A New Approach to Generating Object Reference Frame for Maxillary Dental Arch

Background and Purpose:

One of the important steps in orthognathic surgical planning is to restore the symmetrical alignment of dental arches with the midsagittal plane (MSP) of the whole face. Analyzing dental arch symmetrical alignment requires an object reference frame: midsagittal plane (MSP) should divide the arch evenly into right and left halves; axial (occlusal) plane should evenly pass through incisal edges and occlusal cusps; and coronal plane is mutually perpendicular to the other two. Unfortunately, there is no effective method of generating such object reference frame. In the past, we developed a triangular method for our computer-aided surgical simulation (CASS). It was straightforward and easy to implement. However, this method was only reliable when a dental arch was relatively symmetrical, the teeth were relatively well aligned, and there was no partial edentulous dentition unilaterally. Therefore, in this study, we developed a new method to mathematically generate an object reference frame for maxillary dental arch, and compared it to other methods.

Material and Methods:

Thirty maxillary digital dental models of patients with dentofacial deformities for orthognathic surgery were used [IRB(2)1011-0187x]. Three object reference frames were generated for each arch by: 1) principal component analysis (PCA) based adaptive minimum Euclidean distances (PAMED) [using central incisal edge midpoint (U0) and up to 14 paired and unpaired right and left dental landmarks; they were later automatically resampled to 400 points during the computation]; 2) standard PCA (using U0 and up to 14 paired dental landmarks; unpaired landmarks were excluded from the computation); and 3) the triangular method (using only U0 and mesiobuccal cusps of the right and left molars landmarks). The origin of the reference frame was U0. An oral surgeon experienced in orthognathic surgery evaluated the resulting 90 sets of the reference frames using the best of his clinical knowledge. He was blinded to the methods for generating the object reference frames. A binary visual analog scale (VAS) was used to evaluate the MSP and occlusal plane. In the evaluation of the MSP, ‘acceptable’ referred to a MSP dividing the arch evenly into right and left halves; ‘unacceptable’ referred to an incorrectly generated MSP. In the evaluation of the occlusal plane, ‘acceptable’ referred to an occlusal plane evenly passing through the incisal edges and the occlusal cups; ‘unacceptable’ referred to an incorrectly generated occlusal plane. Descriptive statistics was performed after the evaluation results were un-blinded.

Results:

The results showed that with the PAMED method, 100% of both MSPs and occlusal planes were correctly generated. With the standard PCA method, 46.7% (14/30) of MSPs and 100% of occlusal planes were correctly generated. With the triangular method, 93.3% (28/30) of MSPs and 83.3 % (25/30) of occlusal planes were correctly generated.

Discussion and Conclusion:

Our new PAMED can reliably generate the object reference frame for maxillary dental arch. It is capable of accounting for different clinical situations, e.g. asymmetrical dental arch, crowding, misalignment, or unilateral partial edentulous dentition, which are problematic in generating MSP with other methods. In fact, the unacceptable MSPs generated by the standard PCA are mainly due to these reasons. Triangular method performs reasonably well in generating MSP because the two posterior landmarks are digitized ‘dynamically’. Instead of statically using 2 mesiobuccal cusps of the first molars, surgeons may have to change landmarks for forming an isosceles triangle representing an arch, e.g. using mesiobuccal cusp of the second molar on the contralateral side instead. The triangular method is also problematic in generating an occlusal plane because the computation is based on only 3 landmarks instead of the entire arch.

References:

Xia-JJ et al. IJOMS, 2015;44:1431

Zelditch-ML et al. Geometric morphometrics for biologists. 2012:Elsevier