7 Riverview Drive, Guelph, Ontario, N1E 3R6,
Canada
Mon: 10:00am - 7:00 pm
Tu/W: 8:00am - 5:00 pm
Thur: 9:00am - 4:00 pm
Fri - Sat 9:00 am - 3:00 pm (alternating)
Ask any dentist or any ceramist and they will tell you (at least my ceramist most definitively will) that matching a single front tooth crown, especially a central or lateral incisor to natural teeth, is one of the most challenging tasks in cosmetic and restorative dentistry. Why? It's not because the crown preparation is inherently difficult, this actually becomes fairly straightforward, although it can vary dramatically depending upon the final goal and material choice (more about this in a future blog post).
No, it's because the crown is an artificial tooth which is made from artificial, synthetic, man-made materials such as porcelain, zirconia or lithium disilicate (most commonly e-max).
These materials have to not only reproduce the optical properties and appearance of a natural tooth but they have to match its colour, texture, shape, translucency, opalescence and the way they handle light. Given that the human eye is so exceptionally capable of identifying subtleties in a face, it becomes extremely difficult to completely match a single crown to natural teeth.
In contrast, it is substantially simpler to match crowns or veneers to one another and this is a major reason why most anterior veneer & crown cases are done as sets of 6 to 10.
This patient presented with recurrent decay below an existing crown. The crown was a porcelain-fused-to-metal (PFM) restoration, placed over twenty years earlier.
PFM crowns can last well over 20 years and if it weren't for the recurrent decay below its margin, this crown could have been maintained longer. Nonetheless, if you look at the image below the limitations are clear:
There has to be a clinical basis and diagnosis that supports treatment. In this case there were two:
Interestingly, a common question I receive is whether a fractured PFM crown can be repaired. Can composite or "filling" material be added? Yes it can but it doesn't hold up. Porcelain hydrates over time and the bonding techniques used in modern day dentistry do not produce a bond that allows repair of the crown. It most definitely wouldn't hold up to forces from bruxism and clenching.
This is because grinding produces a massive amount of force. The forces of bruxism and clenching produce well over 250lb of pressure with severe bouts exceeding 500lb of pressure. It's like being hit with a sledge hammer.
For this reason we discussed fabrication of a maxillary guard and using a different material for the crown. Here, a semi translucent porcelain layered zirconia (via the cutback technique) is recommended to provide both a visually aesthetic result and one that maintains its strength and is less likely to fracture given this patient's history of grinding and clenching.
With almost every crown case I get this question. The answer, like many things in life, is that it depends.
If you look at the literature as a whole it will tell you crowns have high survival rates. A 2015 systematic review in Dental Materials reports a 94.7% survival rate for metal-ceramic crowns at the 5 year mark.
I routinely see crowns which pass the 10-15 year mark and have also seen crowns past the 30 year mark.
But, nonetheless it depends. In particular crowns placed in dentition where there is balanced occlusion (the biting force is balanced along multiple supporting teeth), with good amounts of supporting bone and healthy gingival tissue and where the underlying tooth has appropriate volume/ferrule can do well.
This is why it is important to define a prognosis for the tooth. Is it Good? Fair? Guarded or poor? In each case a crown may be possible but the longevity can vary dramatically.
For instance, in this case there is a substantial amount of underlying tooth available, no sites of periodontal pocketing exceeding 3mm, good healthy bone and periodontal attachment levels, but a long term history of bruxism and clenching. The patient also has a very low decay risk with only 2-3 cavities over her entire lifetime.
In her case I would assign a "fair" prognosis and expect the crown to last well over 10-15 years as long as the bruxism can be managed and the oral hygiene maintained.
However, if the tooth had been root canal treated with a post or periodontally compromised, it may not have even lasted half as long. This is why a clinical assessment and assignment of a prognosis is important and something I do for major treatment.
The crown material itself cannot decay. Zirconia, porcelain and metal are not biological tissue, and bacteria cannot dissolve them. What can decay is the natural tooth. The site most susceptible to this is the crown margin, the micron level gap where the crown ends and the tooth begins.
This boundary, if you look in figure 5 below, is sealed with a dental cement and is commonly between 50 - 100 micrometers (0.05 to 0.1mm). This is thinner than a strand of hair and sealed!
Although the cement seals the space, over time the margin can wear from excessive brushing or the cement can wear and debond from heavy forces such as those in bruxism and clenching.
However, the most common reason and clinical finding along a failing margin is caries or decay. If the area is not adequately cleaned and plaque accumulates, it can lead to caries below the crown and into the underlying tooth. For this reason I commonly place my margins above the gumline at sites where it can be cleansed and cleaned unless it is not possible.
A note about cements. There are various types but they generally fall into two categories. Resin-modified glass ionomer cements and resin cements. Although the choice is based upon various pros and cons, in this case it was guided by the location of the margin. Here the margin is located below the gumline. Resin cement is technique-sensitive to clean up in that position and females usually have thin gingival biotypes, which this patient does.
If cement is retained below the gumline, which can happen, it can cause tissue inflammation (especially in thin tissue biotypes) and, around implants in particular, bone loss. A resin-modified glass ionomer cement was used for that reason because in contrast it is much easier to clean and consequently ensure no left over cement.
Most people want to know, can the tooth be fixed?
To assess if decay below a crown can be fixed we need to ask how deep the decay is and the most direct way of determining this is by removing the crown.
Before we do this though, we use indirect measures to estimate the depth of decay, its restorability, and the status of the nerve.
Clinically we can estimate the depth/extent of the decay via visual examination and by using tools such as an explorer ( to assess its depth of penetration ), but this is an indirect measure and can be inaccurate.
Radiographs are also limited because the crown itself is radiopaque so it blocks X-rays and the resulting opacity does not allow you to see the tooth structure beneath it.
Nonetheless radiographs are essential because they can still help us understand the extent of the decay into the tooth structure surrounding the crown and its restorability.
The assessment therefore ran in two stages.
1) Before crown removal: a radiograph, testing of the nerve's vitality (is the nerve alive), percussion (tapping the tooth) and palpation of the surrounding tissue. Direct visual inspection and gentle probing with an explorer, a thin-ended instrument used to feel the decay's depth and texture.
The findings: the decay had penetrated below the crown but was within 1 to 2 mm. This is within the dentin, not likely deep, and not expected to extend into the pulp. The nerve was not likely involved.
2) Reassessment following removal of the crown. This is the most accurate assessment.
The findings: Reconfirmed our initial speculation. Decay was not deep and a simple crown replacement would suffice.
Figure 6 above shows the general course of reasoning. In particular it's the findings and patient decisions that set the direction for treatment. If the tooth is restorable and the decay does not involve the nerve, a straightforward crown replacement can proceed. If decay reaches the pulp, or the pulp is irreversibly inflamed or dead, the options change; root canal treatment before the new crown, or, where too little sound tooth remains, extraction and a discussion about replacement. Note there are exceptions to these.
In most cases the simplest and gentlest way to remove the crown is by sectioning it under magnification. By cutting the crown in half we were able to remove and lift each half. Although tools exist for removing crowns as a single piece if the original crown is well cemented and not already mobile, they can exert considerable force on the tooth and I prefer not to use them.
Once the crown was removed the preparation was refined using principles of tooth preparation. In this case, because the decay was not large, it could be removed and the site of decay was incorporated into the tooth preparation.
What this means is the decayed area was cleaned out and became part of the shape the new crown would fit over.
This saved the individual a secondary procedure and its associated cost, which is commonly known as a core build up.
I do also want to draw attention to the stump. Stumps are naturally warmer than the surrounding teeth and can sometimes be much darker or greyer. Its colour and the amount of available space have a direct impact on the type of materials we choose.
A final impression was then taken digitally. Sunny Dental Centre uses the iTero Lumina scanner as a part of its digital workflow. It creates a 3-dimensional model of the gingiva and teeth, which is then used by my laboratory to create the final crown. For complex cases a secondary manual impression is sometimes also taken to ensure that the lab has both a digital and manual model to confirm the size and fit of the crown. This is commonly done when someone is travelling from afar or being sedated for treatment.
Finally, in the meantime a temporary crown is fabricated using the original tooth as a mould from a tooth colour matched material, polished and cemented with a temporary cement.
Among all the steps, shade matching and fabrication of a crown that will match the natural dentition is the most challenging. Teeth are not a single colour. Teeth are a combination of varying shades, have luminosity, surface texture, translucency, fluorescence, and intrinsic features. These features then change as you progress from the biting edge of the tooth to the gumline.
Remaking this in a lab is inherently difficult but can be done. Doing this to match a specific tooth is even more so!
Firstly, when matching any tooth, you need a reference. In most cases for a lateral incisor it is a central, this is the front large tooth. In most cases I avoid matching to canines because they have a larger pulp, are yellower than central incisors and typically darker.
Now here is the important part. Consistency. And, quite frankly this is the reason dentists invest so heavily in fine tuning systems and equipment. We don't just want a close shade match. We want a close shade match every time.
I use a few techniques and pieces of equipment in order to accomplish this:
The first and most important step is taking high-resolution photographs with shade guides held against the teeth, so that tooth colour is calibrated against a known reference rather than described from memory. The most commonly used guides are the Vita guides; a VITA 3D-Master and additional secondary guides were used here. Roughly twenty to thirty images are taken in total, under varying lighting conditions, including the guide tab that is the closest match plus one shade lighter and one shade darker. Bracketing this way makes the match verifiable rather than a single judgement call.
Colour and lighting can be extremely deceptive. Photographers and cinematographers will tell you that it's all lighting. If you look at the images above, it's the same teeth and shade tab, but, it looks completely different. This happens all the time and in cases with colour complexity or difficulty reproducing the shade, images are taken with flashes outfitted with polarized filters.
This produces cross polarized light which removes surface reflections and allows the internal natural colours and features to come through.
Using this a porcelain recipe is created. In this case a zirconia core was used, the outer layer of zirconia was removed until 0.3-0.4mm of zirconia was remaining, this was layered with various porcelains to recreate the natural features of the tooth as requested in the prescription and shade map. This can be extremely labour intensive.
For materials we had 3 choices, PFM, zirconia or e-max. I chose zirconia because of the patient's history of clenching. She would break a different material, it would just be a matter of time. Also the stump was dark, I did not want the yellow to show through and zirconia is great at blocking out the underlying tooth colour.
In order to make it appear natural the crown was cut back and layered with porcelain, texture and additional features were recreated as requested.
The crown was tried in and assessed under several lighting conditions, and the patient was asked to evaluate its appearance in natural light. If a crown does not meet the functional or aesthetic requirements at this stage, I resubmit it to my lab. This is actually more common than not. There is no substitute for trying a crown in and assessing it in natural lighting conditions.
This is why I send many patients to the lab for a custom try in and modification of the crown prior to final cementation. This is exactly what we did here after the first try in attempt, during which we noticed the crown was too bright. The permanent crown was left in, the patient visited the lab and it was removed, adjusted, layered with porcelain and tried in, until the desired outcome was achieved.
Once we were happy with the appearance and fit (balanced bite, good marginal fit, and tight contacts), the crown was cemented with excess cement removed.
Note: Over time the colour and shade stabilize. Porcelain hydrates as time progresses and this will further improve the colour of the crown.
A thermoplastic night guard was made and the patient was asked to use it nightly.
We also reviewed home care techniques and recommended a Waterpik.
Sunny Dental Centre is a general dental practice. Dr. Mandip Puri, DDS, is registered with the Royal College of Dental Surgeons of Ontario as a general practitioner, not a specialist. Individual results vary; what is achievable depends on your teeth, bite and goals. All patient photos and figures are property of Dr. Puri from Sunny Dental Centre and not permitted for reuse or redistribution.
7 Riverview Drive, Guelph, Ontario, N1E 3R6,
Canada
Mon: 10:00am - 7:00 pm
Tu/W: 8:00am - 5:00 pm
Thur: 9:00am - 4:00 pm
Fri - Sat 9:00 am - 3:00 pm (alternating)