How common is it to have a second bone graft prior to the dental implant?

Some patients do need to have a second bone graft placed before an implant and this can be for a variety of reasons. The goal is to have a solid foundation to place the implant in, if that is not achieved the implant is more likely to fail. 

It is common to make several attempts in order to have a better stability of the implants
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before , during and after implant placement procedure / treatment , bone grafting is very common. 

Actually it is a common procedure and it happens when the patient didn't get enough height or width on the first bone graft, or when a lot of time passed after the 1st bone graft.

It is very common. We place a bone graft when we take a tooth out hoping we won't need more grafting when it's time for the implant to be placed. If removing the tooth was difficult or if a lot of time passes between the removal of the tooth and the implant placement, it's often necessary to graft again when the implant is placed. 

it is very common procedure to receive a second bone graft. Sometimes the first one is not enough. The dentist wants to make sure you have good, sufficient bone prior to the procedure.