Clinically, teeth with .xed restorations may need to undergo endodontic treatment or require replacement of the crown. There are a host of reasons for removal of existing .xed restorations, including replacement of the crown related to recurrent decay, replacement af.ter endodontic treatment, fracture of the existing res.toration or conversion to a .xed abutment for a bridge. There also may be situations where the existing crown will be maintained, but endodontic treatment needs to be performed on that tooth, so preserving the current restoration is required. Various materials have been utilised for .xed restorations, some metal-based, others ceramic and metal or all-ceramic in nature. The direc.tion over the past decade and longer has been to elim.inate metal in .xed restorations owing to the increasing demand for more aesthetic restorations by the patient and the consequent introduction of new stronger and more aesthetic ceramics. Ceramics, whether they are feldspathic, leucite or lithium silicate-based, are fairly easy to cut through when needing to access the pulp chamber to perform endodontic procedures or remove the crown to replace the restoration. Yet, the industry has been steadily moving in the direction of zirconia restorations, be they monolithic or as a base for an overlying ceramic. The increase in zirconia res.torations has become a frustration to the practitioner when these restorations need to be removed or ac.cessed through.
As with any procedure, selection of the correct tools and technique improves performance while decreas.ing the time to achieve the desired results and eliminat.ing frustration on the practitioner’s part. In this article, I will discuss methods to access through and remove zirconia-based restorations, as this is the material that causes the most frustration for practitioners in this regard. Additionally, techniques for metal-based resto.rations will be addressed.
Not all ceramics are the same
Zirconia hardness, as measured by Vickers hard.ness, is reported in the range of 700 to over 1,200, depending on how the material is treated during man.ufacturing and how it is .red after milling.1 Hardness of zirconia restorations can decrease over time with consumption of acidic drinks (e.g. carbonated drinks and citrus juices), but this affects non-zirconia-based ceramics to a greater extent. Flexural strength is also greater for zirconia monolithic restorative materials, decreasing for zirconia hybrid ceramics and further for lithium silicate and leucite-based ceramics. These properties correlate with the abrasiveness of the restorative material.
The typical reasoning is that the harder the material and the more abrasive the ceramic, the more pressure (force) needs to be applied to cut it. Diamond-coated burs are used to cut ceramics because carbide burs are ineffective at cutting ceramics. When a carbide bur contacts a ceramic, the ceramic rapidly wears the car.bide bur’s °utes, creating irregularities on the carbide surface that may lead to microfracturing of the ceramic surface. Microfractures will not present an issue when removing the crown, but if the plan is to preserve the crown after endodontic treatment, it may lead to crown failure related to crack propagation over time. Whereas a diamond-coated bur abrades the surface being contacted, a carbide bur cuts.
Ceramics need to be abraded in order to penetrate the material; they cannot be cut, as it leads to structural damage (microfractures) and is an inef.cient prepa.ration method. Metal needs to be cut, and essentially a carbide bur mills the material it is preparing.
Monolithic zirconia or zirconia-based restorations
All-ceramic restorations constitute the majority of what has been used in the US with increasing frequency over the past ten or more years, and zirconia, especially in the posterior, is the dominant material chosen owing to its higher strength then other ceramic types. As zirconia is being manufactured with greater translucency, its use in the anterior will increase, especially in those patients with parafunctional habits that other ceramics may not withstand long term. The technique and materials that will work on zirconia, being the hardest ceramic mate.rial in use currently, will also work on ceramics that have lower material hardness.
Choosing the correct tool is as important as the tech.nique used with that tool. The belief that a hard ce.ramic material requires a coarser diamond-coated bur is actually not accurate and has driven practitioner frus.tration when attempting to remove or access through zirconia restorations. Additionally, instinctively, when we are preparing a hard material, we tend to subcon.sciously apply greater pressure to the instrument in an attempt to cut through the material. Add to this that, when we want to cut a slot into the crown to allow the crown to be split to remove it, we tend to push the dia.mond-coated bur’s long axis into the crown, directing force towards the centre of the tooth. Conversely, when creating access through the crown to initiate endodon.tic treatment, we push the diamond-coated bur down against the ceramic in an attempt to create a hole in the restoration and contact the underlying dentine. Unfor.tunately, these inef.cient methods lead to practitioner frustration.
Utilisation of medium-grit diamond-coated burs with the correct technique allows faster, more ef.cient ac.cess through zirconia, whether creating a slot to split the crown or performing endodontic access to the pulp chamber. As the technique has similarities and minor differences depending on whether crown removal or endodontic access is being performed, I will address these differently.
The key to removal of a zirconia crown is creating a slot across the buccal and occlusal or incisal surfaces so that torquing force can be applied to the slot and split the crown while preserving the underlying tooth struc.ture. A medium-grit, slightly tapered diamond-coated bur (Great White Z [GWZ] 856-018, SS White Dental; Fig. 1) is used in a high-speed handpiece with copi.ous water with light pressure against the tooth and moved parallel to the bur’s long axis in a sawing motion (Fig. 2). This is begun on the mid-buccal surface, cre.ating the slot from the crown margin to the junction with the occlusal or incisal surface, depending on what tooth is being treated. The slot is periodically checked to avoid penetration into the underlying dentine (Fig. 3). Once the slot has fully penetrated through the zirconia, with a posterior crown, an additional slot is cut across the occlusal surface that is continuous with the buccal slot (Fig. 4). When the crown being removed is on an anterior tooth, the incisal surface of the crown requires a slot deep enough to reach the underlying dentine (Fig. 5).
As the crown may be fairly thick in this area, depend.ing on how the original tooth was prepared or what remained of the tooth prior to preparation, in some cases, this slot may be . 3 mm deep.
A °at-bladed instrument such as a crown remover is inserted into the buccal slot, where the occlusal or incisal slot is continued and a rotational torquing mo.tion is made to split the restoration into two halves and separate it from the prepared tooth (Fig. 6).
It is not advisable to attempt this with a °at-bladed composite instrument, as the torque generated will either break the tip of the instrument or warp it. Crown removal in.struments are better designed for this and can with.stand the forces generated without instrument break.age. Should the crown not split into two pieces, verify that both grooves are continuous with the dentine and deepen them if ceramic remains in the slots. If the slots have penetrated to the dentine, and the crown is not splitting, continue the slot 2–3mm on to the lingual surface. Should that still not allow crown splitting, using the same diamond-coated bur, remove ceramic inter.proximally at the expense of the crown being removed, in order to create space to allow the crown halves to separate when torqued with the instrument.
Dental adhesives are less retentive to zirconia than to tooth structure, and there is debate whether a true adhesive bond occurs to this substrate.5 Upon crown removal, often the luting material is still bonded to the dentine, and this can be prepared off during re-prepa.ration of the tooth. This also applies to zirconia-based restorations that have an overlying ceramic for improved aesthetics, whereas ceramics such as lithium silicate,
leucite-based and other true ceramics, including those containing zirconia particles, as well as ceramic–resin hybrids, have true adhesion between the crown mate.rial and tooth structure.6–8 These may not split with the two slots created or removal of interproximal crown ma.terial. The author has found that this is more frequent in the posterior. When this is encountered, creating an additional slot from the mesial to distal aspect that in.cludes the marginal ridges (creating a plus-shaped slot across the occlusal surface) and placing the crown splitter instrument into the mesial slot and applying rotational torque separates either the mesiobuccal or the mesiolingual cusp portion of the crown from the tooth. This is repeated in the distal slot. If the buccal or lingual crown material remains, the instrument is placed into the remaining slot and that will usually loosen the remaining crown material.
Endodontic access employs similar rotary instruments, a medium-grit round diamond-coated bur being used. The round diamond-coated bur is provided in three sizes, and selection is based on the tooth being treated, determined by the size of the access form needed for endodontic treatment. Mandibular anterior teeth and maxillary lateral incisors will require smaller endodontic access, for which a GWZ 801-014 (SS White Dental; Fig. 1) would be utilised. Maxillary central incisors and canines, as well as both maxillary and mandibu.lar premolars, will require a larger access to unroof the pulp chamber, and a GWZ 801-018 (SS White Dental; Fig. 1) is best suited for these teeth. The round dia.mond-coated bur, as with the technique for crown re.moval, is used with light pressure, constant motion and copious water (Fig. 7). The diamond-coated bur is used to outline the access form desired and not pressed into the ceramic crown material, in an attempt to create a hole through the restoration (Fig. 8). Molars require the larg.est endodontic access owing to the dimensions of the pulp chamber, and widening of the initial access open.ing through the ceramic crown with the GWZ 801-018 can be followed with a football-shaped GWZ 379-023 (SS White Dental; Fig. 1) to achieve the desired ac.cess shape. A carbide bur is utilised to unroof the pulp chamber (Fig. 9). Once the pulp chamber has been penetrated, the outline form may also be re.ned using the GWZ 856-018 and then endodontic treatment of the canal system can begin (Fig. 10).
All-metal or metal-based restorations
As discussed, metal cannot be effectively cut with a diamond-coated bur and a carbide bur is required. When crown removal is planned, the GWZ 856-018 diamond-coated bur is utilised to create the slots through the ceramic layer until the metal substructure is encountered (Fig. 11). A Great White #1557 metal.cutting carbide bur (SS White Dental) is then applied at the buccal margin and stroked in an occlusal direc.tion with the rotary instrument’s tip penetrating to the dentine and continued to the occlusal or incisal sur.face (Fig. 12). Once the slot has been completed to the dentine, the crown removal instrument is placed into the groove and torqued to complete crown removal. The process is similar with all-metal crowns, eliminating use of the diamond-coated bur and completed with the carbide bur alone (Fig. 13).
Endodontic access is also approached in a similar man.ner to when dealing with all-ceramic crown materials. When a porcelain-fused-to-metal (PFM) restoration requires access though the restorative material for end.odontic treatment, one of the round diamond-coated burs (GWZ 801-014 or GWZ 801-018) is used to cre.ate an outline form slightly larger than the needed form through the ceramic to the underlying metal. The Great White #1557 carbide bur is then used to form a hole through the metal into the dentine and then the outline form is widened to the dimensions of the outline cre.ated in the ceramic with the carbide bur and through the dentine to unroof the pulp chamber (Fig. 14).
Typically, access in precious and semi-precious met.als can be performed with a single carbide bur, but non-precious metals may require multiple carbide burs in order to complete the task. This also is true with re.moval of PFM crowns. All-metal crowns require more metal to be removed and usually require multiple car.bide burs to accomplish the task. Light pressure, allow.ing the carbide bur to cut the metal, is more ef.cient and less likely to snap the head off the carbide bur, which may occur when excessive pressure is applied to the restoration with the carbide bur. The carbide bur should be disposed of after a single crown removal or end.odontic access or when it stops cutting the metal during the procedure. The diamond-coated burs may be used multiple times and should be discarded when the dia-mond particles begin to fall off the shaft of the rotary instrument. This can be typically noted by baldness on the tip, as that area takes the most stress when in use.
Crown removal and endodontic access can be accom.plished with ef.ciency and decreased chair time when the diamond-coated burs and carbide bur mentioned are utilised as described. Light pressure with a stroking motion (for crown removal slot preparation) or circular outline formation (endodontic access) with copious irrigation improves ef.ciency while decreasing stress on the rotary instrument as well as the practitioner.