A cavity is a hole in a tooth, a physical defect resulting from tooth decay, also known as dental caries. Decay begins unseen as a microscopic loss of minerals from the tooth structure. It often becomes apparent as a dark spot that progresses from brown to black. This discoloration is a common sign that the decay process has advanced beyond the initial stages.
How Tooth Decay Begins
Tooth decay starts when bacteria in the mouth form plaque, a sticky film on the teeth. These bacteria consume sugars and starches, producing acid as a byproduct. This acid begins to dissolve the mineral content of the tooth’s outermost layer, the enamel, in a process called demineralization.
Enamel is highly mineralized and is the hardest substance in the human body, but acid erosion occurs over time. If this acid attack continues without neutralization from saliva or fluoride, the microscopic mineral loss progresses. This initial damage may appear as white spots, which are areas of weakened enamel.
As decay moves past the enamel, it reaches the dentin, the layer beneath the surface. Dentin is softer and more porous than enamel, with a lower mineral content. Decay spreads much faster once it reaches this layer, forming the physical cavity and setting the stage for the visible color change.
Why Cavities Change Color
The appearance of a black or dark brown color in a cavity is caused by a combination of intrinsic chemical changes and extrinsic staining. The color change occurs because decay has penetrated the white enamel and exposed the underlying dentin. Dentin naturally has a yellowish-brown hue since it is less mineralized than enamel, and the decay process intensifies this color.
Intrinsic discoloration is primarily a chemical reaction where the breakdown of tooth structure creates dark byproducts. As decay destroys organic components of the dentin, such as collagen and proteins, these materials decompose. This decomposition leads to the formation of dark pigments, often indicating a chronic, slower-moving decay that took an extended time to develop.
Extrinsic factors also contribute to the final dark color, as the porous, decayed tissue readily absorbs stains. Common culprits include chromogens found in dark foods and drinks, such as coffee, tea, and red wine. Tobacco use causes external staining that darkens the compromised tooth structure. Furthermore, certain dental treatments, such as silver diamine fluoride (SDF) used to stop decay, intentionally turn the treated area a permanent black color due to the silver component reacting with the tooth.
Stopping and Treating Discoloration
Addressing a black cavity requires professional dental intervention to treat the underlying decay and restore the tooth’s structure. For established decay, the dentist must physically remove all infected and discolored material. The resulting void is then filled with a dental filling, such as composite resin or porcelain, which seals the tooth and prevents bacteria from re-entering.
If decay is extensive or has compromised a large part of the tooth, a crown may be necessary to cover and protect the remaining structure. If decay reaches the pulp chamber—the tooth’s innermost layer containing nerves and blood vessels—a root canal procedure is performed to remove the infected tissue. Once decay is treated, any external discoloration from staining or tartar can be removed through professional cleaning.
Preventing a cavity from reaching deep discoloration focuses on inhibiting the decay process early on. This involves maintaining consistent oral hygiene, including brushing twice daily with fluoride toothpaste and flossing every day. Regular dental checkups allow professionals to identify and treat decay at the initial, often reversible, white-spot stage before it penetrates the dentin and turns dark. Limiting the intake of sugary and acidic foods and drinks is also preventative, as it reduces the fuel source for acid-producing bacteria.