A Novel Method of Locating the Transverse Axis of Rotation of the Mandible in 3D Cone Beam Computed Tomography

Objectives: To assess whether a transverse axis of rotation of the mandible could be accurately located in a 3D CBCT scan of a skull model by using two jaw relationship registrations recorded with the mandible 1mm apart.

Methods: A stylised skull model incorporating a hinge axis of rotation of the mandible was fabricated using 3D printing technology. Two jaw relationship registrations were recorded in silicone with the teeth closed and 1mm apart. The skull model was imaged using 3D CBCT with the first record in situ. Maxillary and mandibular dental casts were fabricated using 3D printing technology. These were digitized using a 3D optical scanner with each of the 2 records in situ. The acquired dental models were aligned to the skull model using landmarks. The mandible model was separated, duplicated and re-aligned to the 2nd mandibular position using a ‘best fit’ registration to the corresponding dental cast. The two models of the mandible were exported to 3D inspection software and surface comparisons of the lateral surfaces of the condyles were made. The location of the hinge axis on each condyle was determined by the area showing no surface deviation between the two mandibular positions.

Results: Deviation of the estimated to the original hinge axis location was less than 1mm. This error could be attributed to 3D printing error, 3D imaging error and registration errors.

Conclusion: This proof of concept study supported the case for a clinical validation of the method described prior to implementation. Advantages of this method include its simplicity, no need for use of any other equipment, little extra clinical time required to record two jaw relationships, and use of a single 3D CBCT scan.