Mandibular Second Molar Impaction-Part II: Etiology and Role of the Third Molar
Round 1
Reviewer 1 Report
The authors has previously published a manuscript regarding this topic (Shapira Y, et al. Mandibular second molar impaction. Part I: Genetic traits and characteristics. Am J Orthod Dentofacial Orthop. 2011;140(1):32-7. ), one of the conclusions drawn from that study was that "deficient mesial root length of the MM2 is the primary impaction factor". While in the present manuscript, the authors tried to focus on the etiology and the role of the third molar regarding the emergence of MM2 impaction. The discussion and the presentation of the early stages of the MM2 impaction process are interesting, however, the results could not yet support the conclusion, which need more evidence to be drawn. The characteristics of the differential length between the mesial and distal root has been discussed in the previously published paper by the author. Regarding the role of the third molar, the presence and the adjacent relationship of the third molar and the impacted second molar need to be evaluated with statistical analysis in addition to the presentation of typical cases.
In addition, the author should also explain why the patients from Chinese-American origin were selected for the study?
Author Response
Dear Reviewer,
Thank you for your comments and suggestions for my manuscript ID applsci-1978203
“Mandibular second molar impaction- Part II: Etiology and role of the third molar”.
1) This manuscript evaluates the radiographic characteristics of MM2 and the possible association with MM3 in the process of MM2 path of eruption and its impaction.
The mesial angulation of the impacted MM2s and the increased angle between the first and second mandibular molars were observed in 92.7% of the cases, even in cases with missing 3rd molars, or where adequate space was observed for the eruption of the MM2, these suggested, at least for the presented cases, that arch length deficiency is not the major factor causing MM2 impaction. The serial radiographs demonstrated the early stages of the process of MM2 mesial angulation, regardless of the presence of 3rd molars. The differential growth of the roots where the shorter mesial root length was suggested as an early sign and probably a major cause for MM2 impaction.
The phenomenon of MM2 impaction has several etiologic factors, both dentally and skeletally. These include posterior arch crowding or arch length deficiency, not timely resorption of the anterior rim of the mandible for the eruption of the 2nd molar, as well as the differential roots growth causing early mesial angulation as suggested by us.
Statistical analysis could not be prepared at this point. Practically it would not make any change in the evaluation of the results, in the conclusions, or in the interpretation of the suggested theory regarding the possible etiology of MM2 impaction.
2) It was coincidental that patients of Chinese-American origin were selected for the study. During my Sabbatical year at NYU several years ago, I was looking for a place to investigate the issue of MM2 impaction. A friend and colleague from NYU proposed his cooperation and suggested using his enormous records from his very large practice in Chinatown, NYC, where the great majority of patients were of Chinese-American origin.
Reviewer 2 Report
Knowing the etiology of a problem represents a matter of great clinical interest that could contribute to better anticipation in its management. Surely the impaction of molars, in this case mandibular second molars, like many other dental problems, obeys a multifactorial etiology of complex interpretation. To make this study more difficult, anecdotal cases that would mean more exceptions than rules can contribute to contaminating sample findings that maintain a line of argument, contradicting them.
Taking these and some other considerations into account, the revised work brings together the aforementioned interest with some doubts and issues arising from its methodology. The most relevant is that taking into account that the MM2 study has been carried out on a sample in impact conditions, this may condition its results and conclusions. Thus, altered (asymmetric) root development, is it the cause or consequence of development in a bad position of the germ? Animal experimentation confirms the strong relationship between dental retention and developmental anomalies. The study in a control sample of dental development and the subsequent consequences of the observed asymmetries could strengthen the hypothesis raised with the findings in the sample with impaction.
Regarding the role of the posterior space in which the molars develop, I believe that it has received less attention in relation to the study of the apical area in segments where tooth replacement processes take place. The latter inform us of a certain conditioning in the intraosseous position of the developing germs, in relation to the size of said apical area. Position that will subsequently condition the pattern and eruptive trajectory followed by the seed in its eruptive process. If we attend to these considerations, could we expect a similar influence in the posterior segments? However, the conclusions of the study point to a minor influence. However, to increase the complexity, we ask ourselves the following question: has the possible discrepancy between dental development (accelerated) versus skeletal development of the posterior segment (slowed) been studied? This temporal discrepancy (continent and content) potential cause of the characteristics of the prenatal development of the dentition and that causes an overlapping of germs, could it also be behind the dental anomaly studied? Obviously, this adds a temporary factor to the potential lack of space, which could be compensated later, but conditioning the position of the seed. Again, I believe that the study of dental development conditions in the context of the “posterior apical area” in a control sample could help clarify the issue.
The iconography collects anecdotal cases and as such should be taken with caution when extrapolating the findings further.
Our experience in the context of a rare disease (osteogenesis imperfecta) in which the impaction of M2 is particularly frequent, also MM2 leads us to think that the anatomical characteristics (globular shape of the crowns, accentuated cervical constriction) may also play a role etiology in a context of altered craniofacial development. Other variables to consider.
Despite these doubts, I believe that the work meets sufficient conditions of interest for its publication.
I allow myself to suggest to the authors that with the extensive casuistry handled (5.575 panoramic radiographs) they could carry out a more exhaustive review of the conditions present in the rest of the sample that did not present the anomaly studied and that would help to better understand the relationship of the variables in the etiology of the anomaly studied.
Author Response
Dear Reviewer,
Thank you for your comments and suggestions for my manuscript ID applsci-1978203
“Mandibular second molar impaction- Part II: Etiology and role of the third molar”.
1) This manuscript evaluates the radiographic characteristics of MM2 and the possible association with MM3 in the process of MM2 path of eruption and its impaction.
Your experience in a rare disease like osteogenesis imperfecta where MM2 impaction frequent is present, the particular crown shape is suggested to be a possible etiological factor in MM2 impaction. Tooth size may be affected by genetic factors. In ethnic Chinese people, tooth size was reported to be larger than in Caucasians. Similarly, our study sample of Chinese- American origin presented a higher prevalence of MM2 impaction which may suggest a possible etiologic factor.
2) Another etiologic factor may possibly be the discrepancy between the accelerated dental development at the end of the dental arch while the skeletal development of the anterior rim of the mandible is slower in its process of resorption.
3) MM2 impaction has several etiologic factors, both dentally and skeletally. These include posterior arch crowding or arch length deficiency, not timely resorption of the anterior rim of the mandible for the eruption of the 2nd molar, as well as the differential roots growth causing early mesial angulation and impaction as suggested in our study.
Round 2
Reviewer 1 Report
Thank the authors for the justification.
