Dental Cements for Bonding Application Dr. Waseem Bahjat Mushtaha Specialized in prosthodontics

Types of cements Zinc phosphate cement Zinc silicophosphate cement Zinc polycarboxylate cement Zinc Oxide- Eugenol cement Resin-based cement

Zinc phosphate cement General description: Zinc phosphate is the oldest of the cementation agents and thus is the one that has the longest track record. It consists of powder and liquid in two separate bottles.

Composition 1) Powder : Zinc oxide (90%) Magnesium oxide (10%). The ingredients of the powder are sintered at temperatures between 1000C and 1400 into a cake that is subsequently ground into fine powders. The powder particle size influences setting rate. Generally, the smaller the particles size, the faster the set of the cement.

b) Liquids : Phosphoric acid, water, aluminum phosphate, and in some instances, zinc phosphate. The water content of most liquids is 33% ± 5% Setting reaction: When the powder is mixed with the liquid, the phosphoric acid attacks the surface of the particles and releases zinc ions into the liquid. The aluminum, which already forms a complex with the phosphoric acid, reacts with the zinc and yields a zinc aluminophosphate cement is a core structure consisting primarily of unreacted zinc oxide particles embedded in a cohesive amorphous matrix of zinc aluminophosphate.

Factors Influencing Working and Setting Time 1) pwder:liquid ratio: Working and setting times can be increased by reducing the powder: liquid (P:L) ratio. This procedure, however, is not acceptable means of extending setting time because it impairs the physical properties and results in a lower initial PH of the cement. The reduction in compressive strength, along with the decrease in the P:L ratio. The initial PH of the mixture also decreases with increasing P:L ratio.

2) Rate of powder incorporation: Introduction of small quantity of the powder into the liquid for the first few increments increases working and setting times by reducing the amount of heat generated and permits more powder to be incorporated into the mix. Therefore, it is the recommended procedure for zinc phosphate cement.

3) Spatulation time: Operators who prolong the spatulation time are effectively destroying the matrix that was forming. Fragmentation of the matrix means extra time is needed to rebuild the bulk of the matrix.

4) Temperature of mixing slab: The most effecting method of controlling the working and setting times is to regulate the temperature of the mixing slab. Cooling the slab markedly retards the chemical reaction between the powder and the liquid so that matrix formation is retarded. This permits incorporation of the optimum amount of powder into the liquid without the mix developing an unduly high viscosity.

Physical and Biological properties Two physical properties of the cement that are relevant to the retention of fixed prostheses are the mechanical properties and the solubilities. The prosthesis can become dislodged if the underlying cement is stressed beyond its strength. High solubility can induce loss of the cement needed for retention and may create plaque retention sites.

Zinc phosphate cements, when properly manipulated, exhibit a compressive strength of MPa and a diametral tensile strength of 5.5MPa. Zinc phosphate cement has a modulus of elasticity approximately 13GPa. Thus, it is quite stiff and should be resistant to elastic deformation even when it is employed for cementation of restorations that are subjected to high masticatory stress.

The recommended P:L ratio for this zinc phosphate cement is about 1.4g to 0.5 ml. the increase in strength attained by addition of powder in excess of the recommended amount is modest as compared with the reduction incurred by decreasing the amount of powder in the mix. A reduction in P:L ratio of the mix produces a markedly weaker cement. A loss or gain in the water content of the liquid reduces the compressive and tensile strengths of the cement.

Zinc phosphate cements show relatively low solubility in water when they are tested in accordance with ADA specification. Retention : Setting of the zinc phosphate cement does not involve any reaction with surrounding hard tissue or other restorative materials. Therefore, primary bonding occurs by mechanical interlocking at interface and not by chemical interaction.

Biologic properties As might be expected from the presence of the phosphoric acid, the acidity of the cement is quite high at the time when a prosthesis is placed on a prepared tooth. Two minutes after the start of the mixing, the PH of zinc phosphate cement is approximately 2. The PH then increases rapidly but still is only about 5.5 at 24 hours. The PH is lower and remains lower for a longer period when thin mixes are employed.

Zinc phosphate cement probably occurs during the first few hours after insertion. However, studies of zinc phosphate cements prepared with liquids containing radioactive phosphoric acid indicate that in some teeth the acid from the cement can penetrate a dentin thickness as great as 1.5 mm. Thus, if the underlying dentin is not protected against the infiltration of acid via the dentinal tubules, pulpal injury may occur.

Manipulation 1) It is probably not necessary to use measuring device for proportioning the powder and liquid, because the desired consistency may vary to some degree with the clinical situation. However, the maximum amount of powder possible for the operation and should be used to insure minimum solubility and maximum strength.

2) A cool mixing slab should be employed. The cool slab prolongs the working and the setting times and permits the operator to incorporate the maximum amount of the powder before the matrix formation proceeds to the point at which the mixture stiffens. The liquid should not be dispensed onto the slab until mixing is to be initiated, because water will be lost to the air by evaporation.

3) Mixing is initiated by addition of a small amount of powder. Small quantities are incorporated initially with brisk spatulation. A considerable area of the mixing slab should be used. A good rule to follow is to spatulate each increment for 15 seconds before adding another increment. The mixing time is not unduly critical. Completion of the mix usually requires approximately 1 minute and 30 seconds. As stated previously, the appropriate consistency varies according to the purpose for which the cement is to be used. However, the desired consistency is always attained by adding more powder and never by allowing a thin mix to stiffen. For a fixed partial denture, additional time required to apply the cement. Therefore, a slightly decreased viscosity should be used.

4) The casting should be seated immediately with a vibratory action if possible, before matrix formation occurs. After the casting has been seated, it should be held under pressure until the cement sets to minimize the air spaces. The field of operation should be kept dry during the entire procedure. 5) Excessive cement can be removed after it has set. It is recommended that a layer of varnish or other nonpermeable coating should be applied to the margin. The purpose of the varnish coating is to allow the cement more time to mature and develop an increased resistance to dissolution in oral fluid.

Zinc silicophosphate cement Zinc silicophosphate cement (ZSP) cements consist of a mixture of silicate glass, a small percentage of zinc oxide powder, and phosphoric acid. The clinical indications for this cement are similar to those of zinc phosphate cement. Its strength is somewhat superior, the other major difference is that set ZSP cement appears somewhat translucent and releases fluoride by virtue of the silicate glass. Aesthetically, it is superior to the more opaque zinc phosphate cement for cementation of ceramic restorations. The use of ZSP cement is declining, as practitioners have choices of other more esthetically pleasing materials, such as resin and glass ionomer cements.