How Far Can I Cantilever From Implants? A-P Spread Explained

Cantilevers can be your best friend or your worst enemy.

When used intelligently, a cantilever can allow you to replace a missing tooth without an additional implant.

But when physics are ignored, a cantilever can be the cause of fractured porcelain, screw loosening, bone destruction, and other nasty surprises.

There are numerous types of cantilevers to consider in implant reconstructions.  And the rules can change depending on the number of implants, their position, the diameter and length of the implants, occlusal forces, parafunctional habits, restorative materials, and so on.  Whew… it’s no wonder that there are few hard and fast rules for cantilevers and implant therapy.

In this post, I will just tackle full arch, implant-supported restorations.  Single unit implants play by different rules and will be addressed in another post.  Also note that I said “implant-supported” restorations;  that means no overdentures, which are “implant-retained” but still “soft tissue-supported”.

Let’s look at a case.  Here is a maxillary arch with five implants to be restored:

Click on Image to Enlarge

Personally, I prefer to have 6-8 implants for a maxillary arch, but the patient’s finances prevented us from doing ideal treatment.  Hey, that’s the real world.  So let’s say the patient wants to have teeth going back to her second molars.  Can we do it?

The answer lies partially within the concept of Anterior-Posterior spread, or A-P spread.  On the cast, draw a line through the middle on the most anterior implant and a line through distal aspect of the distal most implants.  Multiply that measurement by a special number to determine how far distal to the last implant you may extend.

Click Image to Enlarge

What is the special number?  Well that depends on which article you read.  The numbers typically range from 1.5 to 2.5.

In my opinion I think this depends on the other patient factors.  If the patient has fewer than ideal implants (as in the case above), shorter implants, narrower implants, higher occlusal forces, wear facets, bruxes, etc, then I will shrink the special number closer to 1.5.  Use your clinical judgment.

So what if the patient really really wants to have second molars in the final restoration?  Well this must be taken into account before any implants are placed.

This is all a part of the pre-treatment diagnostics.  If Mrs. Jones has a wide buccal corridor and therefore would benefit cosmetically from having second molars restored, AND she bruxes… you will need to engineer the case accordingly to reduce large distal cantilever forces.

Mrs. Jones would best be served by having implants in the second molar positions, large and long implants, more implants, etc.  But what else can we do?

The answer lies with implant position and A-P spread.  In the pre-treatment diagnositcs, the positions of implants should be placed so as to maximize the A-P spread, thus allowing you to cantilever further distally.

And this will largely be determined by arch form.  Usually classified as being either square, ovoid, or tapering, the shape of the arch must be considered.  For example, look at this arch:

Click Image to Enlarge

Now compare to this arch:

Click to Enlarge Image

You can see that the more square the arch, the more implants between the cuspids will be in a straight line and thus the shorter the A-P spread will be.  The first arch form is better suited to supporting a distal cantilever than the second arch form.

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