Preparation Margin Shape and Type—What the Lab Sees at the Margin

The type of preparation margin determines how the technician scans, designs, and perceives the boundary between the tooth and the crown. An illegible margin = the technician interpolates, and every assumption is a potential leakage, misfit, and effort for both the laboratory and you.

Three Main Margin Types—Definitions and Applications

Chamfer: A rounded margin finish—most common and versatile for most materials. The transition angle is approximately 45° towards the inner side of the tooth. Requires precision but is legible on scans and models. Typical width: 0.8–1.0 mm. Shoulder (classic 90°): A sharp edge—maximum space for ceramics. The margin edge is linear, perpendicular to the axial wall. A Rounded shoulder is a shoulder with a rounded internal angle (R ≥ 0.5 mm)—a compromise between space and reduction of stress in ceramics. Both are acceptable, but the rounded one is safer for brittle materials. Feather-edge: A margin without a distinct termination—approaches the natural tooth contour. Ideal for metal (full metal crowns), but forbidden for ceramics—material too thin at the gingiva, difficult to identify on a scan.

Material vs. Margin Type—Recommendation Table

Why a Legible Margin Matters to the Lab

Scanning: The margin must be clearly visible on the XYZ scanner data. An unclear margin = the technician has to deduce the boundary, and every deduction is an error. Designing: In CAD software (Exocad, Cerec), the margin is the reference line for the crown geometry. If the line is unclear, the design becomes a guessing game. Milling/3D Printing: CNC and 3D printers rely on digital instructions. An uneven margin = uneven working surface = crown misfit. Occlusion and Anatomy: A legible margin allows the technician to accurately reproduce the occlusal and lateral anatomy. An unclear margin = technician simplifies, anatomical details are guessed.

Unconscious Errors You Make

Error 1: Heterogeneous margin type on a single tooth Part of the tooth chamfer, part feather-edge, part shoulder. The laboratory does not know how to interpret the transitions. → Maintain one margin type around the entire circumference. Error 2: Too wide a chamfer (>1.5 mm) You waste space and make it difficult for the technician to identify the boundary. → A 0.8–1.0 mm chamfer is standard. Error 3: Sharp angle between margin and axial wall (feather-edge on ceramic) Ceramic too thin at the gingiva, risk of fracture and leakage. → For ceramic, always chamfer or shoulder. Error 4: Margin at the enamel-dentin junction without prior notification The technician might not know that the graphics change color—and thus changes the margin line. → If the margin extends into dentin, describe it clearly in the order. Error 5: Internal rounding at the axial wall edges (only feather-edge) Internal rounding reduces stress concentration. Feather-edge lacks this. → For ceramics: rounded internal angles (R ≥ 0.5 mm) are mandatory.