Pen construction
A post abutment describes a dental restoration. After a tooth that has been severely damaged by caries has undergone a root canal treatment , in most cases only a little of the original hard tooth substance is preserved. Therefore the tooth is strongly weakened against the chewing forces, which can lead to fractures . In addition, there is often not enough substance left to fit the tooth with a crown . To compensate for the lost substance, a post structure is inserted, which stabilizes the tooth and offers sufficient retention for a crown. Although it should actually be called adhesion from a physical point of view, since the adhesive forces of different materials are meant here, this term has not (yet) established itself in dentistry. That is why we will speak of retention in the following .
Preparation of the root canal
After all caries has been removed from a destroyed tooth and the root canal treatment has been completed, a decision can be made as to whether the tooth can be directly restored with a filling or a crown. However, if too much substance is lost, a pin is anchored in the root.
In the case of teeth with several canals, a decision is first made as to where the post should be placed. In most cases, the canal is selected on the X-ray image whose root meets all of the following requirements as far as possible:
- straight course over a long distance
- large diameter
- adequate anchorage in the bone
- correct direction of insertion
If no canal meets all the requirements, it is sometimes possible and necessary to insert several posts in the same tooth.
Once the decision about the canal has been made, it must be prepared for receiving the pin. For this purpose, a part of the root filling is removed in the desired length with various burs. The canal is then excavated. Depending on the system used, this is done with one or more standardized drills. Finally, a plateau can be created, which serves as additional retention and protection against rotation.
Further measures
If the tooth is deeply destroyed, a surgical crown lengthening may be necessary to create the necessary ferrule effect. As a result, the artificial tooth crown can surround the tooth in a stabilizing and tight manner.
Indirect method
The indirect method is used for post abutments, which are made of metal in the dental laboratory. After the canal has been prepared, a prefabricated post made of burn-out plastic can be used, over which an impression of the destroyed tooth is taken. A model can thus be produced in the laboratory, which also shows the exact course of the canal. On this model, the actual wax structure is modeled on the pen and the whole thing is implemented in metal. The post structure is then worked out and inserted by the dentist. After cementing, the abutment can be ground to accommodate a crown.
Direct procedure
With the direct procedure, the post abutment is manufactured and inserted in one session at the dentist. To do this, the channel is prepared and then a ready-made pin is used. The actual structure is then modeled on this pin in the mouth. After hardening, this structure can also be ground to accommodate a crown.
Shapes of the pen
The ideal shape of the post structure has been researched for a long time. Today there are various forms in which ideological views sometimes collide. A basic distinction is made between pins and screws. While pins are suitable for both the direct and the indirect procedure, screws must be processed directly in the mouth. One hopes for stronger retention through the thread. However, the force when screwing in also weakens the tooth, which increases the risk of fracture.
Pins and screws are available in cylindrical, conical and mixed (cylindro-conical) shapes. Here, too, an attempt is made to maintain a high level of retention without weakening the tooth too much. With the cylindrical shape, the retention is very high due to the parallel walls. However, since the root is tapered, either a very thin pin must be used, which bends or breaks quickly, or a thick pin is used in which the root is severely weakened in the lower third and is thus at risk of fracture. With the conical post shape, the course of the root is better traced, which means that the risk of fracture of the root is lower. However, the retention is also greatly reduced in this way, which is why the root post and thus also the structure and the crown can quickly loosen again. The introduction of the mixed (cylindro-conical) shape provided a remedy. The pin or screw is cylindrical over most of its length, the last 2-3 mm then tapering off. In this way, good retention was achieved and the tooth was still adequately protected from fractures.
materials
For the indirect method, the materials of the pins and the superstructures are limited to precious metals and non-precious metals . In contrast, in the direct method, in addition to metal pins and screws, carbon-reinforced or glass-fiber reinforced plastic pins can also be used. This also expanded the range of materials that are suitable for the actual construction to include the various cements and composites .
Attach
Before alternative materials were introduced, post abutments could only be cemented with zinc oxide-phosphate cement or glass ionomer cement. This cement takes 24 hours to fully harden. Only then can the structure be ground to accommodate a crown, otherwise it is possible that the pin will loosen again due to the vibrations of the drill.
Today fastening with composites is becoming more and more popular. While it was initially only used for plastic pens , it is now also used for metal pens , which have been prepared accordingly with silane . Fastening with plastic increases the hold of the post and simplifies further processing, as the composite is completely cured after a few minutes. In this way, the structure can be sanded immediately afterwards in the same session.
Remove
By taking appropriate care with post abutments and crowns, you can create a denture that can remain in the mouth for decades. Nevertheless, it is occasionally necessary to repeat the treatment at a later point in time. If this concerns the superstructure , i.e. the crown or bridge, the post structure can often be left in the tooth. After the old crown has been removed, the direction of insertion of the abutment can be adjusted if necessary. A new crown can then be manufactured and inserted.
Sometimes, however, it is necessary to remove the pin as well, for example because deep caries has penetrated into areas of the pin. In this case, before starting treatment, a decision is made as to whether the remaining root residue is still suitable for accepting a new post. If the post was attached with cement, it can in most cases be removed again with little effort using ultrasound. In contrast, the bond between post and tooth is so strong with composites that it is almost impossible to remove them later. With some post materials you can try to remove the entire post with a burr, otherwise often only the extraction of the root residue remains.
Advantages and disadvantages
There are various advantages in favor of the direct method. Since the treatment time is shorter and there are no metal costs for the gold, the process is usually cheaper. In addition, pins made of glass fiber reinforced plastic can prevent the metal in the area of the tooth neck and the gingiva from shimmering darkly, especially in aesthetically relevant areas . Metallic or carbon-reinforced posts should not be used, particularly if an all-ceramic crown is selected as the final restoration for aesthetic reasons, as otherwise the deceptively real result is prevented. However, these methods are still relatively new. That is why there are no long-term studies that observe the course of treatment over years and decades. In addition, there is increasing discussion about composites regarding their potential to trigger allergies. In the case of particularly sensitive patients, these materials can therefore be tested before treatment.
In contrast, the main advantage of the indirect method is the high breaking strength of the metal pins, especially compared to plastic. This is particularly useful in the posterior region, as this is where the strongest chewing forces occur. In addition, there are various studies for this method that show that these post abutments have a shelf life of several years to decades with appropriate production and regular controls. That is why cast pin abutments made of precious metal are still considered the gold standard despite all the advantages of the direct method .
literature
- Clinical Prosthetics, Vol. 1: Fixed and implant-supported dentures, Wolfgang B. Freesmeyer Karl F. Haug Fachbuchverlag (December 1995) ISBN 3830401264
- Curriculum Prosthetics, Vol. 2 & 3, Jörg R. Strub, including Quintessenz, Berlin (November 2004)