Necessity of 3D visualization for the removal of lower wisdom teeth: required sample size to prove non-inferiority of panoramic radiography compared to CBCT.

The availability of cone beam computed tomography (CBCT) and the numbers of CBCT scans rise constantly, increasing the radiation burden to the patient. A growing discussion is noticeable if a CBCT scan prior to the surgical removal of wisdom teeth may be indicated. We aimed to confirm non-inferiority with respect to damage of the inferior alveolar nerve in patients diagnosed by panoramic radiography compared to CBCT in a prospective randomized controlled multicentre trial. Sample size (number of required third molar removals) was calculated for the study and control groups as 183,474 comparing temporary and 649,036 comparing permanent neurosensory disturbances of the inferior alveolar nerve. Modifying parameter values resulted in sample sizes ranging from 39,584 to 245,724 respectively 140,024 to 869,250. To conduct a clinical study to prove a potential benefit from CBCT scans prior to surgical removal of lower wisdom teeth with respect to the most important parameter, i.e., nerval damage, is almost impossible due to the very large sample sizes required. This fact vice versa indicates that CBCT scans should only be performed in high risk wisdom tooth removals.

Spatial relation between a rigid (digital) intraoral X-ray receptor and longitudinal axes of maxillary teeth.

The purpose of this study was to quantify the existing (inevitable) angle which in intraoral radiology appears between tooth length axis and receptor caused by the anatomical situation. Especially in the upper jaw, due to its arched anatomy, a true "paralleling technique" is not achievable. The angulation necessarily causes distortion and a foreshortening of the image; hence, the foreshortened image leads to misinterpretations in diagnostics. We investigated the effects of the realistic angulation on these image deteriorating factors. Two hundred ninety-four plaster models of the upper jaw were collected, and the angles between a dummy receptor and the axes of the central incisor or the first molar were measured. For evaluation, a rigid dummy of an intraoral charge-coupled device (CCD) receptor (30 mm × 40 mm) was used. The mean angulation evaluated for central incisors was 36.7° (range 19-56°) and for first molars 42.5° (range 26-56°). This leads to a foreshortening of the tooth ranging from 5.4% to 44.1% in the image, when magnification is neglected. Large angles of up to 56°, in both incisor and molar region, result in a relevant underestimation of true tooth length up to 44%. It is important to note that this error cannot be simply corrected by means of local magnification correction. Techniques should be developed that allow for automated assessment of the effective angle to provide information for distortion correction.

Ex vivo radiographic tooth length measurements with the reference sphere method (RSM).

A reference-based radiographic "reference sphere method" (RSM) for accurate length measurements in (dental) projection radiographs for the assessment of tooth length in dry human mandible sections is evaluated. RSM determines the depth coordinates of reference spheres placed in the object plane from the elliptical distortion of their shadows. Two segments (one canine and one molar) of dry human mandibles were exposed 95 times at different angulations (0-40°) on a dental charge-coupled device receptor. Three steel spheres (diameters d (1) = 2.00 mm, d (2) = 3.00 mm) were attached roughly coplanar with the tooth's main axis. Radiographs were assessed once by visual inspection plus manual landmark identification with a mouse-driven cursor. The results were compared to the true tooth length assessed after extraction and to a conventional method (C), i.e., the rule of proportion based on magnification of the sphere shadows. Mean relative length error was 2.28% (d (1)) and 0.46% (d (2)) for RSM and -13.58% (d (1)) and -9.90% (d (2)) for C. For both methods, length errors were significantly (p < 0.0001) correlated with the inclination relative to the receptor. RSM allows for complete a posteriori determination of the imaging geometry under the assumption of a known source-to-receptor distance. One specific application is foreshortening correction of objects coplanar with the reference spheres. Remaining errors are mainly due to incorrect landmark definition. In our setup, these were exaggerated by the visual/manual image-evaluation process. Automated image analysis has been shown for similar tasks to minimize these errors considerably.

Determination of projection geometry from quantitative assessment of the distortion of spherical references in single-view projection radiography.

A method is introduced, inferring the three-dimensional (3-D) location from the 2-D radiographic shadow of an opaque spherical reference body of known radius by considering its elliptical distortion, the 2-D shadow location and a known source-to-receptor distance. Three noncollinear spheres fixed to a rigid object constitute all possible degrees of freedom, i.e., the entire 3-D imaging geometry. The method may be used (a) to determine the 3-D imaging geometry from a single 2-D view and (b) to correct for foreshortening of object distances coplanar with the plane defined by the sphere triplet. Apart from the mathematical background the article describes a small feasibility experiment, performed with four different sphere diameters and a commercial dental ccd-receptor system (pixel length: 0.0195 mm). The mouse-cursor based image evaluation revealed an average underestimation of the critical depth- (x-) coordinate decreasing with increasing radius (-30.3% for r=0.5 mm to 2.8% for r=2.5 mm). Intraobserver reliability (the standard deviation between three single cursor-based assessments) ranged between 0% and 8% of the actual true depth. The main source of the input error is associated with the assessment of the amount of elliptical distortion, where subpixel accuracy is demanded. Consequently, software-based automated image evaluation is required using available methods for pattern recognition and point-spread correction. Provided sufficient accuracy, the method provides an important tool for foreshortening correction, depth assessment, motion analysis, and 3-D reconstruction from two or more 2-D views.