A comparison between in vivo radiographic working length determination and measurement after extraction

The purpose of this study was to compare the difference between the in vivo working length established by viewing a periapical radiograph and the in vitro measurement from the file tip to the apical foramen of the extracted tooth. Twenty-six canals from teeth that had been treatment planned for extraction were accessed under rubber dam isolation. The coronal portion of each canal was flared using Gates Glidden drills, and a K-type file was inserted down the canal until an electronic apex locator indicated that the file had reached the apex. A size 20 K-type file was locked into place with glass ionomer cement at this position. A radiograph was exposed and the tooth was extracted. Each tooth was viewed using a videomicroscope at 30x original magnification, and the distance from the file tip to the most coronal aspect of the major foramen was measured. Six examiners viewed each radiograph and assessed the working length of each canal. Chi(2) analysis of the data revealed a significant difference (p < 0.01) between the estimation of working lengths and the microscopic measurements. The examiners overestimated the distance between the file tip and the apical foramen of the tooth when the file was placed short, and underestimated the distance when the file was placed long. In conclusion, when the file is short it is actually closer to the apical foramen than it appears radiographically; when it is long it is actually longer than it appears radiographically.