The temporomandibular joint : Tomopraphic and CT assesment of its bone demarcations with reference to adjacent organs

Abstract: The wall of the temporal bone separating the temporomandibular joint (TMJ) from surroundings organs, can be very thin and also have development defects. Distortion effects in the radiographic reproduction of these bone walls can result in misinterpretations when exanimating suspected pathologic changes in the region. These areas need to be radiographic identified prior to taking any invasive measures. Incorrect assessment of bone thickness may lead to serious sequelae due to the risk of penetration injury during invasive investigation or treatment of the TMJ or ear. The purpose of this project was to gain more detailed knowledge about the anatomy and topography of the TMJ with special reference to its bone demarcations regarding adjacent organs and to evaluate the tomographic and computed tomographic (CT) depiction of these bone walls. To obtain a basic analysis of the tissue anatomy and tomographic and CT reproduction of the TMJ region, autopsy specimens were studied. After CT and conventional tomography, the specimens were sectioned in a microtom. Three- dimensional orientation systems allowed identification of section depth in the radiograms and in the histologic sections, allowing the radiograms in turn to be correlated with the true anatomy. The angle of inclination relative to the perpendicular to established imaging planes the bone walls studied was examined in three projections in order to identify regions where the bone demarcation showed an unfavorable inclination regarding the possibility of valid radiographic representation. The thickness of the bone wall between the TMJ and the middle cranial fossa, measured in the thinnest part, varied between 0.08 and 3.62 mm, averaging 1.14 mm. The bone wall between the TMJ and the middle ear showed less variation in thickness ranging from 0.00 to 1.80 mm. The thickness of the bone wall separating the TMJ from the external auditory canal varied between  1.50 mm (lateral part) and 1.21 mm (central part), with a range of between 0.21 and 4.10 mm. Development defects of this bone wall were found in 5.2 % of the examined joints. The validity in tomographic depiction of these walls was highly dependent on an optimal orientation of the bone wall in relation to the image plane. The variations in the anatomy and sagittal dimension of the external auditory canal led to variations in tomographic blurring, and suggested the need for examinations after patient repositioning in cases of suspected bone resorbing lesions so that image aberration due to unfavorable inclination of the bone wall relative to the image plane may be excluded. CT of these bone walls was valid (± 10 %) for walls thicker than approximately 1 mm, forming an angle of less than 35® with the perpendicular to the scan plane when the bone wall thickness was determined as the full-width-at-half-maximum (FWHM). For bone walls thinner than 1 mm, and for those thicker than 1 mm and at an angle exceeding 35®, partial volume averaging effects resulted in a progressively increasing magnification of bone dimensions. Observer estimations of bone thickness from images obtained using conventional bone window settings (c=400, W=2000) showed good agreement for bone walls thicker than 1 mm and with an angle of inclination relative to the perpendicular to the image plane of less than approximately 25®. For bone walls thinner than 1 mm and for thicker than 1 mm with an inclination exceeding approximately 25®, the estimations resulted in a progressively increasing overestimation amounting 200% for gracile bone walls with an inclination of 45® to 50®. Determination of width or absence of the central white zone in images obtained with the described parameters could help to reduce the risk of overestimation of bone thickness. A considerable part of the bone walls separating the TMJ from the middle cranial fossa and the external auditory canal/middle ear, respectively, have dimensions and inclinations to established imaging planes used at TMJ examinations that make the depiction of these walls highly susceptible to image distortion.