Creating Endodontic Access through Crowns
As restorative dentists, we are inundated with new materials and techniques daily. Continuing education is vital in order for us to keep current on new and innovative treatment plans and newly developed instrumentation. We also must consider the biological impact of our work. One of the most experienced and least desired side effects of any dental procedure is the need for endodontic therapy. Doctors often must make decisions to perform a precautionary root canal prior to restorative treatment because there is the lingering fear that a root canal may be necessary sometime down the road. That decision is not easy, we often feel strained at condemning a tooth to endodontic treatment. In addition, the patient may not understand the need to perform this precautionary treatment, which although not immediately mandatory, will present them with added cost and anxiety.
Often we choose the route of conservative dental treatment based upon our education and better judgment for the patient’s dental health. The dilemma arises when planning a restoration such as a crown for the tooth. Deep areas of decay may indicate that the pulp has been contaminated by bacteria. This situation forces us to decide as to whether to do endodontic therapy and becomes based on the past experiences we have had in similar situations and our educational training. The aggressive treatment plan calls for either doing the endodontic treatment or sending the patient to a specialist. But we can only make the initial assessment based on subjective criteria, as we have no way of knowing if there has been direct infiltration into the pulp of bacteria.
The real world scenario that causes us to rethink our treatment planning occurs when a tooth, which had recently been restored, now needs endodontic treatment. No one has escaped this dilemma! Let’s consider the case where a tooth that had a large amalgam present, with no pain except that which is provoked by cold fluids. We see clinically a partially broken amalgam with what looks like minimal recurrent decay. As we remove the amalgam, we notice that the decay is not extensive, although closer to the pulp than we feel comfortable with as the initial preparation was deep. The dilemma… What course of action do we take? We must inform the patient of all options. The first option is to explain to the patient that root canal therapy would be in their best interest, but we must also explain the cost and emotional involvement that accompanies this treatment action. In the interest of patient relations, finance, and conservative treatment, we also must present the fact that root canal therapy is not mandatory at this time under these present conditions and that a restoration can be placed in lieu of the root canal. Most patients will elect to choose the latter form of treatment. In this case, we place a bonded core, prepare the tooth, make a temporary crown, and wait for a short time to see if any sensitivity or pain develops. After a re-evaluation we can then decide if RCT is necessary. We have done our patient the humane and ethical service they chose. The crown is cemented sometime later, and everything is fine.
Experience tells us that this treatment planning sometimes is very short term. The beautiful crown we placed now has a tooth under it that has signs and symptoms of pulpal dismay. We must now elect to send this patient to an endodontist for treatment. The dilemma created for us at this point is to figure out what to do after the root canal has been completed. Assuming the crown is intact except for the access opening, we could place a bonded resin and explain to the patient that it should last for a long time without problems, and also can be easily replaced if needed. However, we all know that once the integrity of the crown has been violated it is not the same restoration we placed initially. Creating access through a crown can cause weakening and fracture of the ceramic material. Sometimes it happens upon access; sometimes it happens on a delayed basis.
Access through a crown is an important step to the longevity of that restoration. In the case described, the crown was new and it would be desirable to maintain that crown for an extended time. After all, the root canal was in itself an investment and the patient (or the insurance carrier) is reluctant to pay for a new crown. In the case of an older crown, the longevity might not be such an issue as replacement is planned. The timing of the treatment becomes a factor and the old crown may need to function for a while as a transition.
Since access through a crown is important, there are some considerations that need to be recognized. We will be cutting through crowns that are either all-metal, metal-ceramic, or all-ceramic (i.e., Procera, Cercon, etc.). The basic rule of thumb for cutting through these materials is the following:
Use diamonds on ceramic and precious metal
Use carbides on non-precious metal
Diamonds cut by abrasion, this action is very similar to how sandpaper works. Because ceramics are a brittle material, using a carbide can damage the surface due to the vibration (chatter) associated with the bur design. Diamonds are suited for ceramics as the mechanism of grinding, this is accomplished by essentially scratching the surface with the particles at high speeds, lends itself to a smoother cut with less vibration. The hardness of the diamonds is also less likely to be affected by the ceramic substrate it is cutting.
Carbides on the other hand cut by slicing or shearing small particles of material. This action is more suited to cutting non-precious metal as a more efficient cut can be gained resulting in a smoother surface. Although a diamond is harder than the metal, its use is not desired because a diamond creates slurry as it cuts. This slurry limits visibility and also causes clogging of the diamonds cutting surface, greatly decreasing efficiency. The action of the blades on a carbide bur is more suitable for the cutting of metal.
It is a good idea to mention the term chatter here. Bur chatter causes vibration, and vibration is not good when it comes to drilling. Chatter relates to the burs and also to the handpiece that holds them. If a bur shank is not manufactured concentric, a whipping action may occur as it rotates. This whipping action can cause the tip of the bur to become eccentric and not contact the substrate equally. A handpiece needing repair that does not secure a bur tightly, thus, placing the bur out of concentricity will allow the bur to ‘swim’ and rotate erratically, creating the undesirable chatter.
To gain access through a particular crown, we must consider what material was used to construct this crown. Following are the rules which guide me when cutting different substrates:
Full cast non-precious metal: use carbides only (figures 1, 2)
Ceramo-metal: use diamonds to gain access through the ceramic. Once the metal is exposed, use carbide for penetration through the metal substrate (figures 3, 4)
All-ceramic: use diamonds only. This also applies to the newer bi-ceramics, such as crowns with aluminum oxide and zirconium cores. (Figures 5, 6)
Figure 1: Endodontic access being created with a Great White #6 surgical length bur.
Figure 2: Access hole upon completion. Note the added length of the surgical bur, this allows for proper posterior access in tight cases.
Figure 3: Gaining access through a PFM crowns require two steps. Step 1: Trim away the porcelain with a Piranha #801-016 Diamond to expose the metal coping.
Figure 4: Step 2 creates endodontic access to the tooth using the GW #6 surgical length and a plunge cut technique, drill through the metal coping until the tooth is exposed.
Figure 5: Because of there unique qualities, it is best to use a diamond when creating endodontic access through all-ceramic crowns.
Figure 6: Piranha Diamond #856-016 used to create access without causing undue damage to the crown.
Use a medium grit #801-016 Piranha Diamond (SS White) for cutting through the ceramic portion of PFM and all-ceramic crowns. After access has been successfully created, switch to a #856-016 Piranha Diamond to flare out the canal and create adequate working space with proper visibility (figure 7). Medium grit diamonds cause less vibration due to the smaller diamond particle size as compared to a coarse or supercoarse diamond, which can cause undue chatter. The lower cost of a disposable diamond makes this the best choice for my purposes.
For cutting through the metal, portion of a PFM and full cast non-precious crown, I use a Great White #6 surgical length carbide bur manufactured by SS White (figure 8). It is a carbide that cuts very efficiently and smoothly, even through the hardest metals.
Figure 8: Great White Surgical Length #6
Figure 9: The evolution of carbide burs
Figure 7: Piranha Diamond #856-016
For greater efficiency and less vibration, it is important to use a cross-cut fissure bur (rather than a plain fissure bur) when cutting through metal. The Great White burs have been engineered with sharpened dentates and an increased cutting surface (figure #9). These features reduce vibration while increasing cutting ability significantly.
With both types of burs, the use of water spray is imperative to both clear the cut material from the bur, and of course to keep from overheating.
Identifying the proper instrument to task, even when speaking of rotary instruments can minimize time and effort. As restorative dentists, we can increase the quality of our patient care and become more efficient in our efforts by using the proper diamond or carbide to cut any given substrate. Active communication between dentist and endodontist can also aid in patient care. The need to save a crown is a financial issue for the many patients and one that is desirable for the dentist. By using the proper instrument to task to create endodontic access through the many types of restorative substrates that are in use today, a higher percentage of crowns can be saved and not replaced.