In this third article in my ongoing series about FPDs vs Implants, I want to address two specific risk factors concerning FPDs: the length of the overall span (e.g. number of pontics) and the risk of weakened abutments from endo and post-cores. As in the previous articles, I am relying on well-done literature references to identify what we can expect in these different clinical situations.
Whenever a discussion of length of span comes up in dentistry, it is not uncommon to hear Ante’s Law brought up, which was published by Irwin H. Ante in a thesis in 1926 (1). The thesis stated:
"the total periodontal membrane area of the abutment teeth must equal or exceed that of the teeth to be replaced."
"the length of the periodontal membrane attachment of the abutment tooth should be at least one half to two thirds of that of its normal root attachment"
The net effect of Ante’s law was the utilization of multiple abutments to increase the periodontal membrane surface area of the abutments, something that significantly complicates the FPD and may lead (in some cases) to earlier failure.
Suffice it to say that multiple clinical studies have refuted Ante’s concepts, as was beautifully illustrated in the 2007 systematic review referenced below (1).
The article I am going to use to help us understand the risks of length of span and abutment teeth with posts and cores was published by a group in Belgium in 2007. The beauty of the article is that it looks at the following:
- 20-year survival rates for single crowns on vital teeth or teeth with posts and cores
- Three-unit tooth-supported FPDs with vital abutments vs. one non-vital abutment with a post and core
- Tooth-supported cantilever FPDs with vital abutments or non-vital abutments with post and cores
- Four or more unit tooth-supported FPDs with vital abutments, vs. with one non-vital abutment
As in previous articles, I have color-coded the results. For single crowns at 20 years, whether the tooth was vital or had endo and a post and core, the success rates for either situation were not statistically different, both being near 80 percent. For three-unit FPDs, however, the success rate at 20 years was dramatically effected by the presence of one abutment having endo and a post and core, dropping from 83 percent for vital abutments to 60 percent if one abutment was non-vital and had a post and core.
The impact is equally significant for four or more unit FPDs or cantilever FPDs. Dropping from 77 percent for four or more unit FPDs with vital abutments to 56 percent with one non-vital abutment with a post and core, and from 73 percent for cantilever FPDs with vital abutments to 52 percent if there was one non-vital abutment.
This literature is significant to me in discussing implants vs FPDs to patients, as it provides evidence that allows us to inform them of the future risks of choosing an FPD if one of the abutments is non-vital and has a post and core.
The patient below is an example in which this literature was very helpful. He presented with an existing four-unit FPD from lateral incisor to lateral incisor, replacing the right central. In other words, the left central and lateral are used as double abutments.
At his initial exam, there was chipped porcelain on the left lateral incisal edge, and the left lateral had come loose and could easily be pumped up and down. In addition, a radiograph revealed the right lateral had endo and a large post and core.
This patient had been presented an implant treatment plan by another dentist and refused it, knowing it would require bone and soft tissue grafting prior to implant placement (both an expensive and time-consuming course of treatment in his mind). He was adamant that he wanted a new FPD.
My personal belief is that my role is never to select or dictate a patient’s treatment plan, but instead to provide the options that I think are acceptable and then present the pros and cons of each plan. For him, I presented a new FPD as one option, but used the literature to show him how much risk there is in redoing an FPD that has already failed with a new one on the lateral with a post and core. In addition, I presented the implant option, which in my opinion was far preferable. He wasn’t swayed by my proposal.
Realizing that, for either option, I needed to remove the existing FPD and make a provisional, I convinced him to let me do that. I could then further assess the right lateral condition, but also find out if the left lateral was actually un-cemented or if the prep was fractured. He agreed to let me remove the FPD and place the provisional, assuming we would move forward with a new FPD. Below are photos of his existing FPD pre-treatment and what I found after removal. Note how minimal the ferrule is on the right lateral.
I took the photos of the preps you see above and walked him through what I saw, emphasizing again the high risk of doing another FPD as opposed to the implant. Ultimately, he agreed to see a surgeon for a consultation of what was involved and what the cost was for the implant, and in the end chose to go through with the surgery and implant placement. In my opinion, this would have been a very high-risk case to simply redo the FPD.
Clin Oral Implants Res. 2007 Jun;18 Suppl 3:63-72. Ante's (1926) law revisited: a systematic review on survival rates and complications of fixed dental prostheses (FDPs) on severely reduced periodontal tissue support. Lulic M1, Bragger U, Lang NP, Zwahlen M, Salvi GE.
(Click this link for more dentistry articles by Dr. Frank Spear.)