University of Minnesota
Associate Professor, Division of Prosthodontics, Department of Restorative Sciences
For those data, very, very interesting. And of course, that leads us nicely on to John Schulte’s presentation. John is a prosthodontist, an associate professor of prosthodontics, and co director of graduate prosthodontics at the University of Minnesota. He is active in continuing education, and has presented nationally and internationally. John, please. Thank you very much and good morning. I hope you have enjoyed these presentations as much as I have. I feel that we have been given a wealth of information that we can take back to our offices to better serve our patients. I will be sharing with you the results of a very simple research project that was conducted by one of my graduate students. The title of that project is The Crown Implant Ratio as a Prognostic Indicator of Implant Survival. Now the primary reason that we chose this as a topic is because it is common practice for clinicians to use prognostic indicators that are associated with natural teeth and apply them to a dental implant restoration. For example, the prognostic indicator of the crown root ratio is frequently used to analyze a proposed implant site or existing restoration. As I was saying, the crown root ratio is one of those prognostic indicators. And, for example, when the crown of the tooth, the crown defined as that portion outside of the bone, is one half the length of the root, then the crown root ratio is point five to one. All normal, healthy, natural teeth have a crown root ratio of point five to one. When the crown equals the length of the root, we have a ratio of one to one. And most clinicians would assign a guarded or unfavorable prognosis to a tooth in this condition. When the crown of the tooth exceeds the length of the root, as in this example, in which we have a one point five to one crown root ratio, most clinicians consider this to have an extremely poor long term prognosis. So, the specific aim of this study was to determine if the prognostic indicator of the crown root ratio that we apply to natural teeth is useful when planning or assessing an implant supported crown. The study of the design is retrospective. The study was conducted at the Implant Dentistry Center, Boston, Massachusetts. It included patients who received one single tooth one or more single tooth implants between May of nineteen ninety two and April of two thousand and four. A chart review was done to secure radiographs of single implant supported crowns in which the entire implant and crown were visible on the radiograph. Measurements were made from the top of the implant to the height of the crown and from the top of the implant to its base to calculate the crown implant ratio. We purposely designed this very simple study to look only at one variable and that is the crown implant ratio. The results of this study: There were 889 implants included in the study. And there were only sixteen failures, which made statistical analysis of the data difficult. So what I have decided to do is just to describe it very simply with descriptive statistics. So 889 implants, sixteen failures, for a success rate of ninety eight point two percent. This is a frequency diagram of all 889 implants. On the vertical axis is the number of implants. On the horizontal axis is the crown implant ratio. Those implants that are continuing to function are illustrated in red. The average crown implant ratio of those was one point three to one. Those implants that failed are illustrated in blue. The average crown implant ratio of the failures is one point four to one. The range of those that are continuing to function are point five to one to three point one to one. The range of those that failed was 1.0:1 to 2.1:1. This is a visual example of a crown implant ratio of 1.3:1, which again was the average of those implants, 873 of them, that were found in this study. This is a visual example of the difference in the crown implant ratio of the eight seventy three that are continuing to function and the sixteen failures, which is point one. And from a clinical perspective, I would have to say that there is no significant difference in the crown implant ratio of those that are continuing to function and those that have failed. So let’s return to our original question. And that was, to determine if the prognostic indicator of the crown root ratio that we apply to natural teeth is useful when planning or assessing an implant supported crown. And clearly, the question is no. The average crown implant ratio found in this study grossly exceeds what we would consider favorable for a natural tooth. The most important finding, I believe, is this is that the success rate was ninety eight point two percent. Now, if I were to speculate and guess why that happened, I would say that it relates specifically to the geometric design of the Bicon system and more specifically, the plateau design. As you have heard earlier, there are some very positive biomechanical characteristics associated with the plateau design, And I’m just going to quickly mention a few. Number one, the plateau increases the surface area of the implant. If we start with a cylindrical shaped Bicon implant and mill in the plateaus for any given implant dimension, it increases the surface area by thirty to thirty five percent. Stress is defined as force over a given area. So if we increase the area of the implant, we decrease the stress. Very simple concept. Number two, and this one is really more important than the first, is that the plateau designs optimize the functional surface area for compressive, and I will also add the term shear, load transfer, optimizes the functional surface area. The functional surface area is defined as that area of the implant that actively serves to dissipate compressive, tensile and shear stress. Actively serves. This is a diagram that was recreated from an illustration in an article published by Kim. Kim, using three-dimensional finite element analysis, compared the stress transfer to bone of three different thread designs: the V shaped thread, the reverse thread, and the square thread, which we would call the plateau. What he found was, is that there was a decrease in compressive stress associated with the square or plateau design when compared to the other two. But most importantly, he found that there was a dramatic decrease in shear stress transfer associated with the square thread. So the square thread or plateau design can most efficiently deal with the most destructive of forces or stress, which is shear. Now, the implants in his study had the same pitch, which is number of threads per unit area, and the same thread depth. If we change the pitch or the thread depth, we can change the performance of the implant. And let’s look at the thread depth of the Bicon. The thread depth of the Bicon varies from zero point five millimeter to zero point seven five millimeter. The 0.5mm depths are the four point five and larger diameter implants and the 0.75mm depths are the four millimeter and narrower. Now that compares very, very favorably to the V shaped or screw type implants that are currently on the market. Their thread depth averages from zero point two four millimeters to zero point three seven millimeters. So what is this doing? It’s increasing the functional surface area. That area of the implant that is actively involved in dealing with force and stress. Okay. Number three. The plateau design allows for a broad distribution of stress. When an occlusal load is placed on a plateau designed implant, it is distributed throughout the body of the implant. We saw that earlier. As opposed to stress concentration with the screw type design. The stress concentration with a screw type design, many people believe, is responsible for the bone loss that you see down to the first thread and many times farther during the first year of function. So many believe it is this stress concentration at the crest that results in that bone loss And that’s why you don’t see it with the Bicon. And finally, the last thing I want to mention as far as the geometric design is that the plateaus promote the formation of dense bone. This is a histological slide taken from Dr. Jack Lemons’ work. Dr. Lemons did a wonderful job yesterday explaining to us this biomechanical property of the plateau. So there’s nothing more that I need to say about that. So these are the primary reasons, I think, we recorded a ninety eight point two percent success. There are others, such as the bacterial seal, the sloping shoulder. But I believe that it is the macrogeometry of the implant that allows for such a high success rate. It is understood that there are methodological and other concerns with a very, very simple study design such as this. However, I believe that the ninety eight point two percent success rate minimizes many of those concerns. So, when you see a radiograph like this, do not become alarmed, do not become concerned because the crown implant ratios exceed what we would consider favorable for a natural tooth. Understand that this is normal for the Bicon system and I’ll say it one more time, it is associated with a very high success rate. Now I want to share one more piece of information with you and that concerns the short implant. None of the sixteen failures in this study was a short implant. However, there were not a large number of them in our database. Now we have just learned from Dr. Sung-Kiang Chuang that the clinical performance of the short implant is the same as implants that are longer. And if I were to go into his database and measure his short implants, I would probably find that the crown implant ratio of those short implants is a lot larger than the average found in this study. Now, what does that mean? What that means to me is that the crown implant ratio is not a factor when determining long term success. And that’s my conclusion. I would like to extend a very, very special thank you to the Implant Dentistry Center, Boston, Massachusetts and all the wonderful people that work there. To Dr. Vincent Morgan for allowing us free and open access to any of the charts and records that are in the Implant Center. I also want to thank him for taking real good care of my graduate students. I’d like to thank Mr. Thomas Peterson and Ms. Megan Weed for their technical and organizational support. And finally, congratulations Bicon on your twentieth year anniversary. I, for one, am looking forward to many more. Thank you.