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That is perfect you got rid of all your wisdom teeth at once. Now you are facing some temporary postoperative difficulties that will end soon and you will be relieved. It will take most likely a week to open your mouth properly, so it is fine if you cannot brush teeth at back. After a few days, you will be able to do the proper cleaning. Regarding the white scab, it is the dead tissue formed that is not much harmful, it will shred away itself during this phase of healing if you are doing your rinses properly. Take a cup of lukewarm water and add 1/4th teaspoon of salt in it and do its mouth rinses five times a day. It is mandatory for two weeks. If you will skip it then some re-infection in the bone can happen which is very painful. So, it is better to be safe than sorry. Hopefully, everything will be good soon.