Construction Of Point-based Mandibular Median Sagittal Plane Of Adult Patients

Objective: To develop a new point-based mandibular median sagittal plane(MMSP)by selecting from varied mandibular planes which were formed by combining bony landmarks derived from multiple regions of the mandible,which is for clinical screening and classifying various types of facial asymmetry.Methods: Forty adult patients without any orthodontic treatment experience were randomly selected,and evenly divided into two groups by the degree of chin deviation: mandibular asymmetric group(MA group)and mandibular symmetric group(MS group).Based on cone beam computed tomography(CBCT)data,three dimensional(3D)mandibular models were rebuilt by Mimics 19.0 software.In this software,3D coordinates of 4 central landmarks and 6 pairs of lateral landmarks derived from mandibular surface were collected.Then 120 candidate planes were constructed by point-based method,and compared with the reference plane formed by the iterative closest point(ICP)algorithm.All the planes were reordered and screened by geometric indices such as angle,distance and area in MS group.In addition,the clinical symmetrical indices such aspoint-to-plane distance,surface area difference,volume difference,and 3D deviation ratio of screened planes were compared in MA and MS groups,SPSS 20.0 software was used for statistical analysis.Results:(1)The ranking sequence of primary screening process suggests: in MS group,the planes co-constructed with landmarks from three mandibular regions(symphysis,mandibular angle and coronoid process)were the top 5% in all the planes.The angle between ICP plane and B-Cor-Golat plane co-constructed by supramentale(B point),coronoid superius(Cor point)and gonion lateralis(Golat point)is the smallest(0.589 degrees);the point-to-plane distance between the ICP plane and Pog-Sim-Golat plane co-constructed by pogonion(Pog point),simoid notch(Sim point)and gonion lateralis(Golat point)is the smallest(0.416 mm).(2)The results of further screening process suggests: in both MS and MA groups,the differences of point distance,area difference and volume difference between B-Cor-Golat plane and ICP planes was the smallest,and there was no significant difference(P > 0.05),while there were significant differences between the rest screened planes and the reference plane(P<0.05).(3)The interval ratio index of 3D deviation shows that the difference between B-Cor-Golat plane(MS group:94.77±1.87%;MA group:87.95±6.63%)and ICP plane(MS group:96.26±1.46%;MA group:89.49±6.65%)was the smallest,and the average value was less than 2%.Conclusion:(1)The symmetry of three regions(symphysis,mandibular angle and coronoid process)is relatively good in all regions of mandible,and the location of anatomical landmarks derived from above-mentioned regions is relatively stable and repeatable.(2)The differences of each indices between B-Cor-Golat plane and ICP plane are the smallest,it suggests that the B-Cor-Golat plane can be used as the mandibular median sagittal plane to reflect the symmetrical degree of the mandible well.