canine, in contrast, is over‐retained (extraction advised), since the long root of its successor illustrates delayed eruption.
Permanent tooth with delayed eruption: This term refers to an unerupted tooth whose root is developed in excess of two‐thirds of its expected final length and whose spontaneous eruption may nevertheless be expected within a reasonable time.
Impacted tooth: This refers to a tooth whose root is developed in excess of two‐thirds of its expected final length, but which is not expected to erupt in a reasonable time.
When assessing the dental age of the patient, it is important to emphasize that one should not include the maxillary lateral incisors, the mandibular second premolars and the third molars in this calculation. The development timetable of these teeth is not always in line with that of the patient’s other, ontologically more stable teeth [9, 10]. These are the same teeth that are most frequently congenitally missing in cases of partial anodontia (hypodontia) or oligodontia. Indeed, reduced size, poorly contoured crown form and late development of these teeth are all considered microforms of congenital absence [9–12]. The variation in their timing is, however, always expressed as lateness and they are never seen in a chronologically more advanced state of development than the other teeth. If the individual dental age of any of these variable groups of teeth is advanced, then so too is the dental age of the entire dentition in which they are to be found.
In summary, therefore, we may assert as follows:
All orthodontists must have at their fingertips the ages at which the permanent teeth normally erupt.
Permanent teeth normally erupt when approximately two‐thirds of their final root length has developed.
The remainder of the root reaches apexification approximately 2½–3 years after eruption.
Determining the closed apex of the tooth on a radiograph is usually an easy and accurate parameter to establish.
Determining the completed proportion of the root of a tooth, whose final completed length is unknown, is not an assessment that can be performed accurately. It rather falls into the realm of informed guesswork.
Having now set out the principles upon which dental age may be assessed, we must turn to the practical side of translating these principles into clinical terms in a logical, systematic and didactic manner. The simplistic way of adopting the above principles would be to take the panoramic radiograph or full‐mouth periapical survey and then work around each dental arch from one tooth to the next, individually and from left to right, upper to lower, evaluating each tooth in turn. This would then require coordinating all the individual results and computing a final figure that is the dental age of the patient. The resulting conclusion would have to be compared against the values seen on an idealized chart of the norms for a given population [1, 4, 13]. Although this method delivers accuracy, it requires a considerable time and it is likely to take an hour or so to reach the final conclusion. It is an arduous and tedious endeavour, which does not lend itself to the conditions that are present in a busy clinical orthodontic practice.
What is recommended is a simple, logical but systematic approach that can be employed chairside to reach a similar conclusion in just a few minutes, even at the initial orthodontic consultation visit. This method will rely on the same criteria of establishing the development of crown and root, but must do so in a step‐by‐step manner, starting from a different point of departure. The ‘starting point’ of this systematic approach has, for reasons that will be explained in the following paragraphs, been set at the cut‐off dental age of 9 years.
Assessing dental age in the clinical setting – the Jerusalem method
There are several criteria that are appropriate to the appraisal of tooth development when using full‐mouth periapical radiographs or a panoramic film. The information that is available regarding the ages at which the various stages of dental development occur is based on the classic random studies that have been carried out over many decades of the local populations of the researchers involved. The figures for the mean ages at which these stages occur, in the hypothetical child, are as follows:
1 The first signs of the presence of a tooth are discernible radiographically with the initiation of calcification of incisal edges and cusp tips. Thereafter, one may observe the formation of the completed crown as well as progressive degrees of root formation (usually expressed in fractions), and thence the fully closed root apex. Since orthodontic treatment is largely performed on a relatively older section of the child population, the stages of actual formation of the root become the only relevant factors.
2 The accuracy with which one may assess fractions of an incomplete, immeasurable and merely ‘expected’ final root length is not reliable and is very much a matter of individual observer variation.
3 The stage of tooth development that is easiest to define with confidence and with accuracy is that which relates to the closure of the root apex. So long as the dental papilla at the root end remains discernible, the apex is open and Hertwig’s root‐forming, epithelial sheath is in an active stage of increasing root length. However, once fully closed, the papilla disappears and a continuous lamina dura will be seen on a periapical radiograph, closely following the root outline. These are the specific diagnostic signs of that landmark event. Apexification is therefore the most important single factor upon which a system of assessment may be faithfully and easily made of the dental age of a given patient in the clinical environment.
4 From population studies, we learn that the first permanent tooth to erupt in the mouth is the mandibular central incisor, closely followed by the first permanent molars, and this occurs at the age of 6 years.
5 Root development of the permanent teeth is completed approximately 2.5–3 years after their normal eruption [4]. This allows us to conclude that, at the age of 8.5–9 years, the child’s mandibular incisors will be the first teeth to exhibit closed apices and will usually be closely followed by the four first permanent molars. This being the case, it is clear that the age of 9 years must be the basic starting point from which to commence the evaluation of the child’s dental age. If mandibular incisors or molars demonstrate root closure, then the tentative diagnosis would be that the patient has a dental age of at least 9 years. If the apices are still open, then the conclusion would be that the child has a lower dental age.
It should be emphasized, however, that the exercise is aimed at ranking a specific child’s dental development vis‐à‐vis the above hypothetical mean. Whether or not the evaluated tooth has actually erupted is entirely irrelevant to this equation.
Let us examine the progressive diagnostic path in its correct order (see also Table 1.1):
1 If the mandibular central incisor roots are complete, we may presume that the patient is at least 9 years old (dental age), i.e. this figure is derived from 6 years (the normal age of eruption as determined by two‐thirds root length development), with the addition of 2.5–3 years to apexification.
2 We may then proceed and check for closed apices of the first molars (9–9.5 years).
3 At 9.5 years, the mandibular lateral incisors roots will have completely developed.
4 The next teeth in the expected eruption series are the maxillary central incisors, whose closed apices would indicate a dental age of 10 years.
5 Because their rate of development is variable, it would be wise to bypass the assessment of the maxillary lateral incisors at this point in the diagnostic process and move
