Sexual dimorphism in mesiodistal dentin and enamel thickness.

This study evaluates sexual dimorphism in mesiodistal diameter, enamel thickness and dentin thickness of the permanent posterior mandibular dentition in order to gain a better understanding of variation in mesiodistal tooth size. The results relate to a sample of 59 males and 39 females, 20-35 years of age. Bitewing radiographs of the right permanent mandibular premolars and molars were illuminated and transferred at a fixed magnification to a computer via a video camera. Enamel and dentin landmarks were identified and digitized on the plane representing the maximum mesiodistal diameter of each tooth. The results showed significant sex differences (p < 0.01) in mesiodistal diameter favouring males over females. Dimorphism was more pronounced for the molars than for the premolars. Enamel thickness, which is 0.4-0.7 mm greater for the molars than premolars, shows no significant sex differences. Dentin is significantly thicker in males than females, and is 3.5-4.0 mm thicker in molars than premolars. It is concluded that sexual dimorphism in mesiodistal tooth sizes is due to differences in dentin thickness and not enamel thickness.

Grid versus air gap. A comparison of cephalometric techniques.

This study compares two cephalometric radiography techniques: one that uses a grid, the other an air gap, both of which increase image contrast by reduction of scatter radiation to the film. A 12.0 cm water phantom was used with a modified cephalometric radiography unit operating at 71 kVp. Air gaps (patient-to-film distances) of 0.0, 7.5, 12.5, 17.5, 25.5 cm were compared with a 10:1 ratio stationary focused grid at 0.0 cm. Comparisons were made from the measured patient surface exposure, the calculated contrast improvement factor, nominal magnification, and image resolution at each distance. Radiation exposure to the patient was 2.52 times greater with the grid than without. Using regression formulas, the contrast improvement factor of the grid was calculated to equate to an air gap of 9.5 cm. The 9.5 cm air gap would reduce patient exposure by 59.6% with similar image contrast and resolution as compared with the grid. Overall magnification was increased by 6.6% with the 9.5 cm air gap.

Localization of a displaced root fragment by stereoscopy and pluridirecitonal tomography.

The case reported describes two methods used with the Scanora integrated imaging system to locate a root fragment thought to be displaced into the maxillary sinus--rotational narrow beam stereoscopic scanning and wide angle cross-sectional pluridirectional tomography. While the stereo scans located the fragment within the sinus, a more accurate determination of its site, in relation to an object of known position, was obtained from the tomograms.

Parameters affecting radiographic contrast.

Kilovoltage, in most cases a variable readily controlled by the operator, and half-value layer have come to be regarded as factors defining image contrast. This has led to the assumption that comparable levels of image contrast may be obtained using X-ray units operated at the same kilovoltage and with the same half-value layer. To ascertain the validity of this view X-ray beams generated by seven X-ray units were compared with each other and with those from a Gendex Model 1000 operated at the same kilovoltage. The basis for comparison was the level of image contrast the X-ray beams were capable of producing after passage through selected thicknesses of aluminium absorber. While results showed no correlation between the nominal (stated) operating kilovoltage of any X-ray generator studied and the level of image contrast produced, a correlation (-0.98) was found between the measured half-value layer and image contrast when the units tested were compared with the Model 1000. This information should significantly affect the criteria used for the selection and/or operation of X-ray generators.