THIRD MOLAR ERUPTION PREDICTOR

Abstract: Provided are materials and methods for predicting whether a molar tooth will remain impacted or will erupt based upon anatomical measurements of the position of the teeth in relation to the ascending ramus of the mandible. A transparent template is superimposed on the panoramic tomograms of the patient teeth and mandible. Pre-printed profiles and reference points are aligned on the tomograms with the apexes of the molars. The distance between the profile of the mandible and the pre-positioned reference points allow making a prediction on third molar eruption.

Description: THIRD MOLAR ERUPTION PREDICTOR FIELD OF THE INVENTION The present invention relates to the field of dentistry and specifically to materials and methods for the prediction of eruption or impaction of molars. BACKGROUND OF THE INVENTION The ability to predict the eruption or impaction of molars, and especially third molars, is of considerable value in clinical dentistry because extraction poses a greatly reduced risk to a patient if done when the patient is in his or her twenties as opposed to later in life. However, routine extraction of teeth early in life in order to avoid potential risks of later removal is not considered to be good practice. Means for predicting third molar eruption according to the art are based upon lateral cephalographic measures as reported, for example, in Richardson, et al . , Am. J . Orthod . , 86 : 119-129 (1984) . Other early means for predicting eruption include the use of bite-wings, antero- posterior views and periapical films, as reported, for example, in Sewerin, et al . , Int . Dent . J . , 40 : 24-30 (1990) . Recently, panoramic tomograms have become more readily available to the practicing dentist as the preferred means for visualizing the mandibular region via X-ray. There are several reports of measures used to evaluate the clinical status of molar development. These include measurements of the angle of development of the third molar, mesiodistal space available for eruption of the tooth, development of the root, depth of the tooth in bone, and type of impaction. Of the aforementioned measures, the highest probability of predicting eruption (70%) was reported using mesiodistal width as a predictor. Ganss, et al . , Oral Surg . Oral Med . Oral Pathol . , 76 : 688-693 (1993) . The highest probability of predicting impaction (94%) was reported by Venta, Oral Surg . Oral Med . Oral Pathol . , 76 : 699-703 (1993) . However, no single method was reported to consistently predict eruption or impaction and methods reported in the art generally involve complicated cephalographic measurements. The present invention provides a safe, reliable means for predicting the course of development of the lower third molar. Methods and materials according to the invention also eliminate the need for elaborate cephalographic measurements. SUMMARY OF THE INVENTION The present invention provides materials and methods for predicting the developmental course of teeth. In particular, materials and methods according to the invention allow one to predict whether a lower third molar (wisdom tooth) will erupt or remain impacted. In a preferred embodiment of the invention, methods are provided comprising the steps of obtaining an X-ray, and preferably a panoramic tomogram, of a patient's andibular region; superimposing over said X-ray or panoramic tomogram a sheet, which may be transparent, having a first line and a second line perpendicular to said first line and positioned with respect to the X-ray or tomogram such that said first line is coincident along a superior aspect of an occlusal surface of the first and/or second molars and said second line is tangential to a distal surface of said second molar; and determining a point of intersection of said first line and an anterior border of the ascending ramus of the mandible of said patient. The distance between the intersection of the first line with the anterior border of the ascending ramus of the mandible and the intersection of the first line with the second line provides a basis for predicting whether the third molar will remain impacted or will erupt. If that distance is 9.5 mm or less, there is an approximately 100% probability that the tooth will remain impacted. If the distance is 16.5 mm or greater, there is an approximately 100% probability that the tooth will erupt. Also in a preferred embodiment of the invention, the sheet may contain reference points which demarcate critical points (9.5 mm and 16.5 mm) against which intersection of the anterior aspect of the ascending ramus of the mandible with the first line is measured. Using such reference points, the practioner may visually determine the position of intersection of the ramus with the first line in comparison to the reference points in order to diagnose whether impaction or eruption will take place. For example, if the intersection of the ascending ramus of the mandible and the first line on the sheet is at or mesial to the intersection of a first reference point spaced about 9.5 mm from the intersection of the first and second lines, then it is predicted that the tooth will remain impacted. Similarly, a reference line or point may be drawn at approximately 16.5 mm from the intersection of the first and second lines and eruption may be predicted based upon intersection of the anterior aspect of the ascending ramus of the mandible with the first line at a point at or distal to the 16.5 mm reference point. The reference points are positioned along the first line and may intersect the first line, but it is not necessary that they do so, as the reference points may be lines which terminate above the first line. In the latter case, the reference line may be visually extended to the first line for demarcation of critical areas. Materials according to the invention comprise a sheet having a first line and a second line perpendicular to a point on the first line. The sheet may optionally include one, two, or three reference points positioned such that a first reference point is spaced about 9.5 mm from the intersection of the first and second lines. A second reference point is positioned approximately 14.5 mm from the intersection of the first and second lines, and a third reference point is positioned at approximately 16.5 mm from the intersection of the first and second lines. Other aspects and advantages of the invention will become apparent upon consideration of the following detailed description thereof. DESCRIPTION OF THE DRAWINGS Figure 1 is a histogram showing distances to the intersection of the first line and the anterior aspect of the ascending ramus in patients with finally erupted or impacted third molars. Figure 2 is a weighted histogram showing distances to the intersection of the first line and the anterior aspect of the ascending ramus in patients with finally erupted or impacted third molars. Figure 3 shows a device for predicting third molar eruption/impaction according to the invention superimposed over a schematic diagram of the mandible. Figure 4 is a panoramic tomogram with a device according to the invention superimposed thereon. Figure 5 is a diagram of a device for predicting eruption/impaction according to the invention. Figure 6 is a copy of a panoramic tomogram showing a lower third molar predicted by the invention to erupt. Figure 7 is a copy of a panoramic tomogram showing a lower third molar predicted by the invention to remain impacted. DETAILED DESCRIPTION OF THE INVENTION The present invention provides materials and methods for use in predicting whether a patient's third molar will eventually erupt or remain impacted. Consideration of the methods described herein reveals that such methods may be adapted to the prediction of eruption/impaction of other teeth by simply altering the critical distances on the transparent sheet described herein. A device according to the invention is shown in Figure 3 superimposed over a schematic diagram of a mandible. The device is a sheet having thereon a first line and a second line which is perpendicular to the first line, and optionally three reference points. The first reference point, shown as line A in Figure 3 (long dashes) , is positioned along the first line at approximately 9.5 mm from the intersection of the first and second lines. The second reference point, shown as line B in Figure 3 (short dashes) , intersects the first line at a point approximately 16.5 mm from the intersection of the first and second lines. A third reference point, shown as line C in Figure 3 (solid line between the two broken lines) represents the cut-off point for predicting eruption/impaction. Only the intersections of the various reference lines with the first line are important for purposes of the invention. Thus, it is immaterial whether the reference lines are curved. If the intersection of the first line with the anterior aspect of the ascending ramus of the mandible is mesial to the solid reference line, continued impaction is predicted at a probability of about 76%; whereas if the intersection of the first line with the anterior aspect of the ascending ramus of the mandible is distal to the solid reference line then eruption is predicted at a probability of about 72%. Figure 4 represents a device according to the invention superimposed upon a tomogram showing an ascending ramus of the mandible. Figure 5 shows a diagram of a device according to the invention with reference points at 9.5 mm, 14.5 mm, and 16.5 mm. The only critical position is the point of intersection with the first line of the device. The device shown in Figures 3 , 4 , and 5 is superimposed over a panoramic tomogram of the mandibular region, including the region of the molars. The device is placed on the tomogram such that the first line coincides with the superior (occlusal) surface of the first and second molars and the second, perpendicular line coincides with the distal surface of the second molar. Figure 4 shows proper placement of the device over a tomogram or X-ray. Upon placement over the tomogram, the clinician may either measure the distance from the point of intersection of the first and second lines to the anterior aspect of the ascending ramus or the aforementioned reference points may be used to predict the developmental fate of the lower third molar. Intersection of the first line with the anterior aspect of the ascending ramus mesial to line A in Figure 3 constitutes an approximately 100% probability that the third molar will remain impacted. Intersection distal to line B in Figure 3 results in an approximately 100% probability of eruption of the third molar. Intersection between lines A and C in Figure 3 results in a high probability of continued impaction and intersection between lines C and B indicates a high probability of eruption as discussed below. Figures 6 and 7 are panoramic tomograms which show lower third molars predicted to erupt and to be impacted, respectively, according to the invention. The result of the invention is that the practitioner may advise patients whether to seek removal of the third molar at an age where such surgery is relatively less risky as compared to performing the surgery at a later age. Accordingly, use of claimed materials and methods on individuals in their late teens or early twenties is preferred. The following example provides details of the development of the invention and some of its preferred applications. EXAMPLE A longitudinal study was carried out at the Finnish Student Health Service by the University of Helsinki. Subjects were 40 University students (25% male, 75% female) who were evaluated during a six-year period from age 20 through age 26. All subjects had non-erupted third molars at the beginning of the study. Panoramic tomograms were taken of all subjects and the clinical status of the third molars of all subjects was recorded (i.e. orientation, etc.) . Tomograms were taken with an Orthopantomograph™ OP-3 SE (Palomex, Inεtru, Finland) and with a Cranex™ DC SL-4 (Soredex, Orion Corp., Espoo, Finland). Tomograms were analyzed using a transparent overlay (the device) according to the invention which was labeled as shown in Figure 3. The overlay was placed directly on each tomogram such that the first line on the overlay lay directly over the occlusal (superior) aspect of the first and second molars. As the superior surface of the first and second molars is not always a straight line, one must match the first line on the overlay to the superior surface to the best extent possible. The second line on the overlay was lined up with the posterior surface of the second molar. An example of the manner in which the transparency was placed on a tomogram is shown in Figure 4. Distances from the intersection of the first and second lines and the anterior border of the ascending ramus of the mandible were measured for each patient. Results are presented in Figure 1 which is a histogram showing the distances for patients whose third molars were finally erupted six years after initial measurement (dark boxes) and those for patients whose third molars remained impacted six years after initial measurement (light boxes) . Although clear distinctions exist between finally erupted teeth and finally impacted teeth in terms of the above-noted measurement, Bayes' Decision Theory was used to determine a cutoff for purposes of predicting whether a tooth would ultimately erupt or remain impacted. According to Bayes' Decision Theory, the sum of false negatives (beta error) and false positives (alpha error) is at a minimum when two distribution curves intersect. For the data shown in Figure 1, the sum of the alpha error and beta error was lowest at a distance of 12.5 mm distal to the point of intersection of the first and second lines (i.e., the distal surface of the second molar) . A priori probabilities for eruption and impaction of lower third molars are 30% and 70%, respectively. Because impaction is more common than eruption, the cutoff for predicting impaction or eruption may be more distal from the distal surface of the second molar than would be predicted by straightforward probability analysis. Accordingly, a weighted decision point may be obtained, as shown in Figure 2, by weighing the values in Figure 1 by their respective a priori probabilities (30% and 70%) . When this is done, a new cutoff point is generated 14.5 mm from the point of intersection of the first and second lines based upon the fact that the sum of false negatives and false positives in the weighted histogram of Figure 2 is lowest at a point 14.5 mm from the distal surface of the second molar as shown below in Table 1. TABLE 1. Variables calculated from weighted histograms (in percent) . 13.5 mm 1 .5 mm 15.5 nun False negatives + 31 25 26 false positives Sensitivity 80 95 95 Specificity 45 30 25 Probability of 49 72 68 eruption Probability of 77 76 75 impaction From the above data, lines were established on the transparent sheet which intersect the first line at 9.5, 14.5 and 16.5 mm, respectively (Lines A, C, and B, respectively, in Figure 3) . Those lines were used to predict eruption/impaction in test subjects. Twenty test subjects were given panoramic tomograms at a mean age of 20.7 years (s.d. 0.6) and again at 25.3 years (s.d. 0.7) and the device described above and shown in Figure 3 was used to predict eruption or impaction based upon the point at which the first line of the device intersects the anterior surface of the patient's ascending mandibular ramus. Significant differences between sample means related to mesiodistal space available were found using the t-test with a pooled variance estimate. The device shown in Figure 3 was applied to each tomogram as a transparent sheet superimposed upon the tomogram, wherein the cutoff point (line C in Figure 3) for predicting eruption/impaction was 14.5 mm. If the first line intersects the anterior aspect of the ascending ramus of the mandible at or mesial to a distance of 9.5 mm (line A in Figure 3) there was a 100% probability that the tooth would remain impacted. If the intersection of the first line and the anterior aspect of the ascending ramus was distal to 16.5 mm from the second line there was a 100% probability of eruption. If the intersection of the first line and the anterior border of the ascending ramus is between Lines A and C in Figure 3, there is a greater probability of impaction; whereas if that intersection is at a point between Lines C and B in Figure 3, there is a greater probability of eruption. The relevant predictors are shown in Figure 5. If the intersection between the first line and the anterior border of the ascending ramus occurs in the area marked "F", eruption is predicted to occur 100% of the time. If that intersection occurs in the area marked "G", impaction is predicted to occur 100% of the time. If the intersection is in the area marked "H", eruption is more likely than impaction; whereas if the intersection occurs in the area marked "I", impaction is more likely than eruption. Results indicated that the difference in mean mesiodistal crown width of lower third molars which remained impacted and those which erupted was not significant (15.4 mm and 15.1, respectively) . However, the mean values of the distance from the intersection of the first and second (perpendicular) lines to the anterior aspect of the ascending ramus were 11.1 mm (s.d. 2.7) for impacted third molars and 13.6 mm (s.d. 2.0) for erupted third molars. The difference in those values was statistically significant (p< 0.01). Accordingly, there is significantly more mesiodistal space in the retromolar area when third molars erupted as opposed to when they remained impacted. In fact, use of the device shown, in Figure 3 resulted in 97% predictability in the longitudinal study referred to above. Only one exceptionally small tooth in a male patient in the study failed to meet the predicted outcome (the tooth erupted when use of the device predicted impaction) . The present invention has been described in terms of its preferred embodiments and further aspects of the invention are apparent to skilled artisan based upon knowledge in the art in light of the present disclosure.

Claims: CLAIMS We claim; 1. A device for predicting eruption of third molar teeth, comprising a sheet having affixed thereon a first line, a second line perpendicular to said first line, and a first reference point positioned along said first line and spaced about 14.5 mm from said second line. 2. The device according to claim 1, further comprising a second reference point positioned along said first line and spaced about 9.5 mm from said second line and a third reference point along said first line spaced about 16.5 mm from said second line. 3. The device according to claim 1, wherein said sheet is transparent. 4. A method for predicting eruption of lower third molar teeth in a patient, comprising the steps of: obtaining a panoramic tomogram of said patient's mandibular region, including the region of the second, and third molars; superimposing over said panoramic tomogram a sheet having affixed thereon a first line and a second line perpendicular to said horizonal line such that said first line is coincident along a superior aspect of an occlusal surface of said first and second molars and said second line is tangential to a distal surface of said second molar; determining a point of intersection of said first line and an anterior border of the ascending ramus of the mandible of said patient; and measuring a distance from said point of intersection of said first line and an anterior border of the ascending ramus of the mandible of said patient to the point of intersection of said first and second lines on said sheet. 5. The method according to claim 4, wherein said sheet is transparent. 6. The method according to claim 4, wherein said measuring step comprises the step of comparing the intersection of the anterior aspect of the ascending ramus of the mandible with said first line with reference points on said sheet, said reference points being positioned at approximately 9.5 mm, 14.5 mm, and 16.5 mm, respectively from a point of intersection between said first line and said second line.

Inventors: MURTOMAA HEIKKI, VENTAE IRJA, YLIPAAVALNIEMI PEKKA, MURTOMAA, HEIKKI, VENTAE, IRJA, YLIPAAVALNIEMI, PEKKA
Applicants: UNIV HELSINKI LICENSING, HELSINKI UNIVERSITY LICENSING LTD. OY
Application year: 1996
Main classification: A61C19
IPC codes: A61C19