I should like to carry on the theme of anterior crowns from ‘Bend it like Beckham’ to look at multiple anterior crown preparations, anterior bridges, and aspects of technical support. As previously described, booking in work with your technician is an important first principle.
Let is say you are going to make an anterior bridge to replace a missing maxillary central incisor. The adjacent central and lateral incisors have some restorations, and you have recommended a 3 unit bridge rather than an implant. Which tooth do you prepare for a crown first, the central or the lateral. Always start with the smaller tooth, the lateral. The line of insertion is more critical with this tooth, and if there are problems with a parallel preparation on the central incisor, that tooth is larger and so can better stand additional reduction. If you prepare the central first, you may well find that the lateral incisor could finish with a rather small , and therefore weak core, and there is a greater risk of pulpal exposure.
Your patient has requested, and you have planned and agreed a smile makeover for 6 maxillary teeth, the incisors and canines. However, there is limited posterior occlusion (Fig 1). Your patient (customer) wants to have the anterior teeth restored first, then the posteriors. What would you (a) advise, and (b) do? In my opinion you should always restore the posterior occlusion first to create centric occlusion at centric jaw relation at the correct vertical dimension (Fig 2). If you restore the anteriors first there is a real danger that your patient will then decide that you have given them what they want and not return for the posterior reconstruction. When the crowns fracture or the teeth move under occlusal load, it will all be your fault . In figure 2, I have deliberately left the right lateral incisor untouched to demonstrate to my patient just how much change had taken place. I have also built up other teeth with composite to give him, and his wife, an indication of the final result. More importantly I have tried to create the ideal width relationships between the central and later incisors and the canine, the so-called golden proportion which is stated as 1.617/1.00/0.617, for central, lateral and canine respectively. The canine records 0.617 only because this is the tooth that turns the corner of the dental arch, and it is normal, in a full frontal view to see only the mesial aspect. Whilst there are marketed golden proportion gauges to assist in the correct ratio planning, if you or your technician do not have this aid available, you cannot measure to three places of decimals. A simplification is called golden ratio, that is the width of each central is 25%, lateral 15% and canine 10% of the available space. Always remember that the smile you need to create must be bilaterally symmetric.
Having taken all this trouble to show your patient what the end result is likely to be, the natural tendency is to prepare all six teeth, take your impressions and let your technician work out the widths. If you do that, it is very hard to achieve the correct widths and more importantly, the correct interproximal emergence profiles of the resulting crowns. I use the ‘every other one’ method. This makes life easier for your technician and recreates, as accurately as you can, the golden ratio you have spent time developing.
The method is as follows (Fig 3). Prepare alternate teeth taking great care not to touch the interproximal aspects of the other teeth. In the example shown, I have prepared a right lateral, left central and left canine. Then place retraction cord and take an impression (1) of the three teeth and put this to one side. Remove the cord, prepare the other three teeth, place retraction cord around all six preparations and take a further impression (2). Your technician now has two master impressions on which to work. First the crowns are made on the model from impression 1. This ensures that the correct widths are created in relation to the other teeth. These crowns are then placed on model from impression 2, and if all is well, will fully seat down. The remaining three crowns can then be made to the widths and contacts previously created. I accept that this takes more time, but the results (Fig 4) are well worth the extra effort.
In my hands on courses, I ask delegates where contact points should ideally be placed in relation to these six crowns. It would appear that this aspect of crowns has not been taught in many Dental Schools. The contact points can be placed
- In the incisal third
- In the mid third
- In the gingival third
Where should they be placed?
- Between central incisors in the incisal third
- Between central and lateral incisors in the mid third
- Between lateral incisors and canines in the gingival third.
This is illustrated in Fig 5; the contact points move gingivally from front to back.
Other points to consider are:
Maxillary central incisors together occupy 50% of the width available, the so called central incisor dominance. Their width is normally 75 to 80% of their length.
Lateral incisors can have very variable shape, and this is where you and your technician can be more artful and crafty in developing the smile. However, the long axis of the lateral incisor is inclined 5 to 10 degrees towards the distal, and its neck is similarly inclined towards the palatal. This explains why, the ideal gingival architecture shows the gingival margin of a lateral incisor about 1mm coronal to the line of the gingivae drawn between central incisor and canine.
Canines can also have variable shape. As mentioned above, canines turn the corner of the dental arch, so when viewed from directly in front should show just the anterior aspect.
Shade taking makes or mars the result. Teeth never are one colour, and differ between incisal, body and gingival as well as the marginal ridges. Maxillary canines are the darkest teeth. Aids include involving your nurse and patient, asking the technician to take the shades providing the laboratory is conveniently situated in relation to your surgery, or one of the shade taking cameras available commercially.
So, how good is your result? I apply the supermarket checkout test. This is the place to see strangers when you are in the queue to pay. Have a look at their teeth. You can often see their incorrectly coloured or badly contoured restorations – someone else’s dentistry.