Endodontic Frequently Asked Questions (FAQ)

• How do you pulp test teeth with full crowns?
• Why are some teeth so difficult to anaesthetise?
• How do I anaesthetize a 'hot' tooth?
• What's a good way to remove silver points to retreat the canal?

Electric Pulp Test (EPT)

If you can find an area of tooth exposed beyond the cervical crown margin, most often lingual, you can pulp test the tooth. Some EPT manufacturers have adapters with fine tips to allow contact with these narrow areas of exposed tooth structure of teeth with full crowns. You could also achieve the same result by using a fine metal instrument as a bridge between the tooth and the regular EPT tip:

1. Hold a hand instrument with a fine tip, such as a probe in contact with the exposed tooth.
2. Place the larger standard EPT tip onto the probe and perform the pulp test as usual!

Cold Test

We all know full gold crowns are good at thermal transfer. However, we know porcelain or ceramic are insulators and dismiss the idea of performing a cold test when teeth are covered by full crowns made from these materials. This myth has been busted (Miller et al. 2004)! Apparently thermal transfer through porcelain, ceramic and even porcelain fused-to-metal (PFM) crowns are similar to unrestored teeth — they seem to conduct cold much the same way enamel would.

A comparison of the temperature drop produced by ice sticks, Hygienic Endo Ice Green and CO2 snow through crowns showed that after 5s application, the temperature drop at the pulp-dentine junction was similar (Miller et al. 2004). After 10 s, Endo Ice Green produced the fastest drop in temperature. Using a #2 cotton pellet with Endo Ice Green appeared the most effective means of performing a cold test through crowns. CO2 snow was only 1-2° C higher, and is probably just as effective clinically. Both were superior to the ice stick!

When there is acute inflammation of the dental pulp, for example, irreversible pulpitis, the following develops:

• Activation and sensitisation of nociceptive nerve fibers
• pH shift at inflamed site
• Synthesis of tetrodoxin resistant (TTXr) sodium channels

Activation and sensitisation of nociceptive nerve fibers

Inflammatory mediators cause both activation and sensitisation of nociceptive nerve fibers. In addition, nerve sprouting occurs in response to nerve growth factor, increasing the quantity of nerve endings. This results in hyperalgesia, which is characterized by:

• Spontaneous pain
• Reduced pain threshold (allodynia)
• Increased pain perception

pH Shift

LA dissociates in the tissue according to the reaction:

RNH⁺ RN + H⁺

The pH of inflamed tissue shifts from the normal pH 7.4 to as low as pH 5.6. This pH shift affects the dissociation of the LA solution so that it favours the ionic form, RNH⁺ which is incapable of migrating through the neural sheath. Normally, approximately 75% of the LA converts to the ionic form. In the presence of inflammation, this drops to as little as 1%.

TTX-Resistant Sodium Channels

From the clinical standpoint, the other important change is the shift in sodium channels in the C afferent nerves from TTX-sensitive (TTXs) to TTXr. Unlike the normal TTXs sodium channels present in sensory nerves, these TTXr sodium channels require 5x more lidocaine to block these channels to achieve anaesthesia than the TTXs sodium channels (Scholz et al. 1998)! The result of all these reactions in a tooth with irreversible pulpitis is that it could be difficult to anaesthetise the inflamed dental pulp to a sufficient depth for endodontic treatment to proceed painlessly, especially when lidocaine is used for local anaesthesia (LA). Hence the term 'hot' tooth.

The following methods may be helpful:

• Bupivacaine. This is more effective than lidocaine at blocking TTXr sodium channels (Scholz et al. 1998). For more predictable anaesthesia in cases of irreversible pulpitis, bupivacaine should be the LA of choice.
• Increase the volume of lidocaine to compensate for the lower dissociation and diminished effectiveness because of TTXr sodium channels — a good strategy if bupivacaine is unavailable to you!
• Intra-pulpal anaesthesia. A good, simple, dependable supplemental LA technique. It is important that there is pressure for this to work — the choice of LA is secondary! Useful once you have just penetrated into the pulp chamber and the patient starts to feel. However, your patient is likely to feel the needle penetrate. Ouch! Ouch! Ouch! But after that ..... it usually works. Applying some topical anaesthetic and leaving for two minutes will help alleviate the initial pain from this injection.
• Periodontal ligament anaesthesia. Another helpful supplemental technique, but increases the possibility of post-operative tenderness if not performed slowly. It is easier when using a intraligamental syringe, though the normal syringe will work, but a lot of back-pressure needs to be overcome. Needs one injection at each line angle of the tooth.
• Intraosseous (IO) anaesthesia. Although it can be used as the main LA technique, it has similar success rate to inferior alveolar nerve (IAN) block in lower molars. It is really useful as a supplemental technique where it usually never fails to work! Supplemental IO has a success rate of 83% in lower molars with irreversible pulpitis (Wong 2001). This old technique has recently surged in popularity because of the relative ease with which it can now be performed with the available commercial IO systems. The site of injection is either mesial or distal of the tooth, at the level of the height of the attached gingiva. Some systems (Dentsply MPL Technology) use a double sleeved needle — the outer perforator needle penetrates bone, and also supports the finer inner LA needle as it penetrates. Other systems have a separate perforator attached to a slow speed handpiece to drill into position. Once positioned, a finer needle follows the path established by the perforator needle to deposit the LA solution.