Localizing ectopic maxillary canines--horizontal or vertical parallax?

This study compared the use of horizontal parallax (HP) and vertical parallax (VP) radiography for localizing ectopic maxillary canines (EMCs). The true positions of 43 EMCs were determined using the operative notes and subsequent study models following exposure and eruption. Thirty-four palatal and nine buccal EMCs were included in the study. Six experienced orthodontists examined the radiographs of the EMCs and were asked to localize each EMC using VP and HP on separate occasions. The examiners recorded a diagnosis of 'unsure' in 12 per cent of cases using VP and in 5 per cent of cases using HP. The level of agreement of the diagnoses with the true position of the EMCs was significantly greater for HP. Eighty-three per cent of EMCs were correctly located with HP while only 68 per cent were correctly located with VP (P < 0.05). The diagnostic sensitivity for palatally placed canines was significantly greater for HP (88 per cent) than for VP (69 per cent). Both techniques performed poorly when used to localize buccal EMCs, with HP and VP each having a sensitivity of only 63 per cent. It is concluded that HP is superior to VP in diagnostic accuracy, and that two peri-apical radiographs or one peri-apical and one anterior occlusal radiograph are the radiographs of choice for localizing EMCs.