From Volume 16, Issue 4, October 2023 | Pages 189-194
This is the second article in a two-part series considering the relevance and clinical uses of digital technologies in relation to orthodontics. The aim is to take a closer look at the application of digital technology in relation to joint orthodontic/orthognathic treatment and present two clinical cases that have undergone treatment by means of a digital workflow.
CPD/Clinical relevance: Digital technologies can enhance pre-operative orthognathic planning
For those patients whose orthodontic problems are so severe that neither growth modification nor camouflage offers a solution, surgical realignment of the jaws or repositioning of dentoalveolar segments is often the only possible treatment. Successful management of combined surgical and orthodontic treatment often requires the integration of pre-surgical orthodontic, surgical and post-surgical orthodontic phases of treatment.1
The developments in three-dimensional (3D) clinical imaging and intra-oral scanning in conjunction with advances in computer-aided design (CAD), computer aided manufacture (CAM) and additive manufacturing have led to a digital workflow revolution in the management of orthognathic cases.2
The management of the joint orthodontic-orthognathic patient is a multi-disciplinary team (MDT) approach. Good collaboration between the orthodontist, surgeon and laboratory technician is critical for a successful clinical outcome.3 Traditionally an ideal occlusion is established based on two-dimensional (2D) cephalometric planning of the maxillary and mandibular segments, and by replicating the surgery using gypsum models mounted onto an articulator using a facebow transfer. An orthognathic wafer is then fabricated on the gypsum models in the laboratory using acrylic. The wafer(s) is an occlusal guide used intra-operatively that enables the surgeon to position the jaw(s) in the corrected position(s) following the osteotomy cuts.4 In recent years, the use of digital 3D surgical planning platforms for orthognathic surgery has increased. It is widely accepted that improvements in image acquisition, software capabilities, PC processing power and accuracy in 3D printing have led to precise virtual surgical simulations that are valuable in surgical planning, assessment of surgical outcome and patient communication. The 3D printing of orthognathic wafers, osteotomy cutting guides, repositioning jigs, customised fixation plates and anatomical models allows a virtual surgical plan to be transferred from the workstation into the operating theatre. Although 3D-virtual surgical planning (3D-VSP) requires a relatively high level of investment when compared to traditional planning methods, it offers clear advantages. These include the potential to improve reproducibility, clinical and laboratory efficiency, and surgical precision.3, 5, 6 The benefits and drawbacks of the use of digital technology in the joint orthodontic/orthognathic approach are listed in Table 1.
| Benefits of digital technology | Drawbacks of digital technology |
|---|---|
| Potential for improving pre-operative assessment | Initial expenditure/funding |
| Potential for improving clinical efficiency | On-going financial implication of sustaining the service |
| Potential for enhancing precision of treatment | Recruiting and developing technologists with a specialist skill set |
| Improved consistency and reproducibility | Ensuring clinicians and supporting staff members are also trained |
| Patient data and study models stored safely behind Trust firewall | Reliable I.T. infrastructure required |
| Patient records/models can be called upon with ease | |
| 3D models provide understanding of anatomical structures in advance and improve confidence of the surgeon | |
| Reduce surgical morbidity through an enhanced understanding of the local anatomy | |
| Litigation protection | |
| Decreased plaster usage which promotes a cleaner working environment in the laboratory |
The first stage in orthognathic 3D-VSP is to acquire the relevant clinical imaging of the facial bones, dentition and occlusion. Images of the facial bones are most commonly captured once pre-surgical orthodontic treatment has been completed by taking a computed tomography (CT) or a cone beam CT (CBCT) scan (Figure 1). Initially, 2D data acquisition takes place, the data is then saved in the Digital Imaging and Communications in Medicine (DICOM) format before then being imported into the 3D-VSP software. The software uses a combination of volume and surface rendering techniques to produce a 3D image of the facial structures. The hard tissues are identified manually by setting a grey level threshold, which best separates the hard from the soft tissues. The reconstructed 3D model of the hard tissues can now be segmented into anatomical regions of interest (ROI) (Figure 2). The first stage of segmentation is to detect and delineate the relevant anatomic structures, such as the maxilla and the mandible. Each ROI is then colour coded accordingly to make it easier for the operator to identify them during subsequent digital planning.
The segmentation process should be conducted by a trained engineer or technologist. The process can be carried out manually or by using semi-automated or automated algorithms that form part of the overarching 3D-VSP software package.7 The automated methods of segmentation can be a useful time saving tool. However, the user must rely entirely on the software to accurately detect the relevant structures. This may be problematic when segmenting complex cases (i.e. those with facial asymmetry) or datasets that have a high level of scatter artefacts. Therefore, the use of automated algorithms should be assessed on a case by case basis. The more complex datasets benefit from a joint manual/semi-automated approach.
CBCT scans suffer from limited spatial resolution8 which can result in poor image reproduction of the teeth, as shown in Figure 3. Therefore imaging of the maxillary and mandibular dentition and the bite registration, need to be captured in separate high resolution intra-oral scans. These records are usually captured via direct intra-oral scanning, but can also be captured by traditional impression, casting in dental stone and scanning the gypsum model using a benchtop scanner. Once the CBCT has been reconstructed and the subsequent intra-oral scans have been obtained, the images must be merged to create a final virtual patient model. The images are manually aligned using a semi-automated rigid point registration tool (Figure 4). This model is used for the 3D-VSP workflow.
Before simulating orthognathic surgery, the virtual patient model must undergo pre-operative analysis. First, a cephalometric reference plane must be determined; our cases are set according to the Frankfort horizontal plane. The Frankfort plane is commonly used in anteroposterior assessment and to help identify and calculate maxillary canting9 and is set by manually plotting points on four anatomical landmarks (left and right Orbitale and Porion respectively) on the virtual model (Figure 5). Occlusal reference points are also plotted manually using rigid point registration. Points are typically placed on the upper and lower dental midline, incisal tip of the canines and central pit of the first molar. The points are used to track the distance/degrees of intra-operative maxillary and mandibular movement. The next step is to perform the virtual osteotomies according to the surgical plan.
Virtual osteotomies are performed by plotting points of intersection on the virtual patient model (Figure 6). A cutting plane is simulated and adjusted to coincide with the clinical prescription. Adjustments involve ensuring that the chosen saw blade thickness is correct and that the cutting plane does not intersect vital anatomical structures, such as the inferior dental canal and nerve. Le Fort, bilateral sagittal split osteotomy (BSSO), genioplasty and custom osteotomies can all be replicated in the 3D software.
Once virtually cut, the osteotomised segments are repositioned by making translational and rotational movements in relation to the sagittal, coronal and transverse planes. The previously plotted occlusal reference points are now used to track the distance/degrees of movement for each segment whilst ensuring the movements adhere to the prescription.
When carrying out 3D-VSP for bimaxillary orthognathic surgery, the virtual maxillary osteotomy is usually the first osteotomy to be undertaken. Typically, the ‘maxilla-first’ sequence is the preferred surgical approach when repositioning the bimaxillary complex. Liebregts et al. (2017) reported that maxilla-first sequencing is more accurate and predictable as higher levels of inaccuracies can arise when manipulating the proximal segments during fixation in the mandible-first approach as this procedure is more prone to potential condylar displacement. 10
Once the maxillary segment has been manipulated into the intermediate position, the remaining mandibular segment is positioned into the final post-operative occlusion. Generally, there are two methods that can be used to achieve the final position.
The first method is when the technologist manually replicates the final post-operative occlusion with the surgeon and orthodontist, using handheld dental models. Both, gypsum models taken from traditional alginate impressions or 3D printed pre-surgical intra-oral scans will be sufficient. The predetermined final position is identified on the models and the occlusion is scanned using a benchtop scanner and imported into the 3D-VSP software as an ‘occlusal registration guide’. Using the previously mentioned rigid point registration algorithm, the remaining osteotomised segment is aligned according to the imported occlusal registration guide (Figure 7). Reverse engineering the final occlusion is one way of achieving the optimal post-operative position.
The second method involves utilising a series of embedded tools in the 3D-VSP software that allow the user to autorotate the mandible, detect bony collisions and to achieve maximum intercuspation. However, from personal experience determining the final position solely using the embedded tools is difficult, as the level of on-screen occlusal contact is not always apparent. This issue could be simplified with the introduction of haptic technology. Haptic technology is currently used in surgical simulation, 3D sculpting, data visualisation and computer gaming. This technology provides tactile feedback which recreates a sense of touch. This is achieved by applying vibrations, force or motion to the user. Therefore, virtual objects in a computerised environment will appear real and tangible.11
Once the intermediate and final post-operative occlusions have been established (Figure 8), the occlusal wafer design process is undertaken. A wafer is virtually constructed by marking the points of intersection on the incisal and cuspal regions of the upper and lower dentition (Figure 9). The software auto-generates a digital wafer design according to the plotted points. The level of retention, material thickness and global profile of the wafer can be adjusted according to the clinical need. At this stage the wafers can also be labelled so that they can be identified with ease (Figure 10). The completed wafers are saved in the STL file format and exported to the 3D printer for fabrication (Figure 11).
The 3D-VSP can be transferred from the workstation and replicated in the operating theatre with the use of patient-specific surgical cutting guides and repositioning jigs (Figure 12). The use of such devices has been shown to improve intra-operative accuracy and post-operative results. 12.
The following two clinical cases have undergone joint orthodontic/orthognathic treatment by means of a 3D-VSP workflow.
Case 1 relates to a 17 year old male whose main concern was his bite. He presented with a Class II division 1 incisor relationship on a Class II skeletal pattern with decreased lower anterior face height and competent lips (Figure 13). His malocclusion was complicated by an increased overjet of 8 mm to the UR1, 6 mm to the UL1 and a proclined lower labial segment. The centrelines were coincident and there was mild crowding of the upper and lower arches. At the initial assessment, the patient was given the options of a) do nothing, b) orthodontic camouflage including maxillary extractions to reduce the overjet or c) a combined orthodontic/orthognathic approach. Having considered the options, the patient returned to the Joint orthodontic-orthognathic MDT clinic and opted for the following treatment plan:
The 3D virtual surgical plan (Figure 14) consisted of:
This case relates to a 15 year old female patient who presented with a Class III incisor relationship on a Class III skeletal pattern with average anterior face height and competent lips (Figure 17). Her main concern was that her lower teeth were further forwards than her upper teeth.
Her malocclusion was complicated by severe crowding of the upper arch, mild crowding of the lower arch, a reverse overjet of 2 mm and a crossbite associated with UR652, UL23456 with anterior displacement. There was a centreline discrepancy with the upper shifted 1 mm to the right. Following attendance at the Joint orthodontic-orthognathic MDT clinic, the patient and her family agreed to the following treatment plan:
The 3D virtual surgical plan (Figure 18) consisted of:
The use of digital workflows in joint orthodontic/orthognathic planning has become increasingly popular. 3D-VSP has the potential to improve clinical and laboratory efficiency whilst enhancing surgical precision, consistency and reproducibility of treatment. However, a 3D-VSP service requires substantial investment as well as the development and recruitment of technologists with a specialist skill set. The two clinical cases presented illustrate how 3D-VSP and surgical simulation is used to manage such cases.
