Finish Lines in Tooth preparation – Indications, Advantages and Disadvantages

Finish Line: It is the peripheral extension of tooth preparation or It is the terminal portion of a prepared tooth.

Definition by Schwartz: Refers to the border of preparation where the prepared tooth structure meets the unprepared surface of the tooth.

Tooth preparation: The process of removal of healthy or diseased enamel, dentin, and cementum to shape a tooth to receive a restoration (crown).

Rosenstiel described Finish line as – A line of demarcation, the peripheral extension of tooth preparation, the planned junction of different materials and the terminal portion of the prepared tooth.

Types of Finish Lines in Tooth preparation for FPD or Crown: Shoulder, Shoulder with Bevel, Sloping Shoulder, Radial Shoulder, Chamfer, Heavy Chamfer, Knife edge or Feather Edge, Chisel Edge.

Classifications of Finish Lines based on width:

1. Marginal width less than 0.3mm: Knife Edge or Feather Edge
2. Marginal width up to 0.3mm: Chamfer
3. Marginal width more than 0.3mm: Shoulder

Shoulder:

It is the type of tooth preparation in which the finish line forms a 90-degree angle with the unprepared tooth surface.

Indications: All-ceramic crowns, PFM crowns, Injectable porcelains

Advantages: Less distortion of crown margins, provides adequate bulk, good crown contours, can attain good esthetics

Disadvantages: Marginal adaptation can be affected, more tooth structure needs to be removed

Shoulder with bevel:

Similar to shoulder but has a bevel placed at the border.

Indications: Gingival finish line on the proximal box of inlays or onlays, Occlusal shoulder of onlays and mandibular three-quarter crowns, the finish line for extremely short walls. The facial margin of Posterior metal-ceramic crowns with a supragingival margin.

Advantages: To get proper marginal adaptation, the bulk of the material in border

Disadvantages: More tooth structure is removed, less conservative approach, preparation should be extended apically

Radial Shoulder:

Similar to shoulder in design but has a more rounded border to the tooth surface and not a 90-degree angle.

It is indicated in ceramic restorations

Stress concentration is lesser than the classic shoulder

More tooth structure should be removed

It is indicated in Facial margin of metal-ceramic crowns and All-ceramic crowns

Sloping Shoulder:

Used in the facial surface of a Metal ceramic crown having a 120-degree angle instead of a 90-degree shoulder angle. It decreases the presence of unsupported dentin on the finish line. Sufficient bulk can be provided in metal-ceramic crowns allowing thinning of the crown at the border.

It is indicated in the facial surface of any tooth having a metal-ceramic restoration

The bulk of the crown material can be used

More tooth structure should be removed.

Chamfer:

GPT Definition: A finish line design for tooth preparation in which the gingival aspect meets the external axial surface at an obtuse angle.

TYLMANN Def (1965): A concave extra coronal finish line that provides greater angulation than a knife-edge and less than that of a shoulder.

The Cavosurface margin is 90 degree or greater than 90 degrees.

Indications: Cast metal restorations or lingual margin of metal-ceramic restorations

Advantages: Conservative tooth prep, good marginal adaptation, provides bulk to the restoration

Disadvantages: It is technique sensitive and any defect in fabrication can lead to unsupported tip of the Crown.

Burs used for prep:  Round End Taper

Heavy Chamfer:

Similar to Chamfer finish line but comes with a 90 degrees cavosurface angle with a large radius rounded internal angle. In case of metal restorations, a bevel is added to the finish line.

Indications: It is used in Ceramic crowns and for metal crowns with a bevel.

Advantages: Best finish line for a Ceramic crown

Disadvantages: Technique sensitive, the little discrepancy can lead to the formation of a lip or unsupported fragile enamel

Burs used: Round end taper Bur

Knife Edge or Feather Edge:

As the name suggests it has a knife-edge or thin edge.

Indications: Young patients, Metal restorations, MOD Onlay, Inaccessible areas, when the finish line extends to the cementum, the lingual surface of mandibular posterior teeth, pin ledge 3/4th quarter crown.

Advantages: Conservative, helps in preserving tooth structure, ideal for marginal adaptation, useful in gold and metal crowns  /restoration as it lets you burnish it to a fine finish. It is considered an ideal finish line.

Disadvantages: Distinct or proper finish is not visible, waxing, polishing and casting become critical, Overcontouring of restorations to obtain a bulk is a usual complication.