Connector Height using Zirconia

[charles007]

Anyone know the minimum connector height on posteriors and anterior bridges ?
Seems like I heard its 3mm

We all know these requirements:

Minimum sq Anteriors is 7mm
Posterior is 9mm

What book or website shares this type of info on zirconia ?

PLEASE,,,, just facts, no guessing !

 

[2thm8kr]

9 sq. mm is 3x3 or 4.5 x 2 etc. Never had a fractured connector staying within 9sq. Although I never tried 1mm h x 9mm w.

 

[charles007]

That's what I've heard,
I think we shouldn't use anything less than 3mm height, and the 1 failure I had was a touch less than 3mm.... but the total sq mm was over 9mm on a posterior 3 unit bridge in FC

 

[2thm8kr]

That's what I've heard,
I think we shouldn't use anything less than 3mm height, and the 1 failure I had was a touch less than 3mm.... but the total sq mm was over 9mm on a posterior 3 unit bridge in FC

Did it fracture through the connector only or one of the abutments and connector? Sometimes you just aren't going to have 3mm of height on patients that are vertically challenged.

 

[CoolHandLuke]

when i were but a wee berne in the land of zirconia it was explained to me as follows:

the strength of zirconia is in its height not its width, because of the way the crystals stack in the sintering process. they stack parallel to the plane of occlusion (perpendicular to the direction of applied masticatory force). like cordwood. so more height always means more strength, even if its not very wide.

now, i have also found that to not necessarily be true for all designs of bridge. for example in the anterior region the connector between a lateral and incisor will be Triangle shaped, due to the narrowness near the incisal proximal contact area. leaving the connector thin in this area creates a weak spot, like the pointy part of a teardrop or the tail of a comma. the bridge will be prone to shearing even if you make the wide end of the connector over 12mm sq. this happened to me in January 2014, and we didnt realize what was going on until the 5th remake. it kept shearing in the sintering oven.

so there is a minimum Width to obey, and with that i typically try to keep all connectors around 9-12mm sq. and shaped as evenly as possible.

 

[zero_zero]

I always try to make it as thick as I could get away with esthetically, also watching the wall thicknesses adjacent to the connectors...

 

[2thm8kr]

when i were but a wee berne in the land of zirconia it was explained to me as follows:

the strength of zirconia is in its height not its width, because of the way the crystals stack in the sintering process. they stack parallel to the plane of occlusion (perpendicular to the direction of applied masticatory force). like cordwood. so more height always means more strength, even if its not very wide.

now, i have also found that to not necessarily be true for all designs of bridge. for example in the anterior region the connector between a lateral and incisor will be Triangle shaped, due to the narrowness near the incisal proximal contact area. leaving the connector thin in this area creates a weak spot, like the pointy part of a teardrop or the tail of a comma. the bridge will be prone to shearing even if you make the wide end of the connector over 12mm sq. this happened to me in January 2014, and we didnt realize what was going on until the 5th remake. it kept shearing in the sintering oven.

so there is a minimum Width to obey, and with that i typically try to keep all connectors around 9-12mm sq. and shaped as evenly as possible.

I go for as much of an ovate connector as possible with the long axis being vertical. The chemical structure may be an issue, but I have found staying with the law of beams is predictable for most materials in bridge work.

 

[charles007]

Did it fracture through the connector only or one of the abutments and connector? Sometimes you just aren't going to have 3mm of height on patients that are vertically challenged.

Broke on the connector and I knew it might happen...

Only had I zirconia bridge to break in my experience and it did have 9mm+ sq but the height was just under 3mm.. Basically an experiment since the doc wanted FCZ.
I designed adding as much to the buccal and lingual surrounding connectors and as wide as possible on the lingual and close to the margins.
Old bridge was on 17x19 in fcz and the kind of case you would design with metal rap-around occlusal if making in metal ceramics.

 

[charles007]

May as well ask another question on minimum thickness on occlusals

I've heard of zirconia bridges breaking on the occlusal near the connector. In other words the zirconia on occlusal was to thin.
What would you consider the minimum thickness ? Yes I know our scanners are preset to minimums. Our docs can adjust to below the minimum thickness and have breakage later
whether it be from improper adjusting or to thin.
What say you with this kind of experience from breakage

Anyone have published articles from Dr. McLaren you could post on zirconia, or links to his articles ?

 

[CoolHandLuke]

thin areas don't react to flexing easily, so either the pontic was not correctly adapted to the area under the pontic to prevent flexing or the pt bit down on a rock.

very often however, our hands are tied with pontic areas.

 

[2thm8kr]

I never go thinner than .5mm occlusally, anything less will fracture in the posterior. I do everything I can to keep it thicker than that.

 

[Patrick Coon]

View media item 22

 

[charles007]

Wall thickness is overlooked I believe, and the reason for many zir. failures

Thanks Pat !!

 

[ChrisBWJ]

Connectors - If you have a basement with an open ceiling, go downstairs and take a peek in the center. There will mostly likely be an I-beam that the your floor joists are resting on. Strength comes from the vertical component of a bridge connector, much more so that it does from width. 9 square mm is typically the cross sectional area for a zirconia bridge connector ( I like the chart above Patrick Coon). That 9 sq mm can be 3x3mm, 4.5 (vertical) x 2 (horizonal) or 2 (vertical) x 4.5 (horizontal),or any combination that adds up to at least 9 square mm. That' being said, because of the strength that comes from more vertical content than horizontal, I'd be leaning towards 4.5(v) x 2(h),over 3x3 or anything less. When I get some more time, I'll search for and post some actual "engineering" examples and what it means to the strength of the connector.

 

[ChrisBWJ]

Fractures - The connector is usually around 9 sq mm and actually ends up being the strongest part of the bridge. The weakest part isn't the connector, but where the connector attaches to the abutment teeth. Remember, ceramic materials do not flex. We go from say a 4.5 mm (v) x 2 mm (h) connector to 5-6 mm (h) x 0.6 mm zirconia coping wall thickness. This is the weak spot and most likely location for fractures. The best thing you can do with bridge frameworks is bulk up the area around where the connector comes into contact with the abutment teeth in the bridge. The bulkier the better, as much as you can.

This of course applies mostly to bridge frameworks. Full contour zirconia bridges have other considerations that can cause fractures, but isn't usually related to material thickness.

 

[Patrick Coon]

Connectors - If you have a basement with an open ceiling, go downstairs and take a peek in the center. There will mostly likely be an I-beam that the your floor joists are resting on. Strength comes from the vertical component of a bridge connector, much more so that it does from width. 9 square mm is typically the cross sectional area for a zirconia bridge connector ( I like the chart above Patrick Coon). That 9 sq mm can be 3x3mm, 4.5 (vertical) x 2 (horizonal) or 2 (vertical) x 4.5 (horizontal),or any combination that adds up to at least 9 square mm. That' being said, because of the strength that comes from more vertical content than horizontal, I'd be leaning towards 4.5(v) x 2(h),over 3x3 or anything less. When I get some more time, I'll search for and post some actual "engineering" examples and what it means to the strength of the connector.

Good explanation, Chris. This is a practical explanation of the Law of Beams. Basically if you double the horizontal component you get double the strength, but if you double the vertical component you get eight times the strength. This, of course, relates to vertical deformation of any material; wood, metal, ceramic, etc.

That being said, I would not go with anything less than 3mm vertical (occlusal-gingivally),as you will lose most of your vertical deformation strength. So that would leave out the 2mm (V) x 4.5mm (H) dimensions. ALWAYS err on the taller side of the equation.

 

[charles007]

Thanks for the detailed info Chris & Pat ! what can you teach us about FCZ bridges to prevent failures.
My only bridge failure in zirconia was FC, and the height was not there on short preps.

 

[ChrisBWJ]

Patrick Coon, thank you!! That's the reference I was looking for. I'm an electrical engineer so I can quote all sorts of electrical laws, equations, etc... That mechanical engineering course content was more than I was willing to commit to memory!! But that is exactly it!

Charles, go ahead and another thread on FCZ bridge failures (if there isn't already one)... We'll keep the topics separate since people are likely to search for them separately.

 

[charles007]

Patrick Coon, thank you!! That's the reference I was looking for. I'm an electrical engineer so I can quote all sorts of electrical laws, equations, etc... That mechanical engineering course content was more than I was willing to commit to memory!! But that is exactly it!

Charles, go ahead and another thread on FCZ bridge failures (if there isn't already one)... We'll keep the topics separate since people are likely to search for them separately.

Great idea Chris.

Go to FCZ Bridge Failures and Solutions
Travis just posted a link below..
Thanks Travis !

 

[Travis]

http://dentallabnetwork.com/forums/threads/fcz-bridge-failures-and-solutions.20641/