One of the key components of orthodontic treatment is anchorage control. Many studies focused on linear change instead of on tipping movement of molars [14,15]; however, during orthodontic treatment, crown movement surpasses root movement in speed and extent, signifying the initiation of anchorage loss. Molar mesialization almost always accompanies forward tipping, in other words, mesial tipping is closely related to anchorage loss. Unfortunately, orthodontists rarely take naturally or physiologically tipping of molars into consideration while developing a treatment plan. This cross-sectional study comprehensively analyzed physiological factors and mechanical factors that might trigger anchorage loss, in Class I or Class II patients who underwent extraction and required anchorage control. A few characteristics have been found that are attributable to anchorage loss and we believe that more attention should be given to preserving anchorage on patients who have those signs.
Growth (age) plays an important role in the anchorage loss of upper first molar. The majority of orthodontic patients are teenagers. According to table 2, teenagers exhibited 3.46º mesial tipping of the maxillary first molar while adults 0.05º distal tipping, and it showed statistical significance. The results agree with the finding in Xu’s study , which showed younger adolescents had significantly more molar mesial displacement than older adolescents (mean difference, 1.3 mm). Mckinney  also found similar results that showed teenagers are prone to anchorage loss more than adults. It is reasonable because teenager is a period around the second growth peak, during which the maxillary first molar will tip forward significantly by growth. Plenty of longitudinal studies reported gradual mesial tipping during this period. Iseri and Solow  noticed the maxillary first molar would continuously erupt inferiorly and anteriorly before 25 years old and it would continue erupting with a slower speed afterward. Tsourakis and Johnston  found out a compensatory growth pattern of the maxillary molars in response to the greater and longer mandibular growth, which revealed a close relation between the movement of maxillary molar and mandibular growth. Zhang  studied the longitudinal eruptive and post-eruptive tooth movements using oblique and lateral cephalograms with implants. They found that continuous mesial tipping of the maxillary molars happened from 8.5 to 16 years of age, averaging 8.2° ± 5.5° for the first molars and 18.3°± 8.5° for the second molars. Therefore, we inferred that the anchorage loss before adulthood might spring from two factors: 1. The application of force during space closure; 2. Growth and development of maxillary teeth, the direction of which is down and forward.
According to table 2, sex is obviously another factor contributing to maxillary molar tipping during orthodontic treatment. Male patients tend to undergo more mesial tipping than female patients do, which is in agreement with preview studies [13,16].We believe this phenomenon is attributable to the relatively late arrival of the male growth peak. With similar height, female patients are in average two years in advance in terms of maturation, while male patients grow later and greater.
According to table 2, the maxillary first molar tipped forward greater in Class II patients, indicating Class II patients are predisposed to anchorage loss. Our previous cross-sectional study  found that patients with Class II malocclusion had the most distally tipped UMs. Kim  stated a well-compensated Class II patient tended to exhibit the most distal tipping of the maxillary first molars. For Class patients, the distally-tipped maxillary molars would be leveled and aligned in the very beginning by a flat light wire, causing unfavorable anchorage loss, and reduced extraction space. Mckinney  mentioned the undesirable anchorage loss with the straight-wire brackets, which should be considered iatrogenic and unnecessary.
In order to stop the upper first molar from tipping forward, orthodontists resort to auxiliary appliances (Nance appliance, TPA, and headgear, etc.). However, our results showed that different types of auxiliary appliances used in this study had no statistically significant effect on the angular change of maxillary first molar (table 3). We guess that the flat wires engaged in the upper arch would do the leveling 24 hour a day throughout the whole treatment, while headgears are worn 8-12hour per day. Nance appliance and TPA could reduce the angular change of maxillary first molar.
To compare various factors in regards to the amount of maxillary molar tipping, stepwise multiple regression analysis was performed. According to the result from table 5, the most contributing factor to the regression formula was the pre-treatment angulation of the maxillary first molars. Since the pre-treatment status of molars is determined completely by individual malocclusion and physiological characteristics, and its role is strikingly more important than the traditionally-believed forces from space closure or other mechanics. The negative standardized coefficients suggested that the more the distal tipping of pre-treatment maxillary first molars, the more the mesial tipping would occurduring orthodontic treatment, in other words, the more the anchorage loss would happen.
From table 5, the second most contributing factor was the incisor/molar height ratio (the ratio of the vertical position of incisor to that of molar relative to the palatal plane). The negative standardized coefficients indicated the less the pre-treatment UIE-PP/UMC-PP-1, the more the mesial tipping of the maxillary first molars. The different relative heights of the brackets’ position resulted in different deformities of the wires. The incisor/molar height ratio is rarely mentioned and considered in other studies, but it could be an important indicator of anchorage loss.
The present study has some limitations. There were more female and teenager patients, and in most of the case, the extracted teeth were upper first premolars, which might bring systematical bias into the study .These limitations were due to the availability of data.
In summary, this study is a supplement of the traditionally-believed concept that anchorage loss is merely from the mechanical force used during space closure, and it brings about something new into consideration. We believe the natural compensatory eruption of maxillary molars relative to mandibular growth, as a natural physiological characteristic of the dentoalveolar growth, plays an imperative role in anchorage control. The results of this study show various patient’s responses to the Straight Wire Appliance. For extraction cases with the need for anchorage control, anchorage loss tends to occur in specific groups of patients, like teenager, male, and Class II. When it comes to pretreatment distally-tipped maxillary molars, this status is a type of anchorage preservation. Although the Straight Wire Appliance is renowned for its convenience, it is worth considering how to avoid iatrogenic maxillary molar tipping caused by the insertion of a NiTi wire in the molar tube.