Abutment Selection in Restoring Implants
What types of abutments should I use? This question has come up several times from my students on the topic of implant dentistry and I want to share my thought process I have when it comes to restoring dental implants.
First, there are stock abutments and there are custom abutments. I rarely use stock abutments because they are pre-shaped based on arbitrary assumptions of how an abutment preparation should look like. They may come in different heights but mostly they are an extension of the implant dimensions and often shaped like a cylinder or variations of a cylinder. If you go back to the basic principles of tooth preparation with resistance and retention form, you will soon realize these stock abutments may or may not have the ideal retention and resistance form for your particular implant site. They may be too short, lack anti-rotational element and does not support the full coverage restorations with proper strength depending on the material you have in mind. To me they require some form of modification and it’s never perfect if you want to develop the ideal emergence profile. So I never order stock abutments and prepare them chairside and treat the site like a conventional crown and bridge impression. Most of the time, I take a fixture level impression and make a restorative decision after the case has been mounted and evaluated. Most of the time, I use an abutment that will allow me to customize the shape in ways that will best support the restorative material I want to use.
And these customizable abutments are available for cast metal, milled metal or zirconia. At the beginning of my career, I used a lot of UCLA type of abutments where the lab can wax the abutment to ideal contour to support the porcelain. Whether it is used as a cementable restoration or screw retained restoration, this is my way for many years. Now with the high cost of metal and labour, and the movement to digital dentistry and milling restorations, I find myself using more milled titanium abutment to support the porcelain for my implant restorations. Depending on the size of your case, the bigger case may have to be outsourced to bigger milling centers to have them milled as one piece. It does take a bit more time especially during the pandemic.
I also do use zirconia as the restorative material. It is strong but I would look at the type of abutment that support these zirconia restorations carefully. I look at the base of the abutment and it should be a metal connection to the implants. Historically, abutments that are made entirely of zirconia material or high strength porcelain do not work well. I find that they will eventually break. And the industry has learned that as well. Most of the time, people will speak of zirconia restoration with a titanium base where there will be metal to metal connection to the implant.
It is also interesting to note the process of making these zirconia restorations may have led to a term “screwmentable” restoration where the crown is pre-cemented on the abutment in the lab allowing it to be delivered like a screw retained restoration. Although you can use this design for other material like PFM, this type of design has dominated the market for zirconia implant restoration. I find most labs will push for this type of restorations because of the cost effectiveness on their end and they are priced to encourage many dentists to use them as well. While I do use them for single restorations, I have my reservations for larger cases. For one, this design leaves another interface available for potential problem to occur. Most of these abutments designed for screwmentable design are cylinder in shape with various height and minor surface details. My concern of using these “stock abutment” is whether it has enough surface area to withstand the occlusal forces of the restorations especially on cases with long clinical crown height and susceptible to large lateral forces. And does the type of cement make a difference intra-orally? Although the actual restoration is to be retained by the screw, it still leaves the possibility of this cement layer to fail under loading. Anecdotally, I learned from a prosthodontic colleague who reported that his co-residents from the Grad Prosthodontic program were having to deal with many recently delivered full arch cases (within six months of delivery) where they were debonding from the abutments. There may be many factors responsible for this but it does leave me question about the long term success for this design for full arch cases.
The last thing I want to comment in selecting abutments for your implant restoration is to understand the difference between single unit abutments vs splinted unit abutments. For single units, one should use an abutment that has an anti-rotational element to it. The anti-rotational element will have to depend on the design of the implant. It may be part of engaging the external hex, internal hex or the trilobe shape of the implant at the connection point. By that, the abutment should be engaging the anti-rotational feature of the implant such that the abutment cannot rotate freely on the abutment when the screw is absent. This is an important concept to understand and I have noticed lab using the wrong components when I am dealing with prosthetic complications such as loose or broken screws. My recommendation is look closely at the base of the abutment for your single unit implant restoration and know the type of antirotational feature it has.
I hope the next time you have an implant restoration and want to know what abutments to you, some questions you want to ask are the following:
1/Are you using a stock abutment or custom abutment?
2/What type of restoration are you fabricating? Screw retained or cement retained?
3/What type of restorative materials do you want to use? Porcelain fused to metal? Full contour zirconia? Zirconia with layered porcelain?
4/What type of materials do you want your abutments to be in? Metal? Zirconia? Will it be cast or milled?
5/Is this a single or splinted restoration? Should there be any anti-rotational feature in my abutment?
I hope this post will give you new insight as to what to think about and to ensure that your lab has selected the right component for long term success. Thanks for reading!
For more information, please see my powerpoint presentation on this topic