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.