When the teeth are decayed and it has to be restored the dentist goes for the filling but sometimes the decay is extensive and a large part of the tooth is lost. In such cases pins and posts are used as the simple filling material can not give the adequate strength to sustain the masticatory forces and fractures. The pins and posts can be used separately or together as decided by the dentist.
Pins in Dentistry
There are many types of pins available in dentistry. They are basically thin metallic shafts. They are either cemented in the tooth after making the space for them or they are screwed in the tooth. They are used to give the strength to the filling. First the pins are placed in the tooth above that core built up is done and after that crown is given above that.
- As additional aids of retention in badly broken down or mutilated teeth.
- In teeth with poor prognosis i.e. endodontically and periodontically involved teeth.
- When one or more cusps need capping.
- Increased resistance and retention form is needed.
1. Direct/non parallel pins- are inserted into Dentin followed by placement of restorative material directly over them.
Three major categories of direct pins are:
Cemented pins- pins are 0.001-0.002 inch smaller than their pin channels and the difference in diameter provides space for cementing medium. Are least retentive but virtually place no stress on surrounding dentin during or after placement.
Friction locked pins- are 0.001 inches larger than their pin channels and hence utilize the elasticity of dentin for retaining the tapped pins in a vise like grip. Better retention than cemented pins but generates stresses in dentin in the form of cracks or craze lines.
Threaded pins- are 0.0015-0.002 inches larger than their pin channels and like friction locked pins they are also retained by elasticity of dentin. Provides maximum retention but at the same time generates excessive stresses in the form of cracks in dentin.
Four sizes of TMS pins.
• Regular ( 0.031 inch [0.78 mm]
• Minim (0.024 inch [0.61 mm]
• Minikin (0.019 inch [0.48 mm]
• Minuta ( 0.015 inch [0.38 mm]
2. Indirect/parallel pins- are an integral part of Cast restoration. These pins are placed parallel to each other as well as parallel to the path of insertion of the restoration.
1. Stainless steel
4. Cast gold alloys
PRINCIPLES OF PIN PLACEMENT
Most desirable locations for pin holes are corners of the tooth and least desirable are in the middle of facial, lingual, mesial and distal surfaces of a tooth.
Dentist should try and place pins in locations where they will be surrounded by optimum bulk of dentin and restorative material.
Minikin pins placed in maxillary second premolar and restoration built over pins
- Offer retention without the need for extensive preparation of tooth structure.
- May increase resistance form of the tooth preparation to some extent.
- Less time consuming and less expensive than cast restorations which require multiple appointments.
- Do not increase the strength of the overlying restorative material.
- Induce stresses in dentin in the form of cracks or craze lines, which may increase the potential for the fracture of tooth, micro leakage, pulpal damage etc.
- Increase the chances of perforation into pulp or on the external tooth surface.
Posts in Dentistry
As compared to pins the posts are thicker than the pins and are used in the root canals after root canal treatment where the tooth loss is extensive. In the root canal the post is cemented or screwed and above that core built up is done. After the core built up crown is given. Now many types of posts are available.
- Primary purpose is to retain a core in a tooth with extensive loss of coronal tooth structure.
- If an Endodontically treated tooth is to receive a crown.
- Pulp chambers are too small to provide adequate retention and resistance.
TYPES OF POSTS
1. Active versus passive posts - Most active posts are threaded and are intended to engage the walls of canal, where as passive posts are retained strictly by the luting agent. Active posts are more retentive than the passive posts but introduce more stresses into the roots than the passive posts. Use of active posts should be limited to short roots in which maximum retention is needed.
2. Parallel versus tapered posts -
- Parallel posts are more retentive than tapered.
- Induce less stress into the root.
- Less likely to cause root fracture.
- Have more success rate.
- Tapered posts on the other hand require less dentin removal.
Tapered post on the left side and Parallel post on the right side
3. Prefabricated post and cores- are typically made of stainless steel, nickel chromium alloy or titanium alloy. They are very rigid, and with the exception of titanium alloys, are very strong. The main advantage is that they can be laced in the single visit and are quite economical as compared to the posts which are fabricated in the laboratory.
Prefabricated screw post
4. Custom cast post and cores- These have fallen from favour because they require two appointments, temporization, and a laboratory fees. Offer advantage in certain clinical situations like when multiple teeth require post and core, malaligned tooth, or tooth with minimal crown structure.
5. Ceramic and zirconium posts- These are aesthetic posts that are white and/or translucent but are weaker than metal posts, so a thicker post is necessary, which may require removal of additional radicular tooth structure.
6. Fibre posts- were more flexible than metal posts and had approximately the same stiffness as of dentin. Can be made up of Carbon, Glass, Quartz and Silicon.
How the Post Space is Prepared
-Preservation of radicular dentin is important.
-Gutta-percha can be removed with the help of heat or chemicals.
-Post length equal to 3/4th of root canal length or at least equal to the length of the crown is required.
-At the same time 4-5mm of gutta-percha should remain apically to maintain adequate seal.
-Post diameter should not exceed 1/3rd of the root diameter.
What can be Complications While Inserting Post
- Certain degree of risk to a restorative procedure.
- Procedural accidents may occur during post space preparation that includes perforation in the apical portion of the root or into the lateral fluted areas of midroot, so called STRIP
3. Also increases the chances of root fracture and treatment failure.
For these reasons, posts should only be used when other options are not available to retain a core.