I recently posted a picture of a failed mandibular full arch zirconia prosthesis that got a lot of feedback, especially from lab technicians. The prosthesis was a mess, really. (It not done by me, thankfully). Some implants had screw retained attachments, some cement retained, many screws were fractured, and the zirconium bar was split right in half.

This is the prosthesis:

The thing that strikes me right off the bat is how thin the zirconium framework is. I have seen  several of these bars snap in half when they are not thick enough. (I’ve never seen a titanium framework do that.)

But the interesting thing for me was that I got a few comments stating that: 

“You cannot splint across the mandibular arch with posterior implants due to mandibular flexure.”

Is that true? Is it really contraindicated to do this? This is the topic I’d like to discuss in this short post. 

Mandibular Flexure DOES Occur

We all know mandibular flexure occurs–that fact is not contested. During opening and protrusive movements, the lateral pterygoid muscles cause the mandible to deform slightly. The amount of flexure is very small, but the thinner the mandible–the greater the amount of movement. Also, there is more flexure in the posterior region than the anterior region.

The practical implication of this is that if you are taking final impressions for a longer span prosthesis such as a bridge, a removable partial denture, or a fixed complete denture, then you should take the impression with the patient’s jaws relaxed and not open very wide in order to reduce the distortion. Also, for full arch implant prosthetics, you can improve the accuracy of your cast and passivity of your prosthesis by taking an extra step to verify your master-model. 

The role of mandibular flexure in splinted full-arch implant prosthetics is kind of treated like an urban legend. At a meeting, a speaker might mention to the audience: ‘I once had a patient once who had an inexplicable pain and we determined it must have been due to mandibular flexure.’ There are some case reports that speculate that mandibular flexure prosthetic complications, implant failures, and phantom pain. 

Let’s Look at a Couple of Articles…

Earlier articles claim that mandibular flexure is the cause of implant failure in the distal implants in these prosthetics. A study by Miyamoto et al (2003) compared a group of 7 patients with implants only in the anterior mandible to a group of 8 patients with implants in the anterior mandible and an additional implant placed bilaterally distal to the mental foramen. After 1 year, the implant survival rate was 100% and 60% respectively. They found that only those posteriorly placed implants failed in these patients and they claim that mandibular flexure was the reason. 

But in reality, we don’t have much data to make any definitive statements about this. One thing to consider about the Miyamoto article is that maybe the passivity of the framework was less ideal with the posterior implants. These metal frameworks were cast, so of course, a longer span of the framework will result in more distortion.

Also, this was before the age of CAD/CAM. Hardly anyone casts their frameworks anymore. Scanning and milling is used to create a more accurate framework than was possible almost 15 years ago when this article was published. Here is an example of me scanning my cast with the Nobel Procera Scanner:

An article published in the Journal of Prosthodontics in 2012 by Law et al, reviews the literature on this topic and concludes:

“The clinical significance of mandibular flexure on the success of dental implant treatment is at this time unclear, and further research is needed.”

Closing Remarks

So, is cross-arch splinting strictly contraindicated? Not necessarily. But flexure is something we should be keenly aware of when we are taking final impressions. Realize that flexure can have implications for the passivity of your framework so ALWAYS pay close attention to verifying your models.

Also, I’ll have a little more respect for the limitations. For instance, there is no reason to try to be a hero and place an implant as far back as a second molar site. Our choice is a balance between minimizing the cantilever and mitigating the effects of mandibular flexure. I will still aim for placing my implants around the second premolar site and then allowing myself a maximum of a 10 mm distal cantilever.

I would love to hear your thoughts. Do you have any experience with this? Have you seen any cases fail due to flexure?

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Cheers!
Ivan

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