Hypodontia and oligodontia: how a full-mouth plan is built when teeth are missing from birth

A congenitally missing tooth is not a tooth lost later on; the bud for that tooth never formed. Current classification uses three headings. With wisdom teeth left out of the count, fewer than six missing permanent teeth is called hypodontia, six or more missing teeth oligodontia, and none of the teeth developing at all anodontia. Wisdom teeth are held apart because of how often they are missing: one review gives missing wisdom teeth at 22.6%, and at 29.7% in Asian populations. So someone without wisdom teeth is not counted as out of the ordinary.

With wisdom teeth left out, the frequency of missing permanent teeth ranges between 2.6% and 11.3% across studies. A meta-analysis carried out in white populations gives 4.6% in men and 6.3% in women for Europe, 5.5% and 7.6% for Australia, and 3.2% and 4.6% for North America. The frequency in women is 1.37 times that in men. The most commonly missing tooth is the lower second premolar, followed by the upper lateral incisor and the upper second premolar. The absence being on one side is more widespread than on both, but with the upper lateral incisor the reverse holds.

The same meta-analysis reports that in 83% of people with a congenitally missing tooth, only one or two teeth are missing. The severe end of the picture concerns a narrower group than it appears: the figure given for oligodontia unaccompanied by a syndrome is 0.14%, that is, around one and a half in a thousand. That narrow end is what this piece focuses on. We are talking about situations where the whole mouth has to be rebuilt rather than a single gap filled.

The cause is largely genetic. The genes most often named today are EDA, MSX1, WNT10A and PAX9, with others such as TSPEAR, LRP6, PITX2 and GREM2 added to them. The same gene can cause missing teeth alone in some families and come alongside a syndrome in others. So the line between on its own and part of a syndrome is not always sharp. Even so, the form of missing teeth that does not come with a syndrome is the most common form.

Among the syndromes, the heading that concerns dentistry most is ectodermal dysplasia. Hypohidrotic ectodermal dysplasia is defined by the triad of sparse hair, markedly reduced sweating and missing teeth. Because sweating is insufficient, body temperature can rise in the heat; that is a safety matter which comes before the mouth. In the mouth there are few teeth, often malformed and pointed at the tips, and they come through later than average. The picture is inherited in an X-linked way through the EDA gene; recessive or dominant inheritance through EDAR, EDARADD and WNT10A is also reported.

If there is a syndrome, the plan stops being only a dental plan. The care recommendations written for hypohidrotic ectodermal dysplasia include starting dental examinations at one year of age and repeating them every six to twelve months, reshaping teeth that are pointed at the tips, orthodontics where needed, and renewing the prostheses in line with growth, most often every two and a half years. If there is difficulty chewing and swallowing, dietary advice is on the list as well. So treatment is not a single appointment but a monitoring programme that begins in childhood.

The answer to why the jawbone develops poorly lies in development itself. Unlike the jaw's basal bone, alveolar bone is a structure that forms along with the erupting tooth. Where the tooth never developed, that bone never formed either. The systematic review examining the subject writes it plainly: a site with a congenitally missing tooth is not like the implant site of a patient who lost a tooth later; the width, and usually the height, is missing from the start, and that bone was never there at all. This is where adding bone being an ordinary step in these patients rather than an exception comes from.

The second reason is growth. An implant fused to the bone stays where it is while the jaw around it grows, and over the years it can fall below the level at which the neighbouring teeth bite. The same review records a fall-behind of roughly 1 millimetre developing later in implants placed in the upper incisor region in childhood; in individually reported cases this has reached as much as 2.2 millimetres. That is why the criterion for a permanent solution is not the date of birth but growth being complete.

The distinction below is not for selecting patients but for showing what the subject says to whom. None of it stands in for an examination or for your own dentist's assessment.

The order below is a general framework. It changes with how many teeth are missing, whether a syndrome comes alongside, and where growth has got to.

1. 1

   Early diagnosis and taking stock

   The absence is most often noticed when the baby teeth are changing over: a baby tooth does not come out, or it does and nothing comes through in its place. On the X-ray, which buds are present is counted. In ectodermal dysplasia, the care recommendations ask for dental examinations to start at one year of age and then to be repeated every six to twelve months.

2. 2

   Keeping function going through the growing years

   In this period the aim is not a permanent arrangement but that chewing, speech and appearance carry on in a way appropriate to the age. In the ectodermal dysplasia review, removable complete or partial dentures appear as standard between the ages of 2 and 17; where there are more than six teeth per jaw, resin-bonded bridges are used as well. The dentures are renewed as the child grows, most often every two and a half years.

3. 3

   The orthodontic decision

   Will the gap be closed, or opened to the right width and kept to be filled later? This decision shapes every step that follows. Keeping the gap open has a price of its own: the review in this field writes that in a gap kept for a future implant or opened with orthodontics, the alveolar bone can waste away over time in both height and thickness.

4. 4

   The options for the interim period

   Keeping the baby tooth and, in suitable cases, transplanting a tooth to another position within the mouth come forward while growth continues. In the review, the survival of transplanted teeth is 94.4% with an annual failure rate of 1.061%; for retained baby teeth the figures are 89.6% and 0.908%. Both are below the annual failure rate of an implant (3.317%). This comparison has to be read cautiously: the review itself writes that the follow-up periods of the groups were not equal and that part of the difference may come from there.

5. 5

   Growth being complete

   The criterion standing before the permanent arrangement is not the date of birth but a demonstration that skeletal development has finished. Monitoring that is the orthodontist's job. The figures explain why the wait: in the review, implant survival is 72.4% in children under 13, 93.0% in adolescents under 18 and 97.4% in adults.

6. 6

   The permanent arrangement in adulthood

   The choice is made on the mouth as a whole: the state of the remaining teeth, the width and height of the bone, and what is happening in the opposing jaw. In the literature, fixed or removable arrangements on up to eight implants per jaw are reported in adult patients with ectodermal dysplasia. If you are an adult whose growth is complete and a large number of whose teeth are missing, the dentist who sees the three-dimensional image will tell you whether your picture today suits a fixed solution; if you write in through the form, the clinical team will call you and listen to your situation.

The rates below come from a systematic review examining patients with congenitally missing teeth. Most of the studies in the review are retrospective and their follow-up periods differ; read the figures as a sense of magnitude, not as a firm ranking.

In this picture things usually go wrong not through a single mistake but through the timing slipping. Six headings stand out.

With congenitally missing teeth, treatment is not an event but a programme. The intervals below are a general framework; they vary with the person and the picture.

1. Childhood and adolescence

   The recommendations written for ectodermal dysplasia ask for dental examinations to be repeated every six to twelve months. The dentures are renewed in line with growth, most often every two and a half years. Taking impressions and renewing dentures in this period is not failure; it is the plan itself.

2. Around the end of growth

   This is the period when a permanent arrangement is seriously discussed for the first time. The orthodontics being finished, the bone being measured on a three-dimensional image and the remaining teeth being assessed one by one all happen at this stage.

3. The first year after the permanent arrangement

   Getting used to a new chewing pattern, speech settling and a cleaning routine being established all fall into this period. The baseline measurements and films are taken during this year; they become the ground for comparison in the years that follow.

4. The years after that

   In oligodontia patients followed for more than ten years, peri-implant inflammation was found to be common. So follow-up does not end with the good news on survival; the gum measurements and the bone level are repeated at regular intervals.

You will not find figures here. These are the headings that set the weight when you read a plan:

How many teeth are missing, and in which jaw

A few gaps in one jaw and both jaws being empty at once are not the same job. What is happening in the opposing jaw shapes the plan too.

Whether bone needs adding

At a congenitally missing tooth site the bone can be narrow from the start. Whether an extra procedure is needed can only be established with a three-dimensional image, and it changes both the length and the scope of the plan.

The role and length of the orthodontics

Closing the gaps, or opening them to the right width, is a job that takes months. It has to be finished before the permanent arrangement.

Renewal and follow-up

Studies with long follow-up show a continuing need for maintenance, including renewal of the prostheses. How often follow-up happens and what it covers should be discussed from the start.