Densitometric measurements of the mandible: accuracy and validity of intraoral versus extraoral radiographical techniques in an in vitro study

In this study a comparison is made between intraoral and extraoral radiographical techniques for quantitative measurements of bone density in the edentulous mandible. A phantom model, composed of a dry mandible surrounded by a soft tissue substitute was used. Radiographs were made of aluminium implants of known volumes. Intra- and extraorally placed reference wedges of aluminium or a combination of aluminium and plexiglas were used for the measurement of the aluminium volume of the above-mentioned implants and compared with the real volume. The IBAS image analysis system was used for image processing and measurements. A systematic deficit in the measured volume of 15 to 29% was found. This error was irrespective of the radiographical technique and indicates a constant underestimation of the aluminium volumes. The differential influence of the secondary radiation on the images of the jaw and the wedge is proposed to be a possible cause of this constant underestimation. The intraoral and extraoral techniques do not display significant statistical differences in the levels of validity or accuracy. Obtaining serial intraoral radiographs of adequate quality in the atrophic edentulous mandible can be problematic. It is concluded from this in vitro study that in these cases Oblique Lateral Cephalometric Radiographs may provide a valuable alternative for quantitative image analysis