How Does Local Anaesthetic, or “Freezing”, Work?

What is Topical Anaesthetic, or Superficial Freezing?

Anaesthetic, commonly known as freezing  is “medication that produces the temporary loss of feeling or sensation”. It is used during many dental procedures for pain control. There are two main types used on a regular basis in any dental practice. The first is topical anaesthetic. The purpose of topical anaesthetic is to numb the oral mucosa in a specific area where the injection is going to take place. This creates a temporary numbing of the nerve endings, making the injection less painful. For this type of gel to have maximum effectiveness it should remain on the site for 3-5 minutes.

What is Local Anaesthetic, or Full Freezing?

Local anaesthetic is what actually numbs your teeth, cheek, lip or tongue. It is achieved by injecting a solution near a nerve, which temporarily blocks nerves from generating an impulse. A vasoconstrictor is used in anaesthetic to slow down the intake of the agent and increase the duration, which is the “time from induction to complete reversal of anaesthetist”. This agent prolongs the duration by decreasing blood flow and bleeding. The typical vasoconstrictor used with anaesthetic is epinephrine. Since the solution is absorbed into the body, it can cause strain on your heart. Therefore, patients with heart conditions should receive freezing without a vasoconstrictor. This is why it is important to inform your dentist of any medical conditions and medications being taken.

Do Some Places Take Longer to Freeze Than Others?

When receiving local anaesthetic on the mandible, it lasts longer and takes a little longer to start working because the bone is dense. Therefore, the dentist will perform a block injection. This freezes the entire area, which is approximately half of your mouth. Sometimes you will feel a shock to the lip or tongue. That just means the injection is in the appropriate spot. The second option is to utilize the maxillary arch, which uses infiltration. This area is more porous and reaches the apices and bone of your teeth more easily.

 How Long Does Freezing Last?

According to the Canadian Dental Association, the approximate duration of aesthetic is as shown:
(*) Used in our dental practice.

Type of Anesthetic

Maxillary (Upper)

Mandibular (Lower)

Articaine 4% with epinephrine

3h10m

3h50

Bupivacaine 0.5% with epinephrine

5h40

7h30

Lidocaine 2% with epinephrine(*)

2h00

3h10

Mepivacaine 2% with levonordefrin

2h10

3h05

Mepivacaine 3% plain

1h30

2h45

Prilocaine 4% with epinephrine

2h20

3h40

Prilocaine 4% plain(*)

1h45

3h10

Are There Any Possible Complications?

As with everything in life, it is important to be aware of possible complications. While rare, complications such as the injection of air into a blood vessel are possible. To ensure this does not happen, your experienced dentist will alway aspirate the needle before injecting the solution. Another possible complication is paresthesia, which is freezing that lasts longer then it should. This can be temporary or permanent, but most cases are resolved within 8 weeks. Paresthesia can be caused by contaminated solution, bleeding in or around the nerve sheath or by trauma during injections.

However, the most common complications aren't nearly so severe, and are even more easily avoided. Since local anaesthetic creates temporary numbing you have to make sure you do not bite your lip, cheek or tongue. If you do, you may feel swollen or have a ‘fat lip’, but this is completely normal and should pass in a few days.