Success of Digitally Designed Screw-Retained Implant Restorations

Two factors have a great influence on the success of the digitally designed screw-retained implant restoration. The first is the availability to select the best implant component for the individual restoration. We must understand that the components available for the restoration are driven by the scan body used and library that is tied to the scan body. The other factor of success is the cemented bond of the Ti Base to the zirconia crown. It is never a good day when you receive the dreaded phone call from the doctor saying a Ti Base that you cemented debonded from your restoration.

Fig. 1

The first step, as with all restorations, is proper planning and quality evaluation. After determining that a screw-retained restoration is best, the next step is entering the digital workflow. The restorative control of these implant cases is largely determined by the scan body and library options connected to any given scan body. I have worked with many different libraries and many different implant companies. Sometimes I do not have a choice of what library or Ti Base to use. This can be because the dentist has completed an intraoral scan with a scan body they chose or the dentist prescribes a specific component be cemented in the restoration. Many times, though, my clients ask for my input and guidance. I have created relationships with my dentists and want to communicate restoration expectations before we enter a workflow. One thing I stress to my clients is maintaining flexibility. Preat Corporation’s library has been one of the best resources for maintaining flexibility on implant cases in a digital workflow.

Fig. 2
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The Preat Choice Library (Fig. 1) makes my job easier by creating workflow flexibility while allowing the restorative team to select the best implant component for the specific restoration I am working on. My dentists can scan intraorally with a Preat scan body or take an analog impression and I can scan the model in the laboratory. It does not matter to me. My goal is to allow my dentists to be comfortable and confident in the process. After scanning, I can use 3Shape to evaluate the patient and design the restoration as it should be for the best results. This is why I dig the Choice Library. If my clients want a provisional, the library has temporary cylinders (Figs. 2-3) for that type of material selection. The best part is the height variations available in the library. I get cases with minimal vertical space and cases where I need the tallest implant component possible. Because I have three height selections, I can choose what is best without having to rescan, or worse, ask the dentist to bring the patient in and perform another intraoral scan.

If we are working on a zirconia screw-retained crown or bridge, using the software for a diagnostic communication device ensures that my dentists will not have any surprises when the case is delivered to the office. I can show dentists the expected  results of the restoration with a straight screw access channel versus using an angled access channel (Fig. 4). The difference between the Ti Base selection can be the difference from a disappointed patient and a patient that receives a life-altering restoration.

At the laboratory, we are also operating a business, and want maximum return from a limited investment. This is how I view the Dynamic X 9mm Ti Base (Fig. 5). With one Dynamic X 9mm Ti Base, I can design a restoration with a straight access channel to an access channel angled up to 30 degrees. I can evaluate the intraoral space and choose between a 5mm tall base, 7mm tall base, or 9mm tall base (Fig. 6). I can get all of the solutions from the use of the library and by altering the height of the Dynamic X 9mm Ti Base to any of the markings (Fig. 7).

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Fig. 8a
Fig. 8b

After I have completed the design, it is off to the mill. I will stain the restoration and then sinter the crown or bridge. Now it is time to bond the Ti Base to the restoration. Ti Base debonding seems to be a frequent topic of discussion in the industry. With so many materials and techniques out there, it is hard to know the good from bad and which cements work and which ones do not. At one point, I had no idea and used cements that I thought were good. Sadly, I found out they were not. I asked my colleagues for their recommendations, finding out that most were using different materials than I was. Their choices in cements were more expensive, but I asked myself, what is worth more? Saving a few dollars on cheaper cement that did not meet the needs of the task or losing valuable client accounts? The right choice was clear; get the better cement and use a consistent technique! Some may argue this, but knowing I am using the right material and refining my techniques puts my mind at ease. My process for cementing Ti Bases to zirconia crowns is done using Ivoclar’s Multilink Implant HO 0 zirconia and Panavia’s SA Cement Universal handmix white. For this case, I have designed the crown to a Dynamic Abutment Ti Base using Preat’s Choice Library.

Even with the right cement, the cause of most failures is a loose-fitting relationship between the crown and Ti Base. Make sure that the Ti Base and the crown seat fully; the space between them should be minimal. After the crown is properly fitted, proceed to clean up and polish/glaze with no overglaze in and around the seating area of the Ti Base (Fig. 8a-b).

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Fig. 10

Now we are ready to prep for cementation. Sandblast the inside of the zirconia crown (Fig. 9), and if you like, sandblast the mating surface of Ti Base as well. Since the Dynamic Abutment Ti Bases have horizontal grooves to provide a retentive surface, I leave them untouched. If choosing to sandblast the Ti Base for an added retentive surface, I like to apply White Effect 2 from Origin to the milled zirconia while still in the green state. This applies a white opaque layer on the inside of the zirconia crown, masking out the greyness of the abutment and keeping the value of the crown the same. Next, steam both surfaces and dry with an air hose. If you like to anodize your Ti Bases, feel free to do so at this time. One of the things I like about the Preat Ti Bases, besides the screw access channel correction, is that they are already anodized and this saves me valuable time.

Lay out all of the needed items at your workstation so you are not in a panic trying to find things in the middle of cementing. You will need the crown, Ti Base, lab screw, implant driver, the model, your primer and cement, an application brush (available from most dental supply companies), a mixing slab you love and/or a mixing pad, a knife or tissues, tweezers or crown holder, and a mixing instrument (Fig. 10).

Fig. 11

With the parts prepped and cleaned, you are now ready. Prime the restoration surface so that the cement will adhere as intended. NOTE: Not priming the surface will lead to a debond. Using the application brush, apply the primer to the inside of the zirconia crown and the chamfer area (Fig. 11).

Fig. 12

For the Ti Base, use small tweezers to hold it while applying the primer (Fig. 12). Let the primer set for 60 seconds then blow with air. Side note, if you are cementing PMMA crowns, the cement will work fine, but you must use an acrylic primer like SR Connect from Ivoclar. Otherwise, cement will not adhere to the crown.

Fig. 13

With both surfaces “primed,” you are ready to apply cement. The cement may or may not come with mixing tips. Personally, I do not use them, as in my experience they waste cement. The amount lost in one tip is the equivalent amount to cement eight crowns if not more. Because of this, I dispense equal amounts and hand mix thoroughly (Fig. 13).

Fig. 14

With the cement properly mixed, use an instrument and apply cement to the Ti Base (Fig. 14). Applying the cement to the inside of the crown results in a messier application and a greater chance of cement going into the screw channel. Another advantage in applying to the Ti Base is that it will push the cement down and out, leaving cement in the needed areas and an easier clean-up.

Fig. 15

Once cement is applied to the Ti Base, insert it into the crown, looking for the cement to push down and out (Fig. 15). For now, you can leave the excess cement and simply scrape it off with a knife once it starts to cure. This is where I like to take a tissue or an application brush and quickly wipe the excess cement off. Check to make sure no cement made it into the screw channel from the top of the crown. Application brushes work well for cleaning excess cement out of the screw channel.

Fig. 16

Place the cemented crown with Ti Base on the model to assure that it is properly aligned and tighten it down with the lab screw and driver (Fig. 16). Applying an oxygen air barrier gel at the margin before curing will aid in a better seal at the Ti Base junction. After the crown is seated, you can cure it with a light cure gun or light box, or let it sit and self-cure if dual-curing cement. Finally, clean up and polish the restoration as desired. By knowing your materials and refining your techniques, you have achieved the best bond with a repeatable process.

A successful relationship between dentists and technicians relies on trust, communication, and understanding each other’s expectations. After discussing expectations with my dentists, we choose to use the workflows and materials that provide predictable results. By delivering a restoration I can be confident in, my dentists can have peace of mind that their patients are receiving esthetic, functional implant restorations.

About the Author

Sevan Pulurian, Director of CAD/CAM at Pacific Dental, Inc. has 20 years in the industry. He is a dedicated and innovative individual. Sevan has influenced and improved the dental CAD/CAM technology industry since early 2002. Now a 3Shape Specialist and K.O.L., Sevan manages the CAD/ CAM Department at Pacific Dental, where he strives to provide doctors and their patients with high-quality work and digital craftsmanship and is always pushing the envelope to better digital dentistry.

Sevan Pulurian