A dental crown is a custom-made cap placed over a damaged tooth to restore its shape, size, strength, and appearance. When a crown is repeatedly dislodged, it is an understandable source of frustration, as these restorations are intended to be permanent fixtures in the mouth. The failure of a crown to stay seated is rarely due to a single event and instead points to an underlying technical, structural, or biological problem. Understanding the precise reason for the repeated failure is the first step toward finding a definitive, long-lasting solution.
Causes Related to Adhesive Failure
The most immediate reason a crown falls off is the failure of the dental cement, which acts as the adhesive layer between the crown and the prepared tooth structure. This cement can degrade over time due to a process known as washout, where oral fluids and forces from chewing gradually dissolve the material.
The initial bonding process can also be compromised by technical factors. Saliva or blood contamination during the seating appointment introduces moisture, which can significantly weaken the bond strength of many modern resin-based cements. Furthermore, insufficient preparation of the tooth surface means the cement never forms a strong, durable connection. If the cement layer breaks down, it allows bacteria to infiltrate the space, accelerating the degradation of the bond and potentially causing decay underneath the crown.
Structural and Biological Reasons for Repeated Loss
Structural problems with the underlying tooth are often the true reason a crown cannot be retained, making simple re-cementation a temporary fix. A tooth must have a specific shape, known as the retention form, to mechanically lock the crown in place, with the cement only serving to seal the margin. If the remaining tooth structure is too short or the walls are too tapered, the crown lacks the necessary resistance to leverage forces from chewing.
Retention form relies on the height of the prepared tooth walls and their near-parallel orientation. If the tooth is too short, the forces generated during chewing can easily rotate the crown off the preparation. This inadequate foundation is a common issue for teeth that have suffered significant damage or have undergone prior procedures like root canals.
Biological factors, particularly recurrent dental decay, also undermine the crown’s stability by dissolving the supporting tooth tissue. Decay begins at the crown margin, weakening the base upon which the crown relies for its fit. Once the dentin underneath the crown is compromised, the crown will no longer fit securely, and the decay creates a poor surface for any new cement to bond to. Finally, excessive or uneven forces from the bite, known as occlusal trauma, can repeatedly stress the crown, causing the cement to fracture and the crown to be leveraged off the tooth.
Patient Habits That Contribute to Dislodgement
External forces applied by the patient can place undue stress on the crown. Parafunctional habits, such as clenching or grinding teeth (bruxism), generate immense, repetitive forces that can fatigue the dental cement. These forces are often exerted unconsciously during sleep, gradually breaking the bond and pushing the crown away from the tooth.
Dietary choices are another significant factor, as sticky or hard foods can physically pull the crown free or fracture the underlying tooth. Chewing extremely sticky items like caramel or taffy can engage the crown and exert a tensile force strong enough to overcome the cement bond. Biting down on hard objects such as ice, nuts, or hard candy can also shock the crown and cause the cement to fail. Using teeth as tools, like tearing open packages or biting fingernails, applies shear forces that crowns are not designed to withstand.
Professional Diagnosis and Long-Term Solutions
When a crown repeatedly falls off, the first step is a comprehensive diagnostic assessment to determine the root cause, not just to re-cement the crown. This process typically involves clinical examination, analysis of the patient’s bite, and dental X-rays to check for decay or bone loss beneath the restoration. The dentist must determine if the failure is purely adhesive, structural, or biological before proposing a long-term fix.
If the underlying tooth is healthy and the crown is intact, simple re-cementation with a stronger, permanent luting agent may be successful. However, if structural inadequacy is the problem, more involved treatments are necessary to ensure the crown’s stability. A core build-up can be performed to replace lost tooth structure and create a better foundation for the crown. When the tooth is too short for proper retention, a minor surgical procedure called crown lengthening may be recommended, which reshapes the gum and bone tissue to expose more of the natural tooth surface. This increases the clinical crown height, providing a greater surface area for the new crown to grip. If the tooth is too compromised by decay, fracture, or bone loss, it may be deemed non-restorable, and the dentist may advise extraction followed by replacement options like a dental implant.