Do crowns fall off? If you are a patient who has had a CEREC crown placed, you may be wondering how it stays in place. What type of cement or bonding agent is used? If the crown ever comes off, is there a particular way to re-cement it? This blog post will answer these questions and provide the top 8 reasons crowns fall off from a dentist’s perspective.
Crowns are made from many materials, including gold, porcelain, metal, and zirconia. These materials interact differently with dental cement and other tooth bonding agents. For example, gold and other metals cannot chemically bond to most bonding agents, so these crowns must rely on mechanical bonds.
Let’s look at the two types of bonds when cementing a crown.
Chemical bond – Includes treating the tooth and crown so that both surfaces are primed and able to chemically adhere to each other using bonding cement. Chemical bonds are strong, durable, and lasting; however, the process is technique sensitive. CEREC, zirconia, or different porcelain/ceramic crowns can be acid-etched and treated to chemically bond to the tooth surface. Chemical bonds are indicated for short teeth, including molars. Lastly, chemical bonding increases the strength of the crown material. For example, bonding a weaker, highly-esthetic ceramic crown will increase its strength to prevent fracture or failure. Chemical bonds can be self-cured (cured without a light source) or light-cured (cured with a blue light source).
Mechanical bond – Includes the process of relying on the tooth’s physical properties, including tooth height, width, and crown preparation. For mechanical bonds to be successful, the dentist must prepare the tooth so that the tooth walls are parallel, tall and upright to prevent the crown from rotating or shifting off during mechanical stress. Materials such as gold and PFM rely on mechanical bonds due to their inability to chemically bond to the tooth surface. Mechanical bonds function with traditional cement that mechanically locks into place. The taller and straighter the tooth walls are, the less likely the crown will come off with conventional cement. In addition, specific traditional cement can uptake fluoride in the oral cavity to prevent cavities from forming around the crown margin (edge where the crown and tooth meet).
CEREC crowns & tooth decay
Aside from exceptional esthetics, same-day crown delivery, and durability, CEREC crowns are radiolucent. Radiolucent means that the material is transparent on an x-ray.
This feature is helpful for dentists because it allows the dentist to visualize any abnormality or cavity forming under existing crowns. CEREC materials will enable the x-ray beam to pass through the crown and visualize structures under the restoration.
As a dentist, I greatly appreciate the ability to see a small cavity forming under an existing crown and manage it before the cavity progresses and requires further treatment, such as a root canal.
Unlike CEREC or other porcelain/ceramic crowns, gold, PFM, Zirconia, and metal crowns are radiopaque. Radioopaque means that you can’t visualize anything under the material. Another way to describe it is the inability of the x-ray beam to travel through the metal crown, so it appears white on an x-ray.
Small cavities that form under the crown can be difficult to detect radiographically (with x-ray) until it’s too late and the cavity has caused extensive damage. Dentists rely on instruments and tactile sensation to poke under metal crowns to feel for cavities.
Dental crowns are cemented with traditional cement or bonding cement, and as mentioned, these two methods rely on chemical or mechanical bonds.
Chemical bonds are powerful and lasting; however, the process is technique sensitive, and the bond strength is greatly minimized if any contamination occurs. On the other hand, mechanical bonds are less technique sensitive for the dentist and have been around for hundreds of years—this process relies on the tooth’s shape and other physical properties.
According to a recent study, dentists were more likely to use bonding cement if there was excessive tooth reduction, anterior crowns were being cemented, or cementing a CEREC or other porcelain/ceramic crown. The results clarified that 202 dentists cemented about 3,500 crowns; 38.1% bonded the crowns while 61.9% used traditional cement.
Crown failures can occur due to many issues; however, let’s look at the most common reasons.
Tooth decay – Cavities are incredibly damaging and spread quickly. Tooth decay that forms under CEREC crowns can be visualized on an x-ray and treated by crown replacement before a root canal is needed. Traditional crowns (gold, PFM, Zirconia) are radiopaque, and cavities can brew there for many years. Crown failures that occur due to tooth decay reveal a yellow, brown, or black sticky cavity that is soft and pliable to touch. If a crown fails because of a cavity, the tooth is often non-restorable because of the large size of tooth decay. Therefore, further treatment is required, such as tooth extraction and replacement with a dental bridge, partial denture, or dental implant.
Sticky food – Many foods can stick to the crown and suck it right off the tooth. For example, carbohydrate-rich foods such as bread are a common culprit in dislodging a dental crown. Other more obvious sticky foods include candy such as caramel, gummies, or taffy. In my practice, energy gummies were a common food culprit in removing a crown because they would stick to the crown and opposing tooth and pull it right off. Crowns are less likely to fall off if they are chemically bonded.
Short teeth – Teeth that are short (lower molars) are more likely to have a crown fall off because they lack adequate tooth wall height. The taller the walls of the tooth, the less likely the crown will fall off. As a dentist, crowns cemented on shorter teeth should be chemically bonded to have an increased chance of longevity.
CEREC or other porcelain crowns bond the best however they are weaker than different types of materials. Fragile crowns on lower molars are terrible, especially for patients who grind or clench their teeth. Zirconia crowns are an excellent option for molar crowns because they can be milled very thin, allowing the dentist to remove less tooth structure. Also, they can be chemically bonded, and they are highly durable.
Poor tooth preparation – Ideal tooth preparation parameters are essential for long-term success. When the dentist trims down the tooth structure, adequate tooth removal is necessary to allow the crown to fit between the other teeth and within the bite, so it’s not hitting high. The dentist must also consider the tooth’s height and shape and the crown’s dependability.
For example, if the dentist removes too much tooth structure on top of the tooth, the tooth walls will be shorter, increasing the chances of the crown falling off. Mechanical fit relies on tall teeth and straight tooth walls. The crown lacks adequate retention if the dentist removes too much tooth structure around the tooth and the tooth walls converge. Straight walls provide a more mechanical fit compared to convergent tooth walls.
Cement failure – The dentist much consider what crown cement works best for each case. Most traditional crown cement relies on accurate measurement and mixture of two products. If the products are inadequate or poorly mixed, crown failure can occur. Research and development is a large portion of emphasis for dental materials companies. As a dentist, it’s essential to go with cement with years of clinical success to provide the best opportunity for long-term crown success.
Dentist error – It’s paramount for the dentist to be well-trained on how to use dental cement properly. Traditional cement is less technique sensitive and straightforward to use. Bonding cement is more technique sensitive and relies on chemical bonds between the crown and tooth. If the dentist doesn’t apply the bonding agent, acid-etch, or any other step for the exact time, the dentist will compromise the bond strength. Or, if contamination occurs on the tooth, such as saliva or debris, the dentist will compromise the bond strength. Cementing a crown in a wet environment can be highly challenging, especially if it requires keeping the tooth free of saliva. The process from start to finish takes 2-4 minutes, and the patient, dentist, and dental assistant must work together to give the crown the best opportunity for long-term success.
Faulty crown – A crown that fits the tooth poorly will not adhere to the tooth properly and leave the crown susceptible to popping off or coming loose. For example, if the crown fits loosely and rocks on the tooth before it’s cemented, it’s not well-adapted to the walls of the tooth. Also, if the crown has an “open margin” (opening at the junction between the crown and tooth), bacteria, saliva, and other debris can dislodge the cement seal and lead to crown failure.
Open margins can also lead to tooth decay. This process is known as “marginal leakage” and can lead to extensive tooth decay under a crown. In my opinion, your dentist needs to verify the crown’s fit with your tooth by taking an x-ray before cementing it into place.
Bruxism (grinding/clenching) – Patients that grind their teeth at night or during the day are more likely to pop their crown off because of excessive bite forces. Occlusal (chewing surface) and excursive (side-to-side motion) forces can break the crown cement seal and cause the crown to fall off. Bruxism patients must wear an occlusal guard at night to prevent excessive pressure from damaging teeth. Zirconia crowns are recommended for patients that grind their teeth because they are extremely strong and durable.
In my practice, the most common reasons CEREC crowns fall off are sticky foods or cement failure due to contamination during the bonding process. However, all dental crowns are susceptible to falling off. Understanding why it fell off is the first step to moving forward with treatment. Learn how to temporarily recement your dental crown and visit your dentist as soon as possible for repair.
The more you know, the more healthy habits you can develop, saving you and your family from avoidable and potentially expensive dental procedures. Talk to your dental professional for more suggestions on improving oral health and check back for more blog posts and relevant information. Please share this site and let us know what else you’d like to know!
Dr. Advocate is an actual board-certified dentist with clinical practice experience and a mission to provide accurate dental patient education. He believes everyone should access easy-to-read dental resources presented in layman’s terms with relevant, up-to-date dental research and insight to improve their oral health.