Virtual surgical planning and orthognathic procedures

The standard Houston/computer-aided surgical simulation (CASS) planning protocol (Materialise, Leuven, Belgium) for orthognathic surgery was followed, as described by the manufacturers and in previous investigations. The facial skeleton was digitally recreated from a cone beam computed tomography (CBCT) scan (NewTom 3G; QR s.r.l., Verona, Italy) with standard settings (field of view (FOV) 20 cm × 20 cm; 110 kV; radiation exposure 59 μSV, according to the 2005 International Commission on Radiological Protection tissue weighting factors ). A preoperative CBCT was taken just prior to the surgery, after orthodontic decompensation retained with passive wires. Virtual surgical planning was carried out using Dolphin 3D (Dolphin Imaging and Management, Chatsworth, CA, USA) in an online meeting between the surgeon and a biomedical engineer at 3D Systems–Medical Modeling.

Following planning, surgical splints were produced using CAD/CAM printing techniques ( Fig. 1 ), and the treatment plan was finalized by marking the inferior alveolar nerve and the distances of proposed movements between bony segments.

Surgical splint design. CAD/CAM surgical splint with at height of 4 mm and dental impressions according to the planned final occlusion (white arrows indicate occlusal points without interference from the splint).

Four types of surgery were planned and performed: bimaxillary surgery, bimaxillary surgery with segmentation of the maxilla, bimaxillary surgery with genioplasty, and bimaxillary surgery with segmentation of the maxilla and genioplasty. Orthognathic surgery was carried out according to the treatment plan using surgical splints and surgical calipers. Surgical procedures followed well-recognized protocols for orthognathic surgery, with bilateral sagittal split and Le Fort I osteotomies. Segmentation of the maxilla was performed as a three-piece segmentation between the lateral incisor and the canine bilaterally. The vertical movement was controlled by external measurements from the right medial canthal ligament and internal measurement relative to the osteotomy according to the planned movement. The sagittal split was fixed with three bicortical screws along Champy’s line bilaterally, and the maxilla was fixed with four L-shaped plates. Bony interferences were removed and interposed in the osteotomies. The surgical splint was fixed for 6 weeks in segmented procedures and for 1 week in non-segmented procedures. All surgeries were performed according to the standardized departmental approach by three surgeons (TT, EA, and PT).

Follow-up 1 week after surgery included a clinical examination and a CBCT scan. The surgical splint was still in place during examination and scanning.

Evaluation of accuracy and precision

The main outcome variable of interest was the difference between the virtual planned movement and the surgical movement obtained. Alignment and placement of reference points and measurement of the planned and postoperative movement were performed by 3D Systems–Medical Modeling and inserted into a spread sheet. The first author performed the data analysis.

The following data were collected retrospectively from the patient journal: patient age, surgeon, type of surgery, date of surgery, complications during surgery, and intraoperative alterations to the plan.

After surgery, CBCT scans were taken of the patients, and the planned outcome was superimposed on the postoperative outcome to determine how well the virtual surgical plan translated to the surgical result. The postoperative CBCT scans were converted to 3D surface files using both automatic and manual segmentation techniques (Mimics; Materialise, Leuven, Belgium). The virtually planned anatomy was aligned to the postoperative anatomy using best-fit surface alignment. The maxillary anatomy was aligned using the non-operated midface as its own reference ( Fig. 2 ). For the mandible (as this is a mobile segment), the anatomy was compared by aligning the condylar head position and the vertical position of the anterior mandible. Additionally, the genioplasty was compared using best-fit alignment of the distal mandible. When the condylar head position was assessed, the maxillary anatomy was aligned as described above and the mandible was brought along in the scanned relationship with the midface.

Model alignment. Alignment of the virtually planned model with the postoperative model in a bimaxillary procedure with segmentation of the maxilla. The models are aligned above the maxillary osteotomy and the base of the skull. The differences between the red and green model illustrate the differences between the virtual surgical plan and the postoperative model of the maxilla and mandible. Note a lack of transverse expansion compared to the virtual surgical plan. Green: outcome planned by 3D virtual surgical planning. Red: postoperative outcome evaluated by CBCT at follow-up 1 week postoperatively. X: anteroposterior direction. Y: mediolateral direction. Z: superoinferior direction. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Reference points were placed on the top of the mesiobuccal cusp on the first molar on each side and in the midline at the edge of the central incisors, both in the maxilla and the mandible ( Fig. 3 ). Mean linear differences between the virtual surgical plan and postoperative outcome were calculated as the mean difference of the three reference points in all three dimensions. The linear difference for the placement of the chin segment was calculated at a single point, as the linear difference in menton. All measurements were recorded in relative numbers according to the corresponding axis to evaluate any systematic differences in direction. The positive values of the axis were left (mediolateral axis), anterior (anteroposterior axis), and superior (superoinferior axis).

Reference points and segmentation of the maxilla. Reference points were placed on top of the mesiobuccal cusps on the anterior molars and in the midline of the central incisors. The mean of the coordinates to the reference points created a fourth, central reference point, marked with a blue star. Segmentation of the maxilla was done according to the dotted line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Differences in orientation were calculated as the difference between a representative reference point and the mean linear difference of the maxilla or mandible ( Fig. 4 ). Roll was evaluated around the anteroposterior axis as the individual superoinferior difference of the mesiobuccal cusp of the first molar relative to the roll. A positive difference in roll meant superior movement of the left molar and inferior movement of the right molar. Pitch was calculated around the mediolateral axis as the difference in superoinferior direction between the midline of the incisal edge and the mean linear difference. A positive pitch meant an upward movement of the central incisors and a decrease in the occlusal plane angulation. Yaw was calculated around the superoinferior axis as the transverse difference between the midline of the incisal edge and the mean linear difference. A positive difference in yaw meant a rotational displacement of the incisors to the left. The orientation of the chin segment was not evaluated. This method accounts for all six degrees of freedom in a 3D model, as proposed by Xia et al. and Hsu et al.

Rotational differences. Rotational differences were measured as the difference between the mean linear movement at the central reference point (blue star) and the movement of the relevant reference points (green cross). Pitch: Rotational difference around the mediolateral axis (occlusal plane). Roll: Rotational difference around the anteroposterior axis (occlusal cant). Yaw: Rotational difference around the superoinferior axis. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The transverse expansion of segmented maxillary procedures was calculated between the mesiobuccal cusps on the first molars. The difference between the virtual surgical plan and the postsurgical outcome was evaluated against the planned expansion to visualize correlations.

Statistical analysis

Normality of the overall distribution was evaluated visually with a box-and-whisker plot ( Fig. 5 ). Non-parametric statistical hypothesis testing was performed due to the low number of participants in subgroups. The evaluation of heterogeneity with regard to surgeon was performed using Kruskal–Wallis analysis for the maxilla, mandible, and menton in all three dimensions. If no difference existed, the data were to be pooled and analyzed as one database; if differences existed, the results were to be investigated individually. The mean linear difference was used as a measure of accuracy and the standard deviation as a measure of precision. Additional statistical evaluation was provided using 95% confidence intervals. Analysis of the differences between the virtual plan and the postoperative outcome was performed by Wilcoxon signed-rank test, while the Mann–Whitney U -test was used to analyze differences between the dependent groups when comparing precision between the independent subgroups. Multiple tests were performed, and therefore chance findings may occur. The statistical significance level in all tests was set at P ≤ 0.05. The clinical success criterion was set at a difference of less than 2 mm between the virtual surgical plan and the actual surgical outcome, as proposed in previous studies.

Linear differences. Box-and-whisker plots of linear differences: mediolateral, anteroposterior, and superoinferior.

Complications

Intraoperative complications occurred and were resolved during surgery as close to the planned outcome as possible. In one surgery, the intermediate surgical splint did not fit. The final splint was mounted and the segments had to be placed by hand according to the planned position. To date, this is the only intermediate surgical splint that did not fit. However, one intermediate and one final surgical splint were judged by the surgeons to have a poor fit, but the fit did not interfere clinically with the planned outcome.

Moreover, an unfavourable split (or bad split) occurred during two surgeries, both on the left side during the sagittal split. In one case, the split was repaired using two bicortical screws between the proximal segment and the free segment at Champy’s line, and two bicortical screws between the free segment and the dental segment at the base of the mandible. In the other case, the intermediate segment was fixed to the dental segment with an internal fixation plate, but the condylar segment could not be fixed and was left floating. When the condyle was left floating, the split did not have a negative impact on the lateral placement of either maxilla or mandible. However, the maxilla was positioned 2.7 mm posterior compared to the virtual surgical plan.

Whenever intraoperative complications occurred, the data were compared with mean differences and standard deviations. Intraoperative complications were rectified within one standard deviation of the mean; therefore all procedures were included in the final analysis.

Accuracy and precision

Variation in differences within and between surgeons was calculated by Kruskal–Wallis analysis, but no statistical significance was observed. The P -values were evenly distributed and ranged from P = 0.07 to P = 0.91. No heterogeneity between surgeons was noted. Therefore, the results of the procedures were pooled and analyzed as one group.

All mean accuracies, measured as the mean linear difference for the maxilla, mandible, and chin segment in all three planes, were within 0.5 mm ( Table 1 ). The mean precision, measured as the standard deviation, had the smallest deviation superoinferiorly, followed closely by mediolateral deviation, and finally the largest deviation was found anteroposteriorly. Figure 5 shows the distribution of the mean linear differences between the planned and postsurgical outcomes as a box-and-whisker plot.

Table 1

Linear differences. Overall linear positional differences between the planned and postsurgical outcomes.

Segment	Axis	Linear difference	SD	95% CI Upper limit	95% CI Lower limit	P -value a Planned vs. outcome
Maxilla	Mediolateral	−0.31 mm	1.41 mm	0.20 mm	−0.81 mm	0.284
	Anteroposterior	−0.39 mm	2.38 mm	0.46 mm	−1.24 mm	0.237
	Superoinferior	−0.05 mm	1.37 mm	0.45 mm	−0.54 mm	0.644
Mandible	Mediolateral	−0.14 mm	1.40 mm	0.36 mm	−0.64 mm	0.910
	Anteroposterior	0.28 mm	2.08 mm	1.02 mm	−0.47 mm	0.658
	Superoinferior	0.13 mm	0.73 mm	0.39 mm	−0.14 mm	0.614
Menton	Mediolateral	−0.16 mm	1.89 mm	0.56 mm	−0.89 mm	0.951
	Anteroposterior	−0.16 mm	3.52 mm	1.20 mm	−1.51 mm	0.977
	Superoinferior	−0.41 mm	1.07 mm	0.01 mm	−0.82 mm	0.110

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