Is it true that you need at least 4 mm of mesio-distal ridge space between teeth to place an implant?

That used to be true, but there are now some companies that make small diameter implants of a 3 mm diameter, and there are even mini-implants with diameters ranging from 1.8 – 2.5 mm. Mini-implants are primarily marketed for retention of dentures and overdentures, but they can also be used for mandibular incisors or maxillary lateral incisors.