The current operational plan for the conventional orthognathic surgery is based on analysis of lateral cephalometry to determine the moving amount and posteroanterior (PA) cephalometry to investigate maxillary canting and asymmetry of chin . Occlusal plane angle and incisal inclination based on analysis of lateral cephalometry is an important measurement in the plan for rotational orthognathic surgery with maxillary posterior impaction . And common rotational surgery is focused on anteroposterior movement and aesthetic improvement of lateral profile. However, most patients want not only proper maxillary anteroposterior position, but also aesthetic, slender, symmetric mandibular inferior border line in frontal view, so called V-line. However, the surgical plan for orthognathic surgery based on PA cephalometry to accomplish optimal postoperative frontal profile has a limited application in practical operation, with no guarantee of an aesthetic frontal face [11, 12]. Skeletal investigation based on 2D modality like PA cephalometry tracing is a limited adaptation for surgical planning because of the difficulty in positioning the reoriented natural head position and selecting specific anatomical points in overlay of structure. So, there is no referential measurement for surgical planning of the frontal profile. As a result, most surgeons have done lateral mandibular angle reduction, mandibular body contouring based on their experience and preference. There is no standard measurement.
Many clinicians have suggested several different methods to convert two-dimensional radiograph to three-dimensional image for solving the problem of 2D-based surgical plan [13–16]. But, former methods had several limitations to apply in practical operational procedure because of the radiographic magnification distortion, and the need to measure a reference point repeatedly on various image views. So, the establishment of plan for orthognathic surgery still depends on lateral cephalometry, and there is no practical and predictable planning method for investigating frontal view. As a result, many practitioners decide to operate with one’s own preference or feel. Consequently, revisional surgery may be performed to correct an unsatisfactory postoperative outcome.
With the development of 3D computerized tomography, 3D reconstruction modality and analyzing software program, the use of 3D analysis method for getting more aesthetic surgical result is studied by many clinicians. So, in this research, we try to study a useful and valuable reference measurement required for establishing three dimensional treatment planning by using 3D image of the patient. There are several problems that must be solved prior to establishment of 3D treatment planning. First, it has to reproduce natural head position (NHP) of the patient. Second, an accurate image of the hard tissue and soft body and tooth must be obtained from the low radiation dose. Third, all processes have to be handled in one advanced software . However, 3D analysis based on NHP as a reference plane is still controversial because of its sensitive reproducibility technologically and it is difficult to standardize . So, in this study, the standard plane is horizontal reference plane(HRP) accomplished by both the porion side and right side orbitale and sagittal reference plane(SRP) perpendicular to FH plane passing through basion. CT image of patient was reorganized into 3D reconstructive image and reoriented according to HRP and SRP, and coronal plane perpendicular to HRP and SRP is the practical measurement plane.
In this study, we did not select the anatomical reference point directly on 3D reconstructed image. Instead, we coordinated multiplanar reformatted reference plane that is horizontal, sagittal, and coronal plane to set up the reference point. This has the advantage of simplicity and ease in comparison to setting up a reference point on complicated X, Y, and Z-axes converted from a 3D reconstructed image.
There were several locations to consider. First, it was the gonion location. There was a trouble of deciding the accurate gonion location of patient performing mandibular angle reduction. However, we resolved this isue by setting the most inferolateral point of the proximal segment to gonion. Second, there was a trouble deciding on HRP, including porion and orbitale. Setting up HRP with three points among porions and orbitale of both sides is difficult in asymmetric patients, but we found the nasion and basion to set up SRP, and we set up HRP to the plane perpendicular to SRP which contains three points or passed near four points. Third, the head of condyle (condylon) was located inside zygomatic arch. However, by using 3 type mutiplanar reformatted image, condylon could be easily selected.
In a comparative study of the experimental group, control A, B group, statistically significant measured value in aesthetic mandibular outline was angular measurement of Go’Rt-Me’-Go’Lt, and linear measurement of ratio of Me’-Go’ to Me’-Go’RtGo’Lt, ratio of Go’Rt-Go’Lt to Me’-Go’RtGo’Lt.
Angular measurement of Go’Rt-Me’-Go’Lt in experimental group was shown to have a statistically significant difference with value in control A group, but, not with control B group. Mean value of Go’Rt-Me’-Go’Lt angle of women is 100.74 ± 2.14, that of men is 105.37 ± 3.62 linear measurement. Mean value of Cd’-Go’-Me’ angle is not statistically significant between experimental group and control A group, but there is an apparent difference. So, it may be a useful measurement (134° and 117° in men group, 138°, and 118° in women). Ratio of Go’RtGo’Lt-Cd’RtCd’Lt to Me’-Go’RtGo’Lt, ratio of Me’-Cd’RtCd’Lt to Me’-Go’RtGo’Lt in experimental group had a statistically significant difference with both control A, B group, and it may be the influence of varying position of gonion due to mandibular angle reduction.
The measured value obtained from this study will not become the absolute standard value to evaluate for aesthetic mandibular outline. However, because there are no research that suggest the linear and angular measurement value to analyze for 3D image, measurement of this study will be a valuable measurement to the application of operational plan establishment and intraoperative guidance. Moreover, if long-term study is conducted with a larger population, measuring point and measured values will be standardized as the reference value for 3D morphometric investigation of frontal profile and surgical planning for more satisfying aesthetic appearance. Pitch, roll, and yaw which is difficult to evaluate in 2D radiograph will be easy. Particularly, the volume difference and midline discrepancy of the mandible in an asymmetric patient could be evaluated. And it could be calculated by the quantitative amount of bone reduction and movement for more satisfying frontal profile. Afterwards, it is regarded to provide the reference value which can easily apply clinically and be helpful to the establishment of the diagnosis of orthognathic surgery or treatment planning to achieve a more aesthetic mandibular outline and frontal appearance.
The limitation of this study is that we did not evaluate soft tissue appearance of patient group, but we investigated bony structure. It is difficult to evaluate soft tissue appearance by analyzing bony structure because of personal difference of adaptation of the soft body, lip thickness, adipose tissue and amount of muscle growth, and texture. Soft tissue and bony structure must be studied separately [19, 20]. According to the development of 3D image technology and modality, the soft body can be reconstructed precisely, and the active research including the reaction of the soft body according to the hard tissue change, soft tissue prediction according to operation, overlay of the soft tissue and the hard tissue are being studied. So, this study will be upgraded to investigate soft and bony tissues.