Receiving a dental crown involves a temporary stage that is important for the procedure’s success. When a tooth requires a full-coverage restoration, the dentist first reshapes the natural tooth structure, a process known as the preparation. The edge of this preparation, where the final crown will meet the tooth, is called the margin. Since the permanent restoration must be fabricated in a laboratory, a provisional restoration, or temporary crown, is placed on the prepared tooth to protect it during the waiting period. The precise fit of this temporary piece, particularly where its edge meets the margin, significantly influences the health of the surrounding gum tissue and the eventual success of the final crown.
The Essential Roles of a Provisional Restoration
A provisional restoration serves several functions that maintain the health and alignment of the mouth while the permanent crown is being created. Its primary purpose is the protection of the underlying tooth structure. The preparation process exposes the dentin, the layer beneath the enamel, which contains microscopic tubules leading to the sensitive pulp tissue. Covering the prepared tooth with a temporary crown seals these tubules, preventing sensitivity to temperature changes and the entry of oral bacteria that could lead to infection.
Beyond protection, the provisional crown plays a significant role in maintaining the correct alignment of the teeth. Once the natural tooth structure is reduced, adjacent teeth could drift or tilt into the newly created space. Also, the opposing tooth in the bite might move out of its socket, a process known as super-eruption. The temporary restoration holds the space open and maintains the correct occlusal relationship, ensuring the permanent crown will seat perfectly without requiring adjustments. The provisional also allows the patient to chew and speak normally and provides an aesthetic solution during the fabrication process.
Guiding Principles for Margin Placement
The question of how far below the margin a provisional crown should sit is answered by prioritizing the health of the surrounding gum tissue. The guiding principle is to place the margin supragingivally (slightly above the gumline) or equigingivally (exactly at the gumline), rather than deep below it. This approach minimizes irritation and allows the soft tissues to remain healthy.
The rationale for this placement centers on plaque control and cleanability. When the margin is easily accessible above the gumline, the patient can effectively clean the area with normal brushing and flossing. This reduced plaque accumulation is important because temporary crown materials are not as smooth as permanent materials, making them more prone to attracting bacteria.
By keeping the margin out of the gingival sulcus (the shallow space between the tooth and the gums), the dentist protects the delicate soft tissue attachment. This attachment includes the biological width, which is the vertical dimension of soft tissue that naturally adheres to the tooth surface just above the bone. The biological width averages about 2 millimeters and must not be encroached upon by restorative materials. If the temporary restoration is placed too deep, it can violate the biological width, leading to chronic inflammation and tissue instability. Ensuring the tissue is healthy is necessary for the dentist to capture an accurate impression for the final crown.
Consequences of Marginal Discrepancies
When a provisional crown’s margin deviates from the ideal placement, several adverse consequences can compromise the success of the permanent restoration. A margin that is over-extended, or sits too far below the preparation line and into the gum tissue, is a common issue. This deep placement violates the biological width, which the body attempts to correct by resorbing the underlying bone or causing persistent gum inflammation. This chronic inflammation makes taking a precise impression for the final crown difficult, as the swollen tissue distorts the mold and leads to a crown that may not fit properly.
Furthermore, the irritation can lead to irreversible damage to the soft tissue attachment, potentially causing the gums to recede.
Conversely, if the provisional crown is under-extended, meaning the margin sits too high and does not completely cover the prepared tooth margin, a different set of problems arises. The exposed portion of the tooth preparation is left vulnerable to the oral environment, leading directly to increased tooth sensitivity. The under-extended margin also creates a gap where oral fluids and bacteria can leak beneath the temporary crown, a phenomenon known as microleakage. This leakage can lead to secondary decay or re-infection of the prepared tooth, necessitating further treatment. Precise marginal fit is a foundational requirement for both the temporary and permanent stages of restorative dentistry.