Use Familiar Techniques and Products to Cement Procera® AllCeram Crowns
by Glenn J. Wolfinger, DMD, FACP
Procera® AllCeram crowns have proven to be a great innovation to prosthetic dentistry. Procera® technology involves computer generated copings which provide consistent precision fit well within the acceptable level according to the ADA standards. Prior to Procera® technology all-ceramic restorations had to be bonded into place using resin cements. This was necessary due to lack of strength and marginal integrity of the restorations. Several studies have been done to access the marginal fit of Procera® copings.1 2 3 The results of these studies have all shown that crown restorations produced by Procera® CAD/CAM process were well within the clinical accepted range for marginal opening gap dimensions of less than 100 microns. Flexural strength tests on Procera® AllCeram have shown superior results to other all-ceramic crown systems. The densely sintered high purity alumina (Procera® AllCeram) had consistently higher failure stress than when compared to glass infiltrated alumina (In-Ceram) and leucite-reinforced porcelain (IPS Empress4 ). In another study three new ceramic crown core materials were tested to compare their biaxial flexural strength and indentation fracture toughness. Ten specimens of Empress, In-Ceram and Procera® AllCeram ceramics were prepared according to the manufacturers recommendations. The results revealed significant differences in flexural strength for the three materials. The average flexural strength of AllCeram, In-Ceram, and Empress ceramics were 687 MPa, 352 MPa, 134 MPa respectively. There was no statistically significant differences between the fracture toughness of Procera® and In-Ceram ceramics; however, both ceramics had significantly higher fracture toughness than Empress ceramic.5
With the development of a strong well fitting aluminum oxide core traditional cementation is possible. A study was conducted to test the effect of cement on fracture resistance of Procera® AllCeram crowns. Twenty-seven Procera® cores (0.5mm thick) were fabricated and veneered (0.5mm, 2.1mm occlusally). Cementation was done with zinc phosphate cement, Panavia (resin), Duet (componer), after sandblasting and silanization, under 5Kg. pressure for ten minutes. After storage in 100% humidity for 24 hours the crowns were loaded to fracture in an Instrom machine. The results achieved indicated no significant differences.6
Cementation of Procera® AllCeram crowns requires no additional steps when compared to cementation for traditional ceramometal restorations. The laboratory phase in fact requires less steps, since the application of die spacer is not necessary. A uniform cement space is built into the computer program, which fabricates the coping. Clinically no additional treatment is necessary to prepare the internal surface of the coping nor is any additional steps required in the cementation phase. Cementation is just as simple as it is for ceramometal restorations, and the same traditional cements that the clinician has been using can be used with the Procera® AllCeram system. While these restorations can be cemented with clinicians cement of choice, Procera® has recently recommended RelyX luting cement (3M Dental Products, St. Paul, MN). RelyX (formerly known as Vitremer) is a resin modified glass ionomer cement which can be used to cement any metal or porcelain fused to metal crowns, bridges, inlays, onlays and all-alumina or all-zirconia cores. Resin modified glass ionomers provide a combination of advantages not seen in other traditional cements.
When cementing over vital teeth, post-operative sensitivity is a concern. Some concerns exist regarding post-operative sensitivity using resin and zinc phosphate cements. Glass ionomer cements have provided a great decrease in post cementation sensitivity over vital teeth. Increased solubility issues however have caused great concerns using glass ionomer cements. The resin modified glass ionomer cements, however, have solved the solubility issues while still providing the desirable decreased sensitivity and sustained fluoride release. Clinicians who have used these resin modified glass ionomers cements have seen a dramatic decrease in post cementation sensitivity.
The incredibly high bond strengths shown with resin cements may not be desirable in all cases of crown cementation. If a tooth is prepared properly with good resistance and retention form, then the bond strength of the permanent cement is not a critical issue. Excessive bond strengths may in fact be undesirable since future replacement of crowns may be necessary. If a densely sintered aluminous core is completely bonded to the tooth structure, its removal may be difficult and time consuming. The addition of resin to glass ionomer cements provides the best characteristics for each class of cement; good bond strength, decrease solubility, decreased sensitivity, and sustained fluoride release.
RelyX luting cement is easy to work with clinically. An equal number of scoops of powder to drops of liquid are mixed for thirty seconds. The internal surfaces of the crown or crowns are loaded and seated. The working time of two and one-half minutes is sufficient for multiple crowns to be cemented simultaneously. The self-curing cement requires only three minutes of set time after crown placement, after which time the removal of the excess cement is possible. The excess cement cleans off very easily, unlike resin cements. Since the powder of this cement is radiopaque, a post- cementation radiograph can be used to confirm complete removal of the excess cement.
It has been reported that expansion of resin modified glass ionomer cements may cause fracture of all ceramic crowns. A recent study was done to determine if fracture would occur using Procera® aluminum oxide copings, the substructure for Procera® AllCeram crowns, and two resin modified glass ionomers, Vitremer and Fugi Plus (GC) as luting agents7 . Procera® aluminum oxide cores were cemented to pre-milled titanium standardized dies using the two resin modified luting cements. Fifteen samples were prepared using each cement. The samples were stored for six weeks in 100% humidity, at room temperature at approximately 700°F. Two prosthodontists and the investigator using 2.5X and 20X magnification respectively then inspected the samples. None of the thirty samples showed any signs of fracture by either inspection method. It was concluded that under the conditions of the study there is no evidence to support the hypothesis that expansion of the resin modified glass ionomer cements tested caused fracture of Procera® all ceramic copings.
Clinical Research Associates (Provo, UT) has done extensive independent testing on resin modified glass ionomer cements over the past five years. In March of 1995 the CRA Newsletter provided an extensive “first look” report of physical properties of resin modified glass ionomer cements from CRA lab tests. In the February 1996 CRA Newsletter, they reported that after one year of clinical use without sensitivity, many dentists have began to change to resin modified glass ionomer cements to gain high strength, low solubility, fluoride release, bond to tooth and lack of post-operative sensitivity. In that same newsletter they had stated that resin modified glass iometer cement “may be the best cement categorized to date”. However, we still need years of observation to verify long term success. In the December 1997 CRA Newsletter they had noted that without the use of desensitizers, almost no post-operative tooth sensitivity has been reported in five years of use of resin modified glass ionomer cements.
While resin modified glass ionomer cements have grown in popularity over the past five years, long term research is still lacking. For this reason, I have continued to use the same cement that has proven to be safe and predictable with metal and ceramometal restorations in the past, zinc phosphate (Fleck’s, Mizzy). With over 100 years of clinical success in dentistry, it has been proven to be just as safe and predictable when used with the Procera AllCeram crowns. Post cementation sensitivity can be effectively countered by use of a desensitizing agent (Gluma desensitizer, Miles). The powders come in a variety of shades, which when working with a crown with translucent capabilities, can be an advantage. The ability for the clinician to continue using the materials and techniques that he or she is comfortable and familiar with is a definite advantage when working with the Procera® AllCeram system.
References:
1. May et al. Marginal Fit: The Procera AllCeram Crown. Abstract 2379, IADR 1997.
2. May et al. Precision Of Fit Of The Procera AllCeram Crown. JPD, No. 4 1996.
3. Persson et al. Accuracy Of Machine Milling System and Spark Erosion With A CAD/CAM System. JPD 76, No. 2; 1996.
4. Zeng et al. Flexure Test On Dental Ceramic Int J of Prosthodont Vol 4, 1996.
5. Wagner WC and Chu TM. Biaxial Flexural Strength and Indentation Fracture Toughness of Three New Dental Core Ceramics. JPD 76: 140-4, 1996.
6. Dwan et al. Effect of Cement on Fracture Resistance of AllCeramic Crowns. Abstract 2136, IADR 1996.
7. Snyder et al. Fracture Incidence of Procera Copings Cemented with Resin-reinforced Cements. Abstracts 2979; J Dent Res., Vol. 78 (spec issue abstract papers) 1999: 478.
Printed in DentalTown Magazine November 2000, Volume 1, Issue 10. Address: 10850 S 48th Street, Phoenix, AZ 85044 Phone: 480-598-0001.
