Primary Molar Ankylosis — a Review

Early Discovery

During the formative years at UCLA, the Pediatric Dental Department manned a dental clinic in the UCLA Hospital. It occupied a rather small area in the eastern section of the hospital and was staffed two to three days a week. Those first few years found Pediatric Dental Staff completing many exams in support of the Medical Staff and on some occasions they actually performed some ordinary dental procedures.

During that time the staff was able to examine identical twins that presented with a substantial number of missing teeth and ankylosis of primary molars. The concordance pattern was substantial and noted in a paper that was published in the Journal of the Southern California Dental Association. (1)

The authors were unable to get widespread interest and the paper was not widely quoted in subsequent reports related to ankylosis. Although many papers have been written about primary molar ankylosis, this paper was largely overlooked. The concordance pattern is extensive and is shown below.

                           Twin A

                           Twin A

                            Twin B

                            Twin B

                                                                                 Models of Twin A and Twin B

 Concordance Pattern Twin A & B

I. Congenitally Missing Permanent Teeth

  1. Maxillary right second premolar tooth # 4
  2. Maxillary right first premolar tooth # 5.
  3. Maxillary left first premolar tooth # 12.
  4. Maxillary left second premolar tooth # 13.
  5. Mandibular left second premolar tooth # 20.
  6. Mandibular right second premolar tooth # 29

Ii. Ankylosed Primary Teeth

  1. Maxillary right first primary molar tooth # B.
  2. Maxillary left first primary molar tooth # I.
  3. Mandibular left second primary molar tooth # K.
  4. Mandibular left first primary molar tooth # L.
  5. Mandibular right first primary molar tooth # S.
  6. Mandibular right second primary molar tooth #T.

III. Non - Formation of Third Molars

  1. No evidence of calcification of the third molars.

IV.  Ectopic Eruption of Maxillary First Molars with Mesial Shift


The concordance pattern suggested a genetic etiology and after some additional review in 2015, the authors were able to confirm that most research papers suggest a genetic fault. At this time a Google search of primary molar ankylosis will return 37,300 results in 0.55 seconds.

The current level of interest and the ability of the research community to sequence the genome could present new opportunities for dentistry to employ this emerging science. Over the past few weeks the American Dental Association has announced that they will host a fall conference to explore the opportunities presented in genome sequencing.

Clinical Pathology

While many research papers have attempted to delineate the prevalence of this disorder, the estimates have varied widely from 1% to 16% among those populations studied. Primary molar ankylosis occurs in the younger population with primary teeth that often do not exfoliate at the appropriate time, sometimes causing the permanent bicuspid to erupt out of alignment, and severe cases often require oral surgery and/or orthodontic management. In many cases the costs of oral treatment will require large sums of money to properly manage the pathology.

Figure 2

Figure 2

The diagram above details the mesial movement of the first permanent molar, reducing the arch length and impinging on the mesial-distal distance required for the erupting second bicuspid. This example of arch length loss often requires a substantial amount of orthodontic treatment and surgical intervention. 

The diagram above cannot completely describe the amount of misalignment that can occur in pediatric dental practice. If the ankylosis is severe the eruption of the permanent bicuspids may be completely blocked. In addition the bicuspid may be displaced from its intended location. 

Figure 3 - Ankylosis/Dentigerous Cyst

Figure 3 - Ankylosis/Dentigerous Cyst

Recent Research in Oral Pathology using Genome Sequencing

The most recent research paper that details the genetics of CIPA (Congenital Insensitivity to Pain) also contains an excellent section related to the dental pathology found in this patient. This team of Pediatric Dentists from Xian, China thought it important to add more detail into a report of "Congenital Insensitivity to Pain with Anhidrosis (CIPA)" that has been reported numerous times...but no other researchers carried out any further analysis of the oro-facial defects. (2)

The research article referenced here contains the online version of this important paper. (3) 

Figure 4

Figure 4

Noted in Figure 4 the following:

  1. Many primary teeth missing except lower left 2nd molar...they were loose enough to be extracted by the patient.
  2. Acellular cementum
  3. Periodontal ligament was loosely organized.
  4. Dentinal tubules of greater diameter.
  5. Dentin hypomineralized
  6. Sparse periodontal fibers.
  7. Thinner cementum and fewer attached fiber occupancies.
  8. And more...


NTRK1 is found on the first chromosome, long arm, at 21-22 the notation is 1q21q22. This chromosome has 17 exons and 16 introns 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.(4)

Note the Mutation Analysis of exons 13 and 15 that will predict amino acid substitutions. This patient has two novel missense mutations. This 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.

NTRK1 encodes the receptor tyrosine kinase (RTK) for nerve growth factor (NGF) and is the gene responsible for CIPA. Defects in NGF signal transduction at the TRKA receptor lead to failure to support survival of sympathetic ganglion neurons and nociceptive sensory neurons derived from the neural crest. Thirty-seven different TRKA mutations, identified in patients in various countries, including nine frameshift, seven nonsense, seven splice, and 14 missense mutations, are distributed in an extracellular domain involved in NGF binding, as well as in the intracellular signal-transduction domain.

See: (4)

On the Move Toward a More Defined Genome

Over the past several years there has been a decided movement toward a better understanding of the human genome as evidenced by the huge strides being made in genomic science. Those strides are exemplified by the delivery of a complete genome taken from archeological sites thousands of years old using newer sequencers that enable quick and reliable assembly of human genetic code. The reduction of the amount of time, effort and expenditure of dollars has created a paradigm shift that has enabled researchers to define the exact nature of past and present gene content.

As the total amount of effort and time required to assemble the gene has improved, so has the ability of researchers to further identify the genetic basis of disease. Are we not entering into a new era that suggests a resurgence of investigation into the genetic basis of disease? If our research dollars can be used more wisely and efficiently, we should be able to characterize the molecular basis of disease in a fashion unheard in past decades. We must adopt a new and intensified effort that harnesses the ability of new sequencers to define and characterize the human genome in the pursuit of the genetic basis of oral pathology. The lassitude that has prevailed as a consequence of high cost has fallen by the wayside as corporate America has unfailingly found quicker and cheaper methods to analyze the human genetic system.

Since one of the objectives of genomic research is to identify the specific etiologic factors that underlie disease, the current ability of modern sequencers to drill down to the smallest units of genetic makeup has created a new desire to define specific disease units.

Ankylosis of Primary Molars – A General Research Plan of Action

Present day standards of genetic analysis have often incorporated the use of Microarray-based Comparative Genomic Hybridization or aCGH. The use of this modality has come into general practice as a method to analyze the genomic status of impaired patients. In that respect, many laboratories have become specialized in their approach to seeking a genetic basis for a specific pathology. In response to the demand for answers, some laboratories have designed their micro-arrays around putative genes…creating a microarray that is more specific in nature. As an example, a specific laboratory might cater to physicians and health workers who are interested in impaired cognitive function. The interested partners of a special needs patient might opt for a genetic profile that seeks to define the etiology of their dysfunction. The assembly of a microarray is therefore based on a specific characterization of genes thought to be involved in that etiology. In this instance the microarray is created for a specific set of genes that might harbor the offending is site specific and is titled “targeted.”

If the aCGH is not targeted, it is an array design that looks over the entire genomic sequence, a whole genome or a “shotgun” imperative that seeks to localize the site of a gene malfunction.

If a new study of inquiry was instituted for oral/dental gene malfunction, the use of aCGH to determine copy number variations (CNVs) makes sense because it would deliver an overview of possible areas of interest. This initial characterization of genomic DNA would detail the gains or the losses of whole chromosomes or subchromosomal regions. The use of CGH does allow for the exploration of all 46 chromosomes in a single test and the possible discovery of deletions and duplications. This type of test can also detect single nucleotide polymorphisms or SNPs.

It is apparently true that at some time in the future, a microarray will be custom-created that is “targeted” to specific oro-dental sites. The makeup of such an array will be accomplished when oro-dental implicated genes are more fully identified. Of course, a listing of putative genetic faults that can occur in the oro-dental spectrum is currently available but not yet fully explored.

In the case of Primary Molar Ankylosis, we have many research reports that suggest a genetic etiology, but there is a notable lack of specific papers that seek to directly analyze the genomic status using up-to-date detection methods. Any new movement into the realm of genetic analysis for primary molar ankylosis will likely be based on the creation of an array that is composed of intelligently generated targets.

Our initial efforts now seem to be centered on analysis of the entire genome looking for gains or losses of DNA with a subsequent creation of a targeted array that could define the precise location of primary molar ankylosis. The use of ultra-high density genome-wide oligonucleotide-based microarrays may soon allow the detection of all genomic copy number variations larger than 1 to 10 kb in the human genome. In addition, the possibility that PMA etiology is epigenetic must be taken into account..


1. Stewart, R.E., and Hansen, R.W.: Ankylosis and Partial Anodontia in Twins.  J. So. Cal. Dental Assn. 2:50-52, May 1974

2. Gao L, Guo H, Ye N, Bai Y, Liu X, Yu P, et al. (2013) Oral and Craniofacial Manifestations and Two Novel Missense Mutations of the NTRK1 Gene Identified in the Patient with Congenital Insensitivity to Pain with Anhidrosis.

3. accessed June 14, 2105

4. accessed June 14, 2015