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Retrospective radiographic evaluation of the anterior loop of the mental nerve: comparison between panoramic radiography and spiral computerized tomography.

PURPOSE
This study compares the prevalence and the length of mental loops, which were measured with panoramic radiographs and spiral computerized tomographs (SCT).


MATERIALS AND METHODS
Seventy-three panoramic radiographs and 73 SCTs were taken for preoperative planning of implant placement in the interforaminal region of the anterior mandible. The bone quality of both mental regions in each patient was determined by 1 experienced calibrated clinician, and the cross-sectional images of the SCTs were evaluted for bone quality according to the Lekholm and Zarb classification. Panoramic radiographs and cross-sectional SCT images were examined carefully by the same calibrated clinician to determine the presence and to measure the length of the mental loop in each patient. The relationship between these radiographs was also examined and correlated with bone quality. Paired samples t test and Pearson's correlations were used to examine the agreement between 2 radiographic methods at each bone quality. A 5% level of significance was chosen.


RESULTS
The prevalence of the mental loop in panoramic radiographs and spiral CT images was 28% and 34%, respectively. The mental loop was identified more frequently in spiral CT images regardless of bone quality. The demonstration of the mental loop between radiographic methods was more pronounced in poor bone quality. The mean length of the mental loop in panoramic radiographs was 3.71 +/- 1.35 mm and 3.00 +/- 1.41 mm in SCT. Measurements for panoramic radiographs were higher than those for spiral CT images. There was a correlation of r = 0.66 (P = .01) between the 2 radiographic methods, indicating agreement.


CONCLUSIONS
SCT demonstrated a higher prevalence of mental loops than panoramic radiographs. SCT can be more useful to visualize and measure the mental loop in low bone qualities. Panoramic radiographs significantly overestimate (P = .02 in type 3, P = .01 in type 4) mental loop length, which were measured in spiral CT images in poor bone quality, but there is a close correspondence between these 2 radiographs in higher quality bone.