Locate the Object: September 2013 Answer


Now onto the answers for the September 2013 Locate the Object. Sorry it’s a little late. I will be going over both image shift and SLOB (Same-Lingual, Opposite-Buccal) to determine the location of the left lateral incisor in relation to the left central incisor.

Image shift

Before starting to use the image shift principle it is important to know/remember two key points

  1. Images move in the opposite direction from the movement of the source.
  2. Images of objects farther from the image receptor will move more (aka objects (images) more facial/buccal will appear to move more).

radiolucent area on crown of 9 periapical 1radiolucent area on crown of 9 periapical 2

The first thing to do is pick a stationary object that is seen on both radiographs and appears to move in comparison to the circular radiolucent area on the crown of the left central incisor.  On these radiographs, I am using the root canal space of the left central incisor. The most obvious angle change is to the horizontal angle so will we use those changes. Starting with the central incisors periapical radiograph and moving to the lateral incisor periapical radiograph, the source of radiation (tubehead) moves posteriorly increasing the horizontal angle.

According to point 1 above, this means the images move anterior.

Looking at the second radiograph (lateral incisor periapical radiograph), we need to compare the image movement of the circular radiolucent area on the crown of the left central incisor versus to the root canal space of the central incisor to see which object moved more anterior following point 2 listed above.

The circular radiolucent area on the crown of the left central incisor appears to be more anterior on the radiograph meaning it is farther from the image receptor compared to the root canal space of the left central incisor. This gives us an answer of the circular radiolucent area on the crown of the left central incisor being to the buccal/facial of the root canal space of the left central incisor.

SLOB (Same-Lingual, Opposite-Buccal)

The first thing to do is again pick an object with a known fixed location – the root canal space of the left central incisor.

The second thing to do is determine which direction we are moving from the central incisors periapical radiograph to the lateral incisor periapical radiograph and the answer would be – distal.

Lastly, on the lateral incisor periapical radiograph determine what direction the circular radiolucent area on the crown of the left central incisor appears to have moved in relation to the root canal space of the left central incisor – mesial. Here is where the acronym comes into play. Did the unknown object move in the SAME direction as the radiographs or in the OPPOSITE direction? Our answer is – opposite and the acronym states that opposite is buccal, so the circular radiolucent area on the crown of the left central incisor is to the buccal of the root canal space of the left central incisor.

Another case will be coming later today, October 15th. If you have any questions or comments, please leave them below. Thanks and enjoy!


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