Study On The Stability Of Palatal Bone-supported Rapid Expansion With Protraction Devices

Objective:To establish and use the the three-dimensional finite element model of skeletal Class III patient with maxillary hypoplasia,study and summarize the laws of stress distribution,displacement trend and stability of the craniomaxillary complex during the protraction with several implants in different positions.The aim is to provide theoretical reference for palatal bone-supported rapid expansion with protraction clinical application.Method:Select a 12-year-old male volunteer with skeletal Class ? malocclusion deformity of maxillary hypoplasia.Use a spiral CT to perform a head scan to obtain DICOM format image data.Use Mimics 20 software to read the DICOM format image data and import it.Geomagic Studio 2014 software is used to optimize the model and establish the head curved surface model;use the special 3D mechanical drawing software NX 12 to draw several bone-supported rapid expansion with protraction device models.Import the created 3D model into Ansys Workbench 19.2software.The protraction force of-30° to the horizontal plane is applied at the canine.The magnitude is 5 N.At the same time,put the displacement at the palate perpendicular to the palatal midline by 0.25 mm outward to simulate an instant expansion force.Analyze the stress distribution and displacement trend of the cranial and maxillary complex,as well as the stress on the device and the implant nails,so as to analyze the stability of the device.Result:For the whole model of skull,the orthopaedic force under the three conditions is directly applied to the maxilla,which makes the maxilla suffer the most stress.The maximum stress area occurs at the implant site of maxilla.The dental effect is less.In the second condition,maxilla receives the maximum stress when the sagittal positionof the two front row implant screws placed at the line between the second premolar and the first premolar.The maximum stressed area of zygoma is the orbital process area connected with the orbital surface of maxilla.For the protraction effect,the displacement of anterior teeth area is the largest in the three conditions.The forward displacement trend from anterior teeth cut edge to the top of the frontal bone is gradually reduced,and converts to the displacement trend of the back.The forehead as an anchorage appears slight rotation in the opposite direction.The displacement in the Y direction is also gradually increased with the forward displacement of the implant;in the Z direction,the maxilla are all distribute anti-clockwise rotation of maxilla.There is no significant difference between groups.Secondly,zygomatic bone,which is closely connected with maxilla,has the same direction of displacement trend.While frontal bone and temporal bone,which are far away from the point of action,have small displacement and relatively stable position.Three kinds of palatal bone-supported rapid expansion with protraction devices can be fixed on the palate to provide anchorage.In three conditions,the equivalent stress of the appliance increases in turn with the forward position of the implants.The stress of the front row implant nails decreases in turn.In the third condition,the stress of the appliance is the maximum.The stress difference between the front row nails and the back row nails is the largest,which is not beneficial to the stability of the appliance.The stress of the front row nails in condition 1 is larger than that of the back row nails,and condition 2 is opposite.Conclusion:1.The closer the implant nails are to the front of the palate,the greater the sagittal displacement of the maxilla,which can effectively promote the growth of the maxilla to the front.It improves the degree of the middle of the face depression and the reverse overjet of the teeth.2.The stress of the appliance increases with the forward of the implant location.The force of the implants are also different.The more forward the implant,the greater the dispersion,and the more unstable the device is.