Abstract
In this study, cone beam computed tomography (CBCT) and three dimensional (3D) stereophotogrammetry are used to compare the 3D skeletal and soft tissue changes caused by a bilateral sagittal split osteotomy (BSSO) 1 year after a mandibular advancement. Eighteen consecutive patients with a hypoplastic mandible were treated with a BSSO according to the Hunsuck modification. Preoperatively and 1 year postoperatively, a CBCT scan was acquired and a 3D photograph. The pre- and postoperative CBCT scans were matched using voxel based registration. After registration, the mandible could be segmented in the pre- and postoperative scans. The preoperative scan was subtracted from the postoperative scan, resulting in the hard tissue difference. To investigate the soft tissue changes, the pre- and postoperative 3D photographs were registered using surface based registration. After registration the preoperative surface could be subtracted from the postoperative surface, resulting in the overall volumetric difference. As expected, a correlation between mandibular advancent and volumetric changes of the hard tissues was found. The correlation between advancement and soft tissues was weak. The labial mental fold stretched after surgery. This study proved that using 3D imaging techniques it is possible to document volumetric surgical changes accurately and objectively.
Introduction
With the three-dimensional (3D) imaging possibilities available today, it is time to start 3D documentation of orthognathic surgical interventions. Using cone beam computed tomography (CBCT) imaging and 3D stereophotogrammetry, it is possible to obtain an accurate 3D dataset of the skeletal tissues and the soft tissues of the face. Different datasets can be combined using several image fusion techniques allowing quick, quantitative and objective evaluation of the results of a surgical intervention.
In this study, these imaging techniques are used to accurately compare the 3D soft tissue changes caused by skeletal transformations after a bilateral sagittal split osteotomy (BSSO) 1 year after surgery.
Material and methods
Eighteen Caucasian patients with a symmetrical mandibular hypoplasia without a maxillary hypo/hyperplasia or an anterior open bite (6 males and 12 females) were prospectively enrolled in this study. All patients were treated with a mandibular advancement using a BSSO according to the Hunsuck modification. All patients were older than 15 years with a mean age at the time of surgery of 32 years (range 17–55 years). Inclusion criteria were a non-syndromic mandibular hypoplasia (skeletal class II deformity) and signed informed consent. The exclusion criteria were a history of orthognathic surgery or simultaneously performed other orthognathic procedures, such as a Le Fort osteotomy or a chin osteotomy.
Preoperatively and 1 year postoperatively, an extended height CBCT scan was acquired (i-CAT™, Imaging Sciences International, Inc., Hatfield, USA). Apart from the CBCT scans, 3D photographs were acquired preoperatively and 1 year postoperatively using a 3D camera (3dMDCranial™ System, 3dMD LLC, Atlanta, USA). All 3D photographs and CBCT scans were acquired with the patient in the natural head position and habitual occlusion. During image acquisition, patients were asked to relax their facial musculature and keep their eyes open. Prior to surgery, all patients underwent orthodontic treatment with brackets. At the moment of the postoperative scan there were no fixed appliances in the patients because orthodontic treatment was finished.
Image registration
The preoperative and 1 year postoperative CBCT scans were registered with each other using voxel based registration. After registration of the images, the mandible could be segmented in both scans. By subtraction of the preoperative hard tissue information from the postoperative hard tissue information, the exact volumetric changes caused by the BSSO could be measured ( Fig. 1 ).
To investigate the soft tissue changes 1 year postoperatively, the pre- and 1 year postoperative 3D photographs were registered using surface based registration. After registration, the preoperative surface could be subtracted from the postoperative surface defined by the 3D photograph, resulting in the overall soft tissue volumetric difference ( Fig. 2 ).
The 3D photographs could be registered with the soft tissue information from the CBCT scan, resulting in one combined dataset of accurate hard and soft tissue information.
Comparison of hard and soft tissue changes
To isolate the region of interest (and exclude osteosynthes plates present in the postoperative scan) a limited cephalometric analysis was performed. A hard tissue based reference frame was constructed according to the validated procedure of Swennen et al. Both mental foramina were indicated. These landmarks were defined as the most anterior part of the mental foramen. A new posterior plane was calculated which went through the mean point of both mental foramina and was parallel to the vertical plane of the reference frame ( Fig. 3 ). A final cephalometric landmark was indicated which described the border between the lower dentition and bony structures on the median plane. A new plane was computed as a plane through this landmark and parallel to the horizontal plane ( Fig. 3 ).
The resulting hard and soft tissue volumes in this region could then be isolated and compared. To investigate the interobserver reproducibility of these measurements, this procedure was repeated by a second observer.
To investigate a more clinically relevant measurement the mandibular advancement and bite opening were measured. For measuring these changes a landmark was placed on the pogonion. The advancement could then be measured in mm at the pogonion level. The changes were classified and rated by their main transformation: advancement (translation to front) or opening of the bite (rotation). The relation between the mandibular advancement and the changes in the hard and soft tissue volume could be investigated.
3D curvature changes of the labio-mental fold
To investigate the changes in the labio-mental fold, the 3D curvature of this fold was computed for all patients. The soft tissue sublabiale point was indicated on the pre- and postoperative 3D photographs. The curvature was automatically computed on the sagital slice which went through the sublabial point and was parallel to the median plane of the reference frame. Over a distance of 10 mm in both lateral directions from the sublabial point, this computation was repeated in every sagital slice, with 0.5 mm distance between them ( Fig. 4 ). This measurement was performed by two observers to investigate the observer reproducibility.
Different soft tissue regions
The original volumetric soft tissue difference (the result of subtracting the preoperative 3D photograph from the postoperative 3D photograph) could be divided into specific regions based on specific anatomical landmarks ( Fig. 5 ). A hard tissue reference frame according to the method described by Swennen et al. was set up. Both mental foramina were indicated and formed the left and right borders parallel to the vertical plane of the reference frame. Stomion inferior and the sublabial landmark were indicated, forming the vertical borders of the lip and chin region ( Fig. 5 ). Based on these constructed planes, the volumetric changes in the chin region, the lower lip region, and the labio-mental fold could be investigated.
Statistical analysis
Paired Student’s t -tests were used to calculate the interobserver reproducibility of the volumes and curvature for repeated measurements (mean difference) and to test for statistically significant differences ( P < 0.05). To investigate the measurement errors for the methods used in this study in more detail, the systemic and random errors were calculated as well as the difference in means (95% confidence interval (CI)). Reliability coefficients between correlating measurements were calculated as Pearson correlation coefficients. Statistical data analysis was performed with the SPSS software program, version 16.0 (SPSS Inc., Chicago, IL, USA).
Material and methods
Eighteen Caucasian patients with a symmetrical mandibular hypoplasia without a maxillary hypo/hyperplasia or an anterior open bite (6 males and 12 females) were prospectively enrolled in this study. All patients were treated with a mandibular advancement using a BSSO according to the Hunsuck modification. All patients were older than 15 years with a mean age at the time of surgery of 32 years (range 17–55 years). Inclusion criteria were a non-syndromic mandibular hypoplasia (skeletal class II deformity) and signed informed consent. The exclusion criteria were a history of orthognathic surgery or simultaneously performed other orthognathic procedures, such as a Le Fort osteotomy or a chin osteotomy.
Preoperatively and 1 year postoperatively, an extended height CBCT scan was acquired (i-CAT™, Imaging Sciences International, Inc., Hatfield, USA). Apart from the CBCT scans, 3D photographs were acquired preoperatively and 1 year postoperatively using a 3D camera (3dMDCranial™ System, 3dMD LLC, Atlanta, USA). All 3D photographs and CBCT scans were acquired with the patient in the natural head position and habitual occlusion. During image acquisition, patients were asked to relax their facial musculature and keep their eyes open. Prior to surgery, all patients underwent orthodontic treatment with brackets. At the moment of the postoperative scan there were no fixed appliances in the patients because orthodontic treatment was finished.
Image registration
The preoperative and 1 year postoperative CBCT scans were registered with each other using voxel based registration. After registration of the images, the mandible could be segmented in both scans. By subtraction of the preoperative hard tissue information from the postoperative hard tissue information, the exact volumetric changes caused by the BSSO could be measured ( Fig. 1 ).
To investigate the soft tissue changes 1 year postoperatively, the pre- and 1 year postoperative 3D photographs were registered using surface based registration. After registration, the preoperative surface could be subtracted from the postoperative surface defined by the 3D photograph, resulting in the overall soft tissue volumetric difference ( Fig. 2 ).
The 3D photographs could be registered with the soft tissue information from the CBCT scan, resulting in one combined dataset of accurate hard and soft tissue information.
Comparison of hard and soft tissue changes
To isolate the region of interest (and exclude osteosynthes plates present in the postoperative scan) a limited cephalometric analysis was performed. A hard tissue based reference frame was constructed according to the validated procedure of Swennen et al. Both mental foramina were indicated. These landmarks were defined as the most anterior part of the mental foramen. A new posterior plane was calculated which went through the mean point of both mental foramina and was parallel to the vertical plane of the reference frame ( Fig. 3 ). A final cephalometric landmark was indicated which described the border between the lower dentition and bony structures on the median plane. A new plane was computed as a plane through this landmark and parallel to the horizontal plane ( Fig. 3 ).
The resulting hard and soft tissue volumes in this region could then be isolated and compared. To investigate the interobserver reproducibility of these measurements, this procedure was repeated by a second observer.
To investigate a more clinically relevant measurement the mandibular advancement and bite opening were measured. For measuring these changes a landmark was placed on the pogonion. The advancement could then be measured in mm at the pogonion level. The changes were classified and rated by their main transformation: advancement (translation to front) or opening of the bite (rotation). The relation between the mandibular advancement and the changes in the hard and soft tissue volume could be investigated.
3D curvature changes of the labio-mental fold
To investigate the changes in the labio-mental fold, the 3D curvature of this fold was computed for all patients. The soft tissue sublabiale point was indicated on the pre- and postoperative 3D photographs. The curvature was automatically computed on the sagital slice which went through the sublabial point and was parallel to the median plane of the reference frame. Over a distance of 10 mm in both lateral directions from the sublabial point, this computation was repeated in every sagital slice, with 0.5 mm distance between them ( Fig. 4 ). This measurement was performed by two observers to investigate the observer reproducibility.
Different soft tissue regions
The original volumetric soft tissue difference (the result of subtracting the preoperative 3D photograph from the postoperative 3D photograph) could be divided into specific regions based on specific anatomical landmarks ( Fig. 5 ). A hard tissue reference frame according to the method described by Swennen et al. was set up. Both mental foramina were indicated and formed the left and right borders parallel to the vertical plane of the reference frame. Stomion inferior and the sublabial landmark were indicated, forming the vertical borders of the lip and chin region ( Fig. 5 ). Based on these constructed planes, the volumetric changes in the chin region, the lower lip region, and the labio-mental fold could be investigated.
