How and why a cast post is made?

 Usually cast posts are indicated for some anterior teeth after a root canal treatment,  especially if when these teeth are prepped for crowns the preparations leave minimal tooth structure. Since cast posts are made of metal they can be thinned as much as is needed, without weakening the core portion.  Another reason. that they are used is they can be fit to rather large oval root canal preparations that do not easily adapt to prefabricated round posts. 

Cast posts are more expensive because they involve a laboratory fee and take two visits , while prefabricated post and cores can be completed in one visit and an impression for a crown can also be completed at this visit as well ( assuming that a long enough appointment is scheduled. 

In my practice, I usually opt for a prefabricated post and core , assuming that it can be used without compromising the final result.  That being said, there are times when only a cast post will work  and I am forced to opt for this more labor intensive and costly mode (probably about 25 % more expensive) of restoration. 

There are a number of techniques for fabricating cast post and core restorations. One technique is "direct". This technique involves using GC resin or Duralay and fabricating a plastic version of the post, directly in the patients tooth. When preparing the canal space an attempt is made to remove undercuts from the canal space. Sometimes some small undercuts will remain, especially when removing them would result in a weakened tooth. It is important that the pattern that is fabricated fit passively. A binding fit is extremely undesirable since once the pattern is cast, it will not seat fully because the metal version of the post is not at all flexible. Inserting a cast post that fits too tightly may cause a root fracture and should be avoided. I usually is best to remove any binding spots in the plastic pattern, prior to sending it to the lab to be cast. 

Also it is much easier to shape the plastic pattern that will be the prepared portion of the core prior to having it cast, since shaping the cast post after insertion can be time consuming. This step is sometimes ignored by harried dental students, especially when they are "running out of time" and choose to send the pattern out with a poorly prepared core portion .They usually find out after trying in their cast post, just how much time the  preparation of a non precious post can actually take. This is definitely a case of a stitch in time saves nine,  since it is always better to spend a little extra time preparing a direct cast post and core to have the proper shape and fit prior to sending it out to the dental lab.

Another popular technique is the indirect cast post and core. This technique involve preparing a post hole without under cuts and taking an elastomeric impression for the lab. The lab pours up a model and fabricates the cast post "indirectly" . Usually the elastomeric material is syringed into the post hole and the stump of the tooth. A plastic sprue may be inserted into the post space after syringing the impression material so that it will help keep it stiffer so that it will be less likely to distort. Then a tray full of elastomeric material is placed on top in order to pick up the post pattern.

A third technique that I prefer is a hybrid of direct and indirect technique. I fabricate a plastic post in the canal that is passive and then pick it up with an elastomeric impression. This technique insures that the post will likely fit but save the time spent preparing  the core directly in the patients mouth. Instead the lab waxes this portion on the model and casts the direct and indirect portions together. This works well and can even be used with a prefabricated plastic post that is matched to a parellel sided post that matches the  Parapost drill that was used.

All of the techniques mentioned can work well but like all things in life, practice makes perfect so dentists need to be well versed in cast post fabrication in order to achieve predictably good results. Using prefab post with composite cores is a simpler procedure and easier to master for most dentists.  That being said, there are times when a cast post is worth the extra effort and knowing how to use this restoration should be part of every dentists skill set.