Background
Conventional implant planning is based on clinical examination and 2D radiographic imaging. The adoption of 3D radiographic imaging enables a more precise diagnosis of residual bone dimensions, the intrabony course of the inferior alveolar nerve and neighboring teeth [
1,
2].
Individual patient 3D-imaging data is essential for virtual dental implant planning, computer aided design (CAD) and computer aided manufacturing (CAM) of a drill guide or implant-supported prosthesis. Anatomical data is derived from (cone beam) computed tomography (CT or CBCT) and optical scans of teeth and mucosa.
CBCT has a lower radiation dose (92–118 μSv) than CT (860 μSv) and is therefore often used for dental implant planning [
3,
4]. Both CT and CBCT are stored in the universal format for “Digital Imaging and Communication in Medicine” (DICOM-format). Amongst imaging data, geometric and mathematical information, practical information such as acquisition details and settings are included in the DICOM file.
Volumetric imaging data is displayed in 2D cross-sectional images aligned to the prospective implant position. 3D surface models of CT or CBCT data are displayed using segmentation. Each voxel in the volumetric data set is assigned a grey value following its radiation attenuation, depending on the specific tissue characteristics. The display of a limited range of grey values enables the selective display of specific anatomical structures (segmentation).
CT or CBCT does not sufficiently display the tooth surface for the prosthetic set-up and for drill guide production. Especially in the presence of restorations, artifacts such as streaks and extinct areas occur [
5]. Therefore, CT or CBCT scans and a virtual dental model obtained either from an intraoral optical scan or an extraoral scan of impressions or stone casts are aligned to each other prior to implant planning [
6].
The data of intra– and extraoral optical scans are usually available in the universal stereolithography format (STL). This format contains geometric information of the surface [
7]. Virtual dental models can be displayed in 2D along cross-sections and 3D to assess the mucosal surface from different viewpoints.
The process of aligning multiple imaging datasets with each other is defined as registration [
8,
9]. Different procedures can be used to accomplish an accurate registration of CT or CBCT scans and virtual dental models: The tooth surface as a common structure displayed in both datasets may be used for registration. Custom and standardized reference markers (fiducial markers), respectively, can otherwise be introduced with a radiographic splint [
10].
With standardized markers stored in the software, a single scan of the patient wearing the radiographic splint is performed (single scan protocol) [
11,
12]. In the software, the stored reference marker is registered with the scanned image of the respective marker.
With custom markers a double scan protocol is used: after CT or CBCT acquisition of the patient with the radiographic splint, the radiographic splint alone is scanned [
10,
13,
14]. The images of reference markers in both datasets are registered.
When using the tooth surface as a reference for registration, a splint with fiducial markers is not necessary [
6,
15,
16]. The software uses an algorithm to register corresponding anatomical surfaces (automatic registration) or requires previous selection of corresponding areas by the user to initiate the registration process (semi-automatic registration). The accurate registration of CT or CBCT data and virtual models is crucial for a precise transfer of the prospective implant position to the surgical site [
9].
After data import, segmentation and registration the prosthetic set-up and virtual implant position is planned. The prosthetic set-up combines the ideal position of implant-supported prosthesis and takes the abutment design with its emergence profile, morphology of the tooth, occlusal and proximal contacts into consideration. Using this information, implants can be virtually positioned in cross-sectional images and three-dimensional surface models reconstructed from the radiographic volume.
The design of a drill guide can vary depending on its function. It can either a) only guide the pilot drill (pilot guided) or b) guide every drill of the implant specific drill sequence (fully guided) [
15,
17]. Additional to fully guided drilling, implant placement can be performed through the drill guide [
11]. Guided protocols are preferred to complete free handed drilling and implant placement due to a higher accuracy of the implant position [
14].
Drill guides may either be supported by the remaining teeth, the mucosa, directly by the bone surrounding the implant or by temporarily inserted mini implants [
18,
19]. Especially in edentulous jaws with a mucosal support, the stability may be ameliorated with transitional screws or pins or temporary implants, securing the drill guide to the bone [
20,
21].
In a fully digital workflow drill guides are virtually designed (CAD) and produced using computer-aided manufacturing (CAM). CAD/CAM is either performed by the software user or in a central production facility. The guides are milled from resin blanks [
22,
23] or produced with an additive technique e.g. rapid prototyping [
24]. In a combination of analog and digital techniques, drill guides are adapted from conventionally produced radiographic splints or produced on stone casts.
In this narrative review, the possibilities and limitations of five commercially available implant planning software systems are examined regarding the import of imaging data and the export of the virtual implant planning for the design and fabrication of a drill guide.
Methods
The following commercially available virtual implant planning systems: coDiagnostiX, Version 9.9. (DentalWings, Canada) (CDX); Simplant Pro, Version 17 (Dentsply, Sweden) (SIM); Smop, Version 2.13. (Swissmeda, Switzerland) (SMP); NobelClinician, Version 2.4. (Nobel Biocare, Switzerland) (NC); ImplantStudio, Version 1.6.4.4, (3Shape, Denmark) (IST) were examined.
Study design
The study design included the review of five different planning systems. Data of two patients with different indications for dental implant treatment were used to assess import and processing of imaging data for dental implant planning and drill guide production using CAD/CAM technology.
One patient had a missing single tooth in the region 21 (FDI), a fixed metal-ceramic prosthesis in the first quadrant and a cast post and core and metal-ceramic crown on the adjacent tooth (11 FDI). The second patient presented with a partially edentulous jaw with missing teeth in region 45–47 (FDI), a metal-ceramic crown on the adjacent tooth (44 FDI) and no other restorations in the lower jaw. CBCT data (3D Accuitomo, Scanora) and intraoral optical scans of the first patient (iTero, Cadent, Santa Clara, CA, US) as well as digitized stone casts (D250, 3Shape, Copenhagen, Denmark) of the second patient were available. CBCT data was stored in a DICOM format. Intraoral and extraoral scans and stone cast scans were available in the universal file format (STL). The above-mentioned virtual implant planning systems were evaluated by one examiner with defined assessment criteria as follows:
Data acquisition and registration
Each system was examined regarding its options for the import of radiographic data (CT or CBCT) and virtual dental models. The availability of a proprietary scanner for intraoral scans or extraoral model and impression scans, respectively, and the data format specification for data import were assessed. Settings for the alignment of virtual dental models and radiographic data were evaluated regarding the use of single and double scan protocols and assistance of the system in the registration process (semi-automatic, automatic) (Table
1).
Table 1
Assessment criteria for data acquisition and registration of image data
intraoral scans extraoral scans (CB-)CT | importable data formats proprietary scanner available specifications (data format, image resolution) |
image registration | single scan protocol using reference markers single scan protocol using tooth surface double scan protocol |
Visualization of imaging data
The visualization of CT or CBCT data was compared between the systems, regarding the options to select grey values for the display of distinct structures. Grey values for a segmented display of anatomy were selected manually or with pre-settings for certain structures (e.g. skin, bone, teeth). The selection of three-dimensional display options of CT or CBCT data, the availability of cross-sections as well as their setting and the orientation of models with the help of standard planes and views were assessed (Table
2).
Table 2
Assessment criteria for visualization of imaging data
visualization of dental models | 2D display 3D display manual rotation and translation transparency selective display of initial situation and set-up |
visualization CT or CBCT data | orthopantomographic view 2D cross sectional images 3D model rendering automatic and manual segmentation (grey value adjustment) individual editing of imaging artifacts bone density measurement |
CAD/CAM of drill guides
The spatial coordinates of the planned implant position were used for CAD/CAM of the drill guide. The possibilities for its design were examined for each system. The provided tools for fit, support and material thickness were documented. The options of in-house (individual) or centralized production of the drill guides were assessed (Table
3).
Table 3
Assessment criteria for automatic and manual drill guide design and production
drill guide design and production | supporting structures (teeth, bone, mucosa) guiding protocol (guided pilot drill, guided drill sequence, guided implant placement) export of drill guide design data set individual design and production of drill guide central design and production of drill guide |
Discussion
All tested implant planning systems used CT or CBCT DICOM data for bone diagnostics. None of the systems offered a proprietary CBCT scanner. To the knowledge of the authors, proprietary CBCT scanners are so far not available for any of the systems. Three-dimensional reconstructions and multiplanar cross-sections oriented along the alveolar process in the implant region were available in all systems to review important parameters for the implant position [
25,
26].
With the clinical patient examples chosen in this study, imaging artefacts occurred distorting the tooth surface and bone volume. The examined software systems provided automatic segmentation of bone, teeth or soft tissues; however due to artifacts these default settings could not be used to display specific anatomical structures. Manual segmentation by limiting the window of grey values for the display of three-dimensional models was necessary and possible in all systems. Two systems did not offer tools to manually edit display of imaging data and two of the implant planning software provided tools for bone density measurement. Studies regarding grey values in CBCT data showed that they cannot be standardized and allocated to specific anatomical structures as in CT. Therefore, Hounsfield units used for interpretation of CT data are not applicable for CBCT data and bone density measurements in CBCT are not reliable [
27].
The import, segmentation and pre-processing of radiographic data is crucial for the accurate transfer of the planned implant position to the surgical site. Radiographic data and virtual dental models are aligned with each other using either the tooth surface displayed both in CT or CBCT and in virtual dental models [
9,
28] or with the help of reference markers in a radiographic splint [
11,
12,
15,
29,
30]. Both workflows were available with the tested implant systems. Registration without a radiographic splint appears to be less time consuming as all examinations may be conducted without the preparation of a radiographic splint on a stone cast. However, misalignment between CT or CBCT and virtual models is known to occur after registration depending on the number of existing metal restorations [
9].
The use of either an intraoral optical scan, or an impression or model scan, respectively, to produce a virtual dental model is freely selected by the user if the data is imported in the STL-format. One exception was found for NC, only importing virtual models in a proprietary data format generated by a system-specific model scanner. Intraoral scans including information on the color of teeth and intraoral soft tissue (Trios, 3Shape) were only compatible for IST planning software. The use in a third-party software was only possible after export to an stl-format that does not contain texture information. Therefore, implant planning with consideration of tissue quality is hitherto only possible with one system that includes a proprietary intraoral scanner with texture information (IST).
Intraoral optical scanning reduces the steps and therefore time expenditure to obtain virtual models [
31,
32]
. Besides the promising efficiency of intraoral scans, the accuracy of intraoral optical scanning is still not validated in vivo. In contrast, extraoral optical scanning of stone casts showed high accuracy (10 μm) [
33]. However, the possible inaccuracy of a conventional intraoral impression and stone cast production are not included in the aforementioned studies. Inaccuracies of conventional intraoral impression should therefore be considered, when comparing the accuracy of intraoral optical impressions with extraoral model scans.
Depending on the used implant system either single steps or the full drill sequence and implant insertion is performed through the drill guide [
10,
34‐
36]. The examined software systems allowed guided implant placement for a various number of integrated systems except for one implant system (NC) that only offered guided implant placement for its proprietary implant system. The selection of implant systems for which guided implant placement was provided was restricted and did not correspond to the number of systems offered for visualization. The selection of an implant planning software is therefore dependent on the specific implant systems used in the daily routine.
The support of the drill guide on teeth and mucosa, respectively, allows a more accurate transfer of the implant position than bone support [
19,
37,
38]. The user could choose between the three bearing surfaces with exception of two systems (NC, IST), where no bone support was possible. Furthermore, pins or provisional implants could be inserted with all systems to help the fixation of the drill guide during surgery as suggested previously by other authors [
39,
40]. Individual design of drill guides allowed the user to select bearing surfaces depending on each patient case. Whereas a closed guide design is suggested by most systems (NC, SIM, CDX, IST) an “open frame” design can be advantageous for more visibility, accessibility and less risk for interference with hard or soft tissue. Therefore, the insertion of windows in the closed design becomes important. With central design and production of drill guides, the user has to forward individual information regarding any specialties in the design prior to fabrication. The time consumption for personally designing and/or manufacturing of the drill guide and the cost of the software should be considered by the user, when using or choosing a virtual implant planning software. Two systems did not allow to individually plan nor individually fabricate the drill guides at the time of data collection (SIM, NC). To the knowledge of the authors, more recent versions of both software systems allowed individual production of the drill guide.
It has to be mentioned that user experience plays an important role in every CAD software. Depending on the user’s experience, their affinity to digital products the learning curve can vary. In summary, the authors find one planning software more intuitive than the other, which is very subjective. Before chosing a system it is recommended to test as many as possible to find a satisfactory product.
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