Crown Procedures

In this course, we will compare the three major crown preparations, the shoulderless, or tapering margins, the porcelain margin and the bevel margin porcelain margin. We will be discussing the merits and disadvantages of the three preparations.

The crown preparations will be discussed, but the concepts presented here are not to replace a detail course that one would receive in a dental school. It is assumed here that the crown preparation techniques are general knowledge. This course will give additional practical concepts learned over a forty-year period of practice, especially in the area of possible problems in cutting the preparations, taking the impression, making the temporary and cementing the crown.

 

HISTORY

Forty years ago in dental school, the crowns consisted of the full gold crown, three-quarter gold crown, seven-eighths crown and the full crown with an acrylic facing. The course was completed with the MO gold inlay and a MOD gold onlay.

We used alginate and hydro colloid impression material. The messy, brown, rubber-base impression material was just starting to make the scene. If the patient swallowed it, we knew he was going to die.

The PJC (Porcelain Jacket Crown) was experimental, difficult to construct and usually fractured upon insertion. Our only other choice was the full gold crown with an acrylic facing. The crown was constructed first and then the acrylic was flowed into the facing preparation in the gold crown.

The difficulty was in polishing the acrylic without picking up dark streaks from the gold. Your acrylic temporary usually had better esthetics than the final product. The rule of thumb was to avoid making high polished temporaries. The patient would want the temporary over the finished acrylic gold crown.

The thirty-two dollars per ounce gold was the metal of choice. It had durability, the body tissues tolerated it well, and it was easy to manipulate. Every office had a casting machine to throw the gold into the casting rings to produce their own crowns. During the first few years of my practice, I made my own crowns and yes, wedding rings.

When gold went to eight hundred dollars an ounce, the world changed in dentistry. Gold was no longer the metal of choice. Those who had some foresight made enough money to retire.

Other metals have since been brought into use. All of them were trying to duplicate gold. The metals started with very hard metals loaded with titanium and progressed to softer metals. One can estimate the age of a patient’s crown by cutting it. It will require several diamond burs before a titanium crown comes off.

When the porcelain fused to metal (Ceramco) crown came into popularity, I no longer used the three-quarter, or seven-eighths, crown.

The advantages of porcelain fused to metal crowns:

• All of the margins are below the gingiva.
• The esthetics outmatches the metal.
• The metal can be high noble (platinum-gold), or base metal.

The porcelain jacket crown required a step, or beveled, margin, but no metal was used. The porcelain fused to metal crown allowed a shoulderless margin and the metal would extend to the edge of the gingival margin. The porcelain would feather down to the edge of the metal, allowing the color of the metal to leak through, unless the margin was below the free gingiva. When the gingiva receded, the patient was left with a dark ring around the crown.

Later, the porcelain fused to metal crown developed a porcelain margin. The metal would extend to the base of the step and the porcelain would fill in the step and become the margin similar to the porcelain jacket crown. We will be comparing the advantages of each technique.