A comparison of results of second molar and first premolar extraction treatment Julie Ann Staggers, DDS, MS* Morgantown, ~rV.V(I.
The purpose of this study was to examine treatment results of maxillary and mandibular second molar extraction cases and compare them with treatment results of maxillary and mandibular first-premolar extraction cases. Records of 22 maxillary and mandibular second-molar extraction cases and 22 maxillary and mandibular first-premolar extraction cases were evaluated. For each case, pretreatment and posttreatment lateral cephalograms were traced and several cephalometric parameters were compared. From the pretreatment and posttreatment panoramic radiographs, angulations of the maxillary and mandibular third molars were evaluated. Average treatment time, in months, was recorded for both groups. The data obtained from each group were analyzed for statistical difference. The results showed that the two groups had fewer differences than indicated by advocates of second-molar extractions. Analysis of the cephalometric data demonstrated only a few statistical differences between the groups. The maxillary and mandibular incisors in the premolar group were retracted a significantly greater amount than in the second-molar group. The maxillary and mandibular first molars were protracted a greater amount in the premolar group than in the second-molar group. The lower lips in the premolar group were retracted a greater amount than in the second-molar group. The resulting facial profile after extraction of second-molars appears to be no different from that obtained after extraction of first premolars. The pantographic evaluation of the changes in third-molar angulation were not statistically different. In both groups, the maxillary third molars showed an improvement in third-molar angulation, while the mandibular third-molars showed an undesirable increase in angulation. The average treatment time for both groups was not statistically different. (AMJ ORTHOD DENTOFACORTHOP 1990;98:430-6.)
O r t h o d o n t i c treatment in conjunction with second-molar extractions has been a controversial issue among orthodontists for many years. Although the results of several quantitative studies have been published, most articles pertaining to second-molar extractions reflect the author's opinions and clinical experience, not scientific fact. Indications for second-molar extractions and timing of the extractions vary from one orthodontist to another. Even opponents of secondmolar extractions have different reasons for opposing these extractions. Perhaps the most common reason orthodontists elect to remove second molars, as opposed to first or second premolars, is that they believe better facial esthetics may be achieved after the extraction of second molars. Some orthodontists, including Quinn, ~ Liddle, 2 and Marceau and Trottier, 3 believe that extracting premolars and retracting anterior teeth can lead to a "dished-in" *Assistant Professor of Orthodontics, ttealth Sciences Center North, West Virginia University School of Dentistry. 811116426
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facial profile somewhat similar to that of an edentulous person. However, there has been no published research proving that anterior teeth are retracted more after extraction of premolars than after extraction of second molars. A study by Williams and Hosila 4 did show that less incisor retraction occurred when four first molars were extracted than when maxillary first and mandibular second premolai-s were extracted or when four first premolars were extracted. One might assume that extraction of four second molars might produce results similar to those achieved with the extraction of four first molars, since they are adjacent teeth, but this may not be true. Another reason for advocating the extraction of second molars is that it decreases the number of thirdmolar impactions. In studies by Quinn, t Liddle, 2 Richardson, 5 Gaumond, 6 and Cavanaugh, 7 no third-molar impactions occurred after extraction of second molars. Yet Richardson 8 and Smith, 9 believe that third-molar eruption is unpredictable. Even when third molars do erupt, their position, angulation, occlusion, and contact with the first molar must be considered. Magness I° stated, "Occasionally third molars will erupt with
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proper angulation, but this is the exception rather than the rule." Huggins and McBride t! agree with Magness that root parallelism is not achieved. If a third molar is rotated or excessively tipped at eruption, correction with appliances may be necessary. Patients may not be receptive to a second fixed appliance; moreover, the orthodontist must provide additional time and bear the expense of re-treatment. Re-treatment may be necessary after second-molar extraction in order to correct third-molar position, and how often this occurs varies from practice to practice. Smith 9 evaluated the position of 94 maxillary and 34 mandibular third molars after the extraction of second molars and found that 100% of the maxillary third molars occluded with an opposing tooth, 96% had a mesial contact point, and 97% had an acceptable axial inclination. Looking at the mandibular third molars, he judged that 97% had occlusion with an opposing tooth, 59% had a mesial contact point, and 50% had an acceptable axial inclination. Wilson, j2 examining 178 third molars, reported "good or very good" mesial contact and occlusion in 87% of the third molars. Thirteen percent had "poor" contact and angulation. Huggins and McBride t! examined 50 third molars and found that 90% had "good or excellent" mesial contact. "Fair" mesial contact was seen in 4% of the third molars, and 6% had no contact at all. The existence of occlusion, or lack of it, with these third molars was not reported. Gaumond 6 studied the eruption of 22 mandibular third molars after the removal of second molars. Eighty-six percent achieved a "satisfactory or very satisfactory" position in the arch. The remaining 14% were "imperfectly" positioned. A study by Rindler ~3 involved 118 third molars, of which 77% were judged to have "good or very good" positions, 20% had "adequate" positions, and 3% had "poor" positions. Quinn's conclusion, ~ based on his clinical experience, was that 75% of third molars will achieve mesial contact after extraction of second molars. He further added that maxillary third molars rarely need to be repositioned, but mandibular third molars are more likely to need uprighting and repositioning. When one compares extraction of second molars to extraction of premolars, one finds differences in the amount of tooth structure removed and in the location of the extraction site. When first or second premolars are extracted, approximately 14 mm of space per arch is created, t4 which is a sufficient amount to relieve crowding, since arch length deficiencies rarely exceed 10 mm. ~5 Considerably more space is created--18 to 22 m m - - w h e n second molars are extracted, ~ and the problem o f too much space may be created. Some orthodontists believe that proper anterior retraction and the
Second molar extractions
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natural mesial migration of teeth will close this additional space adequately. Whitney and Sinclair ~6pointed out that successful retraction of the entire dentition into the second molar extraction site relies considerably on patient cooperation. They are in agreement with Haus ~5 that a major disadvantage of second-molar extraction is that space is created in the posterior region of the arch, while the majority of crowding is in the anterior region. Magness ~° does not believe this is a problem. In the absence of second molars, he believes that the maxillary first molars may be moved distally more easily than if second molars were present. There is debate among orthodontists as to whether second-molar extraction cases can be treated in a shorter period of time than premolar extraction cases. Advocates of second-molar extractions believe that the first molars can rapidly be moved distally into the extraction sites and that the total treatment can be completed in less time than if premolars were extracted. Quinn t and Marceau and Trottier, 3 comparing second-molar extractions to premolar extractions, reported a decrease in treatment time, but the actual amount of the decrease was not given. Magness, ~° on the other hand, reported that with extraction of second molars the orthodontist must wait for third-molar eruption and will possibly need a second phase to correct the position of these teeth. This would result in a total treatment time is much longer than that of the average premolar extraction. A final consideration is temporomandibular joint disorders. Many general dentists and some orthodontists, like Bowbeer, ~7 believe that extraction of premolars is a major etiologic factor in temporomandibular joint disorders. They believe that there is a loss in vertical dimension of occlusion following the removal of premolars as a result of the maxillary and mandibular posterior teeth's having been moved forward, thus reducing the Frankfort-mandibular plane angle. This loss of vertical dimension is believed to cause TMJ dysfunction similar to that caused by resorption of the alveolar ridges in an edentulous person. Others have a different theory as to the role first-premolar extractions play in causing TMJ disorders. Farrar and McCarty t8 and Witzig ~9 believe that extraction of premolars leads to over-retraction of maxillary anteriors. As a result, the mandibular condyles are forced posteriorly and TMJ disorders develop. Both concepts are interesting, but there has been no research to support them. However, research has disputed the theory that extraction of four first premolars causes posterior displacement of the condyles and thus leads to TMJ disorders. Gianelly et al. 2° found no difference in condylar position between persons who had been orthodontieally treated with extrac-
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Staggers
tion of four first premolars and those who had not received orthodontic treatment. Furthermore, the results of a study by Blaschke and Blaschke -~ dispute the concept that posteriorly displaced condyles cause TMJ disorders. In fact, 36% of the patients they evaluated had posteriorly displaced condyles, and yet had no symptoms. Pullinger et al. 22 also showed a wide variety of condylar positions in symptom-free patients. It is evident from this review of the literature that there is disagreement among orthodontists and among general practitioners as to the treatment results of second-molar extractions. In view of this disagreement, the purpose of this study was to examine treatment results in cases of maxillary and mandibular secondmolar extraction and compare them with the treatment results of maxillary and mandibular first-premolar extraction. MATERIALS AND METHODS
Records of 22 maxillary and mandibular secondmolar extraction cases were obtained from the files of the Department of Orthodontics at West Virginia University School of Dentistry and from an orthodontist in private practice. Thirteen of these cases were Class I and nine were Class II, Division 1. Sixteen of the patients were girls, and six were boys. Records of 22 maxillary and mandibular first-premolar extraction cases were also obtained from the files of the West Virginia University Department of Orthodontics. Thirteen of these cases were Class I and nine were Class II, Division I. Thirteen of the patients were girls, and nine were boys. The age range for the patients in both groups was 9 to 16 years. For each case, pretreatment ~'nd posttreatment lateral cephalograms were traced, and the following items were compared: 1. the change in the angle of skeletal convexity (nasion-point A-pogonion or N-A-Pg) 2. the change in the anteroposterior position of point A with respect to nasion vertical, a line perpendicular to the horizontal plane through nasion (point A to NV). (Horizontal plane, sella-nasion line plus 7 °, was used as a reference plane instead of Frankfort horizontal plane because of the high variability of the Frankfort horizontal plane.) 3. the change in the anteroposterior position of point B with respect to a line perpendicular to a horizontal plane through nasion (point B to NV) 4. the change in the anteroposterior positions of points A and B, as determined by measuring the distance between the intersection of the occlusal plane and lines drawn perpendicular to
5. 6.
7.
8.
9.
the occlusal plane through points A and B (AB to OP) the change in the horizontal plane-mandibular plane angle (HP-MP) the amount of maxillary anterior retraction, as determined by subtracting the posttreatment measurement from the pretreatment measurement of the distance from the incisal edge of the maxillary central incisor to a line perpendicular to the horizontal plane through nasion (1. to NV) the amount of mandibular anterior retraction, as determined by subtracting the posttreatment measurement from the pretreatment measurement of the distance from the incisal edge of the mandibular central incisor to a line perpendicular to the horizontal plane through nasion (1" to NV) the change in angulation of the maxillary central incisor with respect to the nasal floor, ANSPNS ( ! to NF) the change in angulation of the mandibular central incisor with respect to the mandibular plane
(i to Mr'~ 10. the amount of eruption of the maxillary first molar relative to the nasal floor (6 to NF) 11. the amount of eruption of the mandibular first molar relative to the mandibular plane (6 to MP) 12. tipping and bodily movement of the maxillary first molars, as determined by superimposition of the maxilla on the pretreatment and posttreatment cephalometric tracings and by measurement of the difference in anteroposterior position of the mesiobuccal cusps 13. tipping and bodily movement of the maxillary incisors, as determined by superimposition of the maxilla on the pretreatment and posttreatment cephalometric tracings and by measurement of the difference in anteroposterior position of the incisal edges 14. tipping and bodily movement of the mandibular first molar, as determined by superimposition of pretreatment and posttreatment cephalometric tracings along the mandibular symphysis and by measurement of the difference in anteroposterior position of the mesiobuccal cusps 15. tipping and bodily movement of the mandibular incisors as determined by superimposition of pretreatment and posttreatment cephalometric tracings along the mandibular symphysis and by measuring the difference in anteroposterior position of the incisal edges
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Fig. 1. Measurement of third-molar angulation relative to occlusal plane.
16. the change in the soft tissue angle of facial convexity (glabella-subnasale-soft tissue pogonion or G-Sn-Pg') 17. the change in the anteroposterior position of the upper lip with respect to the subnasale-soft tissue pogonion line (Ls to Sn-Pg') 18. the change in the anteroposterior position of the lower lip with respect to the subnasale-soft tissue pogonion line (L, to Sn-Pg') All cephalographs were taken with the teeth in centric occlusion and the lips in repose. From the pretreatment and posttreatment panoramic radiographs, the angulations of the maxillary and mandibular third molars were evaluated. Angulations of the third molars were measured with reference to the anterior angle between the occlusal plane of the first molar and the second premolar and the long axis of the third molar (Fig. 1). From each measurement obtained from the lateral cephalograms and panoramic radiographs, pretreatment values were subtracted from posttreatment values to reveal the change that had resulted from treatment. These changes were analyzed for statistical difference at the p < 0.05 level of significance by means of Student's t test. Total treatment time, in months, for each person in the second-molar and first-premolar extraction groups was also recorded. The average length of treatment for each group was calculated and compared for statistical difference at the p < 0.05 level of significance with the aid of Student's t test. RESULTS
Statistical analysis showed a significant difference between the premolar and second-molar groups in six of the eighteen cephalometric measurements analyzed.
Table I. Summary of the mean cephalometric
differences (pretreatment values subtracted from posttreatment values)
Measttrement N-A-Pg Point A to N V Point B to N V A-B to O P HP-MP _1 to N V ] to NV 1_ to N F I to hiD
6 to N F to MP 6 superimposition superimposition / superimposition T superimposition G-Sn-Pg' Ls to S n - P g ' Lt to S n - P g '
Second-nzolar extraction group - 3.3
First-premolar extraction group
0
0.0 mm 0.5 mm l.l mm 0.0 ° -0.7 mm 0.7 mm ! .4 ° 4.40
-3.90 -2.5 mm 0.0 mm -0.4 mm - 0.50 - 3.0 -
mm*
0.8 m m 0.60 0.5 o
-
1.70
mm mm mm* ram* - 2.0 ram* - 1.6 mm* - 2.2 o
-
1.4 m m 0.0 mm
-
1.8
mm
-
1.6
mm*
2.4 2.4 0.0 0.3 0.7 1.3
mm mm mm mm mm mm
3.0 3.4 4.8 3.7
*Indicates statistical differences at the 0 . 0 5 level.
The measurements that were statistically different were as follows (Table I): 1. the change in maxillary incisor protrusion, as determined by I to nasal vertical 2. the change in maxillary first molar position, as determined by maxillary superimpositions 3. the change in mandibular first molar position, as determined by mandibular superimpositions 4. the change in maxillary incisor position, as determined by maxillary superimpositions
Am. J.
434 Staggers Table II. Summary of mean pantographic
differences (pretreatment angles subtracted from posttreatment angles) Second-molar group (degrees)
Third molar Maxillary right Maxillary left Mandibular right Mandibular left
14.9 12.6 -5.6 - 6.9
First-premolar group (degrees) 5.8 14.5 - 10.4 - 10. I
Orthod. Dentofac. Orthop. November 1990
lations before and after treatment. In both groups, the maxillary and mandibular third molar long axisocclusal plane angle increased on both right and left sides (Table II). The average treatment time of 38 months for the second-molar group was slightly longer than the average treatment time of 36.8 months for the premolar group (Table III). However, this difference was not statistically significant. DISCUSSION
Table III. Summary of mean treatment time Group Second-molar extraction First-premolar extraction
] I
Time (mo) 38.0 36.8
5. the change in mandibular incisor position, as determined by mandibular superimpositions 6. the change in lower lip protrusion, as determined by Lt to Sn-Pg' The second-molar group showed a mean decrease in the values of N-A-Pg, 1 to NV, G-Sn-Pg', and Ls to Sn-Pg', and a mean increase in the distances from point B to NV, A-B to OP, ]- to NV, 1 to NF, i" to MP, 6 to NF, and 6 to MP. Point A to NV, HP to MP, and L, to Sn-Pg' did not show a mean change in this group. The first-premolar group showed a mean decrease in the values of N-A-Pg, point A to NV, A-B to OP, HP to MP, 1 to NV, and ]- to NV, Gn-Sn-Pg', Ls to Sn-Pg', and Lx to Sn-Pg', and a mean increase in I to NF, i" to MP, 6 to NF, and 6 to MP. The distance from point B to NV did not show a mean change in this group. The position of the maxillary first molars in the second-molar group showed no change, as determined by superimpositions of pretreatment and postreatment cephalograms along the maxilla. The superimpositions in this group also revealed a slight, anterior average movement of the maxillary incisors. Maxillary superimpositions in the premolar group showed an average anterior movement of the first molars and a posterior movement of the incisors. Superimpositions of pretreatment and postreatment cephalograms along the mandible symphysis revealed anterior movement of the mandibular first molars in both groups and the mandibular incisors in the second-molar group. The mandibular incisors in the premolar group, on the other hand, moved posteriorly. Comparison of the pantographic measurements revealed no significant differences in third molar angu-
Facial esthetics is an important consideration in orthodontic treatment particularly when extractions are being considered. Most orthodontists evaluate facial esthetics by the soft tissue angle of facial convexity and by the position of the upper and lower lips to a reference plane, such as the E plane or, as in this study, the subnasale-soft tissue pogonion line. It is accepted in orthodontics that extraction of permanent teeth reduces facial convexity. For patients with protrusive, convex profiles, both reduction in the soft tissue angle of facial convexity and retraction of the lips are desirable. However, others, with straight profiles, would not benefit from such a reduction. This situation produces a dilemma when the patient's dentition indicates that extractions are needed but the profile does not. This study examined the soft tissue changes after extraction of teeth from different areas of the mouth, and the results showed an average decrease in the soft tissue angle of facial convexity of 1.7 ° for the second-molar extraction group and 2.2 ° for the first-premolar group. However, these reductions were not statistically different. Like the angle of facial convexity, the change in upper lip protrusion was not significantly different; yet the change in lower lip protrusion was significant. An average decrease in lower lip protrusion of 1.6 mm was seen in the premolar extraction group, compared with 0.0 mm in the second-molar extraction group. Even though this difference was statistically significant, the overall change in the soft tissue angle of convexity (GSn-Pg') was not. Thus these results do not support the claim that the facial profile is flattened significantly more by extraction of first premolars. The soft tissue profile is influenced by the underlying skeletal structures. Evaluation of the changes in points A and B and the skeletal angle of facial convexity revealed no significant differences between the two groups. The A-B discrepancy was improved by extraction of first premolars, but it was not improved by extraction of second molars. The angle of skeletal convexity decreased in both groups, and this decrease in convexity might account for the decrease in the soft tissue angle of convexity.
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Soft tissue is also influenced by dentition, primarily maxillary and mandibular incisors. However, the results of this study indicate that the influence of dentition is unpredictable. Incisor position can be defined by inclination and bodily position (protrusion). The changes in incisor inclination were not significantly different for the two groups, but there was a difference between the change in maxillary incisor protrusion Q. to NV) for one group, as compared with the other. Dental changes, however, did not affect the lips as would be expected. Lip changes were more than dental changes in some instances and less in others. For example, the maxillary incisor protrusion in the second-molar group decreased an average of 0.7 mm, yet upper lip protrusion decreased an average of 1.4 mm. The mandibular incisor protrusion averaged a 0.7 mm increase in the secondmolar group, but on average there was no change in lower lip protrusion. For the first-premolar group, maxillary incisor protrusion decreased an average of 3.0 mm and upper lip protrusion decreased 1.8 mm. Lower incisor protrusion decreased an average of 0.8 mm, and the lower lip protrusion decreased an average of 1.6 mm. Average incisor procumbency increased in both groups, but lip protrusion decreased, except for the lower lips in the second-molar group, in which average lip protrusion remained the same. Data from both groups indicate little relationship between changes in incisors and changes in lips. Therefore soft tissue changes cannot be predicted by dental changes. Maxillary incisor retraction, as determined by maxillary superimposition, was significantly greater in the premolar g r o u p - - a result that would also be expected to cause a significantly greater decrease in upper lip protrusion in the premolar group; yet this did not occur. Mandibular incisor retraction, as determined by superimposition of mandibular cephalograms, was significantly more in the premolar g r o u p - - a difference that would account for the significantly greater decrease in lower lip protrusion seen in this group. Another important consideration is the change in vertical dimension after extraction therapy. Today, orthodontic treatment that involves extraction of first premolars is commonly blamed for causing TMJ disorders by decreasing the vertical dimension of occlusion. Posterior teeth are believed to move forward after premolar extractions, thus permitting the mandible to overclose and shorten the resting length of the muscles of mastication. This theory disregards the concept that all orthodontic mechanics are extrusive to some degree and that the extrusion maintains or even increases vertical dimension. This study investigated changes only in vertical dimension as determined by measurement of the MP-HP angle after the extraction of first premolars
Second molar extractions
435
and second molars. It did not explore the possibility that a decrease in vertical dimension leads to TMJ disorders. However, this area warrants further research, since articles pertaining to this subject present conflicting views. 17'23 Changes in vertical dimension, as determined by the horizontal plane-mandibular plane angle, were not significantly different between the premolar and second-molar extraction groups. On average, the HP-MP angle remained the same after second-molar extractions and decreased 0.5 ° after premolar extractions. It is questionable that a decrease of 0.5 ° is sufficient to cause TMJ disorders. Changes in vertical dimension can also be evaluated by the amount of tooth eruption and mesial movement of posterior teeth that occurs during treatment. All first molars in both groups were extruded during treatment, and there were no significant differences between the premolar and second-molar groups. The maxillary and mandibular first molars moved farther anteriorly a significantly greater amount in the premolar group than in the second-molar group. On average, maxillary first molars in the second-molar group remained in the same position, and the mandibular first molars averaged a slight (0.3 mm) forward movement. These data indicate that even though the posterior teeth are moved forward more in the premolar group, extrusion prevented any significant loss in vertical dimension. An additional orthodontic consideration explored in this study was the influence of extractions on third molars. Some orthodontists believe that extraction of any permanent teeth will improve the angulation of third molars and thus will improve the chances for third molars to erupt into good positions. Others ~'1° disagree, stating that such improvement will occur only if second molars are extracted and that extraction of first premolars will not prevent the need for extraction of third molars. The change in angulation of third molars relative to the occlusal plane after first-premolar and second-molar extraction treatment was examined in this project. If extraction of second molars does lead to more favorable inclinations of the third molars, one would have expected to see an increase in the angle between the maxillary third-molar long axis and the occlusal plane and a decrease in the angle between the mandibular third-molar long axis and the occlusal plane. In the second-molar group, there was an increase in the maxillary third-molar long-axis-ocelusal plane angle and thus an improvement in maxillary third-molar angulation, but this increase was not significantly different from that in the premolar group. Unexpectedly, the mandibular third-molar Iong-axis-occlusal plane angle increased in the second-molar group, as it did in the premolar group, indicating a poorer mandibular third-
436
Am. J. Orthod. Dentofac. Orthop. November I990
Staggers
molar angulation. The data suggest that there is no advantage to extracting second molars as opposed to first premolars to improve third-molar angulation. One should keep in mind that a favorable change in thirdmolar inclination after second-molar extractions does not necessarily mean that third molars will erupt into the mouth with good inclination, proper contact with the first molar, or acceptable occlusion. The final orthodontic consideration of this study is length of treatment time for premolar and second molar extractions. Length of treatment is dependent on many variables, such as mechanics, patient cooperation, and parent motivation. None of these variables was controlled in this study, and it was assumed that patient cooperation and parent motivation in both groups were similar and that patients were treated with similar mechanics. Length of treatment was not significantly different between the two groups. However, the possibility that there were differences in the level of patient cooperation, parent motivation, and mechanics between the two groups cannot be ignored. Also, third molars had erupted in only a few patients in the second-molar group at the time of debonding. Therefore, the total treatment time for the second-molar group did not take into account any time needed for positioning the third molars after they had erupted. Thus the actual length of treatment for the second-molar group may have been longer than this study indicates. Comparison between patients treated with secondmolar extractions and patients treated with firstpremolar extractions revealed that the two groups had fewer differences than some authors have previously stated. The data presented in this study do not support many of the concepts presented by advocates of secondmolar extractions. Further study in this area is indicated. REFERENCES
1. Quinn GW. Extraction of four second molars. Angle Orthod 1985;55:58-69. 2. Liddle DW. Second molar extraction in orthodontic treatment. AM J OR'roOD1977;72:599-616. 3. Marceau ]E, Trottier BP. Third molar development following second molar extractions. J Pedodontics 1983;8:34-51. 4. Williams R, Hosila FJ. The effect of different extraction sites upon incisor retraction. AM J OR~tOD 1976;69:388-409.
5. RichardsonhiE. The relative effect of the extraction of various teeth on the developmentof third molars. Trans Eur Orthod Soc 1975:79-85. 6. GaumondG. Second molar germectomy and third molar eruption. Angle Orthod 1985;55:77-88. 7. CavanaughJJ. Third molar changes followingsecond molar extractions. Angle Orthod 1985;55:70-6. 8. Richardson ME. Some aspects of lower third molar eruption. Angle Onhod 1974;44:141-5. 9. Smith DI. The eruption of third molars followingextraction of second molars. Dent Practitioner 1958;8:292-5. 10. Magness WB. Extraction of second molars. J Clin Orthod 1986;20:519-22. I 1. Huggins DG, McBride LI. The eruption of lower third molars following the loss of lower second molars: a longitudinalcephalometrie study. Br J Orthod 1978;5:13-20. 12. Wilson HE. Extraction of second permanent molars in orthodontic treatment. Orthodontist 1971;3:i8-24. 13. Rindler A. Effects on lower third molars after extraction of second molars. Angle Orthod 1977;47:55-8. 14. WheelerRC. Dental anatomy,physiologyand occlusion. 5th ed. Philadelphia: WB Saunders, 1974:!96-209. 15. tlaus AJ. Let's take a rational look at permanentsecond molar extractions. AM J ORTHODDEr~rOFACORTHOP1986;90:361-3. 16. WhitneyEF, Sinclair Phi. An evaluationof combinationsecond molarextractionand functionalappliancetherapy. AMJ ORTttOD DEN'rOFACOR'rttoP 1987;91:183-92. 17. Bowbeer GR. The sixth key to facial beauty and TMJ health. Funct Orthod 1987;4:4-20. 18. FarrarWB, McCarty WL. A clinicaloutlineof temporomandibular joint diagnosis and treatment. Montgomery, Ala.: Walker Printing, 1983:84-5. 19. Witzig JW, Sphal TJ. The clinical outline of maxillofacialorthopedic appliances. 1st ed. Littleton, Mass.: PSG Publishing, 1987:161-216. 20. GianellyAA, Hughes HM, WohlgemuthP, Gildea G. Condylar position and extraction treatment. AMJ ORTHODDENTOFACOR"ntoP 1988;93:201-205. 21. Blaschke DD, Blaschke TJ. Normal TMJ bony relationshipsin centric occlusion. J Dent Res 1981;60:98-104. 22. Pullinger AG, Hollender L, Solberg WK, Petersson A. A tomographic study of mandibularcondyle position in an asymptomatie population. J Prosthet Dent 1985;53:706-12. 23. StaeklerH. Clinicalobservationsof cases fiveyears out of treatment. Ast J ORI'tlOD1958;28:108. Reprint requests to:
Dr. Julie Ann Staggers Health Sciences Center North West Virginia UniversitySchool of Dentistry Morgantown, WV 26506
The purpose of this study was to examine treatment results of maxillary and mandibular second molar extraction cases and compare them with treatment results of maxillary and mandibular first-premolar extraction cases. Records of 22 maxillary and mandibular second-molar extraction cases and 22 maxillary and mandibular first-premolar extraction cases were evaluated. For each case, pretreatment and posttreatment lateral cephalograms were traced and several cephalometric parameters were compared. From the pretreatment and posttreatment panoramic radiographs, angulations of the maxillary and mandibular third molars were evaluated. Average treatment time, in months, was recorded for both groups. The data obtained from each group were analyzed for statistical difference. The results showed that the two groups had fewer differences than indicated by advocates of second-molar extractions. Analysis of the cephalometric data demonstrated only a few statistical differences between the groups. The maxillary and mandibular incisors in the premolar group were retracted a significantly greater amount than in the second-molar group. The maxillary and mandibular first molars were protracted a greater amount in the premolar group than in the second-molar group. The lower lips in the premolar group were retracted a greater amount than in the second-molar group. The resulting facial profile after extraction of second-molars appears to be no different from that obtained after extraction of first premolars. The pantographic evaluation of the changes in third-molar angulation were not statistically different. In both groups, the maxillary third molars showed an improvement in third-molar angulation, while the mandibular third-molars showed an undesirable increase in angulation. The average treatment time for both groups was not statistically different. (AMJ ORTHOD DENTOFACORTHOP 1990;98:430-6.)
O r t h o d o n t i c treatment in conjunction with second-molar extractions has been a controversial issue among orthodontists for many years. Although the results of several quantitative studies have been published, most articles pertaining to second-molar extractions reflect the author's opinions and clinical experience, not scientific fact. Indications for second-molar extractions and timing of the extractions vary from one orthodontist to another. Even opponents of secondmolar extractions have different reasons for opposing these extractions. Perhaps the most common reason orthodontists elect to remove second molars, as opposed to first or second premolars, is that they believe better facial esthetics may be achieved after the extraction of second molars. Some orthodontists, including Quinn, ~ Liddle, 2 and Marceau and Trottier, 3 believe that extracting premolars and retracting anterior teeth can lead to a "dished-in" *Assistant Professor of Orthodontics, ttealth Sciences Center North, West Virginia University School of Dentistry. 811116426
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facial profile somewhat similar to that of an edentulous person. However, there has been no published research proving that anterior teeth are retracted more after extraction of premolars than after extraction of second molars. A study by Williams and Hosila 4 did show that less incisor retraction occurred when four first molars were extracted than when maxillary first and mandibular second premolai-s were extracted or when four first premolars were extracted. One might assume that extraction of four second molars might produce results similar to those achieved with the extraction of four first molars, since they are adjacent teeth, but this may not be true. Another reason for advocating the extraction of second molars is that it decreases the number of thirdmolar impactions. In studies by Quinn, t Liddle, 2 Richardson, 5 Gaumond, 6 and Cavanaugh, 7 no third-molar impactions occurred after extraction of second molars. Yet Richardson 8 and Smith, 9 believe that third-molar eruption is unpredictable. Even when third molars do erupt, their position, angulation, occlusion, and contact with the first molar must be considered. Magness I° stated, "Occasionally third molars will erupt with
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proper angulation, but this is the exception rather than the rule." Huggins and McBride t! agree with Magness that root parallelism is not achieved. If a third molar is rotated or excessively tipped at eruption, correction with appliances may be necessary. Patients may not be receptive to a second fixed appliance; moreover, the orthodontist must provide additional time and bear the expense of re-treatment. Re-treatment may be necessary after second-molar extraction in order to correct third-molar position, and how often this occurs varies from practice to practice. Smith 9 evaluated the position of 94 maxillary and 34 mandibular third molars after the extraction of second molars and found that 100% of the maxillary third molars occluded with an opposing tooth, 96% had a mesial contact point, and 97% had an acceptable axial inclination. Looking at the mandibular third molars, he judged that 97% had occlusion with an opposing tooth, 59% had a mesial contact point, and 50% had an acceptable axial inclination. Wilson, j2 examining 178 third molars, reported "good or very good" mesial contact and occlusion in 87% of the third molars. Thirteen percent had "poor" contact and angulation. Huggins and McBride t! examined 50 third molars and found that 90% had "good or excellent" mesial contact. "Fair" mesial contact was seen in 4% of the third molars, and 6% had no contact at all. The existence of occlusion, or lack of it, with these third molars was not reported. Gaumond 6 studied the eruption of 22 mandibular third molars after the removal of second molars. Eighty-six percent achieved a "satisfactory or very satisfactory" position in the arch. The remaining 14% were "imperfectly" positioned. A study by Rindler ~3 involved 118 third molars, of which 77% were judged to have "good or very good" positions, 20% had "adequate" positions, and 3% had "poor" positions. Quinn's conclusion, ~ based on his clinical experience, was that 75% of third molars will achieve mesial contact after extraction of second molars. He further added that maxillary third molars rarely need to be repositioned, but mandibular third molars are more likely to need uprighting and repositioning. When one compares extraction of second molars to extraction of premolars, one finds differences in the amount of tooth structure removed and in the location of the extraction site. When first or second premolars are extracted, approximately 14 mm of space per arch is created, t4 which is a sufficient amount to relieve crowding, since arch length deficiencies rarely exceed 10 mm. ~5 Considerably more space is created--18 to 22 m m - - w h e n second molars are extracted, ~ and the problem o f too much space may be created. Some orthodontists believe that proper anterior retraction and the
Second molar extractions
431
natural mesial migration of teeth will close this additional space adequately. Whitney and Sinclair ~6pointed out that successful retraction of the entire dentition into the second molar extraction site relies considerably on patient cooperation. They are in agreement with Haus ~5 that a major disadvantage of second-molar extraction is that space is created in the posterior region of the arch, while the majority of crowding is in the anterior region. Magness ~° does not believe this is a problem. In the absence of second molars, he believes that the maxillary first molars may be moved distally more easily than if second molars were present. There is debate among orthodontists as to whether second-molar extraction cases can be treated in a shorter period of time than premolar extraction cases. Advocates of second-molar extractions believe that the first molars can rapidly be moved distally into the extraction sites and that the total treatment can be completed in less time than if premolars were extracted. Quinn t and Marceau and Trottier, 3 comparing second-molar extractions to premolar extractions, reported a decrease in treatment time, but the actual amount of the decrease was not given. Magness, ~° on the other hand, reported that with extraction of second molars the orthodontist must wait for third-molar eruption and will possibly need a second phase to correct the position of these teeth. This would result in a total treatment time is much longer than that of the average premolar extraction. A final consideration is temporomandibular joint disorders. Many general dentists and some orthodontists, like Bowbeer, ~7 believe that extraction of premolars is a major etiologic factor in temporomandibular joint disorders. They believe that there is a loss in vertical dimension of occlusion following the removal of premolars as a result of the maxillary and mandibular posterior teeth's having been moved forward, thus reducing the Frankfort-mandibular plane angle. This loss of vertical dimension is believed to cause TMJ dysfunction similar to that caused by resorption of the alveolar ridges in an edentulous person. Others have a different theory as to the role first-premolar extractions play in causing TMJ disorders. Farrar and McCarty t8 and Witzig ~9 believe that extraction of premolars leads to over-retraction of maxillary anteriors. As a result, the mandibular condyles are forced posteriorly and TMJ disorders develop. Both concepts are interesting, but there has been no research to support them. However, research has disputed the theory that extraction of four first premolars causes posterior displacement of the condyles and thus leads to TMJ disorders. Gianelly et al. 2° found no difference in condylar position between persons who had been orthodontieally treated with extrac-
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Staggers
tion of four first premolars and those who had not received orthodontic treatment. Furthermore, the results of a study by Blaschke and Blaschke -~ dispute the concept that posteriorly displaced condyles cause TMJ disorders. In fact, 36% of the patients they evaluated had posteriorly displaced condyles, and yet had no symptoms. Pullinger et al. 22 also showed a wide variety of condylar positions in symptom-free patients. It is evident from this review of the literature that there is disagreement among orthodontists and among general practitioners as to the treatment results of second-molar extractions. In view of this disagreement, the purpose of this study was to examine treatment results in cases of maxillary and mandibular secondmolar extraction and compare them with the treatment results of maxillary and mandibular first-premolar extraction. MATERIALS AND METHODS
Records of 22 maxillary and mandibular secondmolar extraction cases were obtained from the files of the Department of Orthodontics at West Virginia University School of Dentistry and from an orthodontist in private practice. Thirteen of these cases were Class I and nine were Class II, Division 1. Sixteen of the patients were girls, and six were boys. Records of 22 maxillary and mandibular first-premolar extraction cases were also obtained from the files of the West Virginia University Department of Orthodontics. Thirteen of these cases were Class I and nine were Class II, Division I. Thirteen of the patients were girls, and nine were boys. The age range for the patients in both groups was 9 to 16 years. For each case, pretreatment ~'nd posttreatment lateral cephalograms were traced, and the following items were compared: 1. the change in the angle of skeletal convexity (nasion-point A-pogonion or N-A-Pg) 2. the change in the anteroposterior position of point A with respect to nasion vertical, a line perpendicular to the horizontal plane through nasion (point A to NV). (Horizontal plane, sella-nasion line plus 7 °, was used as a reference plane instead of Frankfort horizontal plane because of the high variability of the Frankfort horizontal plane.) 3. the change in the anteroposterior position of point B with respect to a line perpendicular to a horizontal plane through nasion (point B to NV) 4. the change in the anteroposterior positions of points A and B, as determined by measuring the distance between the intersection of the occlusal plane and lines drawn perpendicular to
5. 6.
7.
8.
9.
the occlusal plane through points A and B (AB to OP) the change in the horizontal plane-mandibular plane angle (HP-MP) the amount of maxillary anterior retraction, as determined by subtracting the posttreatment measurement from the pretreatment measurement of the distance from the incisal edge of the maxillary central incisor to a line perpendicular to the horizontal plane through nasion (1. to NV) the amount of mandibular anterior retraction, as determined by subtracting the posttreatment measurement from the pretreatment measurement of the distance from the incisal edge of the mandibular central incisor to a line perpendicular to the horizontal plane through nasion (1" to NV) the change in angulation of the maxillary central incisor with respect to the nasal floor, ANSPNS ( ! to NF) the change in angulation of the mandibular central incisor with respect to the mandibular plane
(i to Mr'~ 10. the amount of eruption of the maxillary first molar relative to the nasal floor (6 to NF) 11. the amount of eruption of the mandibular first molar relative to the mandibular plane (6 to MP) 12. tipping and bodily movement of the maxillary first molars, as determined by superimposition of the maxilla on the pretreatment and posttreatment cephalometric tracings and by measurement of the difference in anteroposterior position of the mesiobuccal cusps 13. tipping and bodily movement of the maxillary incisors, as determined by superimposition of the maxilla on the pretreatment and posttreatment cephalometric tracings and by measurement of the difference in anteroposterior position of the incisal edges 14. tipping and bodily movement of the mandibular first molar, as determined by superimposition of pretreatment and posttreatment cephalometric tracings along the mandibular symphysis and by measurement of the difference in anteroposterior position of the mesiobuccal cusps 15. tipping and bodily movement of the mandibular incisors as determined by superimposition of pretreatment and posttreatment cephalometric tracings along the mandibular symphysis and by measuring the difference in anteroposterior position of the incisal edges
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Fig. 1. Measurement of third-molar angulation relative to occlusal plane.
16. the change in the soft tissue angle of facial convexity (glabella-subnasale-soft tissue pogonion or G-Sn-Pg') 17. the change in the anteroposterior position of the upper lip with respect to the subnasale-soft tissue pogonion line (Ls to Sn-Pg') 18. the change in the anteroposterior position of the lower lip with respect to the subnasale-soft tissue pogonion line (L, to Sn-Pg') All cephalographs were taken with the teeth in centric occlusion and the lips in repose. From the pretreatment and posttreatment panoramic radiographs, the angulations of the maxillary and mandibular third molars were evaluated. Angulations of the third molars were measured with reference to the anterior angle between the occlusal plane of the first molar and the second premolar and the long axis of the third molar (Fig. 1). From each measurement obtained from the lateral cephalograms and panoramic radiographs, pretreatment values were subtracted from posttreatment values to reveal the change that had resulted from treatment. These changes were analyzed for statistical difference at the p < 0.05 level of significance by means of Student's t test. Total treatment time, in months, for each person in the second-molar and first-premolar extraction groups was also recorded. The average length of treatment for each group was calculated and compared for statistical difference at the p < 0.05 level of significance with the aid of Student's t test. RESULTS
Statistical analysis showed a significant difference between the premolar and second-molar groups in six of the eighteen cephalometric measurements analyzed.
Table I. Summary of the mean cephalometric
differences (pretreatment values subtracted from posttreatment values)
Measttrement N-A-Pg Point A to N V Point B to N V A-B to O P HP-MP _1 to N V ] to NV 1_ to N F I to hiD
6 to N F to MP 6 superimposition superimposition / superimposition T superimposition G-Sn-Pg' Ls to S n - P g ' Lt to S n - P g '
Second-nzolar extraction group - 3.3
First-premolar extraction group
0
0.0 mm 0.5 mm l.l mm 0.0 ° -0.7 mm 0.7 mm ! .4 ° 4.40
-3.90 -2.5 mm 0.0 mm -0.4 mm - 0.50 - 3.0 -
mm*
0.8 m m 0.60 0.5 o
-
1.70
mm mm mm* ram* - 2.0 ram* - 1.6 mm* - 2.2 o
-
1.4 m m 0.0 mm
-
1.8
mm
-
1.6
mm*
2.4 2.4 0.0 0.3 0.7 1.3
mm mm mm mm mm mm
3.0 3.4 4.8 3.7
*Indicates statistical differences at the 0 . 0 5 level.
The measurements that were statistically different were as follows (Table I): 1. the change in maxillary incisor protrusion, as determined by I to nasal vertical 2. the change in maxillary first molar position, as determined by maxillary superimpositions 3. the change in mandibular first molar position, as determined by mandibular superimpositions 4. the change in maxillary incisor position, as determined by maxillary superimpositions
Am. J.
434 Staggers Table II. Summary of mean pantographic
differences (pretreatment angles subtracted from posttreatment angles) Second-molar group (degrees)
Third molar Maxillary right Maxillary left Mandibular right Mandibular left
14.9 12.6 -5.6 - 6.9
First-premolar group (degrees) 5.8 14.5 - 10.4 - 10. I
Orthod. Dentofac. Orthop. November 1990
lations before and after treatment. In both groups, the maxillary and mandibular third molar long axisocclusal plane angle increased on both right and left sides (Table II). The average treatment time of 38 months for the second-molar group was slightly longer than the average treatment time of 36.8 months for the premolar group (Table III). However, this difference was not statistically significant. DISCUSSION
Table III. Summary of mean treatment time Group Second-molar extraction First-premolar extraction
] I
Time (mo) 38.0 36.8
5. the change in mandibular incisor position, as determined by mandibular superimpositions 6. the change in lower lip protrusion, as determined by Lt to Sn-Pg' The second-molar group showed a mean decrease in the values of N-A-Pg, 1 to NV, G-Sn-Pg', and Ls to Sn-Pg', and a mean increase in the distances from point B to NV, A-B to OP, ]- to NV, 1 to NF, i" to MP, 6 to NF, and 6 to MP. Point A to NV, HP to MP, and L, to Sn-Pg' did not show a mean change in this group. The first-premolar group showed a mean decrease in the values of N-A-Pg, point A to NV, A-B to OP, HP to MP, 1 to NV, and ]- to NV, Gn-Sn-Pg', Ls to Sn-Pg', and Lx to Sn-Pg', and a mean increase in I to NF, i" to MP, 6 to NF, and 6 to MP. The distance from point B to NV did not show a mean change in this group. The position of the maxillary first molars in the second-molar group showed no change, as determined by superimpositions of pretreatment and postreatment cephalograms along the maxilla. The superimpositions in this group also revealed a slight, anterior average movement of the maxillary incisors. Maxillary superimpositions in the premolar group showed an average anterior movement of the first molars and a posterior movement of the incisors. Superimpositions of pretreatment and postreatment cephalograms along the mandible symphysis revealed anterior movement of the mandibular first molars in both groups and the mandibular incisors in the second-molar group. The mandibular incisors in the premolar group, on the other hand, moved posteriorly. Comparison of the pantographic measurements revealed no significant differences in third molar angu-
Facial esthetics is an important consideration in orthodontic treatment particularly when extractions are being considered. Most orthodontists evaluate facial esthetics by the soft tissue angle of facial convexity and by the position of the upper and lower lips to a reference plane, such as the E plane or, as in this study, the subnasale-soft tissue pogonion line. It is accepted in orthodontics that extraction of permanent teeth reduces facial convexity. For patients with protrusive, convex profiles, both reduction in the soft tissue angle of facial convexity and retraction of the lips are desirable. However, others, with straight profiles, would not benefit from such a reduction. This situation produces a dilemma when the patient's dentition indicates that extractions are needed but the profile does not. This study examined the soft tissue changes after extraction of teeth from different areas of the mouth, and the results showed an average decrease in the soft tissue angle of facial convexity of 1.7 ° for the second-molar extraction group and 2.2 ° for the first-premolar group. However, these reductions were not statistically different. Like the angle of facial convexity, the change in upper lip protrusion was not significantly different; yet the change in lower lip protrusion was significant. An average decrease in lower lip protrusion of 1.6 mm was seen in the premolar extraction group, compared with 0.0 mm in the second-molar extraction group. Even though this difference was statistically significant, the overall change in the soft tissue angle of convexity (GSn-Pg') was not. Thus these results do not support the claim that the facial profile is flattened significantly more by extraction of first premolars. The soft tissue profile is influenced by the underlying skeletal structures. Evaluation of the changes in points A and B and the skeletal angle of facial convexity revealed no significant differences between the two groups. The A-B discrepancy was improved by extraction of first premolars, but it was not improved by extraction of second molars. The angle of skeletal convexity decreased in both groups, and this decrease in convexity might account for the decrease in the soft tissue angle of convexity.
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Soft tissue is also influenced by dentition, primarily maxillary and mandibular incisors. However, the results of this study indicate that the influence of dentition is unpredictable. Incisor position can be defined by inclination and bodily position (protrusion). The changes in incisor inclination were not significantly different for the two groups, but there was a difference between the change in maxillary incisor protrusion Q. to NV) for one group, as compared with the other. Dental changes, however, did not affect the lips as would be expected. Lip changes were more than dental changes in some instances and less in others. For example, the maxillary incisor protrusion in the second-molar group decreased an average of 0.7 mm, yet upper lip protrusion decreased an average of 1.4 mm. The mandibular incisor protrusion averaged a 0.7 mm increase in the secondmolar group, but on average there was no change in lower lip protrusion. For the first-premolar group, maxillary incisor protrusion decreased an average of 3.0 mm and upper lip protrusion decreased 1.8 mm. Lower incisor protrusion decreased an average of 0.8 mm, and the lower lip protrusion decreased an average of 1.6 mm. Average incisor procumbency increased in both groups, but lip protrusion decreased, except for the lower lips in the second-molar group, in which average lip protrusion remained the same. Data from both groups indicate little relationship between changes in incisors and changes in lips. Therefore soft tissue changes cannot be predicted by dental changes. Maxillary incisor retraction, as determined by maxillary superimposition, was significantly greater in the premolar g r o u p - - a result that would also be expected to cause a significantly greater decrease in upper lip protrusion in the premolar group; yet this did not occur. Mandibular incisor retraction, as determined by superimposition of mandibular cephalograms, was significantly more in the premolar g r o u p - - a difference that would account for the significantly greater decrease in lower lip protrusion seen in this group. Another important consideration is the change in vertical dimension after extraction therapy. Today, orthodontic treatment that involves extraction of first premolars is commonly blamed for causing TMJ disorders by decreasing the vertical dimension of occlusion. Posterior teeth are believed to move forward after premolar extractions, thus permitting the mandible to overclose and shorten the resting length of the muscles of mastication. This theory disregards the concept that all orthodontic mechanics are extrusive to some degree and that the extrusion maintains or even increases vertical dimension. This study investigated changes only in vertical dimension as determined by measurement of the MP-HP angle after the extraction of first premolars
Second molar extractions
435
and second molars. It did not explore the possibility that a decrease in vertical dimension leads to TMJ disorders. However, this area warrants further research, since articles pertaining to this subject present conflicting views. 17'23 Changes in vertical dimension, as determined by the horizontal plane-mandibular plane angle, were not significantly different between the premolar and second-molar extraction groups. On average, the HP-MP angle remained the same after second-molar extractions and decreased 0.5 ° after premolar extractions. It is questionable that a decrease of 0.5 ° is sufficient to cause TMJ disorders. Changes in vertical dimension can also be evaluated by the amount of tooth eruption and mesial movement of posterior teeth that occurs during treatment. All first molars in both groups were extruded during treatment, and there were no significant differences between the premolar and second-molar groups. The maxillary and mandibular first molars moved farther anteriorly a significantly greater amount in the premolar group than in the second-molar group. On average, maxillary first molars in the second-molar group remained in the same position, and the mandibular first molars averaged a slight (0.3 mm) forward movement. These data indicate that even though the posterior teeth are moved forward more in the premolar group, extrusion prevented any significant loss in vertical dimension. An additional orthodontic consideration explored in this study was the influence of extractions on third molars. Some orthodontists believe that extraction of any permanent teeth will improve the angulation of third molars and thus will improve the chances for third molars to erupt into good positions. Others ~'1° disagree, stating that such improvement will occur only if second molars are extracted and that extraction of first premolars will not prevent the need for extraction of third molars. The change in angulation of third molars relative to the occlusal plane after first-premolar and second-molar extraction treatment was examined in this project. If extraction of second molars does lead to more favorable inclinations of the third molars, one would have expected to see an increase in the angle between the maxillary third-molar long axis and the occlusal plane and a decrease in the angle between the mandibular third-molar long axis and the occlusal plane. In the second-molar group, there was an increase in the maxillary third-molar long-axis-ocelusal plane angle and thus an improvement in maxillary third-molar angulation, but this increase was not significantly different from that in the premolar group. Unexpectedly, the mandibular third-molar Iong-axis-occlusal plane angle increased in the second-molar group, as it did in the premolar group, indicating a poorer mandibular third-
436
Am. J. Orthod. Dentofac. Orthop. November I990
Staggers
molar angulation. The data suggest that there is no advantage to extracting second molars as opposed to first premolars to improve third-molar angulation. One should keep in mind that a favorable change in thirdmolar inclination after second-molar extractions does not necessarily mean that third molars will erupt into the mouth with good inclination, proper contact with the first molar, or acceptable occlusion. The final orthodontic consideration of this study is length of treatment time for premolar and second molar extractions. Length of treatment is dependent on many variables, such as mechanics, patient cooperation, and parent motivation. None of these variables was controlled in this study, and it was assumed that patient cooperation and parent motivation in both groups were similar and that patients were treated with similar mechanics. Length of treatment was not significantly different between the two groups. However, the possibility that there were differences in the level of patient cooperation, parent motivation, and mechanics between the two groups cannot be ignored. Also, third molars had erupted in only a few patients in the second-molar group at the time of debonding. Therefore, the total treatment time for the second-molar group did not take into account any time needed for positioning the third molars after they had erupted. Thus the actual length of treatment for the second-molar group may have been longer than this study indicates. Comparison between patients treated with secondmolar extractions and patients treated with firstpremolar extractions revealed that the two groups had fewer differences than some authors have previously stated. The data presented in this study do not support many of the concepts presented by advocates of secondmolar extractions. Further study in this area is indicated. REFERENCES
1. Quinn GW. Extraction of four second molars. Angle Orthod 1985;55:58-69. 2. Liddle DW. Second molar extraction in orthodontic treatment. AM J OR'roOD1977;72:599-616. 3. Marceau ]E, Trottier BP. Third molar development following second molar extractions. J Pedodontics 1983;8:34-51. 4. Williams R, Hosila FJ. The effect of different extraction sites upon incisor retraction. AM J OR~tOD 1976;69:388-409.
5. RichardsonhiE. The relative effect of the extraction of various teeth on the developmentof third molars. Trans Eur Orthod Soc 1975:79-85. 6. GaumondG. Second molar germectomy and third molar eruption. Angle Orthod 1985;55:77-88. 7. CavanaughJJ. Third molar changes followingsecond molar extractions. Angle Orthod 1985;55:70-6. 8. Richardson ME. Some aspects of lower third molar eruption. Angle Onhod 1974;44:141-5. 9. Smith DI. The eruption of third molars followingextraction of second molars. Dent Practitioner 1958;8:292-5. 10. Magness WB. Extraction of second molars. J Clin Orthod 1986;20:519-22. I 1. Huggins DG, McBride LI. The eruption of lower third molars following the loss of lower second molars: a longitudinalcephalometrie study. Br J Orthod 1978;5:13-20. 12. Wilson HE. Extraction of second permanent molars in orthodontic treatment. Orthodontist 1971;3:i8-24. 13. Rindler A. Effects on lower third molars after extraction of second molars. Angle Orthod 1977;47:55-8. 14. WheelerRC. Dental anatomy,physiologyand occlusion. 5th ed. Philadelphia: WB Saunders, 1974:!96-209. 15. tlaus AJ. Let's take a rational look at permanentsecond molar extractions. AM J ORTHODDEr~rOFACORTHOP1986;90:361-3. 16. WhitneyEF, Sinclair Phi. An evaluationof combinationsecond molarextractionand functionalappliancetherapy. AMJ ORTttOD DEN'rOFACOR'rttoP 1987;91:183-92. 17. Bowbeer GR. The sixth key to facial beauty and TMJ health. Funct Orthod 1987;4:4-20. 18. FarrarWB, McCarty WL. A clinicaloutlineof temporomandibular joint diagnosis and treatment. Montgomery, Ala.: Walker Printing, 1983:84-5. 19. Witzig JW, Sphal TJ. The clinical outline of maxillofacialorthopedic appliances. 1st ed. Littleton, Mass.: PSG Publishing, 1987:161-216. 20. GianellyAA, Hughes HM, WohlgemuthP, Gildea G. Condylar position and extraction treatment. AMJ ORTHODDENTOFACOR"ntoP 1988;93:201-205. 21. Blaschke DD, Blaschke TJ. Normal TMJ bony relationshipsin centric occlusion. J Dent Res 1981;60:98-104. 22. Pullinger AG, Hollender L, Solberg WK, Petersson A. A tomographic study of mandibularcondyle position in an asymptomatie population. J Prosthet Dent 1985;53:706-12. 23. StaeklerH. Clinicalobservationsof cases fiveyears out of treatment. Ast J ORI'tlOD1958;28:108. Reprint requests to:
Dr. Julie Ann Staggers Health Sciences Center North West Virginia UniversitySchool of Dentistry Morgantown, WV 26506
