A Study On Acid Susceptibility Differences Between Upper And Lower Young Permanent Teeth

Objective:This study aims to explore the distribution of erosion in young permanent teeth through a clinical study and investigate the possible reasons for the differences in the susceptibility of upper and lower young permanent teeth to erosion by means of microhardness change and scanning electron microscopy(SEM)ultrastructural observation before and after erosion to Coke and Gluma respectively.In addition,the optimal etching time of both upper and lower young permanent teeth enamel is also explored.Methods20 young patients with erosive permanent teeth were collected to explore the distribution of erosion.In addition,160 fresh young permanent maxillary and mandibular premolar teeth were extracted for orthodontic reasons.The middle 1/3-neck 1/3 buccal surfaces were exposed as the observation area and the other sides were coated with acid-resistant nail polish twice.Then those enamel blocks were embedded in acrylic resin with bottoms polished by grit grinding paper.The maxillary and mandibular enamel blocks were divided into Coke group,phosphate group and blank control group.There were 9 groups of single jaws with 8 enamel blocks in each group.Coke group was divided into 5min,15 min,30min,60 min groups;phosphate group was divided into 15 s,30s,45 s,60s groups;samples in blank control group were stored in deionized water.The changes of microhardness of enamel before and after erosion were detected.Scanning electron microscopy(SEM)was used to observe the enamel surface ultrastructure changes.Result:1.The distribution of erosive upper and lower teeth varied.69.64% of upper teeth and 47.86% of lower teeth had suffered from erosion,which was significantly different(P<0.05).2.Compared with the blank control group,the microhardness of the maxillary and mandibular teeth both decreased gradually with the etching time.In addition,the change of microhardness in Coke group showed a simple linear regression relationship with the etching time.3.Microhardness results showed that the cola group had significant difference(P <0.05)between the maxillary and mandibular teeth in the other three groups except 5min group.Phosphoric acid group showed that there were statistically difference(P <0.05)in the other three groups except for 15 s group.4.SEM results showed that: enamel demineralization increased with the etching time.Its surface became rough gradually.Enamel rods varied from scale-like and honeycomb-like to crater-like holes of different sizes,and even complete irregular appearance gradually.When the enamel was over-etched,the surface showed a conspicuous structural collapse,while the hollow lesion depth became shallow gradually.At high magnification,the regular hydroxyapatite crystals became blunt and gradually lost their structure and morphology.However,SEM showed that the maxillary teeth in both the Coke group and the phosphate group had more severe erosion compared with the mandibular teeth during equal periods of time.5.When the time of Coke group exceeds 30 min or the time of phosphoric acid group exceeds 45 s,the irreversible damage of maxillary and mandibular young permanent teeth will occur,but the demineralization of maxillary teeth is more serious.Phosphoric acid group: When the maxillary teeth were eroded for 30 s and the mandibular teeth were eroded for 45 s,the SEM showed that the enamel rods were deep and regular,whose structure was the most clear and the degree of enamel erosion was moderate.Conclusion:1.Compared with the mandibular teeth,the maxillary teeth enamel is more susceptible to erosion.2.For better resin adhesive prognosis,30 s and 45 s may respectively be the best etching time for upper and lower young permanent teeth enamel.3.Accumulative drinking time of Coke within a short period of time is recommended to be less than 30 minutes to avoid irreversible damage to the teeth.