Maxillary Nine-year Molars: Prevalence and Associated Dental and Medical Conditions.
OBJECTIVE
The purpose of this study was to determine, from a large number of panoramic images taken in the clinic of a pediatric dentistry residency program, the prevalence of maxillary nine-year molars (9YM), associated medical conditions (MCs), and the presence of coexisting tooth anomalies (TAs).


METHODS
Pantomograms from 13,140 patients were searched for 9YM. Radiographs from identified subjects were examined for coexisting TAs, and charted MCs were noted. For identified 9YM, Nolla's stage of tooth formation was determined and the subjects' dental ages were calculated.


RESULTS
Among 53 subjects (68 percent females), 21 unilateral and 32 bilateral cases of 9YM occurred. Females and males were of similar age (7.98 versus 7.91 years old). Overall dental and chronological ages were significantly different: dental development in bilateral cases was delayed by 4.1 months in females and 2.9 months in both sexes (P=.003). In unilateral cases, it was accelerated by 3.6 months in females and 3.5 months in males. The specific tooth age for all 9YM was 4.95 years for females and 5.56 years for males (P=.018). The average count of associated findings (e.g., missing teeth, peg laterals, other TAs, and other eruption delays) was 0.62 in unilateral and 1.44 in bilateral cases (P=.02). Commonly associated MCs included clefting (<small>N</small> equals five), and Down syndrome (n equals three).


CONCLUSIONS
The prevalence of nine-year molars was one in 248 (0.4 percent), and the average delay in dental development was 2.7 years.
Department of Pediatric Dentistry, Children's Hospital Colorado and School of Dental Medicine, University of Colorado Denver, Aurora, Colo., USA. ulrich.klein@childrenscolorado.org.
Ulrich
U
Klein
Private practice, Lawton, Okla., USA.
Jacqueline A
JA
Alford
Department of Biostatistics and Informatics, School of Public Health, University of Colorado Denver, Aurora, Colo., USA.
Amanda A
AA
Allshouse
Department of Pediatric Dentistry, Children's Hospital Colorado and School of Dental Medicine.
Joel
J
Sain
D002648:Child; D015897:Comorbidity; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D008297:Male; D008437:Maxilla; D008963:Molar / Q000002:abnormalities* / Q000000981:diagnostic imaging; D015995:Prevalence; D011862:Radiography, Panoramic; D014071:Tooth Abnormalities / Q000000981:diagnostic imaging / Q000453:epidemiology*