The main reason to replace missing teeth is to prevent movement of other teeth that can disrupt the harmony of the bite. Teeth are not immoble fixed units, but are susceptible to movement throughout life. The forces providing this movement can originate from the tongue, cheeks or lips. It is this capacity for movement that orthodontists use in a favorable mannaer to improve the alignment and function of the teeth.
When a tooth is lost, the adjacent teeth in the same arch and the tooth which acts as a biting antagonist in the opposing arch lose the support, strength and stabilizing influence which the missing tooth once provided. When a space is created by loss of a tooth, all the teeth adjacent to and opposing it tend to drift into that space. The space created by loss of one or two teeth, if the teeth are not replaced, may cause the loss of more teeth through drifting and periodontal bone loss.