The term "in dentures" refers to something that is fixed or secured within a set of dentures, such as a dental implant or other dental appliances. The term's opposite, or antonym, would be "out of dentures." This can refer to teeth that have not yet been replaced with dentures or dental appliances that have been removed from the mouth. Other possible antonyms for "in dentures" include non-removable, immobile, or unattached. It is important to note that in the field of dentistry, the use of these terms would depend on the specific context and the type of dental treatment being provided.