Now for the answers from today’s earlier post.
Where is the maxillary left second premolar crown (orange arrow) in relation to the maxillary left first molar crown?
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 maxillary left second premolar (#13). On these radiographs, we will use the maxillary left first molar (#14).
The most obvious angle change is the horizontal angle between the two radiographs. Starting with the left premolar bitewing radiograph and moving to the left molar bitewing 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 (left molar bitewing radiograph), we need to compare the image movement of the maxillary left second premolar (#13) versus the maxillary left first molar (#14) to see which object moved more anterior following point 2 listed above.
The maxillary left first molar (#14) appears to be more anterior on the left molar bitewing radiograph meaning it is farther from the image receptor compared to the maxillary left second premolar (#13).
This gives us an answer of the maxillary left second premolar (#13) being to the lingual/palatal of the maxillary left first molar (#14).
SLOB (Same-Lingual, Opposite-Buccal)
We will use the same objects as above (unknown object = maxillary left second premolar (#13) and fixed object = maxillary left first molar (#14)).
Next, we need to determine which direction we are moving from the left premolar bitewing radiograph to the left molar bitewing radiograph and the answer would be – distal.
On the left molar bitewing radiograph determine what direction does the maxillary left second premolar (#13) appear to have moved in relation to the maxillary left first molar (#14) – 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 maxillary left second premolar (#13) is to the lingual of the maxillary left first molar (#14).
Another case will be coming tomorrow. If you have any questions or comments, please leave them below. Thanks and enjoy!