Why would you take your first swing with a foam bat if you have an aluminum bat? When we're selecting an implant to place in our patient's mouth, what size should we choose? Traditional dental wisdom tells us to try to place an implant and if it doesn't stick, get a bigger implant.
This is sometimes called a rescue implant. That's perfectly fine if our goal is to get a piece of metal to stick in the mouth, but that should not be our goal. Our goal is to give the patients an optimal solution. In engineering, optimizations work like Goldilocks and the porridge.
This one is too hot, this one is too cold, and this one is just right. That is how we need to approach implant sizing. In this case, it's okay to be picky.
Let's start with diameter. To choose our implant diameter or girth, we use the one-half rule. The one-half rule says that a good starting point for selecting an implant diameter is to take the standard mesial distal crown width of the tooth and divide it by two. For example, if we're looking at this anterior tooth and its width is 8.5 millimeters, we divide 8.5 by 2 and get 4.25.
Can you guess what implant I use most often at this site? Yep, it's a 4.2. Let's look at a molar. Typically, the mesial distal measurement of a molar is approximately 10 millimeters.
Divide 10 by 2 and 1 gets 5 millimeters. So my starting point for implant diameter in the molar is 5 millimeters. To make things even simpler for myself, I've even made this color-coded diagram that matches the BioHorizons color scheme. At a glance, I can tell my typical starting point for an implant in any position. Now why do I say the one-half rule is a starting point?
Because we can optimize our solution further by maximizing the implant diameter while adhering to the 2B rule. Maximizing the implant diameter helps minimize the mechanical complications. The 2B rule says that we want to have 2 millimeters of buccal or facial bone after placing our implants.
Why? Because research shows that leaving 2 millimeters of bone on the facial reduces bone loss, increases the probability of bone gain, and additionally prevents any discoloration of the gingiva on the facial. So to get the right implant diameter every time, start off with the one-half rule and then adjust as necessary to ensure you have at least 2 mm of bone on the buccal or facial. Keep in mind that bone volume can be a problem in some areas, particularly in the lateral's position.
Laterals often have very little bone that severely limits the size of the implant you can place. Great, so now we have a method for choosing the ideal implant diameter we need. What about length? First, it's important to note that implant diameter or girth is more important than length. While many of the industries will say that length isn't very important, that's only true when the implant is axially loaded.
For off-axis loads, longer implants can resist the forces much better. Why? Think about telephone poles. If you had two identical telephone poles next to each other, one sunk one foot in the ground and the other sunk six feet in the ground, which one would you expect to stand up to wind better? The one that sunk six feet deep, of course.
Why? The wind blows against both poles with exactly the same force and in exactly the same direction. However, if we look at where the resisting forces of each pole come from, we need to go underground.
The buried end of each pole acts as a lever pushing against the dirt. On the first pole, the one sunk only one foot in the ground, our lever is only one foot long. It's only pushing against one foot of dirt.
The second pole, however, has a six foot lever pushing against six feet of dirt. Which lever and which amount of dirt can withstand more force? Exactly, the six foot one. Okay.
So implant length is important for resisting off-axis forces. What else determines the optimal length of our implants? It's actually pretty simple. Make your implant as long as possible without breaking the 2mm rule.
If you're unfamiliar, the 2mm rule simply says that the edge of the implant should not come within 2mm of a vital structure once placed. So choose the longest implant you can without coming within 2mm of a nerve or 2mm of a nerve. sinus, or other vital structure. With diameter and length covered, it should be simple to choose which implant to place in any situation.
This also brings us back to the optimization we talked about earlier. If there's an optimal implant diameter that we can figure out using the one-half rule, that means there's one best diameter and several inferior options. We can compare these options in a graph like this and plot a curve to represent the data.
The best solution sits here at the top of that curve. Likewise, we can compare and graph our potential implant lengths and plot a curve to represent this data as well. If we plot the length and diameter curves together on the same coordinate system, we end up with this umbrella shape.
We know that the optimal diameter is at the top of the diameter curve and that the optimal length is at the top of the length curve. These two points meet here at the global maximum. The global maximum is the point where both diameter and length are most optimized.
This maximum tells us exactly what size we should place. In this example, it's a 3.8 by 10.5 millimeter implant. Now, what if we'd gone with the conventional wisdom? What if we tried to place a 10.5 by 3.8 millimeter implant?
It didn't stick and then we jumped up a size and placed a 4.6 instead. That's a problem. We don't have enough bone to place a 4.6.
If we'd had enough bone, then we would have planned for a 4.6 in the first place. What if we knew from the one-half rule that we could place a 3.8, but we started with a 3.4 in case we needed to go to a size larger? Then we're starting with a suboptimal solution. That's like a pro baseball player going up to the plate for the first pitch and saying, I think I'll use my foam bat for this one.
He takes one swing, the bat shatters, and he says, okay, I'll use my aluminum bat now. If you have an aluminum bat all along, use the aluminum bat. Don't choose a suboptimal solution and plan to fail.
We should never use a rescue implant. If our optimal implant won't stick, we simply can't place an implant in that location right now. If it's a single implant, We need to graft and grow bone before returning to place the optimal implant later. If it's a full mouth case, proper planning dictates that we've built in redundancy in our pros.
We could often lose that failed implant site and still give the patient a fantastic functional solution. In summary, remember, when you're sizing an implant, there's only one optimal solution. That's your only solution.
No rescue implants. The optimal implant size falls at the global maximum. where the optimal length and diameter meet. Use the 1 half rule, the 2B rule, and the length rules to find these maximum and you'll be ready to choose the best implants for each of your cases. And you know what reduces your margin of error and helps you get your optimal implants to stick?
Going fully guided. This has been another episode of Implants Made Simple. SmileEngineer out.