The advancement of technology often provides clinicians and patients better clinical alternatives to achieve optimal treatment outcomes. Computer-guided options allow clinicians to realize the virtual prosthodontically driven surgical plan, facilitating more predictable implant placement. Although the use of technology does not mean the clinicians can forgo the fundamental treatment principles when treating a patient, proper assessment and diagnostic approach from prosthodontic, surgical, and radiographic perspectives are still essential for a successful clinical outcome. The purpose of this article is to review the fundamental concepts for the use of computer-guided surgery to facilitate prosthodontic treatment.
The use of three dimensional (3D) radiographic imaging, such as cone beam computed tomography (CBCT), allows for the collection of accurate pretreatment diagnostic dataset. However, creating airspace around regions of interest is essential to provide clear outlines of varying structures during CBCT imaging.
A static virtual dental patient can be created from the CBCT imaging and optical scanning systems for intraoral (IOS) and extraoral (EOS) imaging to facilitate prosthodontically driven implant planning process.
A radiographic template or a virtual diagnostic wax-up is essential for a prosthodontically driven treatment plan.
Although the use of computer-guided surgery can provide clinical advantages, an objective assessment of proposed treatment difficulty should be undertaken (Straightforward/Advanced/Complex) to reduce surgical complications.
Dental implant-based treatment has developed from a revolutionary option into a widely accepted treatment modality in dentistry. Studies have demonstrated long-term survival rates for dental implants, and success criteria have evolved to include the esthetic outcome of the implant-prosthetic complex. A lifelike implant supported or retained prosthesis, accompanied by healthy peri-implant tissue, has become the gold standard for dental implant treatment. Appropriate patient assessment from a prosthodontic, surgical, and radiographic perspective is now considered to be a prerequisite for the achievement of a satisfactory clinical outcome. Computer-assisted options allow clinicians to realize the virtual prosthodontically driven surgical plan, facilitating predictable implant placement. Various methods, including the use of computer-aided design and computer-aided manufacturing (CAD-CAM) for surgical template design and fabrication are becoming more common place.
The purpose of this article is to review fundamental concepts important to the use of computer-assisted options relating to implant surgery and to prosthodontic treatment.
Three-dimensional radiographic imaging
Thorough pretreatment evaluation of the implant recipient site, and its associated anatomic forms and limitations is required. Visualization of the proposed prosthesis or prostheses and their relationship to the recipient site should be routinely incorporated to improve clinical outcomes and minimize complications. Although two-dimensional (2D) intraoral ( Fig. 1 ) and panoramic ( Fig. 2 ) radiographic imaging remains an option that is used extensively in daily dental practice, access to and use of three-dimensional (3D) radiographic imaging has dramatically enhanced options for data collection. The use of cone beam computed tomography (CBCT) has allowed for the collection of a more consistent and accurate pretreatment diagnostic dataset, with acceptable levels of radiation exposure and cost to the patients ( Fig. 3 ). Unlike traditional fan-beam (medical) computed tomography (CT), CBCT uses a cone or pyramid-shaped beam to scan an entire 3D volumetric dataset in a single rotation using reduced x-ray tube power. The lower power characteristic reduces the radiation exposure and the cost to the patient, but also decreases the contrast resolution of the resulting 3D volumetric dataset. This limits the suitability for the data gathered using CBCT with regard to soft tissue imaging including, for example, facial soft tissue contours. Existing metallic restorations can also result in artifacts (including scatter) and compromise the diagnostic value of 3D volume gathered from CBCT imaging. The reducing initial investment and maintenance costs of CBCT units, and the high spatial resolution of hard tissue structures, have greatly increased the application of CBCT in dentistry.
Creating air space during 3D radiographic imaging
Hounsfield units (HU) are proportional to the degree of x-ray attenuation, are allocated to each pixel, and represent the density of the tissue. HUs provide a universal standard in medical CT imaging systems for scaling reconstructed attenuation coefficients. The degree of x-ray attenuation in CBCT imaging is presented as a gray scale (voxel value), and manufacturers do not have a standard system. However, different studies show a linear relationship between HU (CT imaging) and the gray scale (CBCT imaging), and it has been suggested that the gray scale can be used in determining the radiodensity changes. The different radiodensity, for example, of cortical bone (1700 HU), denture acrylic resin (70 HU), tissue (50 HU), pure water (0 HU), and air (−1000 HU) affords clinicians a method by which a discernible comparison among different structures can be made. The larger the difference between HUs, the easier for clinicians to discern different structures using CBCT imaging. Obtaining a digital volume without proper air space separation in regions of interest can limit interpretation of information relating to hard tissues (such as bone and teeth). Airspace is required to provide a clear visualization of additional structures with similar radiodensity, such as soft tissues and denture acrylic resin, lips, cheeks and gingiva, and occlusal surfaces between opposing arches.
Cotton rolls and soft tissue retractors are useful and simple tools that can be used to create air space around regions of interest to provide clear outlines of varying structures. For instance, cotton rolls can be placed between occlusal surfaces of opposing arches to create separation, and it can allow for clearer visualization of surface detail. Soft tissue retractors and cotton rolls can also be used to isolate the patient’s cheeks and lips from adjacent alveolar ridges and gingiva, and facilitate improved distinction among various soft tissue structures ( Fig. 4 ). When using removable dental prostheses as scanning aids during CBCT imaging, cotton rolls are needed to separate cheeks, lips, and tongue from the acrylic resin of the removable dental prostheses. This can greatly improve the clinician’s ability to visualize the outline of proposed prostheses (the planned prostheses) without the need to duplicate existing prostheses into distinct radiographic templates ( Fig. 5 ).
Creating a virtual dental patient for computer-guided surgery planning
A virtual dental patient can be created in computer-aided design and computer-aided manufacturing (CAD-CAM) and/or virtual implant planning software, thus replicating relevant anatomic structures and functional positions. These include the maxillofacial soft tissues (including muscles of facial expression and facial contour), maxillofacial hard tissues (such as skull and dentition), and intraoral soft tissues (such as edentulous ridge and periodontal or peri-implant soft tissue). Including the aforementioned CBCT imaging, different technologies can be used to collect 3D volumetric data to compose a desirable virtual patient for diagnostic purposes. Current data acquisition technologies include CBCT imaging and optical scanning systems for intraoral (IOS) and extraoral (EOS) imaging. CBCT imaging generates data in the DICOM (Digital Imaging and Communications in Medicine) file format, a general standard format for handling, storing, printing, and transmitting information in medicine (ISO 12052:2017). IOS imaging uses proprietary file formats, or exports the 3D scan in Surface Tessellation Language file format, which consists only of the surface geometries of 3D objects without any color or texture information. EOS imaging can store the scan data in a proprietary format or as OBJ files (developed by Wavefront Technologies for its Advanced Visualizer animation package), an open geometry definition file format capable of storing 3D texture and color information.
The triad of maxillofacial hard tissue, extraoral facial soft tissue, and the intraoral dentition and surrounding soft tissue is key to creating a virtual patient under static conditions. The high-resolution and scatter-free 3D volumetric dataset from IOS and EOS imaging (surface scan) can be used to supplement the diagnostic information from CBCT imaging to create a virtual dental patient for implant planning ( Fig. 6 ). Currently, no readily available system allows for the visualization and duplication of functional movements of maxillofacial structures facilitating a complete 4D (dynamic) virtual dental patient, although this is under investigation. It is of future research interest to improve current technology and develop protocols to do so.
Importance of diagnostic wax-up for implant planning
A prosthodontically driven treatment plan is needed to improve the possibility of a favorable treatment result. The location and position of dental implants should follow the plan for the desired definitive restoration. Comprehensive oral examination, complete periodontal charting, and appropriate radiographic examination are vital for diagnosis and treatment planning. Conventionally, along with a good quality clinical photographs, appropriate diagnostic casts are obtained and articulated. The diagnostic wax-up and tooth arrangement of the desired definitive restoration or prosthesis are traditionally procedures completed in the laboratory ( Figs. 7 and 8 ), and should follow sound prosthodontic principles. In many situations, appropriate soft tissue contours and prosthesis extensions are included in the diagnostic wax-up to reflect the desired outcome. The pink component of diagnostic wax-up represents the deficiency of hard and soft tissue ( Figs. 9 and 10 ). This information is necessary for the designing of implant and tooth supported prostheses, and assessment of any potential tissue augmentation.
The use of radiographic templates
Traditionally, the diagnostic wax-up or satisfactory existing prosthesis can be duplicated and used to fabricate a radiographic template. Radiographic templates contain radiopaque materials (barium sulfate) and/or fiducial markers such as gutta percha or titanium rods, and transfer the proposed prosthesis design and contour, as well as proposed implant position, for radiographic capture ( Figs. 11 and 12 ). The patient wears the radiographic template during imaging (CBCT or otherwise), and the proposed prosthesis can then be visualized on a radiograph or planning software for the purpose of treatment planning. Cross-sectional radiographic images of edentulous areas incorporating radiographic markers also help to determine the need for augmentation procedures during or before dental implant placement ( Fig. 13 ). The implant prosthesis and location of the access for screw retention can also be determined by using the information made available through effective use of radiographic templates. A drawback of radiographic templates is the need for additional clinical and laboratory steps and associated costs needed for fabrication. The clinician is required to obtain duplicate diagnostic casts for the fabrication process and a second clinical appointment is often needed to confirm the fit of the radiographic template before imaging.