The strength of crown cement is a complex measure of how well the adhesive layer between a tooth and its prosthetic cap resists forces over time. This dental cement, or luting agent, plays a defining role in the durability and longevity of a crown restoration. Its primary function is to create a sealed, permanent union between the prepared tooth structure and the artificial crown. The overall strength depends on the material chosen, combining mechanical resistance, chemical bonding, and stability within the harsh environment of the mouth.
The Different Types of Dental Cement
The strength of a crown relies heavily on the specific type of cement used, as dentists choose from several major categories. Resin-based cements are generally considered the strongest, achieving high compressive and tensile resistance through superior adhesion to both the tooth and the crown material. These materials require meticulous isolation from moisture during the bonding process. They offer the best long-term retention, making them a frequent choice for high-stress areas or all-ceramic restorations.
Another common option is the Resin-Modified Glass Ionomer (RMGI) cement, which provides a balance of strength and convenience. RMGIs chemically bond to the tooth structure and also release fluoride, offering a protective benefit against decay around the crown margin. They are more tolerant of moisture during placement than pure resin cements, which simplifies the procedure and makes them popular for metal and certain porcelain-fused-to-metal crowns.
Traditional options like Zinc Phosphate cement are still used due to their long history and high compressive strength. They achieve retention primarily through mechanical locking rather than chemical bonding. Zinc Phosphate is durable but can be associated with post-operative sensitivity due to its initial acidity. The material choice depends on the type of crown and the amount of remaining tooth structure.
The Forces Crown Cement Must Withstand
Crown cement functions in one of the most physically and chemically demanding environments in the body, requiring it to resist three main types of stress. Mechanical stress from chewing is the most obvious challenge, with biting forces on the back teeth routinely generating pressures comparable to standing on a single coin. The cement must withstand these crushing and shearing forces without fracturing or allowing the crown to be pulled loose.
The oral environment also subjects the cement to thermal stress from consuming hot or cold drinks. These rapid temperature shifts cause the crown material and the underlying tooth to expand and contract at different rates, introducing internal stresses that can eventually break the bond. This cycling stresses the cement’s ability to maintain a continuous, sealed margin.
Finally, the cement must endure continuous chemical stress from saliva, bacterial acids, and dietary components. The cement must be insoluble and chemically stable to prevent “wash-out,” where the material near the crown margin dissolves over time, creating a gap. Maintaining this marginal seal is the cement’s most important job, as failure allows bacteria to seep in and cause decay underneath the crown.
Factors That Influence Cement Durability
The longevity of a cement bond is influenced by the technique used during placement, not just the material alone. Achieving the strongest bond with resin cements requires absolute moisture control, as even minute amounts of saliva contamination can reduce the bond strength. If the tooth surface is not meticulously cleaned of temporary cement or properly prepared before final bonding, the restoration may fail prematurely.
The precision fit of the crown itself is another major factor impacting cement durability. If there is a slight gap between the crown’s edge and the prepared tooth margin, the cement layer will be thicker, making it more vulnerable to dissolution and mechanical wear. A precisely fitted crown ensures the cement layer is consistently thin, allowing the material to perform optimally.
Patient habits also influence the long-term success of the cement. Individuals who clench or grind their teeth (bruxism) subject the crown and cement to excessive forces that accelerate wear and increase the risk of bond failure. Consistent consumption of hard or sticky foods can also test the limits of the cement’s tensile strength, potentially dislodging the crown.
Recognizing and Addressing Cement Failure
When crown cement begins to fail, signs can include increased sensitivity to hot or cold temperatures in the crowned tooth. This sensitivity occurs because the deteriorating seal allows fluid movement within the underlying tooth structure. A more distinct sign of failure is a persistent bad taste or odor originating from the crown, which indicates that bacteria are leaking into the space due to cement wash-out.
If the cement bond has weakened, the crown may feel loose or wobbly when chewing or touching it. In some instances, a total bond failure occurs, and the crown comes off completely. This may be a temporary failure if the underlying tooth remains healthy, and the crown can often be re-cemented quickly by a dentist, provided it is intact.
In contrast, slow failure due to marginal leakage can lead to severe decay, compromising the remaining tooth structure underneath the crown. If a crown becomes loose, the priority is to contact a dental professional and store the crown safely. Attempting to force the crown back on or leaving the exposed tooth unprotected can lead to further damage or necessitate a more complex replacement of the restoration.