THE EDITOR'S CORNER
Validity is often the product of repetition. One wonders if this is the case with the interincisal angle, which everyone says should be 130°-135°.
Downs, who may have started it all, developed his analysis initially with 20 white Americans aged 12-17 with excellent occlusions. His measurement of interincisal angle on this group showed a range of 130° to 150.5° with a mean of 135.4°. Riedel reported a mean of 130.98° for adults and 130.40° for children. Several others arrived at means that ranged from 120.8° to 135.3°. Steiner chose 130° with no evidence of sample or range. The 120.8° figure came from a study of 20 American Negroes with normal occlusion. However, there is no need to quarrel with the sampling or speculate on racial mixtures. Both Downs and Riedel showed whopping standard deviations for interincisal angle. Downs had 9.24° and Riedel had 9.55° for adults, 7.34° for children. Remember that 67% of the population is clustered within plus or minus one standard deviation, and 95% between two standard deviations. For purposes of most measurements, two standard deviations is considered the normal range. Let's suppose that the standard deviation for interincisal angle is 9° and that Downs' mean was 135° and Riedel's was 130°. That means that those interincisal angles within a range of approximately 117°-153° are within a normal range for Downs; and those between 112°-148° are in a normal range for Riedel.
You would have to go pretty far to identify an abnormal interincisal angle and you would be incorrect to assume that 130° or 135° was better than 120° or 140° or anything else within the statistically normal range.
Another consideration is whether the measurement is reliably standardized. Do you follow the labial crown surface, or crown angle, or root angle, or some average axis?
Is the interincisal angle a key to esthetics? Not if you consider that the angle--meaning the teeth in their angular relationship--can be canted over a considerable range and yet in the same angular relationship to each other; and they can protrude or retrude bodily together.
What about health? Is the interincisal angle related to the health and long life of teeth and/or supporting tissues? There is no evidence that it is. It is even possible that it is negatively related and that a less traumatic tooth-to-tooth relationship could be a healthier one (i.e. an overjet relationship) .
What about the stability of orthodontic results? This is mentioned a great deal, and many respected clinicians seem to agree that certain interincisal angles contribute to maintenance of bite opening and to stability. This seems to be more of a feeling than a fact and, while one must listen to clinical impressions, they need proof. Perhaps proof will be forthcoming. However, it is a concept that is seriously open to question. It is hard to conceive that anterior teeth, which are movable in response to pressures, would maintain a degree of bite opening by interincisal action or even that they would necessarily maintain a certain degree of interincisal angle. And, of course, they are not constant. Teeth are observed to upright with age. Interincisal angles change following treatment. It may be more correct to assume that this is a variable and not closely predictable, and that interincisal angle is not necessarily related to its own stability, let alone the stability of an orthodontic result.
Some clinicians believe that it is the acuteness of the interincisal angle that permits the lower incisal edge to strike the lingual surface of the upper incisor more directly, that contributes to the maintenance of bite opening and to stability. If this were so, then 90° would be best and marked bimaxillary protrusions would be most stable results.
While some clinicians say that they torque the incisors to get some of that edge-to-surface relationship, most orthodontists are working at retracting the anteriors in a majority of their cases. This is uprighting and it is increasing the interincisal angle. So, it is conceivable that as we approach a more obtuse interincisal angle, the teeth get away from constant striking of lower incisals on upper linguals, closer to a more parallel relationship, and that the nonrelation-ship or minimal trauma contributes to stability? Possibly so, but to carry that to its logical conclusion, we should find that even flatter relationships are more stable and horizontal open bites most stable.
In other words, if it is true, then it should be evident to clinicians that there is a positive relationship between stability and interincisal angle, with stability increasing directly as the angle increases and the incisors flatten. It is doubtful that there is such a definite relationship, and, in addition, you cannot isolate factors so readily.
Interincisal angle has been described as a measure of procumbency of the incisors. This is not necessarily so, and certainly nowhere near as good as other measures. The Steiner analysis makes about as much of interincisal angle as any. One thing Dr. Steiner did was to make use of both linear and angular measurements. Such combinations may be more valid than angles alone which can be misleading.
It might be worth studying whether the lingual anatomy of upper incisors is related to stability and interincisal angle. The point in question there would be whether variations in development of the cingulum can be related to variations in interincisal angle, and they, in turn, related to stability of result or to maintenance of bite opening. On both counts it is doubtful because, by the time the incisal edge of lower incisor reaches the cingulum of upper incisor, you already have a degree of overbite that is conducive to neither.
It just so happens that if you construct a quadrilateral SN-GoGn-1-1and if you have an angle 1-SN of 103°, l-GoGn of 90°,1-1 of 135° and SN-GoGn of 32°, the angles add up to 360° which is correct for a quadrilateral. However, since a range of 115-150° roughly must be considered the normal range, you can see that there can be a great many different such quadrilaterals which, presumably, would be normal.
So, the interincisal angle may be a monument to the thoroughness of the early cephalometricians.
It may end its career as an example of historical minutiae.
Apart from the merits or demerits of the interincisal angle, is the example that it represents of a possible "well known fact" that may have gotten that way by repetition and subsequently avoided examination by the scientific method.
