Introduction
The decision to extract teeth as part of orthodontic treatment is governed mainly by concerns about facial appearance, occlusal stability, arch length deficiency and anteroposterior discrepancies.
Usually, the maxillary first premolars are the most common teeth that orthodontists choose to extract. Despite this overall “trend,” there are other authors who claim that extraction of second premolars (instead of first) provides better results, according to their clinical experience and a number of studies that they performed.
The purpose of this article, therefore, is to summarize the studies and the articles that compare the extraction of the first versus the second premolars and to advise the orthodontist which is the best decision for each case.
Are there any crown morphologic differences between maxillary 1st and 2nd premolar?
Maxillary 1st premolar:
The function of this premolar is similar to that of canines in regard to tearing being the principal action during mastication. There are two cusps on maxillary first premolars, and the buccal cusp is relatively sharper and higher than the palatal.
Maxillary 2nd premolar:
The function of this premolar is similar to that of first molars in regard to grinding being the principal action during mastication. There are two cusps on maxillary second premolars, but both of them are shorter and not as sharp as those of the maxillary first premolars. Furthermore, the 2nd premolar is narrower and shorter than the 1st.
Are there any crown morphologic differences between mandibular 1st and 2nd premolar?
Mandibular 1st premolar:
The function of this premolar is similar to that of canines in regard to tearing being the principle action during mastication. Mandibular first premolars have two cusps. The large and sharp one is located on the buccal side of the tooth. Since the lingual cusp is small and nonfunctional, which means it is not active in chewing, the mandibular first premolar resembles a small canine.
Mandibular 2nd premolar :
The function of this premolar is to assist the mandibular first molar during mastication. Mandibular second premolars have two or three cusps. There is one large cusp on the buccal side of the tooth. The lingual cusps are well developed and functional, which means that the cusps assist during chewing. Therefore, whereas the mandibular first premolar resembles a small canine, the mandibular second premolar is more similar to the first molar. That is why the mandibular 2nd premolar is more often slightly wider than the 1st. (In the maxilla, the 1st premolar is wider than the 2nd.)
Morphologic variations
Variations in size and shape (and anomalies) occur more often in mandibular 2nd premolars than the 1st. Such tooth-size deviations (TSD) have been described by Garib DG, Peck S. [1], who stated that mandibular second premolars are often characterized by a squeezed faciolingual (FL) dimension and an enlarged mesiodistal (MD) dimension (MnP-TSD), reversing the typical premolar proportions. This occurs eight times more frequently to the 2nd mandibular premolars than to the 1st.
How could these morphologic characteristics and variations affect the orthodontic treatment?
Several clinical orthodontic implications become obvious in patients with this (MnP-TSD) anomaly. Decisions regarding orthodontic premolar extractions might be affected if any mandibular premolars are significantly affected. Often, 2nd lower premolars with such teeth size variations are the best choices for removal, if extractions are indicated for a patient’s orthodontic treatment. These teeth tend to resist derotation and torquing because of their imbalanced shapes and unusually broad mesiodistal root surface areas. Moreover, achieving correct intercuspation of that kind of premolar can be especially challenging.
Further considerations in orthodontic treatment
Saatci P, Yukay F.[2] stated that the extraction of four premolars as a requirement of orthodontic therapy is a factor in the creation of tooth size discrepancies (Bolton discrepancies). Patients who exhibit Bolton Index within normal values, after the extraction of 4 premolars (two upper and two lower) often had Bolton discrepancy after the treatment. But it was the removal of the four first premolars that created the most severe tooth-size discrepancy, whereas the extraction of all four second premolars created fewer discrepancies and the smallest range in the size of discrepancies. The results of this study indicate a new point of view to the question of which teeth to extract when evaluated for tooth size aspect only.
In addition, Tong H, Chen D, Xu L, Liu P [3] suggest that usually 4 premolar extractions tend to decrease the Bolton overall values. This means that in a patient with normal values or with small values (maxillary excess), the extraction of 4 premolars tends to aggravate the discrepancy, while in a patient with large values (mandibular excess) tends to normalize the discrepancy.
Specifically comparing the extraction of 1st or 2nd bicuspids, Gaidyte A and Baubiniene D [4] claimed that they have an unequal effect on Bolton Index. While the extraction of 2nd premolars (upper and lower) has little effect, the extraction of 1st premolars tends to increase the Bolton Index (because the remaining mandibular 2nd premolar tends to be wider than the upper 2nd premolar). Therefore, regarding Bolton’s discrepancy, when we have small values, it is better to extract the 1st premolars, while when we have normal or large values, to extract 2nds.
De Angelis [5] according to his clinical experience suggests that extraction of 2nd maxillary premolars are preferable to 1st premolars for esthetic reasons, since the longer and sharper buccal cusp of the first premolar is more esthetically pleasing. Moreover, extraction of the first premolar results in an unsightly gap that will not be closed for many months, which is especially bothersome to some patients.
From a purely physical standpoint, De Angelis suggests that the 1st maxillary premolar is a more valuable tooth than the 2nd since it has 2 roots which are longer than the 2nd premolar single root by 4mm and, therefore, it has a better long-term prognosis. This should be taken into account with the fact that the 2nd premolars have more frequent amalgam restorations and therefore are candidates for extraction. Furthermore, the extraction of 1st premolars is more complicated than the 2nd (2 roots).
In addition, in terms of periodontal conditions after the extraction of the 1st premolars gingival clefts are created that complicate the space closure and influence the periodontium.
Finally, he claims that in upper premolar extraction cases (Class II), it is easier to achieve correct molar occlusion if we extract the 2nd premolar rather than the 1st. Obviously in such cases we should achieve full Class II molar relationship (full Class II: the mesiobuccal cusp of the upper molar occludes in the groove between the lower 1st and 2nd premolar). Therefore, since the mesiobuccal cusp of the upper molar is similar in width to the 2nd premolar, extracting the 2nd premolar and closing the space until the mesiobuccal cusp of the upper molar occludes between the 1st and 2nd lower premolar (where the upper 2nd premolar was before), achieves a more solid occlusion.
Anchorage Considerations in Lingual Orthodontics
Takemoto [6] suggests that the anchorage value of posterior teeth in the lingual technique is higher than that in the labial technique because of the nearness of the lingual brackets to the center of tooth resistance. Additionally, the direction of forces during space closure with lingual appliances creates a buccal root torque and distal rotation of the molar crown, which produces cortical bone anchorage.
In addition, Kurz and Bennett [7] suggest that the smaller arch perimeter increases the rigidity of lingual archwires, which may increase anchorage control during retraction. The larger slot size of the posterior lingual attachments provides an almost frictionless sliding retraction with no energy burning.
In agreement with the previous statements, Geron S, Shpack N, Kandos S, Davidovitch M, Vardimon A [8], in their study showed that the lingual technique provides superior anchorage control compared to the labial orthodontics. In the study, cases treated with labial orthodontics showed more anchorage loss when compared with similar cases treated with the lingual technique. Furthermore, a non-significant difference in anchorage loss was found between first or second premolar extractions in cases treated with the lingual technique.
Good anchorage control can be achieved with lingual orthodontic sliding mechanics when following simple anchorage principles. The “six keys for anchorage control in lingual sliding mechanics” have been suggested as a means of providing maximum anchorage control. “The six anchorage keys” include the following:
• Standard lingual bracket jig prescription for the anterior teeth, incorporating slight extra-palatal root torque and no extra tip for extraction cases; molar tubes placed off – center in a more mesial position and incorporating a mesial tip to encourage molar tip back (uprighting)
• Reduced friction, using sliding mechanics together with bi-dimensional archwires incorporating a rectangular anterior section and round posterior sections or using a standard archwire and placing brackets on the posterior teeth with larger slot sizes
• Posterior bite stops placed on the molar teeth to open the bite
• Light Class I, II, or III forces for retraction or space closure
• Incorporation of the second molars in the anchorage unit
• Incorporation of an exaggerated curve of Spee in the maxillary space –closing archwire.
Providing that the clinician pays attention to the particular anchorage requirements of each case, it is possible, in lingual orthodontics, to achieve excellent anchorage control in extraction cases, even if they include the extraction of the 1st instead of the 2nd premolars.
Conclusion
• Although the 1st and 2nd premolars appear to have same crown morphology, they have slight differences that should be taken into consideration in treatment planning, especially when extractions are involved.
• Particularly, the 2nd mandibular premolars are often characterized by variations in size and shape that could affect the final result complicating the effort of the clinician to achieve correct intercuspation.
• In addition, clinical experience revealed that extraction of upper 2nd premolars results in better and more “solid” buccal occlusion, more esthetic appearance, less teeth size discrepancies and fewer periodontal complications.
• There is no difference in anchorage requirements between the extraction of the first or second premolars, as long as the clinician follows simple anchorage principles.
References
1. Garib DG .Peck S. Extreme variations in the shape of mandibular premolars. Am J Orthod Dentofacial Orthop. 2006 Sep;130(3):317-23.
2. Saatci P.Yukay F. The effect of premolar extractions on tooth-size discrepancy. Am J Orthod Dentofacial Orthop. 1997 Apr;111(4):428-34.
3. Tong H, Chen D, Xu L, Liu P. The effect of premolar extractions on Bolton index. Shanghai Kou Qiang Yi Xue. 2004 Oct;13(5):360-4. Chinese.
4. Gaidyte A and Baubiniene D. Influence of premolar extractions on tooth size discrepancy. Part two: Analysis of Bolton values. Stomatologija. 2006;8(1):25-9
5. De Angelis V. The rationale for maxillary second premolar extractions in adult Class II treatment. Journal of Clinical Orthodontics, 2007 Aug: 8 Vol XLI: 445-450
6. Takemoto K. Anchorage control in lingual orthodontics. In: Romano R. Lingual Orthodontics. Hamilton, Canada: BC Decker; 1998: 75-82
7. Kurz C, Bennett R.: Extraction cases and the lingual appliance. J Am Ling Orthod Assoc. 1998; 3:10-13
8. Geron S, Shpack N, Kandos S, Davidovitch M, Vardimon A. Anchorage loss--a multifactorial response. Angle Orthod. 2003 Dec;73(6):730-7.
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