A dental filling is required when bacteria cause a cavity (dental caries). The drill is used to remove all infected, softened tooth material. The objective is to clean the lesion while preserving the maximum healthy tooth structure possible. This layered removal ensures the remaining tooth can support a lasting restoration.
Essential Tooth Structure for Understanding Drilling
The tooth structure dictates the sequential approach used during drilling due to the varying hardness and sensitivity of each layer.
Enamel
The outermost layer is the enamel, the hardest substance in the human body, composed almost entirely of crystalline calcium phosphate (hydroxyapatite). This dense structure is acellular and lacks nerve endings, meaning drilling through enamel is typically painless. Penetrating the enamel requires a high-speed, precise instrument.
Dentin
Beneath the enamel lies the dentin, a slightly softer, yellowish layer forming the bulk of the tooth structure. Dentin is permeated by millions of microscopic channels called dentinal tubules, which contain fluid and extensions of the cells lining the pulp. The presence of these tubules means the dentin is sensitive, and their diameter increases significantly closer to the tooth’s center.
Pulp
The innermost chamber houses the pulp, a soft tissue containing the tooth’s nerves, blood vessels, and connective tissue. The pulp is the living core of the tooth, providing sensation and nourishment. Maintaining the health and integrity of this delicate tissue is the ultimate factor governing how deep the dentist can safely proceed with decay removal.
The Sequential Process of Removing Decay
The drilling sequence is a controlled process that navigates the differing densities of the layers and the progression of the bacterial infection. The procedure begins with the swift removal of the enamel overlying the carious lesion. Dentists utilize a high-speed handpiece (often over 200,000 rotations per minute) with a diamond or carbide bur. This creates a conservative access point through the tough outer shell.
Once the bur breaches the enamel and reaches the dentin, the approach changes immediately. The dentist transitions to a slower-speed handpiece, typically operating at less than 30,000 rotations per minute, which offers better tactile feedback and control. This shift is essential because decay spreads much faster within the softer dentin layer, often undermining the surrounding healthy enamel.
The goal in the dentin is to selectively remove only the infected, necrotic tissue, which appears soft or leathery. This material must be removed because it is heavily contaminated with bacteria and cannot be repaired by the tooth. Dentists use specialized round carbide burs or hand instruments to gently excavate this infected layer.
Dentists focus on distinguishing the infected dentin from affected dentin, which is merely demineralized but not yet contaminated with bacteria. Affected dentin is firmer and closer to the pulp. It contains collagen that has the potential to remineralize. Therefore, this deeper, affected layer is intentionally preserved to minimize the risk of exposing the pulp chamber.
Recognizing and Managing Proximity to the Pulp
The endpoint of decay removal is determined by the need to maintain the health of the pulp and prevent its irreversible inflammation. As the dentist approaches the deepest part of the lesion, the remaining layer of dentin becomes extremely thin. Visually, this proximity is signaled by a change in the dentin’s appearance, which may become slightly translucent or pinkish due to the underlying pulp tissue.
If the decay is extremely deep but has not yet caused an actual exposure, the dentist will perform an indirect pulp capping procedure. This involves placing a medicated liner, such as calcium hydroxide or Mineral Trioxide Aggregate (MTA), directly over the thin layer of preserved affected dentin. The high pH of calcium hydroxide stimulates underlying cells to form a protective barrier of reparative dentin. This effectively walls off the pulp from the filling.
If the bur accidentally exposes the pulp, or if the decay has already created an opening, the treatment becomes more complex. A small, clean exposure may be treated with direct pulp capping, where the medicated agent is placed directly on the exposed pulp tissue. If the pulp is already infected or inflamed due to prolonged bacterial exposure, a root canal therapy is required to remove the entire infected pulp and save the tooth.