| Objective: The aim of this study was to analyze the quantitative influence of the central incisor crown inclination and angulation on occlusion relationship via three-dimensional tooth alignment study.Methods: Ten volunteers with normal occlusion were selected,being 6 males and 4females,with ages between 18 years old and 25 years old,(mean age =22 years and 6months).Lateral cephalograms and 64-bit CT scans were taken,and the CT data were imported into Simplant pro 11.04 to reconstruct three-dimensional model.The 3D objects of each tooth(except for the second and third molars)were calculated separately defining thresholds for different tissues in terms of the grayscales value.In the CMF,the original three-dimension model of upper and lower tooth with normal occlusion for each sample were reconstructed.The ant/post distances between the cusp of upper and lower canines,and the first molar mesial-buccal cusp in upper and lower arch were measured.In the coordinate system,the original point was set at the central point of the line connecting the incisal edges of the maxillary central incisors.In the reposition of CMF,four groups were set up according to different incisor crown inclination and crown angulation respectively.1.Crown inclination: Group 1: maxillary incisors were moved-10°,-20°,+10°,+20°,while the mandible tooth were unchanged.Group 2: mandible central incisors were moved-10°,-20°,+10°,+20°,while the maxillary incisors were fixed.Group 3: both the tooth of maxilla and mandible were moved simultaneously in opposite direction.(maxillary-10°,mandible +10°;maxillary +10°,mandible-10°.)Group 4: both the tooth of maxilla and mandible were moved simultaneously in same direction.(maxillary-10°,mandible-10°;maxillary +10°,mandible +10°).2.Crown angulation:Group 1: maxillary incisors were moved-5°,-10°,+5°,+10°,while the mandible tooth were unchanged.Group 2: mandible central incisors were moved-5°,-10°,+5°,+10°,while the maxillary incisors were fixed.Group 3: both the tooth of maxilla and mandible were moved simultaneously in opposite direction.(maxillary-5°,mandible +5°;maxillary +5°,mandible-5°.)Group 4: both the tooth of maxilla and mandible were moved simultaneously in same direction.(maxillary-5°,mandible-5°;maxillary +5°,mandible +5°).The upper and the lower arch were aligned again based on the principles of Andrew's six keys according to different changes of central incisors.The ant/postdepositions of before and after motion were recorded and the distances between initial and varied position were measured.The results were analyzed using SPSS19.0 statistical software.Results: 1.Compared with the normal occlusion,there were obvious variations in the matched inter-arch when the incisor was varying from lingual tilting to labial tilting.(1)The upper and lower canine's and first molar's mesio-distal distance increased while the angles of upper central incisors increased.With the decreasing of the angles of upper central incisors,the canine's and first molar's mesio-distal distance decreased.According to the statistical analyse,there were no significant difference(P>0.05)when the angle of central incisor inclination changed 10°,while there were significant difference(P<0.05)when the angle of central incisor inclination changed 20°.(2)The canine's and first molar's mesio-distally occlusal distances decreased when the lower central incisors labially flared than normal.With the lower central incisors lingually inclined than normal,the occlusal distance increased.There were no significant difference(P>0.05)when the angle of central incisor inclination changed 10°,while there were significant difference(P<0.05)when the angle of central incisor inclination changed20°.(3)When upper incisors lingually inclined and lower incisors labially moved(U-10°,L+10°),the canine's and first molar's ant/post distance decreased.When maxillary incisors labially moved and mandibular incisors lingually inclined(U+10°,L-10°),the canine's and first molar's distance increased.There were significant difference in this group.(4)When both upper and lower central incisors moved in lingual inclination(U-10°,L-10°),the canine's and first molar's ant/post distance decreased.When the upper and lower central incisors labially moved(U+10°,L+10°),the canine's and first molar's ant/post distance increased.There were significant difference in this group.2.The ant/post distances between the cusp of upper and lower canines,and the first molar mesial-buccal cusp in upper and lower arch changed when the incisor crown angulation changed.(1)The upper and lower canine's and first molar's mesio-distal distance increased while the angles of upper incisors changed to 5°and 10°mesio-distally.According to the statistical analyse,there were no significant difference(P>0.05)only when the central incisor distally inclined 5°.(2)The canine's and first molar's mesio-distally occlusal distances decreased when the angles of upper incisors changed to5°and 10°mesio-distally.According to the statistical analyse,there were no significant difference(P>0.05)only when the central incisor distally inclined 5°.(3)When upper incisors distally inclined and lower incisors mesially moved(U-5°,L+5°),the canine'sand first molar's ant/post distance decreased.When maxillary incisors mesially moved and mandibular incisors distally inclined(U+5°,L-5°),the canine's and first molar's distance increased.There were significant difference in this group.(4)When both upper and lower central incisors moved in mesial and distal inclination,the canine's and first molar's ant/post distance increased.There were significant difference in this group.Conclusions: Anterior crown inclination and angulation are the foundation to observe ideal occlusion relationship.The varied relationships in canine and first molar deviated from the normal occlusion,with the different angles of incisor inclination and angulation.It will effect the normal and Class I occlusion relationship to some extent when the amount of the deviation value reached a certain degree. |
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