A dental crown is a custom-made cap that fits over a damaged or weakened tooth. Its purpose is to restore the tooth’s original shape, size, strength, and appearance. It provides a protective covering, reinforcing the compromised tooth structure.
Key Requirements for a Crown
For a dental crown to be successful, the remaining natural tooth structure must meet specific requirements. The vertical height of the prepared tooth’s walls, known as axial walls, is crucial for the crown’s stability. Dentists aim for a minimum axial wall height, usually 3 to 4 millimeters, depending on the tooth’s location and the forces it endures. This height provides retention, preventing the crown from lifting off, and resistance against dislodging under chewing forces.
The ferrule effect is another important factor: a band of healthy tooth structure encircling the prepared tooth above the crown’s margin. This band acts like a cuff, providing strength and enhancing retention by bracing against forces. A ferrule of at least 1.5 to 2 millimeters is generally considered ideal. This helps distribute chewing stresses, making the tooth more resistant to fracture, especially if a post and core is used.
The underlying tooth must be free from decay, cracks, or structural weaknesses. Any remaining decay or compromised areas are removed to ensure a clean, healthy foundation for the crown. This step is important to prevent future issues such as bacterial trapping or further tooth deterioration. Without a sound base, the crown’s longevity would be compromised.
Deep fractures or decay extending too far below the gum line can pose a challenge. If these issues reach the bone level, sealing the crown becomes difficult without infringing on the biological width, the natural space occupied by gum tissues around the tooth. Maintaining this space is important for gum health and to prevent inflammation or bone loss. The tooth’s position and biting forces also influence structural needs, with posterior teeth often requiring more robust preparation due to higher loads.
Addressing Insufficient Tooth Structure
When a tooth lacks sufficient natural structure to support a crown, dentists have several techniques to rebuild it. A common approach is a core build-up, where a dental filling material is used to reconstruct the damaged tooth. This process provides the necessary bulk and shape for a proper crown preparation.
For teeth that have undergone root canal treatment and lost much of their coronal structure, a post and core procedure may be necessary. A small post is placed into the root canal to provide an anchor, and then a core build-up material is applied around it to create the crown’s shape and height. This technique offers additional internal support for the compromised tooth.
If decay or fracture extends deep below the gum line, crown lengthening surgery might be performed. This involves removing gum tissue and sometimes bone to expose more healthy tooth structure. This exposes more tooth structure, allowing for a proper ferrule effect and crown margin placement on sound material, ensuring a better seal and fit.
Despite these interventions, some teeth are too damaged to be restored with a crown. If the damage is too extensive, even with these procedures, the tooth may not be a viable foundation. In such cases, alternatives like extraction might be recommended, followed by replacement options such as a dental implant or bridge.
The Crown Preparation Process
Preparing a tooth for a crown involves shaping it appropriately for the crown. The initial step is tooth reduction, where the dentist removes enamel and dentin from all tooth surfaces. This reduction, from the chewing surface, outer surfaces, and sides, creates adequate space for the crown material without making the final restoration bulky.
Following reduction, the tooth is shaped with a slight conical taper. This taper, usually 2 to 6 degrees, ensures the crown can be comfortably seated onto the prepared tooth while providing retention. The precise angulation is important for the crown’s long-term stability and fit.
A distinct finish line, or margin, is created at the base of the prepared tooth. This margin, such as a chamfer or shoulder, is where the crown will precisely meet the natural tooth structure. A clear margin is important for the dental laboratory to fabricate an accurate, effectively sealing crown, preventing bacterial leakage.
Finally, impressions of the prepared tooth are taken. This can be done using traditional putty or digital scanners to create a 3D model. These impressions are sent to a dental laboratory, where a custom crown is fabricated to match the tooth’s dimensions and the patient’s bite. The process prioritizes preserving healthy tooth structure while creating the ideal form for the crown.