Dental X-rays, formally known as radiographs, are a fundamental tool in modern dentistry, providing a view beyond what the human eye can observe clinically. They capture images of the teeth, bone, and soft tissues, revealing hidden conditions like decay between teeth, bone loss, or infections at the root tips. While dentists interpret the visual shades of gray on the film to make a diagnosis, this information is translated into a universal system of numerical codes and terminology for documentation. These standardized systems ensure a patient’s history and treatment plan are clear and consistent across any dental practice. Understanding these numbers is the first step toward comprehending your own dental records.
Decoding Standard Tooth Numbering Systems
The foundation of any dental record is the identification of individual teeth, and three main numbering systems are used worldwide to achieve this precision. The Universal Numbering System (UNS) is the predominant method utilized in the United States, assigning a unique number to each of the 32 permanent teeth. The count begins with the upper right third molar (wisdom tooth) as number 1 and proceeds sequentially around the upper arch to the upper left third molar, which is tooth number 16. The numbering then drops to the lower left third molar, starting at 17, and continues along the lower arch to the lower right third molar, numbered 32.
For primary, or baby, teeth, the UNS employs the letters A through T in the same clockwise pattern around the mouth. This provides a single, unambiguous code for every tooth, which is easily transcribed in digital records and used for all charting purposes. In contrast, the FDI World Dental Federation Notation (FDI) is the most widely used system globally, particularly outside of the US. This method uses a two-digit number where the first digit identifies the quadrant of the mouth and the second digit specifies the tooth type within that quadrant.
For permanent teeth, the quadrants are numbered 1 (upper right) through 4 (lower right), moving clockwise from the patient’s upper right. The teeth within each quadrant are numbered 1 (central incisor) through 8 (third molar). For example, the upper right second premolar is designated as 15, while the lower left first molar is 36, making the system highly logical and systematic. For primary teeth, the FDI system uses quadrant numbers 5 through 8, followed by a tooth number from 1 to 5.
A third method, the Palmer Notation Method, is often favored in orthodontics and oral surgery due to its graphical nature. This system divides the mouth into four quadrants using a cross-like symbol, and then numbers the permanent teeth 1 through 8 from the midline outward. The quadrant symbol placed around the number immediately identifies the tooth’s location, such as when the symbol is used with the number 6 to denote the upper right first molar. The Palmer system uses the letters A through E for the primary teeth, corresponding to the central incisor through the second molar in each quadrant.
Interpreting Radiographic Charting Codes
Beyond simply naming a tooth, dentists use standardized alphanumeric codes to document existing conditions and procedures visible on the X-ray image. The Current Dental Terminology (CDT) codes, for instance, are primarily used for administrative and billing purposes, but they also serve as a documentation shorthand for procedures seen on the film. A code like D2150 signifies a two-surface amalgam restoration, while D7140 denotes a simple extraction, providing an instant summary of the tooth’s history. These codes translate the radiographic findings into universal terms understood by all practitioners and insurance providers.
The appearance of restorative materials on the X-ray is a primary source of this coded information, as materials absorb X-rays differently. Metallic restorations, such as amalgam and gold, appear highly radiopaque, meaning they show up as bright white areas because they block most of the radiation from reaching the sensor. Gold restorations exhibit smooth, well-defined margins, whereas amalgam fillings may have more irregular outlines. These distinct visual characteristics allow the dentist to chart the material used for the restoration.
Composite resin restorations, which are tooth-colored, have a varying degree of radiopacity. They appear less intensely white than metal, though they are still brighter than the surrounding tooth structure. This variation is due to the different filler particles used in the material’s composition. Recognizing the varying levels of brightness and characteristic shapes allows the dentist to confirm existing restorations and assign the appropriate charting code. Documentation will also indicate which tooth surfaces contain the material, such as MO for a mesio-occlusal filling, detailing the extent of the restoration seen on the radiograph.
Numerical Scales for Condition Grading
Radiographs are also used to quantify the severity of specific dental diseases using numerical scales, which helps track disease progression over time. Periodontal bone loss, a common finding on X-rays, is graded based on the amount of alveolar bone that has been lost around the tooth roots. The severity is calculated as a percentage of the total root length, measured from the cemento-enamel junction (CEJ) to the existing bone crest. This measurement is then used to assign the patient a Stage, from I (mild) to IV (advanced), which describes the current severity and complexity of the disease.
A Stage I classification is assigned when the radiographic bone loss is limited to the coronal third of the root, typically less than 15% of the root length. As the disease progresses, Stage II is characterized by bone loss extending from 15% to 33% of the root length. The current periodontal classification also uses a Grade (A, B, or C) to estimate the rate of disease progression. This is often calculated using the ratio of bone loss percentage divided by the patient’s age. A ratio below 0.25, representing a slow rate of progression, falls into Grade A, while a ratio above 1.0 indicates a rapid rate, falling into Grade C.
Dental decay, or caries, is similarly quantified using a grading system that describes the depth of the lesion radiographically. The common D-scale classifies decay based on its penetration into the dentin layer of the tooth. A D1 lesion indicates the dark shadow (radiolucency) has penetrated only into the outer third of the dentin. A D2 lesion signifies that the decay has progressed into the middle third, often requiring restorative intervention. The most advanced stage, D3, means the decay has reached the inner third of the dentin, close to the pulp tissue.