In this lecture we will go through and give recommendations for when to work on abutment level and when on implant level. So, the three main pathways are: The reconstruction that the screw retained on abutment level or cemented on an abutment, or screw retained on implant level. There are many types of abutments: different lengths, widths, profiles, angulations, materials and custom made ones. So, why use an abutment at all? When building real bridges, the engineers always use abutments. This is something that the constructors have known for a really long time, a bridge will hold longer when using abutments. In short, it transfers loads from a superstructure to its foundation elements, it resists lateral loads and wind loads, and this can easily be translated to the mouth and the forces a bridge can experience while chewing. In cemented reconstructions, independent of a single crown or a bridge, an abutment is always needed. You can't cement directly on an implant! In single crowns, you can choose if you want to use an abutment or not. And today, most dentists choose to make a crown directly on implant level. On bridges, there are many factors that affect the choice to use abutments or not. One, if there is an angulation problem. In these cases, there are two options. you can either use screw retained or cement retained. In the last years, companies have presented a solution with angulated screw holes and special screwdrivers are needed in these cases. Another issue is the implant platform. In short, there are external and internal connections. Of course, there are many other subcategories. The internal connection is a very precise connection which demands a perfect fit closest to the implant. In this case, you can see a bridge done at fixture level without abutments. In order to solve the paralleling problem, the path that entered the implant has been shortened, resulting in cracks in the implant after it has been loaded. To the right, a similar bridge correctly done with abutments. Note that the abutments have a much longer contact area to the inside of the implant, transferring the loads much more advantageous. Removing the bridge reveals the cracks in the implant neck. In the situations that we have seen implants with cracks most often is bridge done on implant level. Now this is a clinical view from this. Let's go through one of the reasons for these cracks. Let's say that you try a framework, it fits perfectly. You send the framework to the dental technician to fuse porcelain, you will get an oxidation layer that the technician has to remove. The framework would get a little smaller and in this animation, a small layer has mathematically been removed. Now, when placing the framework back there's a misfit that will give tension to the implants. In a Finite Element Analysis Model, we can clearly see the difference. In both models, there is a misfit. To the left, the misfit is on implant level and to the right, on abutment level. When the same load is applied, we can clearly see that the misfit on abutment level has much less stress on the implant. The abutment transfers a load in a more favorable way to the implants. There are exceptions to this, for instance, Nobel Active recommends an abutment with what you can see on the image to the left but allows a reconstruction on implant level to the image to the right. But then the reconstruction has another design resting on the implant shoulder. Also, there has to be a little gap on the inside in order to compensate for the misfit. The last group is a risk patient group. In risk patients, it is generally recommended to use abutments both on single crowns and on bridges. This is because it's much easier to repair on abutment level than it is on implant level. Examples on risk patients are bruxers, patients with deep bite, few occluding contacts, and major reconstructions. So in summary, always follow the recommendations of the company. In most cases, the following is recommended: if you cement, use it on abutment level; single crowns, you can choose if you want to do it on abutment or fixture level; on bridges, if you have an internal connection, especially the conical system, abutment is recommended. If you have a flat to flat system, you can choose if you want to do it on abutment or fixture level. If you have an angularity problem, you will most often have to use an abutment and in risk patients, abutment is recommended.