A dental filling is a common procedure designed to repair minor to moderate damage in a tooth. The process involves removing the decayed material and replacing it with a restorative substance, such as composite resin or amalgam, to seal the affected area. This halts the progression of decay and restores the tooth’s original form and function for biting and chewing. While effective for small cavities, the suitability of a filling diminishes significantly as the structural or biological damage to the tooth increases.
When Decay Compromises Tooth Structure
A filling functions by being bonded directly to the remaining healthy tooth structure. The structural limitation for a filling arises when decay has destroyed a large portion of the tooth, typically affecting multiple surfaces or a significant percentage of the crown. If the cavity is too wide or deep, the remaining walls of the tooth become thin and brittle.
A general guideline suggests that if a cavity has destroyed more than 50% of the tooth structure, a filling is unlikely to provide long-term success. Placing a filling in such a large defect creates a weak point, making the tooth highly susceptible to fracture under the pressure of mastication. The chewing forces on the filling can act like a wedge, causing the thin, remaining cusps of the tooth to break away.
In these situations, the tooth is still viable but requires a stronger, full-coverage restoration. An indirect restoration, such as an onlay or a crown, is fabricated outside the mouth and cemented over the tooth to encase and protect the structure. This approach redistributes the biting forces, providing the mechanical strength that a simple filling cannot offer.
When Infection Reaches the Tooth Pulp
A filling is limited to treating the external layers of the tooth, the enamel and dentin, but it cannot address biological damage to the innermost tissue, known as the pulp. The pulp contains the blood vessels, connective tissue, and nerves, and is housed within the pulp chamber and root canals. When decay progresses deeply through the dentin, the bacteria and their toxins can reach this central, sterile environment.
The initial stage of inflammation, called reversible pulpitis, might still be managed with a deep filling, as the pulp tissue can heal once the irritant is removed. If the bacterial invasion is extensive, the inflammation becomes irreversible, leading to necrosis. Irreversible pulpitis marks a point where a standard filling is no longer adequate because it cannot remove the infection from the internal structures.
Once the pulp is irreversibly infected, the only way to save the tooth is through root canal therapy. During this procedure, the infected or necrotic pulp tissue is completely removed, and the internal chamber is disinfected and sealed. If the infection is left untreated and an abscess forms, or if the tooth’s internal anatomy is not suitable for a root canal, the only remaining option is extraction.
When Damage Makes the Tooth Non-Restorable
Severe damage can compromise a tooth beyond the scope of repair, including fillings, crowns, or root canals. In these scenarios, the tooth is considered non-restorable, and extraction becomes the only viable treatment option. These conditions generally involve structural failures that extend far below the gumline and into the supporting bone.
One example is a vertical root fracture, a crack that runs longitudinally down the tooth root. This fracture creates a pathway for bacteria to contaminate the bone and ligament, and the segments of the tooth cannot be permanently sealed, often leading to deep, isolated periodontal defects. The presence of such a fracture makes restoration futile because the tooth is mechanically split and perpetually infected.
Similarly, decay that extends significantly below the gum line and into the bone attachment often renders the tooth non-restorable. A successful restoration requires the dentist to create a clean, dry field for the filling or crown to adhere to, which is impossible when the cavity margin is covered by gum tissue or bone. If the tooth has also suffered severe periodontitis, resulting in extensive bone loss, the tooth may be too loose and unstable to support any restorative procedure.