As guided surgery continues to grow in the dental industry, it is becoming common knowledge that a CBCT and diagnostic cast are the basic requirements to have a 3D fabricated surgical guide created.
However, digital dentistry demands digital files and today's technology has provided numerous ways to acquire that. But how is a clinician to decide which route to take? Should the clinician take a traditional impression or invest in an intraoral scanner? If the clinician does choose to take a traditional impression, should he/she then invest in an optical scanner? Or try to utilize model scanning features that many CBCT units are starting to offer? To identify the pros and cons of each option, one first needs to understand the role a diagnostic cast plays in surgical guide design and the nuances involved in that process.
A common (and fair) question is why a diagnostic cast is even needed for surgical guide design? A CBCT provides extensive anatomical information and the latest CBCT units can create impressive 3D renderings of the patient’s anatomy. Can a surgical guide not be created from that information? The short answer is unfortunately, no. While CBCT imaging has advanced substantially, the accuracy of the 3D renderings is still not fine enough for the millimeter precision that is needed for implant surgery. That combined with the fact that one can usually manipulate the threshold of the 3D renderings to adjust the thickness/density of the 3D images creates way too many variables for a consistent and accurate guided solution. Therefore, for now, we are still to rely on the diagnostic cast and the integral role it has in the fit and the accuracy of the surgical guide.
It is obvious that an accurate diagnostic cast will lead to a well-fitting surgical guide. However, it is less obvious how a well-fitting surgical guide can still lead to an inaccurate implant placement. That is because the accuracy of the diagnostic cast is only half of the equation. The other half is the merge between the diagnostic cast and the CBCT itself. The combination of these two pieces of information has many names: merge, overlay, superimposition, registration, etc. Essentially the process involves superimposing the two images and inspecting for discrepancies to ensure the diagnostic cast is accurate before proceeding to surgical guide design. The registration plays a crucial role in verifying the diagnostic cast is true to the patient’s existing anatomy and any discrepancy between the anatomy on the CBCT vs the anatomy on the diagnostic cast lets the designer know that a new impression needs to be taken. Unless the CBCT needs calibration, the inaccuracies of the registration generally arise from the diagnostic cast. However, that is not to say that lack of calibration is the only factor that can cause discrepancy coming from the CBCT. Movement, lack of definition, and excessive scatter are all examples of CBCT concerns that negatively affect the accuracy of the registration.
The 2nd purpose of the merge between the CBCT and diagnostic cast is to ensure the accurate translation of the virtual treatment plan to the actual live surgery. The implant placement is planned on the CBCT but the surgical guide is designed on the diagnostic cast. The merge is the bridge between these two pieces of information that ensures that where the implant has been planned in the CBCT will be where the physical implant will be delivered during live surgery via the surgical guide. If an accurate diagnostic cast is submitted but the merge is off during the design process, the result is the clinician receiving a surgical guide that fits beautifully but the implant position is completely off. This is where the value of an experienced designer cannot be understated. While software has advanced tools to provide impressive assisted merging, it is still not a perfect science. Scatter, distortion, and partial scans are examples of factors that can interfere with the software’s ability to accurately merge. This is where the experienced designer can either correct the merge or make the judgement call to acquire a new diagnostic cast.
So, we have established that the registration determines the accuracy of the surgical guide in terms of the delivery of the osteotomies and implant. We have also established that an accurate diagnostic cast results in a well-fitting surgical guide. Now the question is which method of capture yields the most accurate diagnostic cast: intraoral scan or traditional impression? Let’s explore the pros and cons of each!
As you can see, both options have the potential to yield an equally accurate result. However, from our experience, intraoral scans have proven to be superior due to their consistency and efficiency. For a clinician who has not yet incorporated intraoral scanners into their practice, the following options are available to digitize a physical diagnostic cast:
Ultimately, the technician designing the surgical guide will be the final evaluator of the quality of the diagnostic cast. That may be your local lab, a 3rd party company such as Implant Concierge, or even yourself as the clinician should you choose to take on that responsibility! Even if you are planning to outsource the surgical guide design, it is beneficial to be familiar with common issues to look out for when evaluating a diagnostic cast. This will help avoid secondary appointments for new impressions and will save both the office and the patient time and money. Here are some common problems we encounter at Implant Concierge:
Problems with the stitching or triangulation will need to be addressed to the intraoral scan manufacturer for calibration and troubleshooting. From our understanding, stitching can also be caused by the quick transition from one surface to the other by the technician. Missing some information in that quick transition causes the software to “stitch” one surface to the other causing that line that is commonly seen in the occlusal/incisal edges.
Traditional Impression
The concavities that result from bubbles, chips, etc. will be filled in with resin during the guide design step. Since these voids are not actually present in the patient’s dentition, the resulting artifacts on the surgical guide will prevent the surgical guide from seating properly or at all.
The protrusions created by drag or excess material on the diagnostic cast can result in a loose-fitting surgical guide. If minor, sometimes this can be negligible but when the artifacts are extensive the guide can be loose to the point of rocking.
In conclusion, there are many variables to consider when taking an impression with a guided solution in mind. Those variables will be dependent on the method of capture, physical vs digital. Regardless of the route chosen, the accuracy and registration of the diagnostic cast will be the ultimate determining factor of the success of the surgical guide. At Implant Concierge, we have several steps of quality control checks to ensure stable and accurate surgical guides. Implant Concierge is here to help support clinicians in education, guided surgery evaluation, and to serve as a comprehensive digital dentistry concierge service to help offices integrate and succeed with guided surgery. Please do not hesitate to reach out to us at any time for more information on this topic or anything related to digital implant dentistry!