Study On The Characteristics Of Molars And The Relationship With Maxillary Sinus Floor In Patients With Unilateral Scissor Bite Of Second Molar

Objective:In this study,CBCT was used to study the tooth inclination,alveolar bone thickness,cortical bone thickness and density,relation between root and maxillary sinus floor in the molar area of patients with unilateral scissor bite of second molar,to understand the inclination of the molar in the alveolar bone,the characteristics in periodontal hard tissue and the relation between teeth root tips and the maxillary sinus floor,to provide more reference on the movable range of teeth and predict the possible risks in the process of orthodontic tooth movement for the clinician to make the treatment plan.Methods:Among the patients admitted to the Department of Orthodontics of the First Hospital of Shanxi Medical University from December 2018 to December 2021,according to the inclusion and exclusion criteria stipulated in the experiment,patients with unilateral scissor bite of second molar were selected as experimental group,and patients with normal posterior occlusion were selected as control group,with 30 cases in each group,the experimental group was divided into scissor bite side and non-scissor bite side according to the posterior occlusion.Measure the buccal lingual inclination of upper and lower molars,alveolar bone thickness at the level of 4mm,6 mm,8mm at the buccal and lingual(palatal)apically to the cementoenamel junction(CEJ),apical bone thickness of upper and lower molars,the cortical bone thickness and density at 4mm from the top of alveolar ridge,the distance from the maxillary posterior teeth root tips to the maxillary sinus floor,and the types of contacts in scissor bite side,non-scissor bite side and control group.The measurement results were statistically analyzed using SPSS22.0 software.One-way analysis of variance was used to investigate the differences of molar inclination,buccal alveolar bone thickness,lingual(palatal)alveolar bone thickness,apical bone thickness,the cortical bone thickness and density,and the distance from the maxillary posterior teeth root tips to the maxillary sinus floor in the scissor bite side,non-scissor bite side and control group,and LSD-t test was used for pairwise comparison between groups.Results:(1)In the comparison of tooth long axis inclination of upper and lower molars among all groups,the long axis inclination of maxillary second molars in scissor bite side was significantly higher than that in non-scissor bite side and control group(P<0.01),the long axis inclination of mandibular second molars was lower than that in non-scissor bite side and control group,statistically significant(P<0.05),there was no significant difference in long axis inclination of the upper and lower first molars in scissor bite side compared with non-scissor bite side and control group.In the comparison of long-axis angle of the second molar among all groups,the long-axis angle of the second molar in scissor bite side was significantly lower than that in control group(P<0.05),there was no significant difference in long-axis angle of the first molars in scissor bite side compared with non-scissor bite side and control group.(2)The buccal alveolar bone thickness at the level of 6 mm of maxillary second molars at the apically to the cementoenamel junction(CEJ)in scissor bite side decreased significantly compared with the control group,palatal alveolar bone thickness at 4mm and8 mm was larger than that in control group,statistically significant(P<0.05),the alveolar bone thickness of 6mm buccal side,4mm lingual side and apical level of mandibular second molars decreased significantly compared with the control group(P<0.05),The alveolar bone thickness of upper and lower first molars at the level of 4mm,6 mm,8mm at the buccal and lingual(palatal)apically to the CEJ,apical bone thickness showed no significant difference among all groups.(3)The thickness of cortical bone in the maxillary buccal side at 6-7 parts in scissor bite side was significantly lower than that in the control group(P<0.05),the cortical bone thickness and density in the mandibula buccal side at 6-7 parts in scissor bite side were lower than those of control group,statistically significant(P<0.05),there was no significant difference in the comparison of maxillary buccal side and mandibular buccal side at 5-6 parts among all groups,and there was no significant difference in the comparison of maxillary palatine side parts.(4)Significant difference of the cortical bone thickness and density was found among maxillary buccal area,maxillary palata area and mandibular buccal area in each group(P<0.05),with the largest cortical bone thickness at mandibular buccal area and the smallest cortical bone density at the maxillary palatine area.(5)In scissor bite side of the experimental group,86.6% of the root tips contacted the maxillary sinus floor to different extents,and 70% of the root apex extending inside the maxillary sinus,the palatal root of the first molar,mesiobuccal root and distobuccal root of maxillary second molar to the maxillary sinus floor was significantly shorter than that of the control group(P<0.05),and in scissor bite side,the lowest point of maxillary sinus floor was more in the buccal side of maxilla at the second molars.Conclusion:1.The second molar has obvious buccal or lingual inclination in scissor bite side in patients with unilateral scissor bite of second molar,and the buccal inclination of the maxillary second molar is greater than the lingual inclination of mandibular second molars.2.Alveolar bone thickness in buccal,lingual and apical parts of mandibular second molar decreased to different degrees in scissor bite side in patients with unilateral scissor bite of second molar,it is suggested that mandibular second molars have a higher periodontal risk,we should pay special attention to the periodontal state during the treatment.3.The position of maxillary sinus floor was lower in scissor bite side in patients with unilateral scissor bite of second molar,and the proportion of contact with molar root increased,therefore,appropriate orthodontic force should be applied to the molars in scissor bite side in the process of moving,to reduce root absorption and tooth movement difficulties.