An unusual case of SMMCI syndrome: supernumerary midline central incisor in siblings

From Volume 13, Issue 1, January 2020 | Pages 10-14


Hannah Morison


DCT2 in Orthodontics and Paediatrics in Glasgow, UK

Articles by Hannah Morison

Evelyn Dunbar


Consultant Orthodontist StR in Orthodontics in Dundee, UK

Articles by Evelyn Dunbar


This is the first reported case of siblings with Solitary Median Maxillary Central Incisor (SMMCI) syndrome presenting with an erupted single symmetrical central maxillary midline incisor between normal central incisors. Usually only one central incisor is present. This case is also interesting as the tooth described could have been mistaken for a mesiodens, however, a mesiodens is not symmetrical and usually erupts between the maxillary central incisors and sits slightly to the right or left of the midline.

CPD/Clinical Relevance: This case highlights the fact that SMMCI should not be considered as a simple dental anomaly because it may be associated with more complex craniofacial malformations.


Solitary Median Maxillary Central Incisor (SMMCI) syndrome is a complex disorder and a rare dental anomaly.1 SMMCI occurs more in women and prevalence is estimated at 1:50,000 live births, with a higher incidence in stillbirths and aborted foetuses. The aetiology is uncertain, but is thought to be associated with a missense mutation in the Sonic HedgeHog (SHH) gene.2 It is a unique developmental abnormality arising from an unknown event or events occurring between the 36—38th days in utero.3

SMMCI was initially described as an isolated defect, but has since been associated with multiple, mainly midline defects and HoloProsEncephaly (HPE). HPE is a developmental defect of the forebrain where the cerebral hemispheres fail to separate into distinct halves, leading to midline defects.1 These include defects of the cranial bones, the maxilla, nasal bones, facial bones, the dentition and the brain. It can also cause defects in other midline structures of the body and can be associated with short stature.1,2

Register now to continue reading

Thank you for visiting Orthodontic Update and reading some of our resources. To read more, please register today. You’ll enjoy the following great benefits:

What's included

  • Up to 2 free articles per month
  • New content available