Accuracy of analytic model planning in bimaxillary surgery

Abstract

The purpose of this study was to assess our method of analytic model planning in achieving a planned maxillary advancement for the correction of a dentofacial deformity. A consecutive series of 20 patients who underwent bimaxillary orthognathic surgery, at a minimum, were included in the study group. For each study subject, consistent analytic model planning with splint fabrication was used to establish the desired horizontal repositioning of the maxilla. Using preoperative and 5-week postoperative lateral cephalometric radiographs, an analysis was designed to assess the difference between the planned and actual advancement of the maxilla. The average difference between the planned and actual 5-week postsurgical advancement of the maxilla was 0.6 mm (range 0.2–1.0, P > 0.05). There was a strong correlation between the two data sets ( R = 0.96). The results of the study indicate that the described method of analytic model planning is reliable (within 1 mm) in achieving the planned level of maxillary advancement in bimaxillary orthognathic procedures.

Precise surgical repositioning of the maxilla according to patient-specific functional and aesthetic objectives is essential in orthognathic surgery. This is more problematic in bimaxillary procedures when three-dimensional repositioning of the maxilla is required and there are not adequate biologic landmarks at operation. Any inaccurate maxillary repositioning is then compounded when the mandible is surgically repositioned to the newly located upper jaw. Therefore, in bimaxillary surgery, accurate repositioning of the maxilla is fundamental and must reflect the desired presurgical objectives.

The most reliable method to transfer the planned maxillary movements at surgery is through the use of a prefabricated splint. The effectiveness of the prefabricated intermediate splint in turn depends on accurate model planning methods that capture the desired maxillary reorientation (pitch, yaw, roll), and movements (vertical, horizontal), according to surgical objectives. One of the essential parameters from which to judge the effectiveness of model planning is the accuracy of the horizontal (sagittal) repositioning of the maxilla.

Published studies in which an intermediate splint is utilized to achieve the preferred horizontal maxillary repositioning in the operating room, report a wide range of variation between the planned and the actual outcome. Therefore, the reliability of analytic model planning remains in question. The purpose of this study was to assess our method of analytic model planning used in everyday practice to achieve the desired horizontal advancement of the maxilla.

Materials and methods

A consecutive series of patients ( n = 20) who underwent, at a minimum, Le Fort I osteotomy with advancement and sagittal ramus osteotomies of the mandible for correction of a dentofacial jaw deformity, were included in the study group. The sample consisted of females ( n = 9) and males ( n = 11), with a mean age of 21 years (range 14–45 years). The surgery was carried out by a single surgeon (JCP) at a consistent university hospital. Perioperative orthodontic treatment was completed to remove dental compensation and to align the arches in all patients. Consistent surgical technique, and inpatient and outpatient management through the initial 5 weeks of convalescence was carried out. All study subjects underwent: (1) preoperative and 5-week postoperative lateral cephalometric radiographs of acceptable quality with a metric reference frame for analysis; (2) consistent presurgical facial analysis by the surgeon, including millimetre decisions concerning planned horizontal and vertical repositioning, and reorientation (pitch, roll, yaw) of the maxilla; (3) consistent analytic model surgery planning with splint construction (intermediate and final); (4) use of the prefabricated intermediate splint to establish the maxillary positioning after Le Fort I osteotomy prior to plate and screw fixation; and (5) use of consistent intraoperative landmarks to confirm vertical orientation (medial canthus to maxillary central incisor).

Determination of the preferred surgical repositioning of the jaws was based on a combination of direct visual examination, analysis of the lateral cephalometric radiograph, and analysis of the profile facial photograph taken during a presurgical office visit. The extent of preferred maxillary horizontal movement in millimetres was ultimately assessed by the surgeon with the patient in the natural head position (NHP) during the direct visual examination. Fabrication of an intermediate splint that captures the preferred maxillary horizontal and other vector movements (i.e., vertical, pitch, yaw, and roll) was made using consistent analytic model surgery planning methods.

Methods of analytic model planning

Our routine analytic model planning requires: face-bow registration of the maxillary plane in relationship to the NHP (the #8645 Quick Mount Face-Bow was used; Whip Mix Products, Louisville, KY, USA), a chin point guidance technique to capture the occlusion in centric relation (CR), and alginate impressions of the maxillary and mandibular dentition. In the laboratory, the maxillary cast is mounted with the two face-bow arms parallel to the upper arm of the semi-adjustable articulator (the #8500 Articulator was used; Whip Mix Products, Louisville, KY, USA) to establish the maxillary plane in relationship to the NHP. The mandibular cast is then mounted to the maxillary cast using the CR inter-occlusal registration obtained in the office setting. The vertical height of the articulator pin is established and then maintained during the model planning. In six of 20 cases, inferior maxillary repositioning was also planned (from 1 to 4 mm). Therefore, the height of the articulator pin was increased as needed prior to intermediate splint construction.

Four consistent reference points are marked on each maxillary cast. These reference points are used for baseline measurements on the Erickson model platform (Erickson Model Block and Platform Model Measuring Kit – SAM, Great Lakes Orthodontics Products, Tonawanda, NY, USA). The reference points used are the mesial buccal cusp of each maxillary first molar and the midpoint of each maxillary central incisor edge. The prescription for maxillary movements determined by the surgeon is then added to these baseline measurements. The maxillary model is released from the articulator and repositioned according to the new measurements. The desired surgical repositioning of the maxilla is confirmed using the Erickson model platform. The intermediate splint is then fabricated using the static mandibular position on the articulator as a platform. Once the maxilla is repositioned on the articulator and the intermediate splint has been made, the mandibular cast is separated from the articulator and then resecured to the maxilla in the ‘final’ occlusion. A final splint is fabricated for use at surgery to control the ultimate position of the mandible.

At operation, the planned vertical facial changes are confirmed using external reference landmarks (e.g., medial canthus to mid-maxillary central incisor distance in mm on each side).

Methods used to measure results

For each study patient, a presurgical standard cephalometric radiograph was obtained for later analysis. At 5 weeks after surgery, a similar standard cephalometric radiograph was also taken. This was timed to coincide with removal of the surgical splint.

For each study patient, the maxillary cast was mounted on the articulator with respect to the NHP. The horizontal axis of the NHP has been shown to closely approximate the radiologic landmark of Frankfurt horizontal (FH). Therefore, for the purposes of this study, a cephalometric analysis using a reference plane parallel to FH was designed to measure the linear change achieved at the maxillary incisal edge.

Four landmark points (sella, nasion, porion, and orbitale) that were unchanged from the orthognathic surgery were used in the analysis. Therefore these points could be used to create a consistent reference line from which a linear measurement (e.g., change in horizontal position of the maxillary incisors) could be made prior to and after surgery. To confirm the accurate marking of these landmark points by the investigators, we used a reference angle between sella–nasion (SN) and FH in the sequential radiographs (pre- and postsurgical lateral cephalograms). In this way, we were able to confirm the consistency of the sella, nasion, porion, and orbitale landmark points. When comparing the angles on the sequential cephalograms, variation was less than 0.5° for each patient’s sequential radiograph.

A perpendicular reference line (perpendicular to FH) passing through sella, called the sella perpendicular, was then dropped. From sella perpendicular, a direct linear measurement was made parallel to FH and extending to the maxillary incisal edge (U1) ( Fig. 1 ).

Fig. 1
Lateral cephalometric measurements showing horizontal maxillary movement in millimetres, before (A) and after (B) surgery (SN, sella–nasion; FH, Frankfurt horizontal; S , sella perpendicular; U1, maxillary incisal edge).

To assess accuracy of the horizontal repositioning of the maxillary incisors at operation, the distance in mm (sella perpendicular to U1) was measured on both the preoperation and 5-week postoperation lateral cephalogram. The measurements were then adjusted using a metric reference frame to correct for magnification. Any difference in length of these two data points will correspond to variation in the preoperative and postoperative (at 5 weeks after surgery) horizontal position of the maxillary central incisors.

Comparison between the planned and actually obtained horizontal position of the maxillary incisors at 5 weeks was made using the paired t -test. Statistical significance was set at P < 0.05. Associations between the planned and postsurgical results were investigated with Pearson’s correlation.

Using the paired t -test, method errors of cephalometric measurements taken from the radiographs were defined by repeating the measurements for all 20 patients in a random order.

Results

The data collected for all study patients are presented in Table 1 . The patient example (patient 1 in Table 1 ) from whom cephalometric radiographs are used to demonstrate the study methods of measurement ( Fig. 1 ) is shown in Fig. 2 . In all cases, the surgically achieved and maintained (at 5-week post-operation) horizontal advancement measured at the maxillary incisors was within 1.0 mm of that planned. There was a strong correlation between the two data sets ( R = 0.96) ( Fig. 3 ). Using the absolute measurement values, the mean difference between the planned and actual 5-week postsurgical horizontal advancement of the maxilla was 0.6 mm (range 0.2–1.0 mm). Using the paired t -test, this mean difference (0.6 mm) is not considered statistically significant ( P > 0.05).

Table 1
Planned maxillary vertical change and plane change, and planned and actual horizontal change.
Patient Planned vertical change a Planned plane change b Horizontal change
Planned (mm) Actual (mm) Difference (mm)
1 −6 0 8 8.5 0.5
2 −3 −3 4 3.2 −0.8
3 −2 0 6 5.3 −0.7
4 −3 0 10 9.0 −1.0
5 0 0 10 9.4 −0.6
6 0 2 5 4.8 −0.2
7 −1 3 6 5.2 −0.8
8 −6 0 7 6.2 −0.8
9 2 2 5 4.8 −0.2
10 0 3 8 7.5 −0.5
11 −3 0 7 6.4 −0.6
12 −2 −3 8 7.6 −0.4
13 2 2 5 4.2 −0.8
14 4 1 7 6.8 −0.2
15 1 4 9 8.5 −0.5
16 −5 −4 6 5.2 −0.8
17 −4 0 6 6.5 0.5
18 −5 −2 8 7.2 −0.8
19 3 0 8 8.8 0.8
20 4 0 10 9.1 −0.9
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Jan 24, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Accuracy of analytic model planning in bimaxillary surgery
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