Is an implant necessary to replace tooth 18?

Unless there is infection or some cause of pain or severe alignment issues, you want to avoid removing a healthy tooth. From the information you provided it sounds like there is room for the implant and if you have a healthy tooth above the implant be worth the investment for chewing purposes. Good luck!

HIIf tooth #18 does not have an opposing the risk of teeth movement is low.  As long as you can continue to chew on both sides there is little long term problems.  However, most people tend to chew on the side with most number of teeth as a result over use one side.Best,Dr. Maddahi

Hello! Thanks for your question. I recommend you visit a good specialist, because you have to first check your physiological, osseous, gingival state to be able to define the behavior to follow and depending on this you will define what is your best treatment option. 
Good Luck.