TBU # 010: Deep caries management: Peripheral seal zone (PSZ)

Have you ever drilled a tooth then spend the rest of the day wondering if you have excavated all caries that are left there? Well, I did. That is before I got to know the concept of caries endpoints and the usage of caries detecting dye, In this blog, I will discuss everything I know about the Peripheral seal zone.

Let's say you are suffering from a certain condition and every time you visited a health care professional he\she provided you with something to treat the symptoms rather than removing the causative factor of the symptoms, what would that make you feel if you go back and release that your condition is gotten so much worse because the doctor just treated your symptoms rather than the condition itself? Treating decay without treating the cause of decay is a problem that needs to be solved!

In a deep lesion, we have to put two things under consideration while excavating caries, the first is preserving tooth structure and the second is removing the infected tissues without exposing the pulp. using the old traditional methods doesn't come in handy. The fact that you will be avoiding pulp exposure and conserving the tooth structure which will lead to better bond strength and a better prognosis is the reason that pushes you as a clinician to apply this approach in your everyday practices. So what is a peripheral seal zone (PSZ)? Excavation of caries inside the tooth as needed but very deep caries can be left, that to be said, enamel, the dentino-enamel junction (DEJ), and 1-2mm of superficial dentin should all be carious free. Caries excavation should be limited to a depth of 5mm from the peripheral surface of a tooth while measuring from the proximal tooth, the depth of excavation is ideally 3mm. The caries-free zone is a 2-3mm circumferential area around the cavity which does not expose the pulp. In other words, we have different layers of the carious dentin, an outer contaminated infected layer which should be removed during cavity preparation and an inner demineralised affected layer which shouldn't be removed but should have a periphery of a completely carious free margin. The complete absence of caries detecting dye staining is the confirmation of the carious free margin or as it's called the peripheral seal zone.

Adopting the caries removal endpoint approach and the creation of a peripheral seal zone in treating deep caries lesions will help achieve a bond strength of approximately 45-55 MPa. Also, By leaving the inner demineralised affected dentin which can be confirmed by light pink staining can help you obtain a bond strength of approximately 30 MPa. so in short, applying this will help you get a stronger bond in the long term.

As I mentioned previously there are two layers in carious dentin: The outer layer, the infected dentin. And the inner layer, the affected dentin. These two layers differ from each other by the collagen fibrils and the level of infection and demineralization. The outer infected layer is completely demineralised and has denatured collagen fibrils and can not be remineralised. This layer should be completely removed. The inner affected layer however is partially demineralised and infected, here the collagen fibrils are still intact therefore this layer is very sensitive, this layer can be remineralised.

That to be said, how can we differentiate between these layers, unlike enamel, it is quite hard when it comes to dentin because it's already soft! And the instruments we use doesn't provide a guide to where we can stop excavating. Takao Fusayama made it easier for us after finding the caries detecting dye which stained the two layers of carious dentin differently. The outer layer is stained dark red, and the inner layer is stained light pink. However, dentists were confused about when exactly should they remove the light pink layer. because there is no evidence of the amount of bacteria in it.

Later on, DIAGOdent was invented which is based on laser fluorescence technology and this device gives a numeric value that corresponded approximately to the amount of bacteria present. besides other functions of this device such as the diagnosis of pit and fissure caries, this device was found beneficiary for determining the caries excavation endpoints. Readings are the following: superficial dentin endpoint was approximately 12 or less, while intermediate deep dentin was 36 or less.

Combining the caries detecting dye and DIAGOdent help in guiding the clinician on the proper way to excavate the infected dentin while keeping the affected dentin inside the peripheral seal zone. We should also keep into consideration the depth of removal of tissues which can be measured with the periodontal probe as the usual known cavity depth measurement. To avoid pulp exposure excavation should stop after passing a 5 mm depth occlusally or 3 mm if the measurement is done from the dentino-enamel junction.

As I mentioned above that caries endpoint and achieving a peripheral seal zone will help us obtain a bond strength of 45-55 MPa. In biomimetics, we try to mimic the sound tooth structures as much as we possibly can. It's known that the tensile strength of the DEJ is 51.5 MPa. With bonding to the caries endpoint (sound dentin), you can achieve this strength or maybe even exceed it!.

Now that we understood the protocol, the reasons behind it and the benefits of applying it, let's go through the application technique. First, as with any other restorative treatment, we start by diagnosis. Diagnosis is the key to any treatment. After we check the pulp vitality and make sure that endodontic treatment isn’t needed, we proceed with anaesthesia and rubber dam isolation. Second, we remove old restorative material if any to access the lesion and stain it with caries detecting dye, wait for 10 seconds and rinse. Then we start the drilling process (round diamond bur-fine/medium grit) starting near the DEJ to create the peripheral seal zone area that should be completely free of stains. Which will be around 1 to 2 mm wide. You should also notice the inner pink stained caries. This is repeated until we get a stain-free peripheral seal zone. You can also re-check using DIAGNOdent with a reading of 12.

Now we have bacteria-free superficial dentin, let's dig into the inner layer, here we start by measuring the cavity depth using one or two periodontal probes, then we proceed by removing the red-stained lesions carefully and putting into consideration the pulp horns. After removing the red-stained tissue we will be left with pink dentin, we then confirm with DIAGNOdent of a reading of approximately 24 (it can range from 12 to 36). Small areas of circumpulpal (at the areas of pulp horns) carious dentin could be left to prevent exposure. Here your excavation process is complete.

Now we start the bonding process: We start with 0.2-2.0% chlorhexidine for 30 seconds to inactive the remaining bacteria and the matrix metalloproteinase. Chlorhexidine could come before acid etching or after depending on the bonding system you are using.

After sealing in and deactivating any remaining bacteria we continue with the adhesive restorative techniques that will maximise the bond strength.

So all you need to perform this technique is a good magnification, caries detecting dye and DIAGNOdent. After applying this conservative restorative technique you would never spend your day wondering if you did a decent job with your deep restoration. This technique will not only allow you to sleep at night but will also aid in achieving great bond strength. That to be said it is important to follow the proper bonding protocols afterwards to get the maximum longevity of the restoration.