Bone demarcation of the temporomandibular joint. Validity of clinical assessment of bone thickness by means of CT.

• PURPOSE: To study the CT depiction of bone demarcations in the temporomandibular joint, using conventional window level and window width; and to evaluate observer performance in estimating bone thickness in these images.MATERIAL AND METHODS: Seven joint specimens were imaged by CT and then cryosectioned. The measurements of bone wall thickness in the images were compared to the true bone thickness at each cutting level. In addition, 4 experienced radiologists estimated the thickness of the bone walls in the images.RESULTS: The relative difference between the CT reproduction and the true bone thickness was small for bone walls thicker than 2 mm. This difference increased with the decrease in bone thickness and the increase in the inclination of the bone wall from the perpendicular to the image plane. Bone walls thinner than 1 mm were reproduced as considerably thicker than their true thickness. This resulted in a clinical overestimation of bone thickness.CONCLUSION: Both the CT representation and the interpretation of bone demarcation in the temporomandibular joint may constitute a problem. Partial volume averaging effects can result in an overestimation of bone dimensions amounting to 200% for thin bones. The central white zone in images of thin bone walls obtained with the parameters described here could serve as an indicator that could help to reduce the risk of overestimating bone thickness.

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oral and maxillofacial radiology

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