PG Cert Operative Dentistry – Shenker Posts and Split Cast Post and Core (just a blog!)

There are 5 criteria for post success.

• Length of post – as long as possible but retaining apical GP seal (usually between 5-10mm)
• Width – No more than ½ the width of the root – maintain as much dentine as possible.
• Type of material – ideally yellow gold but depends on post width
• Ferrule – minimum of 2mm and also consideration of thickness of ferrule. Where a ferrule is 0.5mm thick it is useful to request a silver/copper die to prevent chipping when casting up.
• Quality of RCT – ensures success of result preventing bacterial leakage and periapical infection.
• Occlusion –ensuring or considering/planning any guidance on the post crowned tooth.

In this practical, UL3 was prepared for a PFM crown. It was then cutback to 2mm supragingival and the buccal dentine was bevelled. A root canal was prepared with a thin diamond bur and preparation for a shenker post performed. This is a stepped post preparation and useful when internal resistance and retention from is required particularly when adequate ferrule is not present.

The stepped preparation is a compromise between a tapered and parallel sided post that increases resistance and retention form. This stepped shape was created using progressively larger post preparation burs following a 1-2mm setback.

A suitable piece of dentally approved material (remember! A Paperclip is not a dentally approved product. No paper clips were harmed unnecessarily in the making of this blog) was modified with serrations and a tapered/bevelled end to fit passively into the canal with a hooked end 2mm out of the post space. The suitable piece of dentally approved material was sprayed with adhesive. Retraction cord placed in a double cord tech.  Light body impression material was spun into the canal with a spiral paste filler and the suitable piece of dentally approved material inserted. Further material was syringed over the prep and impression tray with heavy body PVS seated. Once checked for detail the impression can be sent to lab requesting a cast-post and core. Disclaimer!: this a just one suggested technique I practiced in the phantom head room and I document here as my own personal blog – there are plenty of other techniques for post and core impressions ( parapost; duralay tech and indeed direct methods using fibre systems) – use the established hierarchy of published evidence in the peer reviewed literature to make your informed decisions on techniques you utilise.

A provisional can be fabricated with an additional prepared paperclip. This time the canal is coated in Vaseline. Quicktemp is applied into the post space and a crown form/putty matrix is load onto prep with paperclip inserted into canal. Once set it is removed and tidied up and then cemented with temp bond.

When cementing the definitive post/core it is useful to cut off the tip of the post and bevel to leave a reservoir of cement apically. Equally, venting the post laterally allows cement to escape and reduces pressure/stress apically. Zinc phosphate is utilised to cement.

The remainder of the session focused on utilising duralay to fabricate a direct post/core made from acrylic. This custom design can be sent direct to the lab requesting a cast/post and core. Duralay was placed into the pre-vaselined canal with a flat plastic or using a syringe technique. A plastic serrated burn out post is placed into the duralay in the post space and additional material used to build up the core. Once set it is removed and checked for airblows and reseated after addition of small amounts of duralay if required. The core is built up further using the bead-on technique with a horse hair brush. The set duralay post/core can be prepared to the ideal crown preparation dimensions thus delivering a custom unit.

Fibre posts were covered along with caution to their limitations. There must be a ferrule present when considering using this type of system. Research suggests 98% success at 30 moths but 65% at 6.7 years (Donavan 2011). This low success rate may indicative of their poor performance under loading and non-custom fit. They can be useful when requiring a tooth coloured core where a thin material is being fitted particularly using porcelain or empress and where the decreased thickness may mean greater translucency. 7^th generation bonding can be used (self-etching systems). RelyX offers good results.

The provision of split cast/post and cores were also discussed. Sornenson and Martinoff highlighted that the long term success rate for posterior root filled teeth doubles with cuspal coverage.

The usual techniques of incorporating a nayar core in root filled molars is useful where some supragingval tissue/ferrule is present. In the occasion of little ferrule and where  it is considered possible and pertinent to save a root filled molar tooth in treatment planning,  then the split cast post and core method can be useful.

For a lower first molar – The mesial two canals are prepared to receive posts to a depth of 5-7mm.

The distal canal is prepared to 10mm. Aluminium temporary posts are size matched into the mesial canals and a plastic burn out post placed in the distal canal. The duralay teachnique is performed in the distal canal and a duralay core built up around the mesial posts. This is removed and refined as described above. The duralay post core is removed and the aluminium temp posts placed back. The unit is sent to the lab tech. Graphite burn out posts replace the aluminium posts and the unit invested in yellow gold. The graphite posts are later removed. The cast post and core is cemented with zinc phosphate into the distal canal and definitive para posts cemented back into the mesial canals. This delivers a divergent post and core and can provide excellent resistance and retention form for a subsequent crown.