| Objective:The morphological characteristics of pulp chamber were measured by anatomical methods, in order to provide a reference to the preparation of vital teeth. The residual dentin thickness of single-canal or two-canal first maxillary premolars and single-canal first mandibular premolars were measured before and after post preparation, the data from which can help clinicians choose the proper sizes of peeso drill, making sure the roots have enough fracture strength.Methods:First premolars extracted for orthodontic reason were collected in Shandong, and 30 first maxillary premolars and 30 first mandibular premolars were choosen from them as specimens. All the teeth were vertically cut along buccal-lingual direction from occlusal surface to cervix, and transected at the cemento-enamel junction and 4mm short of the apex. The thickness of hard tissue at the cervix, and that adjacent to buccal and lingual pulp horns, and that from fossa to pulp chamber roof were measured by vernier caliper with the accuracy of 0.02mm. Four walls of 4mm short of the apex were measured before and after post preparation with respective peeso drills 1#～3#.Results:1. The results of pulp chamber in first maxillary premolars(X±S):A:buccal wall of pulp chamber at the cervix:(2.379±0.216)mmB:palatal wall of pulp chamber at the cervix:(2.448±0.203)mm C:mesial wall of pulp chamber at the cervix:(1.710±0.199)mmD:distal wall of pulp chamber at the cervix:(1.863±0.174) mmE:buccal cusp to buccal pulp horn:(5.087±0.496)mmF:palatal cusp to palatal pulp horn:(5.740±0.348)mmG:fossa to pulp chamber roof:(4.665±0.490)mmL:buccal pulp horn to buccal wall:(3.183±0.136)mmM:buccal pulp horn to mesial wall:(2.822±0.386)mmN:buccal pulp horn to distal wall:(2.815±0.391)mmO:palatal pulp horn to palatal wall:(3.195±0.184) mmP:palatal pulp horn to mesial wall:(2.453±0.317)mmQ:palatal pulp horn to distal wall:(2.627±0.317) mm2. The results of pulp chamber in first mandibular premolars(X±S):A:buccal wall of pulp chamber at the cervix:(2.412±0.255)mmB:lingual wall of pulp chamber at the cervix:(2.328±0.140)mmC:mesial wall of pulp chamber at the cervix:(1.902±0.124)mmD:distal wall of pulp chamber at the cervix:(1.916±0.129) mmE:buccal cusp to buccal pulp horn:(4.682±0.573)mmF:lingual cusp to lingual pulp horn:(5.129±0.691)mmG:fossa to pulp chamber roof:(4.627±0.603)mmL:buccal pulp horn to buccal wall:(3.302±0.273)mmM:buccal pulp horn to mesial wal:(3.002±0.442)mmN:buccal pulp horn to distal wall:(2.960±0.446)mmO:lingual pulp horn to lingual wall:(2.518±0.244) mmP:lingual pulp horn to mesial wall:(2.227±0.345)mmQ:lingual pulp horn to distal wall:(2.310±0.367) mm3. For two-canal first maxillary premolars and single-canal first mandibular premolars, the average residual mesial and distal thickness of root walls 4mm short of the apex were less than 1mm after post preparation by 3# peeso drill. The buccal root had nonsignificant differences (P>0.05). The palatine root and the first mandibular premolars had significant differences (P<0.05). For single-canal first maxillary premolars, the average residual mesial and distal thickness of root walls 4mm short of the apex were less than 1mm after post preparation by 2# peeso drill. The distal wall had significant differences (P<0.05).Conclusions:1. In first premolars, buccal pulp horns were high, closed to the middle 1/3 of the crown, and lingual pulp horns were low, closed to the neck 1/3 of the crown. The distances from buccal and lingual pulp horns to mesial and distal walls were less than those to buccal and lingual walls. The contact areas were near to the occlusal surface buccally. we should pay attention to the middle 1/3 of proximal walls, near the buccal areas, in order to avert pulp exposure.2. In post preparation, in order to ensure the buccal root of two-canal maxillary premolars to have enough fracture strength at root canal 4mm short of the apex, the maximum size of peeso drill was 3# (1.1mm), and for the palatine root and first mandibular premolars, the maximum size was 2# (0.9mm), and for single-canal maxillary first premolars, the maximum size was 1# (0.7mm).
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