Accuracy of angular measurements and assessment of distortion in the mandibular third molar region on panoramic radiographs.

Objective: The objective of this study was to examine distortion on panoramic radiographs by assessing the accuracy of angular measurements in the mandibular third molar region on panoramic radiographs using computed tomography (CT).

Methods: Images from patients referred to a radiology practice for low-dose CT scans of their lower third molars were used in this study. The angle between the long axes of the second and third molars was assessed on panoramic radiographs and the corresponding CT images. Apparent tilting of the tooth across the arch on the panoramic image and the bucco-lingual inclination on CT were also recorded and compared.

Results: Eighty-eight patients had 163 mandibular third molars. The mean difference between the panoramic measurements and the CT measurements was -1.44 degrees, indicating that on the panoramic radiograph the third molar appeared less mesially inclined on average. This pattern was present in most cases, but in 64 cases the third molar appeared more mesially inclined on the panoramic radiograph. It was not possible to predict the direction of the discrepancy based on the radiographic findings. The discrepancy between the 2 measurements was greater if the third molar appeared tilted across the arch on the panoramic image, and tilted teeth showed a higher bucco-lingual inclination on reformatted CT images.

Conclusions: Distortions inherent in panoramic imaging because of projection geometry produce discrepancies in the angular measurements in the mandibular third molar regions on panoramic radiographs. Interpretation of third molar angulation from panoramic radiographs is often unreliable and may not accurately reflect the true orientation of the tooth. Apparent tilting of the tooth across the arch on the panoramic radiograph exacerbates the problem and appears to correlate with the bucco-lingual inclination of the third molar as visualized on reformatted CT images.