I have a retainer and a bone graft, do I really need a dental implant?

If your concern is how you would look or appear in photos, missing a second molar won't have any affect.  It MAY affect how you chew, but not by much.  Most chewing/function is at the first molar and second premolar (tooth in FRONT of the molar), so the effects will be minor.  Having the graft will help preserve bone volume, but without a tooth or an implant, the grafted volume will be lost over time.

A missing lower back tooth that is all the way in the back shouldn’t have any effect on your facial aesthetics.It also shouldn’t have any great effect on your ability to chew properly. There are, however, two things that you do want to take into account.If there is an upper tooth directly opposing the space where the tooth was extracted, then you will start to see that upper tooth overerupt over time. Eventually, this may lead to the upper tooth needing to be extracted.  It’s also very important to not develop tunnel vision and to take into account  the condition of the rest of your mouth. If the rest of your mouth is extremely healthy, then it is far less important to consider replacing this missing tooth. If, however, there’s a possibility that you will be losing more teeth in the future, then you May want to put more consideration into replacing this missing tooth.I hope you found this information to be helpful,Dr. Champagne 

Typically for a 2nd molar like #31, I'll leave that decision up to the patient. I've found that most patients function well without replacing a 2nd molar. If you are having any issues properly chewing your food, I would suggest replacing it. A downside of not replacing the tooth is sometimes the upper tooth will shift down into the empty space and end up needing to be extracted. If the upper tooth is biting against a lower tooth and being kept in place, you don't necessarily need to replace #31.