I had a molar pulled on the upper left. One Dentist says must have implant, another says not true don't need it, who is correct?

Your upper left tooth has now been missing for a year and your dentist tells you he/she has seen no shifting or tooth movement in the last year.  The bigger picture here is that tooth movement and shifting can occur quickly, but also very slowly.  There is no way to tell if you will or will not have shifting occur.  If you are grinding or clenching your teeth, you are definitely more likely to have shifting and tooth movement.  You have two options:1. No treatment.  No Implant.  Shifting may occur and may not occur.  You can also chose to wear an appliance to hold the position and alignment of your teeth to help prevent tooth movement/shifting or take your chances and not wear the appliance.2. Have the Implant placed.  Implants have a very high success rate. Be informed that if you do have the implant placed, and you are grinding your teeth, the implant has a higher risk of failure.  A tooth and an implant are not the same.  Your tooth is held in the bone by ligaments that absorb forces.  Under heavy forces, a tooth will flex and crack and you will feel it as pain and soreness to make you aware of the heavy forces you are putting on your teeth.  An implant does not have these ligaments.  The bone integrates around the implant surface.  Implants can only withstand forces down the long axis of the implant.  It will not hold up to excursive or lateral forces, like grinding forces.  These lateral forces will eventually either fracture the implant, fracture the screw holding the crown, and/or break down the bony interface around the implant causing Peri-Implantitis, which is an infection around an implant.  As I stated, Implants do have a very high success rate when the occlusion or bite is correct.  If your bite is not in the correct position and you are grinding, you have the option to have your bite corrected to stop your grinding or wear an appliance to relieve forces on the dental implant.  The most important thing is to have a dentist be able to inform you of all treatment options, benefits and risks, so the you will be able to make the most informed decision regarding your dental health.

There are several reasons to replace the tooth that was extracted. As was mentioned by several dental providers, the adjacent teeth may shift over time causing misaligned positioning of the bite that may cause discomfort with n the TMJ. Also, the opposing teeth may rise up to fill in the gap from the extraction, described as hypererruption which may also cause issues with the bite. Lastly, if no grafting or implant is placed in the area of the missing tooth, then the floor of the sinus may drop described as pneumatization of the sinus. Placing grafting and/or Implant at the time of extraction usually preserves the 3 Dimensional bony architecture. 

Thank you for your great question. In general I have to agree with NY dentist, extracted tooth should be replaced to prevent teeth shifting in a long term and a bite interference and, posible, bite collapse. It can be achieved using multiple options like implant, a fix or removable bridge. If you would wear your night guard (bite plate) and it is made correctly it might also serve as a space maintainer, so it could help both problems. Because most patients do not wear night guard every night, ideally though you should have something like implant done.