Accuracy of a computer-aided surgical simulation protocol for orthognathic surgery: a prospective multicenter study

Abstract

Purpose: The purpose of this prospective multicenter study was to assess the accuracy of a computer-aided surgical simulation (CASS) protocol for orthognathic surgery.

Materials and methods: The accuracy of the CASS protocol was assessed by comparing planned outcomes with postoperative outcomes of 65 consecutive patients enrolled from 3 centers. Computer-generated surgical splints were used for all patients. For the genioplasty, 1 center used computer-generated chin templates to reposition the chin segment only for patients with asymmetry. Standard intraoperative measurements were used without the chin templates for the remaining patients. The primary outcome measurements were the linear and angular differences for the maxilla, mandible, and chin when the planned and postoperative models were registered at the cranium. The secondary outcome measurements were the maxillary dental midline difference between the planned and postoperative positions and the linear and angular differences of the chin segment between the groups with and without the use of the template. The latter were measured when the planned and postoperative models were registered at the mandibular body. Statistical analyses were performed, and the accuracy was reported using root mean square deviation (RMSD) and the Bland-Altman method for assessing measurement agreement.

Results: In the primary outcome measurements, there was no statistically significant difference among the 3 centers for the maxilla and mandible. The largest RMSDs were 1.0 mm and 1.5° for the maxilla and 1.1 mm and 1.8° for the mandible. For the chin, there was a statistically significant difference between the groups with and without the use of the chin template. The chin template group showed excellent accuracy, with the largest positional RMSD of 1.0 mm and the largest orientation RMSD of 2.2°. However, larger variances were observed in the group not using the chin template. This was significant in the anteroposterior and superoinferior directions and the in pitch and yaw orientations. In the secondary outcome measurements, the RMSD of the maxillary dental midline positions was 0.9 mm. When registered at the body of the mandible, the linear and angular differences of the chin segment between the groups with and without the use of the chin template were consistent with the results found in the primary outcome measurements.

Conclusions: Using this computer-aided surgical simulation protocol, the computerized plan can be transferred accurately and consistently to the patient to position the maxilla and mandible at the time of surgery. The computer-generated chin template provides greater accuracy in repositioning the chin segment than the intraoperative measurements.

Copyright © 2013 American Association of Oral and Maxillofacial Surgeons. Published by Elsevier Inc. All rights reserved.

Figures

Fig 1

Chin template includes a set of 2 surgical guides. The first guide is used to predefine the screw holes prior the osteotomy. The second guide is used to bring the chin segment to the planned position and orientation using predrilled screw holes. Both guides use the mandibular dentition as a reference to predrill the screw holes and to position the chin segment. A) A computer model of the 1st guide to define the screw holes. B) The use of the 1st guide at the time of surgery. C) A computer model of the 2nd guide to bring the chin segment to the desired position. D) The use of the 2nd guide at the time of surgery.

Fig 2

During the landmark digitization, the landmarks on the planned models are marked in green, and the landmarks on the postoperative models were marked in red. A) For the maxilla and the mandible, 3 landmarks were digitized on the occlusal surface: the midline between the 2 central incisors (central incisal embrasure), and the right and the left mesiobuccal cusp tips of the first molars. B) For chin segment, 3 landmarks were initially digitized on the chin segment of the planned model: menton and 2 points located at the right and left lower borders of the chin segment. They were then “glued” to the planned chin segment and duplicated. The first set was hidden and used later as the planned position of the chin segment. C) The chin segment in the second set was registered to the postoperative chin model using the surface-best-fit method, bring the 3 landmarks with it. D) Once registered, the chin segment of the 2nd set is deleted. The 3 landmarks from the planned model were thus transferred and “reglued” onto the postoperative model.

Fig 3

Registration for the evaluation the maxillary and the mandibular positions. A) During the registration, the planned models were kept statistic and served as a reference. In addition, we hided all the landmarks, and the bony segments that were moved during the planning, i.e. Le Fort I segment. Only the bones that had not been moved, i.e. cranial region, were visualized. B) The postoperative CT model was registered to the planned model using the surface-best-fit method. C) Once the midface was registered, all hidden landmarks for the maxilla were displayed and their coordinates were recorded. D) After the postoperative midface model was registered to the planned model, all the planned models were hidden. Only the postoperative models were visualized. E) The mandible was autorotated around the center of the condyles until the maxillary and mandibular teeth were touched. F) All the hidden landmarks for the mandible were displayed and their coordinates were recorded.

Fig 4

Registration for the evaluation of the chin segment position. A) All the chin landmarks were also displayed and recorded for the first evaluation of the chin position in relation to the entire CMF structure. B) The planned chin segment and all the landmarks were hidden. C) The postoperative mandible was registered to the planned distal segment. D) Once registered, all the hidden chin landmarks were displayed again and recorded for the second evaluation of the chin position in relation to the mandibular distal segment.

Fig 5

Computation of the angular difference between planned and postoperative outcomes. A) From frontal view: Pitch was defined as rotation around × (mediolateral) axis, roll as rotation around the y (anteroposterior) axis, and yaw as rotation around the z (inferosuperior) axis. B) From lateral view: Before computing the angular differences, the centroid O'(x',y',z') of the postoperative object was translationally registered to the centroid O(x,y,z) of the planned object. C) From lateral oblique view: The angular difference in pitch was defined as the angle between the projected x'- and the x- axes on the sagittal (y-O-z) plane. By the same token, the angular difference in roll was defined as the angle between the projected z'- and the z'- axes on the coronal (x-O-y) plane, and the angular difference in yaw was defined as the angle between the projected y'- and the y- axes on the axial (x-O-z) plane.

Fig 5

Computation of the angular difference between planned and postoperative outcomes. A) From frontal view: Pitch was defined as rotation around × (mediolateral) axis, roll as rotation around the y (anteroposterior) axis, and yaw as rotation around the z (inferosuperior) axis. B) From lateral view: Before computing the angular differences, the centroid O'(x',y',z') of the postoperative object was translationally registered to the centroid O(x,y,z) of the planned object. C) From lateral oblique view: The angular difference in pitch was defined as the angle between the projected x'- and the x- axes on the sagittal (y-O-z) plane. By the same token, the angular difference in roll was defined as the angle between the projected z'- and the z'- axes on the coronal (x-O-y) plane, and the angular difference in yaw was defined as the angle between the projected y'- and the y- axes on the axial (x-O-z) plane.

Fig 5

Computation of the angular difference between planned and postoperative outcomes. A) From frontal view: Pitch was defined as rotation around × (mediolateral) axis, roll as rotation around the y (anteroposterior) axis, and yaw as rotation around the z (inferosuperior) axis. B) From lateral view: Before computing the angular differences, the centroid O'(x',y',z') of the postoperative object was translationally registered to the centroid O(x,y,z) of the planned object. C) From lateral oblique view: The angular difference in pitch was defined as the angle between the projected x'- and the x- axes on the sagittal (y-O-z) plane. By the same token, the angular difference in roll was defined as the angle between the projected z'- and the z'- axes on the coronal (x-O-y) plane, and the angular difference in yaw was defined as the angle between the projected y'- and the y- axes on the axial (x-O-z) plane.

Fig 6

Statistical schematic charts. A) For the maxillary and the mandibular evaluation. B) For the chin evaluation.

Fig 6

Statistical schematic charts. A) For the maxillary and the mandibular evaluation. B) For the chin evaluation.

Fig 7

Unwanted bimaxillary retrusion was caused by a combination of incorrect recording of central relation and the maxillary surgery first. A) In this unilateral bimaxillary retrusion patient, the midline was shifted to one side (>2 mm). B) From the preoperative CT model, the condyle appeared in the central relation. C) From the postoperative CT model, the condyle also appeared in the central relation. D) When the two models were registered, it clearly demonstrated that the preoperative condyle (yellow) was protruded then the postoperative condyle (blue).