25.2 Margin Design and the Supracrestal Attachment

2. Marginal Configurations and Finishing Lines

The choice of margin profile dictates the marginal fit, mechanical strength of the restoration, biological response of the periodontium, and laboratory seating accuracy.

Finishing Line Profiles
    Light Chamfer         Heavy Chamfer         Flat Shoulder         Radial Shoulder
     (Cast Metal)          (Ceramics)          (PFM Butt Margin)       (All-Ceramic)
       │     │               │     │               │     │               │     │
       │     │               │     │               │     │               │     │
       │     ) 0.5mm         │     ) 1.0mm         │     │ 1.0mm         │     ) 1.0mm
       │____/                │____/                │_____│               │____/
       (Curved)           (Curved with 90° CS)   (Sharp 90° angle)     (Rounded 90° angle)

Detailed Analysis of Finishing Lines

1. Light Chamfer (0.3–0.5 mm depth):
   • Geometry: Distinct curved cavosurface margin produced by a round-ended tapered diamond bur.
   • Indications: Full cast gold alloy restorations, metal collars of metal-ceramic crowns, and the lingual finish line of resin-bonded bridge retainers.
   • Advantages: Low stress concentration, clear demarcation on dies, preserves maximum tooth structure.
   • Contraindications: All-ceramic restorations (leads to thin, friable ceramic margins prone to chipping).
2. Heavy Chamfer (1.0–1.2 mm depth):
   • Geometry: Concave finish line terminating in a 90° cavosurface margin with a wide, rounded internal radius.
   • Indications: Monolithic zirconia, lithium disilicate crowns, and facial metal-ceramic margins.
   • Advantages: Provides sufficient bulk for ceramic strength while avoiding sharp internal stress risers.
3. Flat 90° Shoulder (1.0–1.2 mm depth):
   • Geometry: A horizontal 90° ledge meeting the axial wall at a sharp internal line angle.
   • Indications: Porcelain butt margins for metal-ceramic restorations.
   • Disadvantages: Sharp internal angle acts as a severe stress concentrator for all-ceramic crowns, causing radial crack propagation.
4. Radial / Rounded Shoulder (1.0–1.2 mm depth):
   • Geometry: A 90° cavosurface shoulder with a rounded internal line angle (0.5 mm radius).
   • Indications: The gold-standard finish line for all-ceramic crowns (lithium disilicate, infiltrated alumina, zirconia cores).
   • Advantages: Distributes occlusal stresses evenly across the margin; eliminates ceramic tension spikes.
5. Knife-Edge / Feather-Edge:
   • Geometry: Tapered axial wall fading indefinitely onto the unprepared root surface without an internal shelf.
   • Indications: Historically used for tilted mandibular molars or lingual surfaces of periodontally involved teeth.
   • Clinical Traps: Strictly contraindicated for all ceramics and PFM crowns. The dental technician cannot identify the true margin, leading to over-contoured restorations that trigger chronic periodontal inflammation. Only permissible in cast gold alloy when conservative axial reduction is mandated.

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3. Biologic Width (Supracrestal Tissue Attachment)

In the 2018 EFP/AAP International Classification, the historical term "biologic width" was officially renamed the Supracrestal Tissue Attachment.

Anatomical Dimensions of the Dentogingival Complex
  ┌────────────────────────────────────────────────────────┐ ── Free Gingival Margin
  │ Gingival Sulcus Depth: ~0.69 mm (Histological)         │
  ├────────────────────────────────────────────────────────┤ ── Base of Sulcus
  │ Junctional Epithelium: ~0.97 mm (Hemidesmosomal)       │  
  ├────────────────────────────────────────────────────────┤ ── Cemento-Enamel Junction
  │ Supracrestal Connective Tissue: ~1.07 mm (Sharpey's)   │
  └────────────────────────────────────────────────────────┘ ── Alveolar Bone Crest
   └─────────────── Supracrestal Tissue Attachment ────────┘
                         (~2.04 mm total)

Clinical Significance of Supracrestal Encroachment

• The Classical Metric: Based on Gargiulo et al. (1961) and Vacek et al. (1994), the average physiological dimensions are: $\text{Junctional Epithelium } (0.97\text{ mm}) + \text{Connective Tissue Attachment } (1.07\text{ mm}) = 2.04\text{ mm}$ Adding the mean sulcus depth (0.69 mm), the total distance from the crest of alveolar bone to the free gingival margin is approximately 2.73 mm to 3.0 mm.
• Pathological Consequences of Encroachment: If a crown margin is placed subgingivally within 2.0 mm of the alveolar bone crest, the supracrestal tissue attachment is violated. This induces:
  1. Chronic, refractory gingival inflammation and hyperaemia;
  2. Continuous bleeding on probing (BOP) unresponsive to standard oral hygiene measures;
  3. Unpredictable osteoclastic alveolar bone resorption as the biological attachment attempts to re-establish its 2.0 mm dimensions apically; or
  4. Severe gingival recession with exposed root surfaces and restorative margins.
• Surgical Correction (Crown Lengthening): When subgingival caries, fracture, or retention demands margin placement near the bone crest, surgical crown lengthening must be performed. A minimum distance of 3.0 mm between the planned restoration margin and the alveolar bone crest must be surgically established via ostectomy and osteoplasty.

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Placing the Margin

Margin position is a compromise between aesthetics, access and periodontal health, and the examinable default is that supragingival margins are preferred wherever aesthetics permit, because they are easier to prepare, to record in an impression, to finish and to clean, and they do not irritate the gingiva. Subgingival placement is justified only by aesthetic demand in the visible zone, by the need to extend beyond existing caries or a previous restoration, or by the need for additional retention on a short preparation. Where a margin must be subgingival, it should remain within the gingival sulcus and not encroach on the supracrestal attachment.

Choosing the Finishing Line

Each finishing line pairs with a material. A chamfer suits full metal crowns and the lingual surfaces of metal-ceramic crowns, conserving tooth tissue while giving a defined margin. A shoulder, usually 1 to 1.2 mm, or a shoulder with a bevel, suits the labial surface of a metal-ceramic crown where bulk is needed for porcelain. A rounded deep chamfer or shoulder suits all-ceramic crowns, which require a defined margin of adequate thickness and cannot tolerate a knife edge. A knife-edge or feather-edge margin is difficult to identify in an impression and on a die and is generally avoided except on tilted or periodontally compromised surfaces where tissue conservation is paramount.