There are two basic mechanisms in creating the condition that we refer to as dental crowding. By definition, crowding is a discrepancy between the cumulative widths of the dental units or teeth, and the available space to accommodate these teeth within the alveolar bone. Thus, one can represent a ratio between the tooth size and the supporting bone size. When this ratio exceeds unity, we define such a condition as dental crowding. If the ratio is one or less than unity, we don’t speak about dental crowding, but may, in fact, identify the condition as dental spacing. Arguably, crowding exceeds spacing by a large margin, perhaps as large as 100 or more to one. The ratio of exact unity or one is so rare in real life that it can, for all practical purposes, be ignored.

Calculating this ratio is not an exact science. The numerator part of the ratio is relatively easy to measure, whether it is done directly in the patient’s mouth or on a dental study cast. Numerous studies have demonstrated that the error of measurement for human dentition is typically within the calibration characteristics of the measuring device. Consequently this error is assumed to be so insignificant that it can be ignored. Calculating the denominator of the ratio, the size of the available arch perimeter, is substantially more prone to error and inconsistency. Because various investigators hardly agree on definition of the dental arch characteristics and, therefore, are not in agreement about what should be measured, substantial intra- and inter-investigator errors are often reported. Interestingly, the stated errors do not change the ratio’s outcome in a sense of its direction [i.e. positive to indicate crowding and negative to indicate spacing], but tend to add to the severity of the discrepancy or the ratio’s departure from unity.

The real question then is to analyze which of the two components of the larger than one ratio is more likely to be deviant from the average: the tooth size or the arch perimeter. While reviewing the literature on the subject, seemingly convincing arguments for either component can be found. However, one must not ignore the most logical possibility; the one suggesting that both components contribute to some degree, albeit not coequally. In order to illustrate the two separate possibilities, a study published by this writer and his coworkers in the American Journal of Orthodontics (AJO)1 demonstrated that both individual and cumulative sizes of crowded dentitions were larger than in the non-crowded arches. This could be interpreted as favoring the numerator to be at fault. On the other side, McNamara and coworkers have also reported in the AJO that the arch perimeters of crowd- ed dentitions were significantly smaller than their non-crowded counterparts.2 Their conclusions would suggest that it is the denominator that makes the basic difference. A classic example of allowing the two factors to contribute coequally is the Moyer’s Mixed Dentition Space Analysis, where the calculated space is needed because of either overly large teeth or too little space within the dental arch.3

Because in most of the nature’s systems there is no bias toward any one factor, this writer currently believes that dental crowding can be equally caused by too large teeth and by inadequate bony support within the dental arch. This, however, does not preclude the use of certain “rules of thumb” in assessing the severity of dental crowding. For instance, a relatively simple summation of the 20 permanent teeth mesio-distal widths should, in average, add up to 140 mm [or a mean width of 7 mm per permanent tooth, not including the molars]. If that sum exceeds 140 mm, one can safely state that the dentition is larger than normal and some degree of dental crowding is almost certain to ensue.1 Conversely, if one can measure the intermolar width of less than 36 mm one could safely deduce that the bone is not sufficiently developed and, thus, it is primarily responsible for the encountered dental crowding. Finally, it should be noted that the presence of one of these Orthodontics rules does not exclude the other from being present.

It is interesting to explore the secular changes in both components of the ratio. When one studies these long-term changes, which are probably a result of adaptive modifications to our diets and certain environmental influences, one can observe only modest variations. Individually these variations are insignificant and probably would not be able to explain the increased prevalence of dental crowding. However, because they had been occurring in the opposite direction, namely the size of dentition had increased while the available bone and thus the size of the dental arch had decreased, the ratio has steadily changed in favor of dental crowding.

This last statement brings yet another intriguing question: how does nature react to this? Should the selective adap- tation mechanism not respond and make a provision to resolve this apparent inconsistency? The responsive mechanism appears to exist and it is the cover of yet another dental development theory, the so-called ‘Field theory of missing teeth.’ The theory, in its sim- plest form, states that as dental crowding increases over the time, the last unit of the most numerous morphologic group will be gradually eliminated. Because we have three permanent molars, the theory speculates that the last in that group, or the third molar [“the wisdom tooth”] is the one that will be missing. Because there are two premo- lars and two incisors the picture is not that clear in terms of which tooth will be missing. It appears that in the maxillary dental arch it is the distal [or second] incisor that is likely to be the next most frequently missing tooth. In the mandible that distinction belongs to the second or distal premolar. A slight inconsistency of the Field’s theory choice is observed with the mandibular central incisors, which are congenitally missing more often than the mandibular lateral incisors.

It is interesting to explore the ways that clinicians approach and attempt to resolve the dental crowding. If one accepts the premise that either the teeth are relatively large or the alveolar bone is relatively small, it will follow that in the former case one would consider reducing the tooth substance. This is accomplished by either extracting teeth or by reducing their natural size. Reapproximating or “stripping” enamel from the interproximal surfaces of selected teeth achieves a reduction in the natural size. Clinically, one identifies these approaches as extraction or tooth substance reduction methods. On the other hand, one can attempt to increase or “grow” the available alveolar bone and develop the needed arch periphery. This approach is clinically identified as the functionalist approach. This implies that some form of the functional appli- ance or method is utilized. It should be noted, however, that a great deal of con- troversy still exists in terms of whether or not the functional appliances can pro- duce a real, net gain of the jaws or the alveolar bone. The alert reader readily recognizes that one method [e.g. extraction approach] is not exclusive of the other [e.g. lateral arch expansion]. Indeed, in many severe cases of dental crowding, both methods are needed and are thus employed.