Primary molar ankylosis (PMA) has a long history of research activity that may be reviewed on the web using the query…primary molar ankylosis. The resulting list of articles that span many decades will include over 40,000 articles retrieved in less than one second. The multitude of research papers very often mention that there is a genetic pattern among those with this oral pathology. This author has reported on a pair of identical twins with 16 locations of concordance. That paper is reviewed in this blog and is certainly very suggestive of a genetic/epigenetic etiology. (Stewart & Hansen) A more recent report concerning impacted primary teeth and agenesis of permanent molars in monozygotic twins is also very supportive of a genetic basis to PMA. (Zengin, Sumer, Karaarsan)
Primary molars with a loss of periodontal ligament (PDL) are ankylosed and attached by a cementum to alveolar bone that confers modified treatment protocols. Extraction of ankylosed primary molars is not easily accomplished since the PDL is absent and a normal luxation of the offending tooth is impossible. The crown will fracture and leave rudiments of the root structure embedded in the alveolar bone. A surgical removal of the remaining root structure creates an added facet to the management of PMA.
Radiographic Evidence
Periapical x-rays of PMA will show a PDL that is absent. A more defined title for this mixed dentition oral pathology might be, “Agenesis of the Periodontal Ligament in Primary Molars.” In many cases the succedaneous bicuspid is congenitally missing.
Figure 1 Ankylosed Second Primary Molar
An intraoral photograph of ankylosed primary molars shows infraocclusion of the offending molars. They appear to submerge, however the continuing growth of the alveolar structures leaves the ankylosed molar in its original position. The increase in alveolar height coupled with a primary molar that is immoble, creates the distinctive appearance of ankylosis.
Figure 2
Primary Molar Ankylosis showing partial loss of PDL
A review of the early histology of the formation of the primary molars may be illustrative of time period when the primary molar PDL is being created.
The crown is complete on the first primary molar at about 6 months of age. The crown of the second primary molar is complete at about 11 months. At that time the periodontal ligament (PDL) begins formation. The maturation of the pluripotent cells that form the PDL in the permanent dentition never fail to produce that organ. But a lack of the PDL in the primary dentition is fairly common. Recent research has indicated that 6% may be near the mean of all recent papers that report PMA pathology. That relatively large percentage of incidence results in a cohort of patients that is astounding in the USA and abroad. While we have seen mild forms of the pathology, there are very difficult pathologies in other patients that result in the need for oral surgery and orthodontic treatment.
Our interest should be focused on the primary molars since PMA pathology begins when the root begins formation. The time period of periodontal formation can be reviewed on the chart above. As an example: the Mandibular Second Primary molar begins initial calcification at 18 weeks I.U. The crown is complete at 10 months and at that time the PDL begins formation. The root is complete by 3 years of age providing us with a 2 year and 2 month time frame for PDL formation. In PMA the PDL does not form and there may be additional histologic pathologies connected with the cementum and the alveolar bone. see:http://en.wikipedia.org/wiki/Tooth_development
Progressive Pathology of PMA
The earliest symptom of PMA is often a loss of eruption that allows the primary molar to appear to be submerging. The eruption of the adjacent permanent molar will remain in occlusion with the opposing dentition, but the ankylosed primary molar will not continue to erupt as the alveolar height increases with vertical growth. The most common primary teeth involved with this pathology are the first and second primary molars. The bicuspid(s) may be displaced from their normal position and the resulting pathology requires oral surgery and very likely a long period of orthodontic management. The Pediatric Dentist or the General Practitioner must be alert to the initial signs of ankylosis and create a treatment plan that includes extraction of the primary molar at an early date.
The PDL of PMA is often selectively missing. A few PDL fibers may be found histologically, but the attachment is essentially missing when a periapical x-ray is taken to ascertain the condition of the “submerged” primary molar. The x-ray will often define the extent of the missing PDL and the bony attachment of the root structure to the alveolar bone.
Recent Research
An article reported online several months ago is of importance because it is an addendum to a report by Chinese dentists concerning Congenital Insensitivity to Pain with Anhidrosis (CIPA). See: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0066863
A team of Pediatric Dentists from Xian, China thought it important to add more detail into a report of CIPA. This pathology has been reported numerous times...but no other researchers carried out any further analysis of the oro-facial defects. NTRK1 is found on Chromosome 1 the long arm, at 21-22...notation is 1q21q22. This chromosome reportedly has 19 exons with about 25 kilobases of DNA. CIPA has been identified in 60 cases in the U.S. and about 300 worldwide, but only one report (this one) of oral-dental defects...this one article written by Chinese Pediatric Dentists at the Fourth Military Medical University in Xian, the site of the terracotta warriors.
Note several interesting facts...
Primary teeth missing except lower left 2nd molar...they were loose enough to be extracted by the patient.
Acellular cementum
Periodontal ligament was loosely organized.
Dentinal tubules of greater diameter.
Dentin hypomineralized
Sparse periodontal fibers.
Thinner cementum and fewer attached fiber occupancies.
And more... Check out the Mutation Analysis and exons 13 and 15 that predicts amino acid substitutions. This patient has two novel missense mutations. So this research article notes the aplasia of cementum and the lack of periodontal attachment. Keep in mind that the primary first molar begins calcification in the fourth fetal month and the crown is complete by 5.5 to 6 months of age...then the root formation begins. There is one additional fact of utmost importance. The parents are first cousins, meaning that they are consanguineous.
It seems likely that our profession will see fit to pursue PMA from a genetic stand point. The evidence points to a genetic etiology that could be solved by using genome sequencing that could identify the genetic/epigenetic etiology.
Bibliography
Stewart, R.E., and Hansen, R.W.: Ankylosis and Partial Anodontia in Twins. J. So. Cal. Dental Assn. 2:50-52, May 1974
Zengin AZ, Sumer AP, Karaasian E. Impacted Primary Tooth Agenesis: A Case Report of Monozygotic Twins. Eur J Dent 2008; 2:299-302. 14 See: citeseerx.ist.psu.edu