1. A doctor performs an operation on a patient.
2. A patient is going to have an operation on his/her body part.
Sometimes, you can replace 'operation' with 'procedure' or 'surgery'. However, 'surgery' does not usually take the article 'a', so:
A doctor performs surgery on a patient.
3. This sentence is very unnatural. 'seam' is used in sewing or welding (fusing metal together) most often. In medical terms, we use the word 'stitches'. So:
I just had surgery on my toe. I got ten stitches.
I just got ten stitches in/on my toe.
This is much more natural.
4. Either 'bandage' or 'bandages' is okay. However, the sentence is a bit unnatural as is. Try this:
I went to the hospital to have my bandage(s) changed.