Many Class I treatments the malocclusion may be corrected by tooth alignment only, accepting the position of the upper and lower incisors in the face. This is so-called 'tooth alignment' orthodontics, and it can be straightforward using the preadjusted bracket system. However, the majority of orthodontic cases require changes in incisor position. In addition to 'tooth alignment', most cases require more challenging 'denture-positioning' procedures.

Systemized Orthodontic Treatment Mechanics. McLaughlin, Bennett,Trevisi. 2001.

Nance believed that excessive labial movement of anterior teeth leads to eventual relapse and possible tissue damage: “to line up the teeth in an arch to normal contact point relationships … is downright easy provided one ignores the relationships of teeth to supporting bones.”

Leeway Space and the Resolution of Crowding in the Mixed Dentition. Gianelly. 1995.

Position the maxillary incisors at 5mm ± 2mm and 22º ± 5º to NA for all skeletal patterns. The measurements increase as the ANB angle decreases, and decrease as ANB increases.

Position the mandibular incisal edges at .5mm ± 2mm to NA and 25º ± 5º to NB , with values decreasing as ANB decreases and increasing as ANB increases.

Where Teeth Should Be Positioned in the Face and Jaws and How To Get Them There. Creekmore. 1997.

Most (93%) of the adult white females with harmonious profiles examined in this study had maxillary central incisors positioned anterior to the forehead’s FFA point and posterior to glabella.

AP Relationship of the Maxillary Central Incisors to the Forehead in Adult White Females. Andrews. 2008.

The most attractive inclination of a tangent to the labial face of the maxillary incisor crowns in profile view in relation to the true horizontal line was 85°, i.e. 5°retroclined from a perpendicular 90°inclination. The most attractive range appears to be between 80 and 90°. Excessive proclination appeared to be less desirable than retroclination.

The maxillary incisor labial face tangent: clinical evaluation of maxillary incisor inclination in profile smiling view and idealized aesthetics. Naini et al. 2019.

The most preferred smile matched with a maxillary incisor inclined 93 degrees to the horizontal line

Aesthetic evaluation of profile incisor inclination. Ghaleb et al. 2011.

If a threshold at a 50% probability of developing dehiscence was set, then a threshold for L1-NB would be limited to 2.00 mm and, equivalently, for IMPA, a threshold of 8.028. However, it appears that bone loss may begin to increase even earlier, and, thus, these 50% thresholds would be predicted to decrease ABH by about 1.92 mm and ABT by 0.50 mm, on average. For a more conservative threshold of a 10% probability of developing dehiscences with little predicted bone loss, thresholds of L1-NB movement of less than 0.86 mm or an IMPA change of less than 48 could be used.

A cone-beam computed tomographic evaluation of alveolar bone dimensional changes and the periodontal limits of mandibular incisor advancement in skeletal Class II patients. Matsumoto et al. 2020.

Allow 0.5 mm space for each mm posterior arch width change. An increased amount of space creation can be recorded in cases of rapid palatal expansion.

The Royal London Space Planning: An integration of space analysis and treatment planning. Kirschen et al. 2000.

The average gain in arch perimeter by 1 mm molar expansion was 0.73 mm in maxilla and 0.74 mm in the mandibular arch.

Maxillary and Mandibular Arch Perimeter Prediction Using Ramanujan's Equation for the Ellipse-In vitro Study. Aghera et al. 2016.

Rapid palatal expansion with the Hyrax appliance produces increases in maxillary arch perimeter at the rate of approximately 0.7 times the change in first premolar width

Arch perimeter changes on rapid palatal expansion. Adkins et al. 1990.

The ellipse is an accurate geometric model of the maxillary arch form. The average amounts of maxillary arch perimeter gained were 0.73 mm per millimeter of intermolar expansion […]

Maxillary arch perimeter prediction using Ramanujan's equation for the ellipse. Chung et al. 2015.

Regression analysis indicated that arch perimeter gain through the treatment could be predicted as 0.65 times the amount of the posterior expansion for the RME group and 0.60 times the amount of posterior expansion for the SME group.

Comparison of dental arch and arch perimeter changes between bonded rapid and slow maxillary expansion procedures. Akkaya et al. 1998.

A 1 mm increase in arch width resulted in an increase in arch perimeter of 0.37 mm. This result would be of value clinically for prediction of the effects of mandibular expansion.

An experimental study on mandibular expansion: Increases in arch width and perimeter. Motoyoshi et al. 2002.

In order to establish proper occlusion in maximum intercuspation and avoid balancing interferences, there should not be a significant difference between the heights of the buccal and lingual cusps of the maxillary and mandibular molars and premolars.

Objective grading system for dental casts and panoramic radiographs. Casko et al. 1998.

A flowchart for determining the appliance used for treating individual or segment of teeth in crossbite.

Orthodontic diagnosis and treatment planning in the primary dentition. Ngan and Fields. 1995.

Patients with maxillary transverse deficiency usually have a narrow palate and a posterior crossbite. If the maxilla is narrow relative to the rest of the face, a diagnosis of transverse maxillary deficiency is justified. […] a narrow maxilla accompanied by a narrow mandible and normal occlusion should not be considered a problem just because the jaw widths are below the population mean.

Contemporary Orthodontics. Sixth Edition. Proffit.

[…] the palatal cusps of the maxillary molars should intercuspate with the fossae and marginal ridges of mandibular molars, the buccal cusp of the mandibular premolars should intercuspate with the marginal ridges of the maxillary premolars, and the mandibular canines and incisors should intercuspate with marginal ridges of the maxillary canines and incisors. The maxillary teeth should be upright and centered in the alveolar/basal bone and coordinated with the mandibular teeth, which should also be upright and centered in the alveolar/basal bone to obtain a proper intercuspation.

Orthodontics, Current Principles and Techniques. Sixth Edition. Graber et al.

Rule of thumb: Canine extractions net approximately 15mm. The entire extraction space can be used for incisor alignment and retraction, since the posterior teeth will not move forward at all.

Rule of thumb: Ordinarily, when mandibular first bicuspids are extracted, you can expect the posterior teeth to come forward about one-third of the space (about 2.5mm on each side), leaving two-thirds of the space (about 5mm on each side) for correction of crowding and for incisor retraction.

Rule of thumb: Ordinarily, when mandibular second bicuspids are extracted, you can expect the posterior teeth to come forward about half the extraction space. This nets about 7.5mm for correction of the crowding and retraction of the anterior teeth.

Where teeth should be positioned in the face and jaws and how to get them there. Creekmore. 1997.

If all bone offered the same resistance to tooth movement, the anchorage potential of maxillary and mandibular molars would be about the same. Clinical experience shows that maxillary molars usually have less anchorage value than mandibular molars in the same patient. A common example is space closure in a Class I four premolar extraction case; it often is necessary to use headgear on the maxillary first molars to maintain the Class I relationship. The relative resistance of mandibular molars to mesial movement is a well-known principle of differential mechanics.

Orthodontics, Current Principles and Techniques. Sixth Edition. Graber et al.

All other factors being equal, the amount of incisor retraction will be less the further posteriorly in the arch an extraction space is located.

Contemporary Orthodontics. Sixth Edition. Proffit.

The 4/4 and 4/5 subgroups had mean incisor retractions of 4.2 mm and 3.7 mm, respectively, whereas the 5/5 group had a mean retraction of 2.3 mm.

Anteroposterior changes in the maxillary first molar position are shown in Table 8 […] Mean changes in the estimated molar movement were not found to be significantly different among the groups.

An Occlusal and Cephalometric Analysis of Maxillary First and Second Premolar Extraction Effects. Ong & Woods. 2001.

There is generally more forward movement of the lower molars than incisal retraction with the extraction of lower second premolars than with the extraction of lower first premolars, although a specific extraction pattern does not necessarily guarantee certain amounts of incisor retraction or lower molar forward movement.

The 4/4 group had the greatest mean incisor retraction of 2.4 mm, whereas the 4/5 and 5/5 had mean retractions of 1.4 mm and 0.5 mm, respectively.

An occlusal and cephalometric analysis of lower first and second premolar extraction effects. Shearn & Woods. 2000.

Most orthodontists agree on second-premolar extraction in cases presenting some crowding when they deliberately want to move the molars forward more than 2.5 mm. on each side, perhaps, from 3 mm. to 4.5 mm. or, in other words, they want to slip or lose part of the anchorage.

Second-premolar clinical practice. de Castro. 1974.

Group 2 [second premolar extractions] showed more mesial movement of the maxillary and mandibular first molars and less retraction of the upper and lower central incisors than group 1 [first premolar extractions].

First or Second Premolar Extraction Effects on Facial Vertical Dimension. Kim et al. 2005.

The results indicate that, on average, maxillary retraction in relation to the facial plane (N Po) differed only slightly between group 55 (mean 4.2 ± 2.4 mm) and group 44 (mean 4.7 ± 2.3 mm), with relatively more retraction for group 45 (mean 6.6 ± 2.5 mm; p < 0.05). In contrast, the mandibular incisors were retracted slightly more in group 44 than in the other two groups (p < 0.05).

Differential premolar extractions. Steyn & Harris. 1997.

The mean incisor movements were 3.3 and 2.9 mm lingually in the maxilla and the mandible, respectively. The first molars were moved mesially an average of 3.2 and 3.4 mm in the maxilla and the mandible, respectively. […] Zigzag elastics (Class II elastics and intra-arch elastics) were applied to close the remaining extraction spaces and harmonize the molar relationship.

Tooth movement after orthodontic treatment with 4 second premolar extractions. Chen et al. 2010.

Exclusion of the second molars in a maxillary 1 st premolar extraction scenario results in 50% anterior to posterior anchorage ratio, and reciprocal space closure with equal movements of the anterior and posterior segments is expected.

The Use of CBCTs to Determine Relative Anchorage Values by Measuring Maxillary Root Surface Areas within Bone. Stateson, 2019.

A four first premolar extraction pattern in Class I patients provides space that is consumed almost equally by the retraction of canines and the mesial movement of the buccal segments in the maxilla.

Bowles, Ryan Gregory , "First Premolar Extraction Decisions and Effects" (2005). Theses and Dissertations (ETD). Paper 30.

Extraction pattern is the most significant, where mean canine retraction was 3.4 mm in the first-premolar extraction sample compared to the 2.3 mm observed in the second-premolar extraction sample.

The first-premolar extraction sample’s first molar moved forward an average of 4.0 mm, whereas the first molar moved forward an average of 4.7 mm in the second-premolar extraction sample.

Turner, Robert A. , "Quantitative Analysis of First- versus Second-Premolar Extraction Effects in Orthodontic Treatment" (2007). Theses and Dissertations (ETD). Paper 268.

In the maxillary and mandibular first premolar extraction cases, […] it was found that approximately 66.5 per cent of the available extraction space was occupied by the retracted anterior segments (Table IV).

The cases involving extraction of the maxillary first premolars and the mandibular second premolars […] there was slightly less mean anterior retraction in this type of treatment as compared to the maxillary and mandibular first premolar cases. Approximately 56.3 per cent of the available extraction space was taken up by retraction of the anterior segments (Table V).

The effect of different extraction sites upon incisor retraction. Williams & Hosila. 1976.