In the first part of this series on implant occlusion, we examined how a tooth and an implant respond differently to occlusal forces. Since they are different animals, they should be treated differently. If I gave dog food to my cat she would probably eat it, but it’s not a great idea and I may run into some problems.
"Okay, I'll stop giving you dog food. Just stop standing like that. It's weird."
It’s the same with designing occlusion for an implant crown. If you treat your implant restoration like a natural tooth, it might work but you may run into some problems, namely fractures, re-cements, loose abutment screws, etc.
Why? Recall from the first part of this series that occlusal forces are distributed around a tooth but concentrated in two main locations around an implant. So the stronger the occlusal forces, the stronger the stress concentrations and the greater the chance we’ll run into the complications listed above.
What can make occlusal forces stronger? Bruxism, clenching and other parafunctional habits are some uncommon examples. But there is a far more common offender: cantilevers. Recall from physics that a cantilever is created whenever a force is applied at a distance from a fulcrum. The greater the distance the force is applied from the fulcrum, the more mechanical advantage is generated and the more the original force becomes amplified. Translation: biting forces further off of the long axis of a tooth or implant creates greater stress for the system and implants are worse at absorbing that stress.
So we must be on the lookout for hidden cantilever forces in our implant restorations. There are three dimensions of cantilevers:
Three cantilever dimensions: buccal-lingual, mesial-distal, and apical-coronal.
Before reading further, if you are not familiar with my favorite occlusal philosophy for natural teeth, please quickly review cusp-fossa occlusion in this post, “Occlusion 101: What are Those Dots Supposed to Mean?”
Natural teeth occluding with a cusp-fossa design.
For opposing natural teeth I would have both supporting cusps occlude in the central fossa or mesial/distal marginal ridges of their counterparts.
Maxillary implant, mandibular natural tooth.
In this case, only the natural mandibular tooth’s supporting cusp does the occluding. The maxillary implant’s supporting cusp does not occlude at all! This removes a significant cantilever from the equation.
Mandibular implant, maxillary natural tooth.
The same rule applies in the opposite case. The maxillary natural tooth’s supporting cusp occludes and the mandibular implant’s supporting cusp does not.
For natural teeth in a cusp fossa relationship, the opposing tooth’s supporting cusp occludes in the central fossa or mesial/distal marginal ridge. With implants, I only want those forces loading directly over the long axis of the implant, which should be under the central fossa of the crown.
A mandibular, screw-retained implant restoration. Loads further from the central axis of the implant gnerate greater cantliver forces.
I do not place any occlusal contacts on mesial or distal marginal ridges of an implant crown because they are off of the long axis of the implant.
Wouldn’t it be nice if there was no vertical bone loss? In implant dentistry we often see loss of vertical bone height due to (a) edentulous areas left for a long time without a tooth or implant and (b) periodontal disease.
The implant on the right has less vertical bone height and therefore will have greater cantilever forces.
The greater the loss of vertical bone height, the longer the vertical cantilever will be. Our only option to reduce this effect would have been to promote vertical bone height before the implant is placed through ridge augmentation. Sadly, bone grafting to gain bone height is more expensive than it is predictable.
This is the basis of the theory of Implant Protective Occlusion. For more information on this subject, check out Carl Misch’s book, “Dental Implant Prosthetics.”
In the next post in this series, we’ll look at how splinting adjacent implants changes the rules of the game and how cantilevers become our best friend.