Now for the answer for todays locate the object (February 2015-3).
Where is the second premolar crown (orange arrow) in relation to the first premolar?
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).
The first thing to do is pick a stationary object that is seen on both radiographs and appears to move in comparison to the mandibular right second premolar (#29). On these radiographs, we will use the mandibular right first premolar (#28).
The most obvious angle change is the horizontal angle between the two radiographs. Starting with the mandibular right premolar periapical radiograph and moving to the mandibular right canine periapical radiograph, the horizontal angle decreases meaning the source of radiation (tubehead) moves anteriorly. According to point 1 above, this means the images move posterior.
Looking at the second radiograph (mandibular right canine periapical radiograph), we need to compare the image movement of the mandibular right second premolar (#29) versus the mandibular right first premolar (#28) to see which object moved more posterior following point 2 listed above.
The mandibular right first premolar (#28) appears to be more posterior on the mandibular right canine periapical radiograph meaning it is farther from the image receptor compared to the mandibular right second premolar (#29).
This gives us an answer of the mandibular right second premolar (#29) being to the lingual/palatal of the mandibular right first premolar (#28).
SLOB (Same-Lingual, Opposite-Buccal)
We will use the same objects as above (unknown object = mandibular right second premolar (#29) and fixed object = mandibular right first premolar (#28)).
Next, we need to determine which direction we are moving from the mandibular right premolar periapical radiograph to the mandibular right canine periapical radiograph and the answer would be – mesial.
On the mandibular right canine periapical radiograph determine what direction does the mandibular right second premolar (#29) appear to have moved in relation to the mandibular right first premolar (#28) – 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 – same and the acronym states that same is lingual, so the mandibular right second premolar (#29) is to the lingual of the mandibular right first premolar (#28).
Another case will be coming tomorrow. If you have any questions or comments, please leave them below. Thanks and enjoy!