The terms describe the same family of conditions according to the number of missing teeth: hypodontia (one to five missing teeth, excluding third molars), oligodontia (six or more), anodontia (total absence, rare). In everyday language one speaks of tooth agenesis.
How common it is
In European populations the prevalence of congenitally absent permanent teeth is estimated between 3% and 11%, with values around 9% in an Italian orthodontic sample. In most cases only one or two teeth are missing.
The causes
Agenesis has a strong genetic component: it derives from alterations in the processes that guide the development of the tooth germ, and it can run in families. Several developmental genes are involved. In some cases agenesis, especially oligodontia, is associated with systemic developmental conditions (for example ectodermal dysplasias) or with situations such as cleft lip and palate.
Why identifying it matters
Beyond the appearance of the missing tooth, agenesis affects occlusion, bone development of the site and the position of neighbouring teeth. Recognising it early allows correct planning: from the choice between space closure and opening to the management of the bone deficit at the implant site.
Related reading
References: prevalence in an Italian sample PMC5641500; hypodontia, aetiology and classification PMC5376450. Informational disclaimer. Author: Dr. Gaetano Calesini.