Finishing Line of the Preparation PDF

Lec.3 Crown & Bridge Dr.Ruaa Khalil Finishing line of the preparation The finishing line of the preparation (or the so called "The preparation margin") is the final margin that separates between the prepared and the unprepared tooth structure...

Lec.3 Crown & Bridge Dr.Ruaa Khalil Finishing line of the preparation The finishing line of the preparation (or the so called "The preparation margin") is the final margin that separates between the prepared and the unprepared tooth structure. This line should be smoothly continuous from one surface to another; otherwise, it will interfere with the seating of the crown if it is poorly done. The margin between the prepared and unprepared tooth structure is a very critical area as most failures start from this margin. Types of finishing line according to its design or configuration The following designs for finishing line could be used depending on the type of the crown restoration: 1. Knife edge (also named "feather end") 2. Chamfer 3. Heavy chamfer 4. Shoulder 5. Radial shoulder 6. Shoulder with bevel 1 1. Knife edge or feather end finishing line  We use pointed ended taper fissure bur to provide this type of margin. Advantages of knife edge finishing line 1. It is the most conservative type of finishing line because of least amount of tooth structure is removed 2. It is easy to prepare. 3. It is a burnishable type of finishing line. i.e. it provides a burnishable margin. Burnishing: is the further adaptation of the margin of metal restoration to the tooth structure. Disadvantages of knife edge finishing line 1. Difficult to be identified by the laboratory technician. 2. It provides a thin margin that is difficult to accurately wax and cast. 3. The margin of the restoration is susceptible to distortion since this type of margin design does not provide enough thickness. 2 Indications of knife edge finishing line It is mainly used for: 1. Full Metal Crown (All the surfaces). 2. The lingual and proximal surfaces of full veneer crown, three-quarter crown and post crown. 2. Chamfer finishing line  We used a round end tapered fissure bur to obtain this preparation margin.  It is a well-defined finishing line that has an adequate space in the cervical region so we can make the contour of the crown restoration within the contour of natural tooth.  Since the restoration margin obtained with this type of finishing line is thick, so it is unburnishable. This type of finishing line is indicated for areas to be covered by metal only as the knife edge finishing line, so it is mainly used for: 3  1. Full Metal Crown (All the surfaces). 2. The lingual and proximal surfaces of full veneer crown, three- quarter crown and post crown. 3. Heavy chamfer finishing line This type of finishing line provides a 90° cavo-surface line angle with a large radius internal angle, so it provides better support for the ceramic crown. It can be used with porcelain fused to metal (PFM) crown and All Ceramic crown. 4. Shoulder finishing line (Butt shoulder)  Shoulder finishing line is the least conservative type of finishing line due to the excessive amount of tooth structure removed to obtain this type of finishing line.  In the "butt" type of shoulder finishing line, the axial walls meet the finishing line at a right angle.  A flat-end tapered fissure bur is used to obtain this finishing line.  This type of finishing line is used when bulk is required for strength or esthetic, that's why it is almost used with jacket crown since jacket crown is made of either porcelain or acrylic resin, which are brittle materials and 4  require enough thickness to withstand the occlusal forces without fracture. On the other hand, the increased thickness provides better shade of the material and so better esthetics. 5. Radial shoulder finishing line Radial shoulder is a modification of the shoulder finishing line. It is a shoulder finishing line with rounded internal line angles. This will reduce the shoulder slightly and minimize stress concentration on the tooth structure from one hand and on the restoration itself from the other hand. This type of finishing line was introduced with the ongoing development in all ceramic materials in an attempt to increase the fracture strength of all ceramic crowns by decreasing stress concentration. 6. Shoulder with bevel finishing line  It is a modification of shoulder finishing line by adding a bevel to the shoulder. The bevel is at 45-degree angle.  The bevel provides burnishable margin for the metal that may extend subgingivally (The thinner it is, the more adaptable to the tooth surface) and to remove the unsupported enamel. 5 Indications of shoulder with bevel finishing line 1. It is indicated when we use a combination of metal with facing material (acrylic or porcelain) as in full veneer crown, where it is used for the labial surface. 2. Shoulder with bevel is recommended for extremely short walls. 6 Margin placement (finishing line placement): The finishing line of the preparation can be placed either supra-gingivally, sub-gingivally, or equi-gingivally (with the level of the gingiva). Placing the margin of the preparation above the gingival tissue offers the following advantages: a- can be easily prepared and finished by the operator. b- to provide good vision for the operator during preparation. c- the impression can be easily made. d- the patient can keep the area clean more easily. e- most of the time such a position is situated on hard enamel, whereas subgingival margins are often on dentin or cementum f- less destructive So, as mentioned above and for the reasons formerly mentioned, it is better to place the margin of the preparation supra-gingivally whenever possible. However, there are some situations which require sub-gingval placement of the finishing line as listed below: a- for esthetic. b- when we need extra retention as in teeth with short crowns. 7 c- when there is caries or filling at the area of finish line (the preparation margin should be placed on sound tooth structure). 8