A look into the Tooth. Conceptual Basis for a Successful Post and Core Reconstruction

A post and core restoration is indicated when a significant amount of a root canal treated tooth’s original coronal structure has been lost, and there’s not enough left to support and retain an indirect restoration. And while not every tooth is a candidate for a post and core restoration, those that are require very specific steps that must be followed to ensure success. It’s important to keep in mind that these steps start long before post placement – and mean paying careful attention to the key areas, including:

• Tooth measurements and preparation
• Defining post length and cutting to size
• Placing the post correctly
• Building up the core
• Selecting the right materials – including post choices, cements, and composites

Measure twice, cut once – the importance of accurate measurement

Before undertaking a post and core restoration, it’s critical to measure all dimensions of the tooth and its affected root. These measurements (in four key areas) help ensure a proper fit and adequate support for the restoration and reduce stresses on the tooth (fig. 1). From the root to the core, they are as follows (fig. 2):

1. You must leave a sufficient apical seal of gutta percha (minimum of 4-5mm). If you don’t, there is a risk of leakage, which could lead to infection and loss of the whole tooth.
2. You must adequately prepare the post space, meaning:
   • The space should ideally be only slightly larger than the post itself and not exceed a third of the total root diameter
   • The post prep should allow for the post to span at least half the length of the root that is supported by bone (to ensure the overall stability of the restoration)
   • The post length in the root canal should be at least half the crown height
3.  There must be a ferrule of remaining tooth substance at a minimum height of 2mm around the entire embrasure of the tooth to ensure the stability of the composite core build up
4. There must be positive seating of at least 1mm on either side of the post for the dentin to support a safe core buildup

Figure 1: Key areas of post and core prep.

Figure 2: Required measurements for each key area.

Preparation makes perfect

For a proper fit, the canal must be prepared correctly. The post cannot be loose inside the canal: if the cement shrinks, the post may become loose and the whole restoration may come off, and if the restoration moves, it can fracture the tooth. Many systems utilize a guide that you can superimpose over an x-ray to look at the size of post for prep, but this system can cause distortions that in turn can lead to choosing the wrong post size and, ultimately, an inaccurate prep.

The correct way to approach prep is to use a bur 50 microns larger than the post (25 microns on each side) to create the ideal cement gap (fig. 3). Start by removing some of the gutta percha with a Gates Glidden bur to the desired length, while maintaining a sufficient apical seal (4-5mm). Prepare the canal with a universal drill, cleaning the walls and finish for adequate post length. From there, start with the smallest bur size in relation to the fiber post kit. Increase the bur size consecutively until the bur and corresponding post engage with the wall of the root canal. Try-in the post between each drill to ensure you don’t take away more than you need. Endodontically-treated teeth are fragile and more prone to fracture, so be careful not to make the tooth even more fragile by removing too much tooth structure. With this in mind, you don’t necessarily have to use every drill. Stop when you reach the drill that engages the canal to create optimal cement space. Make sure to use the smallest possible post after you obtain engagement, as the post diameter should not exceed 1/3 of the entire root diameter.

Figure 3: A tooth being prepped. In this instance, after the Gates Glidden bur and thin universal drill are used, the length of the prep is measured – 10mm is sufficient for an anterior tooth. From there, the tooth was prepped from the smallest bur (#0 white) upward until the drill engaged the canal – in this case, #3 blue.

Defining post length and proper shortening

Post length can have a significant impact on retention, making this step in your procedure critical to long-term success. When trying-in the post, move the colored ring down to mark the point to make the cut. The crown will be the height of the neighboring teeth – the post length should be 2-3mm less than neighboring teeth. Be sure to not cut the post too short, as the top needs to be fully embraced by the composite. Once the right length has been determined, the fiber post needs to be shortened perpendicularly to the fibers to avoid fiber damage (fig.4). A diamond disc does this very efficiently.

The post needs to be thoroughly cleaned after having been touched during measuring and cutting to remove contamination like skin oils. Wipe the post with alcohol, and make sure not to touch it again. With some posts, you’ll need a primer to prepare for cementation, however, modern options, like 3M™ RelyX™ Fiber Posts, do not require that step. They are made with a resin that enables a direct bond to 3M self-adhesive cements, saving time and simplifying the procedure.

Figure 4: Shortening of the fiber post, followed by cleaning.

Put posts in their place

When it comes to post cementation, it’s important to choose the right materials. A combination of the right pretreatment, a self-adhesive cement and fiber post that doesn’t require pretreatment can help reduce steps and the chance of mistakes, as well as simplify the procedure.

Some clinicians still use adhesive together with cement, but this can cause issues. Adhesive needs to be air dried, which is tricky in a root canal and adds an extra step to your procedure. Plus, adhesive residue, uncured material or remaining propellant can create a risk of voids and interfere with post seating. Plus, the application microbrush is often too thick to go inside the canal, meaning a thinner brush is needed to apply adhesive in a controlled way. Self-adhesive cements like 3M™ RelyX™ Unicem 2 Self-Adhesive Resin Cement or 3M™ RelyX™ Universal Resin Cement, on the other hand, have standardized application tips which allow for good void-free distribution from the bottom up.

Before placement, use ≤5% sodium hypochlorite solution to clean the canal. It’s important to rinse the prep with water thoroughly – do not let the hypochlorite solution remain in the canal, as it could weaken the bond. Use a paper point to remove excess water without overdrying the canal. You want the dentin to remain moist for an optimal bond. After cleaning the treatment surfaces, there are two easy steps to follow for successful post placement.

1. Apply the self-adhesive cement using the provided root canal mixing tip attachments, keeping the tip immersed in the cement at all times as you gradually work your way up from the bottom of the canal to the top (fig. 5). This technique enables void-free application and an optimal bond. If you simply place cement on the post and place it directly into canal, the air in the canal has no way to get out and you can end up with voids. Using an elongation tip, such as those featured with RelyX Unicem 2 Self-Adhesive Resin Cement or RelyX Universal Resin Cement, can help reach the bottom of the canal. RelyX Universal Resin Cement’s longer and thinner elongation tip, in particular, offers an easier way to apply cement into the root canal (fig. 6).
   Do not overfill the canal – if you place too much cement, it will overflow when inserting the post and get onto the dentin, which will interfere with adhesive application. Only place enough cement so that you can easily remove excess after the post is inserted. Do not use a lentulo bur, the cement will set too fast.

   
   Figure 5: Application of self-adhesive cement.

   
   Figure 6: Side-by-side comparison of the standard elongation tip attachment (left) to the 3M™ RelyX™ Universal Resin Cement elongation tip (right).

2. Insert the shortened fiber post once – do not move up and down, it could create voids (fig. 7). Once placed, either tack-cure excess cement and then remove it or remove excess cement with a microbrush and hold post down to light cure. Light cure from the top through the post, according to the cement manufacturer’s instructions. Use a good, well-tested curing light with a stable output that to ensure a thorough cure, such as 3M™ Elipar™ DeepCure-S LED Curing Light.
   Always use a dual-cure cement to ensure thorough polymerization, even in areas where the light does not sufficiently reach. Cements like RelyX Universal Resin Cement have a powerful self-cure component, providing additional insurance of adequate polymerization – no need to worry about potential voids or uncured material as with the application of a separate adhesive in the root canal.

   
   Figure 7: Insertion of shortened fiber post.

   From cementation to buildup, make sure carefully time your process – do not prepare the buildup too early. Wait a minimum of 6 minutes, ideally 10, for the cement to polymerize to avoid disrupting bonding. Don’t just place the post and start preparing. Cement, wait 10 minutes, then buildup.

Final core build up

As in the previous step of post placement, there are two main steps to follow for the core build up:

1. Rub self-etch universal adhesive onto the tooth and post for 20 seconds, paying careful attention to the tooth (fig.8). Air thin the adhesive layer, and then light cure. 3M™ Scotchbond™ Universal Adhesive and 3M™ Scotchbond™ Universal Plus Adhesive allow for a direct bond between the tooth, the fiber post, and the composite, without need for additional primers.
2. Apply your preferred composite (fig. 9). Bulk fill composites are ideal for this procedure because you can place the composite in thicker layers, avoiding problems that occur when placing multiple layers. Multi-layer problems typically involve voids created by the difficulty in back-filling composite around the post. It is better to fill around the post only once.
   An anterior tooth takes two layers of bulk composite, making it very easy to build up the core. 3M™ Filtek One Bulk Fill is my material of choice, due to its low shrinkage, good hardness, improved opacity, and ease of placement. It can be placed in 4-5mm increments without interfering with esthetics – which is especially beneficial in the anterior. As you build up the core, bear in mind that you will still need space for your restoration, anywhere from 1-2mm depending on your restorative material. Light cure each increment following manufacturer’s instructions and work into shape with finishing and polishing tools (fig. 10). If you want to use a resin cement to bond the restoration, the surface should be smooth: make sure to give special attention to finishing and polishing steps.

   
   Figure 8: Application of adhesive to tooth and fiber post.
   Figure 9: Application of the bulk fill composite in one or two increments for the build up.
   Figure 10: Complete core build up before preparation.

Final summary of a solid post and core procedure

Post and core procedures can be challenging, but they don’t have to be. By paying attention to the little details and key areas listed below, you can set yourself up for success (fig. 11):

• Apical seal: Maintain 4-5mm of gutta percha
• Ferrule: Keep 2mm of intact tooth on all sides for stable core build up
• Positive seating: Ensure 1mm on either side of the post for proper adhesion of composite
• Fiber post: Use a post that doesn’t require pretreatment, measure accurately, and cut perpendicularly with a diamond bur, safely outside the mouth
• Cement: Utilize a self-adhesive cement for its simplicity and make sure to keep the tip immersed in the cement at all times as you gradually work your way up from the bottom of the canal to the top
• Composite build-up: Use a bulk fill to reduce the chance of voids between layers, and a dual-cure cement to ensure thorough polymerization even at depths the curing light may not reach

Figure 11: Anterior and posterior tooth post and core build-up summary.