As it turns out, not all cantilevers are evil. A cantilever can actually be our best friend by allowing us to replace a tooth without having to place an additional implant. But beware! We must follow certain rules when using them to our advantage. Read on, my friends…
Recall from the preceding post in this series that we must be vigilant in removing hidden cantilever forces from our implant restorations. Check out “Implant Occlusion Part 2: Cantilevers are Your Worst Enemy” for a discussion of the theory of implant-protective occlusion.
To summarize, we eliminate any occlusal marks that are not directly over the long axis of the implant. Mesial and distal marginal ridges? Gone. Mesial buccal cusp of mandibular molar implant? History. We only load the opposing natural tooth’s supporting cusps in the central fossae of the implant restoration, which is where the implant should be.
But let’s check out this patient’s restoration:
Splinted implant restoration replacing lower right molars.
Hmm… there appears to be occlusal marks on the mesial aspect of the the first molar crown, which is unsupported. Isn’t that a violation of implant-protective occlusion? Shouldn’t that lead to abutment screw loosening and fractured porcelain? Actually, this patient has had this restoration for almost 20 years! Aside from the small chip on the distal of the second molar, there have been no problems with this restoration. Why?
The answer lies in the power of splinting. Splinting adjacent implant restorations shares their occlusal burdens and helps fight dangerous cantilever forces. This is why I will always splint adjacent implants. Always. If we splint, we will have more freedom in our occlusal designs.
Unfortunately there really aren’t any hard and fast rules in splinting two or three implants together. The distance of cantilever that you gain is controversial in the literature so it’s best left up to your own judgement. Without question, more implants, wider diameter implants, and smaller force factors will all help us cantilever with confidence.
A classic cantilever that has been around since the dawn of implant restorations is the traditional Branemark design. Five or six implants in the mandible between the mental foramina will allow a nice bilateral distal cantilever. If the patient is missing a lot of bone in the mandibular posterior, we may still be able to give them molars without the need for extensive surgery.
A mandibular "hybrid" style restoration on six implants. Note the first molars are distal cantilevers.
This is a tried and true design; very predictable usage of cantilevers. We benefit from a phenomena called cross-arch stabilization. As our restoration design becomes supported on both sides of the arch, we gain a significant amount of support. This situation introduces the rule of anterior-posterior spread, also known as A-P spread.
Anterior-posterior spread of implants (A-P spread).
For a full discussion of A-P spread, check out this post: “How Far Can I Cantilever From Implants? A-P Spread Explained.” As a quick refresher, we begin by measuring the distance from the middle of the anterior most implant to the distal aspect of the distal most implant. Next we multiply that value by a certain number, which varies in the literature typically between 1.5 and 2.5. In my opinion, this number depends upon other patient factors, including bite strength, presence of parafunctional habits, etc.
One of the most important patient factors is arch form. More ovoid arch forms inherently have greater distance between an anterior and posterior landmark comparing the same landmarks to a more square arch form.
An ovoid arch form (left) and a square arch form (right).
So more ovoid arch forms will permit a longer distal cantilever than a more square arch form.
Now that we’ve reviewed the rules, we can look at some cases. In the final post of this series, we’ll see cases that failed miserably because implant occlusion was ignored. We’ll also see how cases were saved by designing the correct occlusal scheme for the patient.