The digital workflow for computer-aided implant surgery includes a range of steps leading to generation of a prosthetically driven, 3-dimensional virtual plan, which is transitioned into the patient’s mouth by the surgical guide and protocol. Guided implant surgery is believed to be accurate and reliable compared with free-handed implant surgery. However, deviation between implant virtual plan and implant real position may occur as a result of accumulated errors throughout the digital workflow. This article reviews the digital workflow of static computer-aided implant surgery. Factors that may affect the accuracy and clinical outcome of the guided surgery are also reviewed.
Key points
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The benefits of guided implant placement include increased predictability, and decreased surgery time and complication rate.
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Static guides remain the most commonly used guide for computer-assisted implant surgery.
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The digital workflow can be divided into 6 steps. Each step may produce deviations from the virtual plan.
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The accuracy, efficacy, and complication rate of computer-guided implant treatment remain within acceptable clinical limits.
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Proper execution of each digital workflow step must be carefully verified to reduce inaccuracies.
Introduction
Guided implant surgery simplifies the execution of implant placement procedures and renders optimal clinical outcomes. Digital implant planning allows the accurate diagnosis of an implant site and virtual visualization of the final prosthetic restoration. Additional clinical benefits include reduced surgical time and a lower complication rate leading to increased patient acceptance and satisfaction. However, all the assumed advantages of guided implant surgery over traditional surgeries depend on the precise execution of the virtual implant plan.
Types of guided surgery
Guided implant surgery can generally be classified as dynamic or static. Dynamic guided surgeries involve the use of a computer-aided navigation system to allow for real-time implant surgery. The major advantage to the dynamic design is the ability to intraoperatively adjust the planned implant positioning. Although navigated surgeries are gaining popularity, static guides remain the most commonly used method. This article focuses on static guide–facilitated implant surgeries.
The static guided surgery approach is based on the 3-dimensional (3D) data obtained from cone-beam computed tomography (CBCT) and optical surface scanning, and computer-aided design/computer-aided manufacturing (CAD/CAM) technology for virtual implant planning and guide fabrication. The fabricated surgical guide can be supported by tooth, mucosa, or bone. Additional stabilization and support can be achieved using mini-implants, screws, or pins. Once the guide is fully seated, the planed drilling protocol beings. The drilling protocol can include using the guide for the pilot drill only, or a partially or fully guided drilling protocol. The implant insertion can be executed without the surgical guide or through the guide via a fully guided approach. Proper case selection and planning throughout the digital workflow allow for accurate execution.
The digital workflow
The digital workflow can generally be divided into 6 steps: (1) patient assessment, (2) data collection, (3) data manipulation, (4) virtual implant planning, (5) guide and prosthesis manufacture, and (6) execution of surgery and delivery of an immediate provisional prosthesis ( Fig. 1 ). It is worth mentioning that a combination of analog and digital steps may be applied. In addition, different virtual planning implant software may have some variation in the digital workflow.
Step 1: Initial Patient Assessment
During patient assessment, a comprehensive esthetic and functional evaluation should include the following.
Dentition status
Both periodontal and restorative status for the remaining teeth must be assessed. Evaluation of the existing denture must be performed in the case of edentulism.
Initial radiographic assessment
The quantity and quality of the bone should be assessed to determine whether grafting or a graftless approach is appropriate; this can be performed on 2-dimensional (2D) radiographs.
Occlusion
Occlusal assessment is essential for acceptable esthetics and function. Adequate mouth opening must be assessed because guided surgery requires extra access, especially in posterior regions.
Aesthetic evaluation and prosthetic consideration
The prosthetic design should ensure appropriate lip support and white/pink display. The prosthetic plan will mandate bone-reduction or augmentation procedures.
Step 2: Data Collection
Data collection includes CBCT acquisition and surface optical scanning. The process of each is reviewed here.
CBCT acquisition
A CBCT is obtained with or without a radiographic guide in dentate patients. A dual-scan technique is the primary method for the edentulous patient ( Fig. 2 ); however, direct mucosal scanning techniques are currently being explored.
Limitations to CBCTs include poor soft-tissue contrast and distortion. Distortion can be caused by patient movement and beam-hardening artifacts caused by high-density materials such as composite filling, metal restorations, and implants. The distortion affects the quality of the image and hence influences the accuracy of the guided surgery.
Surface optical scanning
Soft-tissue and teeth inaccuracy depicted by CBCT can be compensated by obtaining a surface optical scan, which represents the teeth surface and soft-tissue contour. Hard-tissue and soft-tissue changes must be avoided after the surface scan is captured, otherwise the fit of the surgical guide will be affected. The scan can be created via direct or indirect methods. The patient’s impression or stone model is scanned using a lab scanner in the indirect method. In the direct method, an intraoral scanner is used to scan the area of interest of the patient’s dental arch. Each arch should be scanned individually and then together in occlusion to represent teeth articulation.
The CBCT data are saved in Digital Imaging Communication in Medicine format (DICOM) and the surface optical scan is saved and transferred in a standard tessellation language format (STL). In addition, new CBCTs have the ability to merge facial photographs with the CBCT to obtain an accurate representation of the digital smile.
Step 3: Data Manipulation
Data (the DICOM and STL files) are imported into the digital implant planning software. Data manipulation consists of virtual dissection and orientation of the DICOM file, identification of panoramic curve, tracking of inferior alveolar nerve, and merging the CBCT and surface datasets.
Virtual dissection (segmentation)
CBCT shows both soft and hard tissue, and segmentation of the raw data allows for the differentiation and colorization of anatomic structures and areas of interest. In addition, segmentation reduces the image distortion caused by the metal scattering and motion artifacts. The first step of segmentation is to obtain the appropriate density threshold (also coined as gray-value threshold) to clearly visualize the hard tissues (bone and teeth). Manual setup is the preferred method. Any portion of the scan (known as a voxel) at a selected threshold with the same or larger density will be visible in the selected volume ( Fig. 3 A–C ).
CBCT gray values are often not reliable compared with multidetector-row computed tomography. Furthermore, different CBCT machines provide variable gray values. The selection of the threshold is subjective because it can be affected by the bone irregularities and maturation. Gray-value thresholds can also influence the 3D reconstruction and the fitting of surgical guides.
Orientation and panoramic curve definition
The rendered reconstructed volume has to be correctly oriented into the 3 planes ( Fig. 4 ), after which the panoramic arch needs to be defined.
Nerve tracking
The software provides a nerve-tracking tool to detect the inferior alveolar canal by placing dots along its path. Most software programs automatically join the dots and provide a nerve pathway ( Fig. 5 ).
Merging of CBCT and surface datasets
The files are merged by selecting identical anatomic landmarks of teeth surfaces or fiducial markers ( Fig. 6 ). Misalignment between DICOM and STL data sets can be a possible source of error.
Step 4: Virtual Implant Planning
Once an accurate virtual patient model is obtained, the wax-up of the future prosthesis will allow for the virtual placement of the implants. The surgical guide and prosthesis are designed according to the virtual plan.
The wax-up
The future prosthesis is based on the scanned actual or virtual wax-up (crown-down approach) ( Fig. 7 ).
Virtual implant planning
The implant type and size can be chosen from the implant library in the software. Implant position and axis are adjusted according to the available bone. A paralleling tool may be used in the case of multiple implants. Most systems provide an option to set a safety boundary around and between implants (see Fig. 7 ); accordingly, the system will alert the user if these boundaries are violated. In addition, the possibility of a flapless approach or any need for bone augmentation are determined at this time.
Surgical guide design
Once the virtual plan is finalized, the user can design the surgical guide including the type of support (tooth, tissue, bone, or any combination). A minimum of 2 teeth are recommended to support the guide. If additional support is needed, mini-implants may be considered.
The sleeve size (length and diameter) and height (the distance between sleeve and implant platform) may vary among different systems and according to the implant site and plan requirements ( Fig. 8 ). In general, decreasing the sleeve height and using a shorter implant may reduce the angular deviation of guided implant placement. In addition, this will enable the clinician to perform the guide surgery in cases with limited mouth opening.
Prosthesis design
The wax-up (actual or virtual) can be used as a blueprint to fabricate the custom-made provisional or definitive prosthesis. The restorative space should be assessed during the prosthesis design. Virtual abutments can be inserted to ensure a proper emergence profile and access holes. The fully guided protocol will provide the appropriate implant timing and depth for a prefabricated immediate provisional prosthesis ( Fig. 9 ).
The concept of stackable guides was introduced to reduce surgery time and improve the quality of the provisional full-arch prosthesis in implant-retained full-arch cases. A foundation guide can be oriented using the occlusion, or anatomic landmarks and may serve as a bone-reduction guide. The implant guide is stacked onto the foundation guide to perform guided implant placement. Finally, a prefabricated, enhanced provisional prosthesis with premade holes allows for rapid conversion of the provisional ( Fig. 10 ).
Surgical and prosthetic report
Following completion of the planning phase, the designed guides and prosthesis are exported into an STL format for fabrication. A detailed report is generated, which includes the drilling protocol with corresponding implants and prosthetic components.
Step 5: Guide and Prosthesis Manufacturing
The fabrication of guide and prosthesis can be carried out via conventional or CAD/CAM methods. Digital methods for the fabrication of the surgical guide, jaw models, and the prosthesis include additive (rapid prototyping) or subtractive (milling) techniques. Rapid prototyping involves the use of a 3D printer to cure photosensitive resin in layers to generate the surgical guide and the stereolithographic models of jaws. After the surgical guide is printed, implant system-specific metal sleeves are incorporated.
The CAM milling systems offer many material options to produce the provisional and final prosthesis or abutment.
Step 6: Surgical Execution
Proper fitting and seating of the surgical guide should be verified before surgery. The surgical protocol, which includes the implant size and drilling sequence, is followed. Each guided implant kit is specific to the implant system, and the clinician should be familiar with the components before undertaking the surgery ( Fig. 11 ).
Adequate irrigation throughout the surgery is crucial. The surgical guide may prevent sufficient irrigation and therefore induce more heat. Recent in vitro studies found that guided surgery generates more heat than the free-handed drilling protocol. However, the additional induced heat was within the acceptable temperature threshold. Jeong and colleagues have found that proper external irrigation in an up-and-down pumping motion may reduce the risk of overheating the bone during the guided drilling protocol. The authors recommend the use of additional irrigation underneath the surgical guide, specifically an irrigation syringe.
The digital workflow
The digital workflow can generally be divided into 6 steps: (1) patient assessment, (2) data collection, (3) data manipulation, (4) virtual implant planning, (5) guide and prosthesis manufacture, and (6) execution of surgery and delivery of an immediate provisional prosthesis ( Fig. 1 ). It is worth mentioning that a combination of analog and digital steps may be applied. In addition, different virtual planning implant software may have some variation in the digital workflow.
Step 1: Initial Patient Assessment
During patient assessment, a comprehensive esthetic and functional evaluation should include the following.
Dentition status
Both periodontal and restorative status for the remaining teeth must be assessed. Evaluation of the existing denture must be performed in the case of edentulism.
Initial radiographic assessment
The quantity and quality of the bone should be assessed to determine whether grafting or a graftless approach is appropriate; this can be performed on 2-dimensional (2D) radiographs.
Occlusion
Occlusal assessment is essential for acceptable esthetics and function. Adequate mouth opening must be assessed because guided surgery requires extra access, especially in posterior regions.
Aesthetic evaluation and prosthetic consideration
The prosthetic design should ensure appropriate lip support and white/pink display. The prosthetic plan will mandate bone-reduction or augmentation procedures.
Step 2: Data Collection
Data collection includes CBCT acquisition and surface optical scanning. The process of each is reviewed here.
CBCT acquisition
A CBCT is obtained with or without a radiographic guide in dentate patients. A dual-scan technique is the primary method for the edentulous patient ( Fig. 2 ); however, direct mucosal scanning techniques are currently being explored.
