August 6, 2019
by Amarjit Rihal, DDM
The evolution of implant treatment planning has changed over the course of history. From the start, implant dentistry was considered to be surgically driven. Our current standard of care is three-dimensional placement of implants being gingivally driven. By using the latest in intraoral imaging, we can now virtualize the ideal gingival zenith position and appearance of the implant restoration. From that position, we can then plan and manufacture the surgical guide to place the implant to support that prosthetic outcome. The key to this planning comes from knowing the final gingival zenith position in our gingivally drive treatment plan. Technology has advanced rapidly and the level of integration now available allows us to fabricate surgical stents and produce them chairside with in-house milling or 3D printing with the same degree of accuracy as 3rd party manufactures. Surgical procedures then become less invasive in nature resulting in less patient morbidity and fewer complications. This article will explore the digital work that is found with CEREC guide 2, from digital wax up to manufacturing to surgery to placement of the final screw down restoration.
Rational and Accuracy
The increase in accuracy that is achieved from guided surgery has also been found to have a degree of error associated with it. When looking at a review of literature reporting deviations in preoperative implant planning and postoperative implant locations, the comparison of implants showed a mean linear deviation of the implant head of 0.56 mm (standard deviation [SD], 0.23), a mean linear deviation of the implant apex of 0.64 mm (SD, 0.29), and a mean angular deviation of the long axis of 2.42° (SD, 1.02) (Fig. 1).1,2
Computer-aided guided implant surgery seemed to provide several advantages to the clinicians as compared to the standard procedure; however, linear and angular deviations are to be expected. Therefore, accurate pre-surgical planning, considering anatomical limitations, and prosthetic demands is essential to ensure a predictable treatment. Variations were found to be clinically significant and measurable in most studies involving computer-aided guided surgery. When compared to non-guided implant surgery, that degree of error was far less even for an experienced surgeon. As a result, computer-aided guided surgery has become a common standard of care for both novice and experienced surgeons alike.
Conventional vs. digital workflows
When looking at computer-aided planning, the main advantage over traditional analog techniques are:
• The ability to scan patient/model and digitally ‘wax-up’ the proposed prosthetic position of the final implant restoration.
• The ability to take a 3D CBCT showing the bone volume and anatomic structures
• The ability to combine the patient/model scan and 3D CBCT showing accurate tissue and tooth anatomy superimposed over the 3D bone volume
• The ability to manipulate this model 3 dimensionally over a variety of different planes
• The ability to measure locations and relative distances of anatomic structures
• The ability to virtually place an implant in its ideal location while viewing the above structures
• Having the software design the surgical guide at a preset drill length to ensure accurate placement of the implant in its virtually desired position.
DentsplySirona’s CEREC guide 2 has several distinct advantages when it comes to their
digital workflow. It allows you to remain in a digital environment without using conventional models and provides you with two options for in-office manufacturing: milling of the surgical stent and/or
3D printing. See Figure 2 for the CEREC Guide 2 digital workflow.
Let’s look at an example of the digital workflow. Our patient Bonnie had a non-restorable #24 and requested an implant-supported restoration as a replacement. The treatment plan was to do an extraction and simultaneous bone graft using 0.5 cc Symbios autogenous bone graft with a dual membrane technique using a Geistich Bio-Guide absorbable collagen and a Cytoplast non-absorbable membrane on top. The membranes were secured with cross and independent sutures and were removed after three to four weeks along with the external Cytoplast membrane. The advantage of this technique is that it forms a mucopolysaccharide fibroblastic barrier, and there isn’t a need to attain primary closure. The site was then left to heal for a period of five to six months. After healing was complete, the treatment plan was to do a CEREC Guide 2 placement of a Nobel Parallel CC 3.75 mm x 10 mm implant with a Sirona Ti-Base Emax screw down crown.
1. Implant treatment plan:
With complete healing of the surgical site a 3D CBCT image was acquired and using the measuring tool in the Galaxis software the bone volume was measured and an appropriate implant size was planned. See Figure 3 for cross-sectional measurement of the bone volume.
2. Intraoral scanning and digital models:
After the CBCT was measured an intraoral scan was then acquired on a CEREC OmniCam in the chair side SW4.6 software. Note that this step can be done by scanning a study model as well and we typically use both options interchangeably. The importance of this step is that it helps define accurate tooth anatomy and tissue contours and allows the intaglio of the surgical stent to be modelled to this anatomy. When stitched with the CBCT volume it also acts as a reference for where the final gingival form of the final restoration should be. This is important since the correct three-dimensional placement of the implant is based on this final gingival zenith position. When setting up the CEREC scan in the administration window, we prescribe a crown on #24 then proceed on to the acquisition window where we scan the maxillary arch, mandibular arch and the buccal bite when the patient is in maximum intercuspation (Fig. 4).
When scanning in the chairside CEREC software, there is a workflow protocol when fabricating a digital wax-up. The different phases are acquisition and model phases where the digital images are scanned and the design and manufacturing phase where the digital wax-up is generated. The practitioner has to follow the steps in the top window of the program and as shown below.
3. Digital Waxup:
Traditionally a wax-up involves fabrication of regular models. The advantage of a digital environment is that you don’t need to fabricate any models and the digital wax-up can be done immediately after you scan the arches. The intent in the acquisition phase when specifying a crown preparation on tooth number #24 is to fabricate the crown digitally. When fabricating a regular crown, we specify the margin by tracing it with our cursor on the prepared tooth, here we simply draw a circle on the edentulous area much like a crown margin. CEREC will propose a crown within the line you draw, thus completing your digital wax up. It is important to note that the buccal line you specify represents the final gingival position for the proposed final implant retained crown. This line will become important when placing the implant three dimensionally (Fig. 5).
In the Model Phase, after the gingival margin is traced, we advance to the Design Phase. CEREC will propose a digital wax-up of a tooth within the margins the practitioner specified. This digital tooth is in full occlusion and represents what our end screwed down crown should look like in both form and function. If there are any adjustments that need to be done, there is a variety of digital tools that could be used to modify the shape and form of the crown proposal. Care must be taken to ensure that this proposal is as close to perfect as possible since we will be using it to reference the ideal position of our implant.
The final stage is the Manufacturing Phase. Instead of milling the proposed crown, we are going to export the case as an .SSI file. This is CEREC’s prioritized file type that contains the tooth/gingival morphology and the digital wax-up as separate layers. This file is then going to be imported into our CBCT in the next step.
4. Pre-operative CBCT acquisition
With the advent of CBCT technology, the ability to render a bone volume three-dimensionally has changed the treatment planning protocol with guided surgeries. The overall intent to avoid anatomy and vital structures can now be measured, forecasted and surgically approached with less overall complications and morbidities. The advantage with CEREC and Galaxis software is the seamless integration or ‘stitching’ of virtual wax-ups to the CBCT volume through the import of the .SSI file. Every step stays in a model-free digital environment and there are no applications that need to be sent to a third-party lab, thereby increasing the practitioner’s efficiency. For Bonnie, we began by taking a small field of view (FOV) CBCT of the second quadrant, ensuring that we capture the full edentulous space, adjacent maxillary sinus, and adjacent teeth (Fig. 6). In the Galaxis software suite, we can adjust the three-dimensional volume and perform measurement calculation directly on any plane. In Bonnie’s example, we performed buccal-lingual width measurements and coronal axial length measurements, looking at the axial slice view.
5. Data merging
As we stated earlier, the correct three-dimensional placement of an implant is dictated by the desired buccal gingival position in the digital wax-up .SSI file (Fig. 7). We begin this analysis by clicking the CAD/CAM icon and importing the .SSI file. Once the file is located and the volume is loaded the Galaxis software will ask you to define a minimum of three like points on a panographic rendering of the CBCT digital wax-up file as shown below.
Once a minimum of three like points are defined, the software algorithmically will ‘stitch’ or superimpose the two objects in the best-superimposed fit. The practitioner confirms this by looking at the lines to ensure the digital wax-up lines match to the teeth from the CBCT. If the stitch doesn’t seem to correlate correctly, the practitioner can go back and use additional or different reference points. Once confirmed, you would see the CAD/CAM data superimposed in the CBCT volume as shown in Figures 8 & 9.
As you can now see in the cross-sectional view, there is a line that represents the digital wax-up and the exact tissue outline which we will use to place the virtual implant. We have successfully merged that digital wax-up data to the CBCT.
6. Virtual implant placement
Many compatible implant systems work with CEREC. In Bonnie’s example, we will be placing a Nobel Parallel CC conical implant. In Galaxis there is a dropdown menu where you select your implant system and type of implant that you would like to use as shown in the graph below (Fig. 10). You can also specify the diameter and length of the implant.
Once the implant selection is confirmed, it is a matter of the practitioner merely using his/her mouse and dropping the implant on any one of the sectional view windows (Fig. 11). The software will automatically render the implant in all planes and with the addition of an ‘implant align’ button, the CBCT will be automatically orientated to show the implant in full view in each of the sectional view windows. Now the practitioner can grab the implant in any window and manipulate its position in three dimensions.
Using all the merged digital data, virtual implant placement can now be completed by the practitioner. The correct 3D placement would allow enough room for the abutment to have an emergence profile that is similar to the anatomic emergence of a natural tooth. The inclination of the implant would allow the screw access hole to emerge from the center of the restoration. The resulting dimensions are measured against the gingival complex from the stitched digital wax-up resulting in an implant placement that is 3.0 mm apical to the optimal gingival zenith point For a description of what the lines in the CBCT mean, see Figures 12 & 13. If the implant is placed greater than 3.0 mm to the desired gingival zenith, there may be a possibility of mucosal recession as the bone remodels. The result can be a crown with a longer incisal-gingival dimension compromising our aesthetic outcome.3 When looking at the mesio-distal spacing of an implant to a natural tooth, 2.0 mm of space would be ideal, so there would be sufficient mucosa present to fill the cervical embrasure with papillae.4 The proximal contact between the natural tooth and the implant restoration also plays a vital role in papilla formation. In a study of 288 implant sites over 30 patients, if the proximal contact was 5 mm or less from the height of bone, the papilla was present almost 100% of the time. The papilla was present 56% of the time if the distance was 6 mm and 27% of the time if the distance was 7 mm.5 In the Galaxis software, the complete relationship can be easily viewed by the practitioner, as shown below.
Now that the implant is positioned in the correct 3D position to achieve an optimal aesthetic result, we have to decide on the drill length that we will be using for our guided surgery. In this particular case, we will be using Nobel Biocare’s surgical kit with a drill length of 23 mm. To enter this information into Galaxis, the ‘collar’ button is selected (Fig. 14). A selection window will pop-up and you have to select two choices. The first is the diameter of the CEREC guide hole; small, medium, or large. In this case, we are doing a narrow diameter implant (3.75 mm) so the small setting would be appropriate. The next item to enter is the drill length as the D2 value. Since we are using sleeves with the Nobel drill kit, we have to compensate for the height of the sleeve (1 mm); therefore, the D2 value is the height of the drill minus 1 mm. The following pictures show the selection window and the corresponding drill lengths.
Once the collar selection and D2 value have been made, the planning file is ready to be exported for the fabrication of the surgical guide. The saved file is a .cmg.dxd file and contains the virtual position of the implant, digital wax-up, and the scanned patient model. Some additional benefits at this time include a digital treatment plan that can be incorporated into the patient’s digital chart. Figure 15 shows a 3D rendering of the above information which is a great communication tool for your patient.
7. Digital surgical stent design
The next step would be to open the exported cmg.dxd file in the chairside CEREC software on the OmniCam. Here the practitioner can custom design the surgical stent for fabrication and include a variety of different elements into the design. When the file opens, the practitioner goes through the same elements of design as he/she did with the fabrication of the digital wax-up. It begins with an administration, model, design and manufacturing phase; you have to follow the steps. Figure 16 shows the administration window.
In the administration phase, the practitioner will select a CEREC guide bloc to be milled in the Chairside MXCL mill as the following picture illustrates.
In the model phase, the practitioner simply needs to orientate the model as if he/she was mounting a stone cast on a conventional articulator (Fig. 17). The importance of this phase is to establish the horizontal level of the digital scan so the guide can be correctly oriented. The next option will be to set the desired thickness and virtual die spacer for the surgical stent. In Bonnie’s case, we chose a thickness of 3 mm and a spacer of
As you advance forward, the next step will be to outline the extension of the surgical stent. This is achieved by clicking and creating an outline on the screen. The practitioner will also have the ability to view the drill key outline and add drill slots on the buccal or lingual to help with drill access (Fig. 18).
The final step in the design window will be the fabrication of the CEREC guide. At this time the practitioner may choose to redefine the borders and include inspection windows for ease in seeing if the guide is fully seated during surgery. When we advance to the manufacturing phase, you will see the virtually designed surgical stent placed in the block ready to be milled. At this time the practitioner needs to set the block into the mill, close the door, and hit the start button. The mill will take care of the rest and the milled fabrication of the stent will be completed in approximately 40 minutes.
Once the surgical guide has been milled, it will be separated from the block and any sharp edges will be smoothed. Conventional disinfectants/serializing techniques may be used. In CEREC guide 2 digital workflows, the drill key support will be a small, medium or large size and Sirona will have a corresponding key set that will intimately fit the drill key support for your compatible implant system. Once the key is engaged, the practitioner has to choose the drill size on the key set and drill to the corresponding length marker on the drill as specified in our D2 value + 1 mm. The apical position of the drill at that length will correspond to the apical position of our virtual implant position. This helps ensure that our surgical placement of the implant will be as identical as possible to our virtually planned position. Figure 19 is an example of surgical guides and key sets.
8. Discussion on surgical options
With Bonnie, we were pleased with the outcome from our GBR and we were then faced with the question of whether to perform a flap or flapless guided surgery. When looking at conventional surgeries that compare a surgical flap that includes two interdental papillae to a modified flap that maintains the papillae, a reduction in crestal bone loss was noted when the papilla was maintained.6 As a result, more desirable aesthetic outcomes may be achieved with a more conservative surgical protocol. The maintenance of blood supply to our integrated bone graft is of primary importance, so a flap-less surgical approach was used. A simple tissue punch was used at the time of surgery instead of incisions.
When looking at a 10-year retrospective analysis of 770 implants, survival rates varied from 74% to 100%.7 Most of the failures occurred in the early years of this study and a variety of problems such as bone fenestrations, dehiscences, and complete implant loss was noted. When looking at surgical outcomes between flap and flapless surgeries, it was noted that the flapless approach had a ’learning curve’ but worked well with more experienced practitioners. One of the main concerns with guided implant placements with a flapless surgical procedure is the ability to have adequate irrigation to the surgical site. Care must be taken to ensure the proper drill rotational speeds combined with copious irrigation between and during the drilling sequence.
9. Flapless implant surgery
With Bonnie, we performed the guided surgery flapless and placed a 3.75 mm x 10 mm Noble Parallel CC implant with a 6 x 3 mm healing cap to allow for osseointegration. The torque value for the implant at placement was 50 Ncm. Although this was stable enough for an immediate provisional the patient wanted to allow for full integration to avoid any complications. The following radiographs (Fig. 20) are from the guided surgery.
10. Digital workflow
As you work in a model-free environment and by harnessing integrated digital technology, the digital workflow becomes very efficient. For Bonnie, the total time from beginning to end, including the manufacturing of the surgical guide, was 80 minutes. As you can see, the versatility of this workflows makes it possible to make guided surgery a same-day procedure. An alternative to milling the surgical guide is to 3D print the object. Many different 3D printers have entered the dental landscape and there are a variety of sterilizable resins that can be printed and used surgically. With any form of 3D printing, the post-print processing affects the quality of manufacturing of the surgical guide. Typically, this involves a wash in an isopropyl alcohol bath followed by a post-curing process in a light ‘oven.’ With 3D printing, the time to print is typically longer than milling (1-3 hours), and the post-print treatment prevents it from being a same day procedure. Figure 21 illustrates the digital workflow for single implant placement from virtual planning to placement using a milled surgical stent.
11. Time for the crown
After three months of integration, the implant was torque tested for Bonnie and healing was uneventful. With CEREC, there is an excellent chairside solution using Sirona’s Ti-base and Emax meso blocks for the scanning and fabrication of the implant restoration. As with the manufacturing of our surgical guide, the practitioner would follow the same design phases, the difference being in the Administration phase where we tell CEREC that we are doing a screw down Emax Crown (Fig. 22).
With Bonnie, we entered the chair-side software and entered the Acquisition phase. As part of our treatment plan, we are using a Nobel compatible Ti-base that come in a package containing the Ti-base, screw, and scan body that is used in the Scan phase. In the material window, we say we are using an Ivoclar Emax material.
In the Acquisition phase, we will capture the maxillary volume with the healing cap out, the mandibular volume, the 3D bite, and the scan body in the implant volume. CEREC will know how these volumes relate and will stitch them together to help us with our design.
In the model phase, we set the digital model to get ready for designing. We segment out the implant and make a virtual die from the digital model, and, we also click on the head of the scan body. The purpose of the click is to allow CEREC to determine the exact timing and orientation of the implant from the scan body geometry. You will see a graphical representation of the implant in the maxillary volume after this step (Fig. 23).
In the design phase, we can perform any tissue modifications or we can get a proposal for an implant crown, and by using our digital tools, we can ‘free hand’ our emergence. Before this happens, we need to decide on the path of insertion for the implant crown. When doing any screw down restoration, there are two paths of insertions; one follows the implant screw channel and the other the proximal contact surfaces of the adjacent teeth. For the restoration to be seated predictably, they need to be parallel. Since we had a virtual wax-up to dictate our guided implant position, you can see that these paths were parallel to each other. CEREC will then give you a proposal, and with the ‘tool’ wheel, we can change the morphology and contouring of the restoration. The ‘tool’ wheel contains digital tools that allow you to change the position and angulation, scale, anatomy and smoothness of the restoration and with a little practice, you can master the concepts of digital restoration design (Fig. 24).
Once the restoration has been finalized, you advance to the manufacturing phase to visualize the restoration in computer rendering of the Emax block. For Bonnie, the Emax was milled and crystallized and then bonded to the Ti-base ready for insertion (Fig. 25).
After the Emax restoration was bonded on the Ti-base we delivered the crown. After initial placement, there was some gingival blanching since the emergence we created was larger than the healing cap. Since we followed Dr. Tarnow’s principles and we made an allowance for enough gingival room and controlled proximal contacts you can see that the final gingival position of the healed implant crown was identical to our original digital wax-up (Fig. 26).
When looking at the versatility of the CEREC system, one can see that there is an efficient digital workflow that allows you to 3 dimensionally predict the final placement of the implant and forecast the aesthetics of the final restoration. Care must be taken to follow aesthetic guidelines and quantify the desired final gingival zenith when determining the 3.0 mm apical position of the implant. When using digital planning technology, guided implants can increase the accuracy and aesthetic outcomes that prior were more challenging to achieve.
About the Author
Dr. Rihal obtained his DMD degree from the University of Manitoba, Faculty of Dentistry, in 1995. His professional interest has involved all aspects of implant and CAD/CAM dentistry. He has additional training in both hard and soft tissue grafting as well as placing and restoring implants. In addition to implantology, Dr. Rihal has acquired expertise in CAD/CAM based prosthetic dentistry, involving numerous CAD systems and materials. Utilizing his CEREC ac, Sirona InLab system, 3Shape software and Nobel Procera scanner he has designed over 4000 restorations. He currently manages operations of his inhouse lab Lodge Dental, and is the owner of his own digital lab called Rihal Digital Designs.