How to avoid breaking stone teeth off the model

It has been countless times that I or the technician have broken teeth off the stone model. Gluing the tooth back onto the model leads to unnecessary fragility and inaccuracy of the model. I am breaking them off less frequently these days but this involves identifying the risk factors for breaking teeth and employing strategies to avoid this occurence.

Risk factors: Essentially, anything that introduces an undercut or a thin isthmus of stone.

 - Tilted teeth especially proclined or retroclined incisors. Breaking off molars is less frequent as they are short and wide and have less severe undercuts

-Teeth with reduced periodontium. This introduces black triangles for material to lock into and increases the undercut as the root diameter is less than the crown diameter. The more severe there perio, the thinner and weaker the root will become.

-Triangular teeth: Same issue with black triangles

-Porous stone: If the stone is poured with lots of bubbles, the void in the stone will act as a weak spot. Most of the time I have broken a lower incisor off a model, there is a fair sized bubble right under the surface. The thin profile of the incisor makes it more difficult to flow the stone into without introducing a bubble. Using a vacuum mixer and stone vibrator will reduce the risk of this. 

-Incorrect ratio of stone: Stone that is too thin or too thick will be weaker. Stone too thick will not flow as well and risk introducing voids

-Stiff impression materials: The stiffer the material the higher the risk. Greenstick in soft tissue undercuts will lock the tray in place. In removing the tray excess force can break areas of stone with deep undercuts. Soak the tray and model in hot water to melt the greenstick before removing the tray. Polyether is probably the next stiffest. Undercuts need to be blocked off prior to impression and negative undercuts need to be trimmed away prior to pouring. PVS putty and heavy bodied pvs have the same concept applied.

Strategies to avoid breakages: 

- Take the impression off carefully: Haphazardly pulling the impression off is sure to overload the teeth. Stone teeth are much more brittle than real teeth and excess tension on the model will cause chipping. Snap removal is not required as you aren't worried about distorting the impression. Instead, slowly pull the tray up from different areas and give time for the material to deform and flex around the stone.

-Trim the impression prior to pouring. If the impression does not require you to capture sulcus extensions e.g crown and bridge impressions you can trim the sulcus back with a scalpel and trim the depth of the palate flat to make a nice flat surface in the centre of the model. Using a sharp pair of scissors you can cut away any tags of black triangles distant to the working site as they are not necessary. This will make the impression easier to remove without debonding it from the tray and lowers the risk of breaking teeth.

-Block out undercuts prior to the impression: Using light bodied material, wax, flowable composite etc to block out black triangles and space under bridges does help when it comes to removing the impression from the mouth but it will also assist in avoiding breaking stone teeth as the blocked out areas will be translated to the model. 

-If you have forgotten to remove an alginate impression overnight and it has dried up and shrink, soak the model and impression prior to removal. The dried alginate will grip harder onto the model and in some areas can adhere to the model. Soaking it will loosen the alginate up and allow you to remove the tray from the impression material. From there you can peel the alginate off without any pressure on the teeth.

-Remove the tray off the impression material. This is a useful trick as once you remove the tray, you can flex the impression material off around the undercuts and tear it into pieces. Soaking the model and tray can break alginate adhesive bonds. Using a wax knife to cut off tags of impression material from tray perforation sites, get a wax knife in between the impression material and the tray in the posterior area and lever the tray off. If the stone is very fragile or the model is very critical you can use a bur to section the tray off the impression material. This is more critical for very stiff polyether. Unfortunately, this method sacrifices the impression but if you have poured it up well it doesn't make a different. This may not be possible for crown and bridge impressions where multiple pours are needed so you need to employ the other strategies mentinoned.