and anterior face height (P < .05; P < .05, respectively). Minimal cross-sectional area was positively associated with maxillary and mandibular width (P < .01; P < .001, respectively) and negatively associated with sagittal jaw relationship (AnPg, P < .05). Mandibular width and age were the most relevant factors for airway volume (r2 = 0.36). Mandibular width and sagittal jaw relationship were the most relevant factors for minimal cross-sectional area (r2 = 0.16). CONCLUSION: Pharyngeal airway dimensions were significantly associated with age, skeletal maturity, and craniofacial morphology in all three planes. Children with a reduced mandibular width and increased sagittal jaw relationship are particularly at risk of having small pharyngeal airway dimensions.

Accuracy of angular measurements and assessment of distortion in the mandibular third molar region on panoramic radiographs.

OBJECTIVE: The objective of this study was to examine distortion on panoramic radiographs by assessing the accuracy of angular measurements in the mandibular third molar region on panoramic radiographs using computed tomography (CT). METHODS: Images from patients referred to a radiology practice for low-dose CT scans of their lower third molars were used in this study. The angle between the long axes of the second and third molars was assessed on panoramic radiographs and the corresponding CT images. Apparent tilting of the tooth across the arch on the panoramic image and the bucco-lingual inclination on CT were also recorded and compared. RESULTS: Eighty-eight patients had 163 mandibular third molars. The mean difference between the panoramic measurements and the CT measurements was -1.44 degrees, indicating that on the panoramic radiograph the third molar appeared less mesially inclined on average. This pattern was present in most cases, but in 64 cases the third molar appeared more mesially inclined on the panoramic radiograph. It was not possible to predict the direction of the discrepancy based on the radiographic findings. The discrepancy between the 2 measurements was greater if the third molar appeared tilted across the arch on the panoramic image, and tilted teeth showed a higher bucco-lingual inclination on reformatted CT images. CONCLUSIONS: Distortions inherent in panoramic imaging because of projection geometry produce discrepancies in the angular measurements in the mandibular third molar regions on panoramic radiographs. Interpretation of third molar angulation from panoramic radiographs is often unreliable and may not accurately reflect the true orientation of the tooth. Apparent tilting of the tooth across the arch on the panoramic radiograph exacerbates the problem and appears to correlate with the bucco-lingual inclination of the third molar as visualized on reformatted CT images.