Methods for Removal of root filling material in preparation for posts
Gutta-percha is today the universally accepted core material used for root canal obturation. However, the techniques for placement differ, but all require the use of sealing cement. Obturation techniques include cold lateral compaction of gutta-percha points, compaction of gutta-percha that has been heat softened in the canal and compacted (eg System B), thermoplastisised gutta-percha which is injected into the canal (eg Obtura and UltraFil) and finally compaction of gutta-percha which has been placed in the canal and softened by mechanical means (eg McSpadden compactors). These obturation techniques are unlikely to have an impact on the final apical seal once post space preparation has been carried out. An alternative obturation technique also exists which involves heated gutta-percha surrounding a plastic or metal carrier (eg Thermafil). The carrier ensures that the gutta-percha passes to the correct working length and is left in situ with the gutta-percha. During mechanical gutta-percha removal of this system, there is greater potential to disrupt the apical gutta-percha and a number of dye leakage studies have supported this hypothesis. It would therefore be prudent not to use such obturation techniques if placement of a post is anticipated.
Solvents such as oil of eucalyptus, oil of turpentine and chloroform have been used to soften gutta-percha for removal, with the latter two being the most effcicient. However, some of these materials and especially chloroform are hazardous to use as they are toxic and potentially carcinogenic. Oil of turpentine is less toxic, but there is concern that solvents in general lead to a dimensional change in the gutta-percha, leading to increased microleakage. This together with the fact that it is difficult to control the depth of softening of the gutta-percha and potential leakage of the solvents into the periradicular tissues should be sufficient to discourage their use for gutta-percha removal for post placement. They are however a necessary adjunct in root canal re-treatment cases.
A heated instrument such as a lateral compactor can be inserted into the gutta-percha to the desired length to soften and remove the gutta-percha. However, in narrow canals, fine instruments lose their heat quickly and gutta-percha removal can be difficult. A System B spreader is ideal for removal of gutta-percha.
|System B with heated plugger (200°C) in foreground, with rubber stop placed at the desired length for gutta-percha removal|
From a pre-operative radiograph a plugger should be chosen of the correct dimensions that is likely to bind at the desired post length and this position should be marked on the plugger with a rubber stop. The tip should be placed in the gutta-percha and with the heat applied driven slowly to the desired post length in about 2–3 seconds. The heat should be removed and the plugger allowed cooling for about 7–10 seconds, twisted and then removed with the coronal gutta-percha. Alternatively, a short burst of heat to the plugger will allow for easy removal. It is important that the plugger is sufficiently hot to completely soften the gutta-percha. If too cool, it will result in the gutta-percha remaining sticky with the risk of dislodging the apical gutta-percha. An instrument such as a Buchanan plugger can then be used to vertically compact the softened gutta-percha. Such a technique is useful in removing old gutta-percha which can become quite hard.
Some authors would suggest that gutta-percha should be removed with heated techniques as a routine and mechanical removal only used if heat is insufficient.If mechanical removal is used, a heated instrument can be used to soften the most coronal gutta-percha, so that it can be vertically compacted and adapted to the canal walls to create a seal.
Mechanical removal of gutta-percha is efficient and probably the most commonly used technique, but it is a technique that can result in the most damage to tooth tissue. If done incorrectly, it can weaken the root unnecessarily, damage the periodontium and in some cases lead to root perforation.
|Periapical radiograph showing teeth 13 and 12 used as double abutments for a fixed-fixed conventional bridge|
|Periapical radiograph of tooth 23 with a post that poorly fits the prepared post hole|
A non-end cutting bur such as a Gates-Glidden or Peeso reamer should be used for gutta-percha removal, as these will cut and remove the relatively softer gutta-percha preferentially to the dentine of the canal walls.
The sequence in which the burs are used is important so that a rise in temperature at the root surface, which could damage periodontal cells, is avoided and the risk of preferentially cutting away root dentine to one side of the root canal is reduced.
|Periapical radiograph of root filled central incisor teeth (top left) from which the diameter of the post to be used can be estimated|
Temperature rise on the root surface has been investigated in a number of studies. A Gates-Glidden bur rotating at 8,000 rev min-1 results in a small rise in temperature at the root surface. However, both tapered and parallel-sided post drills produce a significant increase in temperature in excess of 17°C. Peeso reamers also generate significant rises in temperature, higher than that reached with Gates-Glidden burs and Parapost twist drills. To reduce this temperature increase, which could potentially damage cells in the periodontal ligament, it is important that the smaller sized Gates-Glidden burs are used first, working up through the sizes in turn, until no gutta-percha is removed apically. At this stage the smallest post drill can be used, again working up through the size sequence until the final post size is reached.