The answers from todays earlier post.
Where is the second premolar (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 left second premolar (#20). On these radiographs, we will use the mandibular left first premolar (#21).
The most obvious angle change is the horizontal angle between the two radiographs. Starting with the mandibular left canine periapical radiograph and moving to the mandibular left premolar periapical radiograph, the horizontal angle increases meaning the source of radiation (tubehead) moves posteriorly. According to point 1 above, this means the images move anterior.
Looking at the second radiograph (mandibular left premolar periapical radiograph), we need to compare the image movement of the mandibular left second premolar (#20) versus the mandibular left first premolar (#21) to see which object moved more anterior following point 2 listed above.
The mandibular left first premolar (#21) appears to be more anterior on the mandibular left premolar periapical radiograph meaning it is farther from the image receptor compared to the mandibular left second premolar (#20).
This gives us an answer of the mandibular left second premolar (#20) being to the lingual/palatal of the mandibular left first premolar (#21).
SLOB (Same-Lingual, Opposite-Buccal)
We will use the same objects as above (unknown object = mandibular left second premolar (#20) and fixed object = mandibular left first premolar (#21)).
Next, we need to determine which direction we are moving from the mandibular left canine periapical radiograph to the mandibular left premolar periapical radiograph and the answer would be – distal.
On the mandibular left premolar periapical radiograph determine what direction does the mandibular left second premolar (#20) appear to have moved in relation to the mandibular left first premolar (#21) – distal.
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 left second premolar (#20) is to the lingual of the mandibular left first premolar (#21).
Another case will be coming tomorrow. If you have any questions or comments, please leave them below. Thanks and enjoy!