In this article I wish to look at some aspects of crowns for posterior teeth. When I qualified in 1968 the principle material for posterior crowns was gold, which then cost the princely sum of $35 per ounce, with various porcelain facing techniques for improved aesthetics. The first porcelain fused to metal (PFM) system, Microbond, was being introduced from the United States at about this time, but was costly and unreliable. Sometimes the metal framework distorted when large span bridges were being constructed.Fortunately, the technical problems of matching the coefficients of thermal expansion of both the underlying cast metal shell and the secondarily applied porcelain have been overcome, and these crowns are in widespread, successful use.
However, they do have limitations. Ideally the buccal surface tooth reduction to allow for a sufficient thickness of both metal and porcelain should be 1.5mm – a lot of tooth. This may be acceptable if there has been loss of buccal tooth substance, but if the loss has been lingually or palatally and this amount of tooth is removed, the resulting core will be both small and weak. It follows that we need to look at where there has been tooth substance loss, and the size of any amalgam or composite core material before advising our patients. Fortunately, in many cases when models are working casts are examined, the dental 1.5mm is less than the international 1.5mm measurement unless definitive width burs have been used to create the shoulder. Less preparation results in bulky crowns with an incorrect emergence profile; one study showed that the bucco-lingual width of a PFM crown increased by on average 2mm when compared with the natural tooth. This will make adequate plaque control harder and increase the risks of periodontal inflammation around the crowns especially if the margins have been placed subgingivally. A further disadvantage of this material is the limitations it gives to reflection and refraction of light. It often results in lifeless, unnatural crowns especially if only one shade has been chosen and can often fail the ‘checkout test’ as described in ‘Alternatives’.
So what in my opinion is the best posterior restorative material? Definitely gold, but only where aesthetics are not an issue. Why gold?
- It is strong in thin sections and therefore the tooth can be finished to a definite chamfer margin rather than a shoulder thus preserving tooth substance.
- there is no risk of porcelain fracturing from the underlying metal.
- The wax pattern made prior to casting the crown can be shaped to the original tooth contours thus reducing the risk of overcontouring.
The latest alternative is zirconium oxide reinforced crowns. These have much better light transmitting qualities and are thus more lifelike, but are expensive when compared with basic PFM crowns. There save tooth substance, as some types require only a 0.7mm reduction. Great care is required to avoid undercut preparations (see bend it like Beckham), as the die made from the impression is scanned either mechanically or optically and the core of the crown made by a CADCAM process. Smooth contours are essential and the shoulder needs to be prepared to a definite well marked chamfer. Again I like the Martignoni bur system for this stage of the preparation.
So far we have concentrated on the materials and aspects of the axial reduction. What about the occlusal reduction. Traditionally there is a lingual face and a buccalface which meet in the centre of the occlusal surface, but when has sufficient tooth been removed? My tip is to get the patient to close teeth firmly together into centric occlusion, then place a CPITN probe (because it has a blunt tip) over the preparation until the first bend is reached. Then move the probe mesially and distally over the preparation. If this happens very easily, too much tooth has been removed; if it catches then too little has been removed. The aim is to just have easy clearance.
Following this stage the occluso-axial junction will be quite sharp, and if left as is, risks a portion fracturing from the model on which the crown will be made. The crown will seat on the die, but not in the mouth and require a timewasting remake (inefficiency, but see below). The effectiveness of your preparation can be improved as follows:
- Create a functional cusp bevel (FCB) 2mm wide. This allows for sufficient space for your chosen restorative material in both centric occlusion and border excursions. The functional cusps are mandibular buccal and maxillary palatal cusps (Figs 1 & 2)
- Create a bevel 1mm wide on the opposite cusps.
- Join the two bevels from (a) and (b) interproximally with about 0.5mm wide reduction and ensure all junctions are smooth (Fig 3).
Unfortunately your last preparation had an insufficient occluso-axial reduction. When you look at the fit on the die its looks perfect and the occlusion is correct, but the crown just does not quite seat down on the tooth. The immediate reaction is to get out the articulating paper, mark the high spots and start grinding in. Don’t! Stop and ask why, if its correct on the model but not in the mouth what has happened. If you suspect that there has been a fracture of part of the model, this may be apparent if you view the area using loupes. Various methods have been described to mark an internal high spot on a crown, for example placing thin articulating paper inside and trying to seat down the crown, or using special marking dies. I have never found that they work. My tip is, if not already used, anaesthetise the tooth with LA. Then enhance very slightly the bevels described above around the preparation using a fine finishing bur or a green stone, then, try to seat the crown again. I would only do this two, or at most, three times. If the crown still does not seat down, then the best use of your time is to cut your losses, redefine the bevels, retract the gingivae, new impression and start again. A simple explanation to your patient that the impression may have distorted slightly and you want to be sure the crown is a perfect fit, will enhance your reputation as a caring, careful dentist who works to a high standard.
The last point to consider when making posterior crowns is the type of border movements, whether canine protected or group function. If the occlusion is canine protected then there will be disclusion in working side movements and the principal consideration for the occlusion will be to ensure it is correct at centric occlusion. If the occlusion is group function, then allow more time for occlusal adjustments, and be prepared when using PFM crowns to try in the biscuit bake, refine the occlusion and then ask the laboratory to carry out a final glazing process.