Measurement of mandibular kinematics for virtual articulation using a structured-light 3D scanner

Abstract

Mechanical articulators have been used for a long time in dentistry for simulating the temporomandibular joint movements. It is recommended to use a facebow transfer and fully/semi-adjustable articulator for the extensive reconstruction cases with vertical dimension change. The mechanical articulators have been replaced and supplemented with the virtual articulator in dental CAD/CAM systems. However in most cases the virtual articulator has been used with average-value settings. The aims of this study were to develop a new digital workflow that can be used to examine the mandibular kinematics and to determine the patient’s motion defining parameters at once by using a structured-light 3D scanner. The digital 3D image of the maxilla and the whole mandible of the three subjects were taken using a CBCT. The segmentation and 3D reconstruction of the jaws from the CBCT data were performed using an image processing software (OnDemand3D, Cybermed, Korea). In order to supplement the inaccuracy of teeth in the CT scan data, more precise digital dental casts were obtained using a model scanner (Identica, Medit, Korea). Using an image registration tool, the original 3D surface model was designated as the underlying reference model and the scanned digital casts were applied to the reference for geometric alignment. Place a lip and cheek retractor and attach four sticky non-reflective targets onto the incisors or adjacent gingiva with non-linear arrangement. Scan the oral cavity by using a facial scanner (Rexcan CS2, Medit, Korea) to obtain the 3D spatial relationship of each dental arch, and align the dental arches to the reference 3D model. In vivo tracking of the mandibular movements using the facial scanner was performed onto the Frankfort horizontal plane and the mid-sagittal plane. On the basis of the least total traveling distance during the minimal opening and closing movements, it is possible to determine the true hinge axis points. By determining the true hinge axis points, we could decide the accurate Bonwill triangle and calculate the Balkwill angle. And the patient-specific motion parameters were obtained through protrusion and lateral movements. The respective characteristics of the subjects such as deviation and occlusal interference were evaluated with the trajectory chart onto the FH plane and sagittal plane. The measurement error of this method was evaluated in static and dynamic situation on the mechanical articulator, ranged from 0.044 mm to 0.076 mm. The mandibular border movements such as maximum mouth opening that lead to conceal the tracking targets behind lip and cheeks may result in the tracking failure. Despite these limitations, we can examine easily the functional motions of the mandible within the general clinical situations. In summary, the proposed tracking method using the facial scanner offer some advantages such as more reliable due to the accuracy, simply applicable and accessible than the existing methods with transoral and extraoral devices, and most importantly, the expandibility to entire fields of dentistry without additional physical equipments.