Bend it Like Beckham
In this month’s tips I should like to return to clinical practice and consider some aspects of anterior crowns, the common faults that arise and how they may be prevented.
I think one of the hardest restorative challenges that we dentists face is to make an aesthetically pleasing crown on one maxillary central incisor. The preparation has to have the correct line of insertion and withdrawal, sufficient tooth reduction for the chosen restorative material and the correct shade. There will be more about shade taking later but many crowns fail on this point alone. I am aware that sometimes patients are advised to have both maxillary central incisors crowned ‘as it will be easier to get a better match’. Some experts regard this as porcelain overkill, and should a complaint arise later, may well write an adverse report about your treatment.
But let us move on. The treatment plan has been agreed, your patient understands that all crowns have a finite life, and that although the tooth is vital, perhaps one in three crowned teeth will become non-vital following preparation, either because of previous restorations compromising long term tooth vitality or overheating the pulp during tooth preparation. So, by how many degrees centigrade does the pulp have to be heated beyond the normal 37degrees C of body heat for it to irreversibly degrade?
Porcelain fused to metal (PFM) crowns are perhaps the most commonly provided anterior restoration, usually because of they avoid the risk of conventional porcelain crown (PJC) fracture. However, the aesthetic results can be poor and lifeless, not least because of their limitation in light transmission properties made worse by the choice of one shade only. PJC’s usually fracture palatally, and this raises the question why? The answer lies in the starting point for crown preparation.
There are 4 choices:incisally, labially, interproximally, palatally, so where do you start your preparation? If you wish to achieve consistent results, like Beckham, who was able to score goals from free kicks because he had in his mind’s eye before he started what he wanted to achieve, we need to follow his example.
Interestingly, the determinant of where to start seems to depend on undergraduate teaching rather than logic.As mentioned above, the preparation aim is:
- sufficient reduction for the chosen crown material,
- keeping the preparation in the long axis of the tooth with no more than a 10 degree taper towards the incisal edge,
- providing sufficient occlusal clearance,
- no undercuts,
avoiding the pulp chamber whilst retaining the marginal placement in relation to the gingivae as discussed “at the margins”, retaining two thirds of the length of the clinical crown for optimum retention.
Quite a tall order, but let’s consider what happens in practice. These notes are based on my observing many colleagues who have attended my crown preparation courses over the years.
If you reduce the incisal edge first down to the two thirds ideal length, thenwhen the other parts of the preparation are completed it is almost inevitable that less than two thirds crown length will remain. Tip one is therefore to leave the incisal reduction to last when it is usual to find very little needs to be taken off.
When the labial aspect is reduced first, for the first few minutes of preparation all seems to be going to plan, then the idea of shoulder preparation kicks in and the handpiece and bur are rotated 15 degrees anteriorly and the shoulder cut to the required width. This act changes the line of insertion and withdrawal, leaves an under-reduced area at the incisal edge and most probably an undercut at the gingival margin. This shape may be likened to a banana and is guaranteed to result in a bulky inaesthetic labial shape to the finished crown, and an incorrect emergence profile at the gingival margin. This is illustrated in Figures 1 and 2. I have removed an old crown, Fig 1 shows the line of insertion of the core; Fig 2 the rotated position to establish the shoulder whilst Figure 3 shows how this affects the emergence profile at the gingival margin.
When the interproximal reduction is made first, because of the way the dentists hold their handpieces and gain finger rests, the long axis of the preparation will be inclined towards the right for a right handed dentist and vice versa for a left handed dentist, that is outwith the correct axial inclination When teeth are closely aligned together this can resulting a finished crown short horizontally of the prepared margin.
We know that PJC’s fracture on the palatal aspect, most commonly because there has been an inadequate tooth reduction and the resulting crown is very thin in this area. It follows that this is the critical reduction in crown longevity and I suggest this is the area to reduce first. It is easy to check that the reduction has been sufficient. If you get the patient to close into centric occlusion then place a mirror in the lower labial sulcus anterior to the lower teeth and observe the space created both at rest and during protrusive movements, it is immediately obvious if the reduction here is correct. This direct observation can be supplemented in cases of doubt by taking a wax wafer and examining the clearance created.
I next reduce interproximally using a narrow bur and avoiding touching the contiguous teeth. This reduction defines the line of insertion which must be in the long axis of the tooth. In order to overcome the tendency to stray off line because of the way that your handpiece is held, it has been suggested thatby placing the index finger of your other hand on the top of the handpiece a correction can be applied. This is fine where you have direct vision, but cannot be used when the labial reduction is carried out simply because it is necessary to see in a mirror that the angle of cut towards the palatal will not result in pulpal exposure. The next reduction is labially, avoiding the banana outcome then finally incisally.
If the burs you are using have brown bits on the tips after tooth preparation, then there is insufficient water cooling and the pulp will have been overheated. By the way, the answer to my question about how many degrees is 6 degrees centigrade, yes six!
The best method I have found to prevent tooth overheating is the two stage technique described by Professor Martignoni and his set of crown preparation burs (North Bel International, Paderno Dugnano, Italy). Here, a tapered tip bur with cross cut water channels is used first to roughly outline the preparation (Fig 4), then a second bur is used to refine the gingival margins, (Fig 5) either to a shoulder for PJC’s, a chamfer for metal or a round internal angle for zirconium oxide reinforced crowns.
Fig 4 Fig 5
As always, be prepared to try something different, and if it works for you adopt the technique into your clinical practice.
Figure 6 shows porcelain jacket crowns on UR1/UL1 made using the above preparation techniques.