Anatomy of the tooth crown

Crown surfaces and their preparation

During the preparation of the abutment tooth for an artificial restoration, the dentist works with various crown surfaces. It is important that the prepared tooth (incisor, canine, premolar, or molar) retains its original anatomical shape, albeit in a reduced size.

The following surfaces and preparation rules are distinguished:

• Vestibular (facing the oral vestibule) and oral (facing the oral cavity) surfaces. During their preparation, a cervical groove is formed 0.3–0.5 mm above the gingiva. The depth of tissue removal depends on the tooth: up to 1 mm is removed on the central upper incisors and canines, 0.7 mm on the upper lateral incisors, and 0.3–0.5 mm on the lower incisors.
• Masticatory surface and incisal edge. In this zone, as well as on the adjacent third of the vestibular side, a special bevel is created. It is necessary to impart an oral inclination to the stump and thicken the ceramic veneer layer. Such a bevel is not made on the oral side to maximize the preservation of hard tissues.
• Proximal surfaces (mesial and distal). Transitions between them and the vestibular/oral walls are smoothed out, removing sharp angles and undercut areas.

Alternative method: a vertical groove can be created on the vestibular surface along the tooth axis (retreating 0.3–0.5 mm from the gingiva). Its depth (0.3–1 mm) serves as a precise guide—tissues are prepared precisely down to the bottom of this groove.

Radiographic anatomy of the coronal portion

Radiography is used to assess the condition of the tooth crown and detect hidden carious cavities (on contact surfaces or under old crowns). On the radiograph, anatomical structures differ in their degree of radiopacity:

• Enamel—the outer layer of the crown, producing the most intense and dense shadow.
• Dentin—the main bulk of the crown, visualized as a less dense shadow compared to enamel.

In addition to natural tissues, the density of materials restoring tooth anatomy is evaluated on the radiograph. Phosphate cement has good radiopacity, whereas plastic, silicate cement, and some composites weakly block x-rays, producing an indistinct image.

Restoration of anatomical shape with inlays

In case of significant crown destruction, its shape is restored using microprostheses—inlays. Fitting requires precise recreation of occlusal relationships:

1. Interference elimination: identification via impressions and grinding of areas preventing prosthesis seating.
2. Marginal adaptation control: checking the joint between the inlay margin and the hard dental tissues with a sharp explorer.
3. Occlusal adjustment: checking tooth closure in central occlusion using articulating paper. Premature contacts (excess metal) are removed until the closure becomes identical to that before prosthetics.
4. Final processing: grinding off excesses along the margin for a smooth transition of the material into the dental tissues, finishing and polishing.

Preparation of the cervical shoulder

An important element of the modified crown anatomy during preparation is the shoulder. Using the end part of a diamond cylindrical bur, it is smoothed and brought close to the gingival margin.

Variants of the final shoulder contour:

• Shoulder with a bevel (formed by a bur truncated at a 45° angle).
• Shoulder at a 135° angle (created by a pencil-shaped bur).

Normally, the preparation boundary is located strictly at the level of the gingiva. Subgingival placement of the shoulder (to the middle of the gingival sulcus) is permitted only as an exception—in patients with a short upper lip and a wide smile. Such a location often provokes marginal periodontal inflammation (gingivitis or periodontitis).

Cementation of restorations to the crown

For metal inlays glass ionomer cement is used, and for ceramic ones — dual-cure composites. The inlay is disinfected with alcohol and dried with ether. The abutment tooth is isolated with cotton rolls, the cavity is treated with alcohol-soaked cotton pellets and dried with an air syringe.

Cement is applied to the internal surface of the prosthesis and into the tooth cavity. After inserting the inlay, the patient closes the jaws in central occlusion. Excess material is removed with a plastic instrument after 5–7 minutes. To avoid mucosal trauma, the doctor's hand must firmly rest on adjacent teeth. Interdental spaces are cleaned with an explorer or metal matrix.

Class and exam questions

What else to study on this topic

• Black's classification of carious cavities (features of Class I and V).
• Method for calculating the true root canal length from a radiograph.
• Anesthesia zones and potential complications in infraorbital nerve block.
• Technique of palatal anesthesia (greater palatine nerve block).
• Indications for dental radiovisiography (RVG).

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.