The following factors should be considered when deciding on an orthodontic extraction: 1. Tooth crowding is derived from direct measurement of the jaw model to derive the degree of crowding. Each 1mm of crowding requires 1mm of arch space to be released. The greater the crowding, the greater the possibility of tooth extraction. 2. Arch protrusion moves the anteriorly protruding incisors lingually and requires an arch gap to restore them to their normal position. For every 1mm movement of the incisal edge of the lower incisor to the lingual side, an arch gap of 2mm is required. The more anterior the incisors are, the greater the possibility of extraction. 3, Spee curve height The vertical distance between the buccal cusp of the second bicuspid and the plane formed by the buccal groove of the lower anterior teeth and the second permanent molar is measured on the mandibular arch model as the Spee curve height. For each 1 mm Spee curve leveling, 1 mm of arch clearance was required. 4. The anterior displacement of the supporting molar should be taken into account when determining the extraction gap occupied by the anterior displacement of the molar. If extraction is used for orthodontic treatment, the anterior displacement of the supporting molar is inevitable when closing the gap. Orthodontists use different measures to control the amount of anterior displacement of the molar: when using strong support, the anterior displacement of the molar takes up no more than 1/4 of the extraction gap; when using moderate support, it is 1/4-1/2; when using weak support, it is at least 1/2. 5, vertical bone facial type There are three cases of vertical facial development, and the steepness of the mandibular plane is usually used to distinguish between the three. The normal vertical osteofacial type has an average SN-MP angle of 34.3° (±5°) and an average FH-MP angle of 27.2° (±4.7°). When the SN-MP angle is greater than 40° or the FH-MP angle is greater than 32°, the vertical development is excessive, and the case is called “high angle”. The SN-MP angle is less than 29° or FH-MP angle is less than 22°, reflecting the underdevelopment of vertical development, which is called “low angle” case. On the issue of orthodontic extraction, there are different considerations for high-angle cases and low-angle cases: the extraction criteria for high-angle cases can be relaxed appropriately, while the extraction for low-angle cases should be strictly controlled. This is because: (1) The chin of high-angle cases is mostly retracted, and it is advisable for the incisors to be more upright at the end of treatment to maintain a coordinated nose-lip-chin relationship; more upright incisors can also compensate for the vertical skeletal imbalance and establish a suitable morphological and functional relationship between the upper and lower incisors. In low-angle cases, the situation is just the opposite, most patients have anterior chin protrusion, and the incisors should be compensated with some lip tilt, which is not only beneficial to the facial shape but also to the function of the incisors. (2) In high-angle cases, the masticatory muscle force is weak, the bone density of the jaw bone is low, the supporting molar is easy to move forward and elevate, and the closure of the extraction gap is easier; at the same time, the forward movement of the molar is beneficial to the correction of the anterior teeth opening tendency that often accompanies high-angle cases. On the contrary, in low-angle cases, the chewing force is strong, the bone density is high, the supporting molar is not easily moved forward and elevated, and the closure of the extraction gap is mainly completed by the distal and middle movement of the anterior teeth, while the excessive inward movement of the anterior teeth is not conducive to the correction of the deep overlap of the anterior teeth that often accompanies low-angle cases. (3) When aligning the teeth by pushing the molar backward or expanding the arch, it can cause the opening of the mandibular plane angle, which has a negative impact on the facial shape and anterior overlap of the high angle cases, but is more favorable for the low angle cases. There is also a difference between high angle and low angle cases when deciding the position of the extracted teeth: if the teeth are extracted from the back of the high angle cases, it is beneficial to control the anterior opening of the anterior teeth; if the teeth are extracted from the low angle cases, it is advisable to extract the teeth from the front of the arch, which is not only easy to close the extraction gap, but also beneficial to open the occlusion. 6. Sagittal bone facial type When the sagittal relationship between the maxillary and mandibular arches is coordinated and the ANB angle is normal, if tooth extraction is needed, usually the upper and lower arches are extracted symmetrically at the same time (unless the Bolten index is not adjusted). If the sagittal relationship between the upper and lower arches is not harmonized, the difference between the upper and lower arches should be considered when deciding whether to extract the teeth. class II malocclusion has a relatively anterior maxillary arch and a relatively posterior mandibular arch, and the ANB angle is large. to compensate for this skeletal malocclusion, the lower incisors can be slightly labially inclined at the end of treatment, and the mandibular extraction should be cautious. class III malocclusion, on the contrary, has a small ANB angle due to the relative underdevelopment of the maxilla and the relative overdevelopment of the mandible. At the end of treatment, the upper incisors are allowed to be slightly labially inclined and the lower incisors are slightly lingually inclined to compensate for Class III skeletal malocclusion, and extractions in the maxilla are particularly cautious. 7, facial soft tissue lateral appearance in determining the extraction or non-extraction orthodontic treatment, the analysis and evaluation of the soft tissue lateral appearance, especially the nasal-lip-chin relationship, should not be neglected. The following two measurement indicators are commonly used. (1) The distance from the upper and lower lips to the aesthetic plane The aesthetic plane is composed of the line between the tip of the nose and the front point of the soft tissue chin. (2) The angle formed by the nasolabial angle nasal small column point, the lower nasal point and the upper lip convexity. Another factor that must be considered when determining whether or not to extract teeth with crowded teeth, especially complex crowding, is growth and development. The growth and development assessment should determine the patient’s current stage of development and select the appropriate treatment. Simple crowding can be treated during the rapid growth period of youth; complex crowding with intermaxillary disorders should be treated 1-2 years prior to the rapid growth period if jaw growth control is considered. Growth assessment also includes prediction of the patient’s craniofacial growth during orthodontic treatment. The use of average growth data from normal fits for growth prediction in misaligned individual patients can be biased due to differences between normal fits and misalignments and interindividual differences.