Fit of PFM bridgel framework on models

[designasmile]

I would like to hear opinions about the fit of a PFM bridge framework on a solid model as compared to the pinned working model.
If the bridge is waxed on a pinned model, and the casting fits perfectly back on the pinned model, should it also fit the solid model?
Will the fit on the solid model be dependant on the length of the bridge, number of abutments, angle of preps and other factors?
I have heard that when the dies are sectioned on the pinned model there is a micro dimentional change in the die stone and the
casting may or may not fit the solid model. I am also aware that there are many pinning systems, and the stability of the dies
in the stone or plastic base can very.
I am finding the bridge that fits the pinned model also fits in the mouth, but may not seat fully on the solid model.
I have also heard that a bridge casting will only fit the model it was waxed on

 

[Marcusthegladiator CDT]

I've built plenty of large frameworks that fit the working model, solid model, and patients mouth. From bunsen burin wax to cad cam zirk and cad cam wax milled frames that were casted. They all fit the way they were spose to. Just got to build the working model properly...

 

[GAP]

I've built plenty of large frameworks that fit the working model, solid model, and patients mouth. From bunsen burin wax to cad cam zirk and cad cam wax milled frames that were casted. They all fit the way they were spose to. Just got to build the working model properly...

Ditto

i haven't done many long span br's (ie roundhouses) but Dr's have sectioned them for me.

 

[Car 54]

I agree, when all the things we can cover in getting the model work is right, i.e. a good stable dual/bi pin, matching low expansion die stone, and base stone, etc., then everything should transfer correctly, and the sawing process that relieves die stone/base stone expansion/tension, should be minimal, if at all. (I use stone base on all my bridge cases)

The thing that has worked for me in more predictably getting good fits to transfer over to both the working and solid cast, is the way I section, lute, sprue and invest a bridge. If you have the model work covered, this is the place (at least for me) that had the greatest impact on success for the wax and cast technique in getting things to fit both casts. As far as a pier abutment case, larger 8-10+ unit cases, divergent abutments where I can see I'm going to have problems, I tend to consider I will have to solder it, or make it in 2 sections, and solder them together.

Like GAP, I don't really don't do to many roundhouse bridges, and if I did, would do it in 2 sections, maybe using an interlock.

 

[cloe]

I have to sit the bridge frame work on the solid model before i go ahead with the porcelain.

 

[Marcusthegladiator CDT]

I once waxed a round house all on the working model, casted it in PG, and finished it... then it went out for a try in. Which was actually upstairs, so I went with the framework... And nervously watched the doctor try it in and probe the margins, everything was terrific, i was nervous... but all was well... Good doctor too...

 

[martintay]

Always check the imp hasn`t slightly "pulled" from the tray after withdrawing from first casting-this is common thus the second pour (solid model) will never be accurate ! Beware this is not always noticed by the model tech !!!! Also allow time for imp "memory" to re-establish.

 

[Car 54]

Good point, especially if it was hard in separating it, as I've seen distortion even on a single when checking a coping on a solid pour up.

 

[Big Guy]

Always check the imp hasn`t slightly "pulled" from the tray after withdrawing from first casting-this is common thus the second pour (solid model) will never be accurate ! Beware this is not always noticed by the model tech !!!! Also allow time for imp "memory" to re-establish.

Exactly what I was going to suggest...... Also I have has clinician choice trays bend after removing the first pour distorting the impression. I have been able to cut away the impression material form the tray and then repour with some success. Bottom line is that the first pour is allways the most accurate.

 

[martintay]

Good point, especially if it was hard in separating it, as I've seen distortion even on a single when checking a coping on a solid pour up.

Years ago (when there was more time and money) we would die trim the master -replace the imp and fill saw cuts with sticky wax to hold preps accurately and then duplicate and pour again. this gave a very accurate model with no soft tissue interference.

 

[kimba]

I agree with all the above. A castng should fit the same on the working model as the solid check model. Any differences are due to impresion distorting sfter taking out first cast, or a die pin not seating 100% at wax up/scan. Yes in theory each subsequent pour of the impression is less acurate but I would think it would be very hard to measure the difference in the first 3 or 4.

 

[user name]

Both models need to be poured with the same powder/water ratio and mixing time, or theyre not going to be the exact same size. Also, if you pour a stone base on your sectioned model, you should be using the same stone as the die pour. Same ratio, same mix time.