Feature
Casting Large Gold Implant Units
I believe it can be argued that a gold crown restoration on a posterior implant is the most reliable and compatible restoration for this area. Producing a nice casting this large can prove to be difficult. We have developed a technique to lighten the pattern that produces a dense, accurate casting.
Lightening the pattern produces a more accurate casting. Where there is less mass affected by the shrinkage of gold that happens as the gold solidifies in the invested pattern, there is less error. Pure gold shrinks by 1.64 percent. When you are working with a pattern of this size, you really notice the improved fit of the casting.
The difficulty in hollowing these patterns, especially for a stock abutment, is due to the narrowness of the neck. It does not provide very good access to hollow the pattern. This technique allows increased access to lighten the pattern.
Technique:
It will take a large bulk of wax to create a restoration that is full contour. Start by waxing the case to full contour. Sometimes when you wax a gold implant you will make the occlusal table narrow or you might under contour the pattern in order to save on gold and make a better casting. With this technique, you can wax patterns to full contour and have confidence in your casting.
After waxing, use a wax bur to cut through the buckle surface through to the lingual, so the abutment is exposed. Exposing the abutment is how the investment will get into the hollow cavity. When removing this much wax, it is best to use a wax bur. The burs we use are available at Stullar. {Stuller wax burs can be found at stuller.com | Phone number: 1.800.877.7777}
The weight of this pattern was reduced by 20%, nearly a pennyweight in the final casting. Once the cavity is hollowed, begin to close the buckle and lingual openings. Carefully add wax to the perimeter of the openings, moving in a spiral direction to close the hole. Be sure not to encroach upon the hollow space. Once these are closed, you are ready to put the casting on a sprue former.
We use a number eight round sprue, approximately 8 mm long. For spruing, be careful to leave yourself a thick area and place the sprue well within the confines of the thick area. Placing your sprue too close to the hollowed area is a recipe for suck back and will produces a porous casting. Investing can be a little bit tricky. The investment has to fill the cavity from the inside of the crown. We use a long camel hair brush to be sure that bubbles do not form in the cavity.
Casting:
When casting, be sure to throw plenty of gold. To get an accurate assessment of how much gold will be in your restoration, you can weigh your wax pattern in grams and multiply is by the density of the gold you are using. I would use at least 2.5 times this number. Lightening any pattern will benefit the fit and finish. A gold implant crown is an extreme case.
