Regional Anesthesia in Small Animal Dentistry

Why Should You Incorporate Regional Anesthesia into Your Dentistry?

Regional anesthesia is a critical method of pain management and is also known as local nerve blocks. When properly performed, regional anesthesia provides complete local analgesia, which markedly decreases minimum alveolar concentration (MAC) of inhalational anesthetics and improves recovery.  However, regional anesthesia is not without its risks.  For example, the caudal maxillary block allows for the possibility of eye puncture.   However, when performed correctly, the benefits of regional anesthesia far outweigh the risks.

What Medications Should You Use for Regional Anesthesia?

The two agents commonly used for regional anesthesia are lidocaine and bupivacaine.  Lidocaine has a fast onset of 1-2 minutes but a relatively short duration, lasting only 1-2 hours, and therefore, it does not provide adequate analgesic duration for lengthy procedures. Conversely, bupivacaine’s analgesic effect is significantly longer in duration, lasting 6-8 hours. Concerns with its use were based on a longer onset of action, but human studies reveal it may take effect in as little as 4.4 to 7.7 minutes.  Maximizing the benefits of short onset with longer duration made combining these products a popular option in the past. However, recent research has shown that combining them may result in decreased efficacy.   

The addition of epinephrine to local analgesics can increase the activity by 50%.  Furthermore, the addition of an opioid (typically buprenorphine) to bupivacaine may result in a duration of effect almost twice that experienced with bupivacaine alone.  

I utilize pre-mixed carpules of bupivacaine combined with epinephrine, providing approximately 12 hours of pain control.  This provides sufficient efficacy for patient comfort, without lasting too long into the postoperative period, which may compromise eating. The pre-loaded carpule system allows for smaller gauge needles to be used (e.g. 27g in dogs and 30g in cats). Finally, an aspirating syringe is much easier to employ than TB syringes when it comes to ensuring that the medication is not injected in a vessel.  Lastly, catheters have recently become popular, especially for the infraorbital block.  

While there is some concern with toxicity, avoiding intravenous injection and overdosage should eliminate this complication.  For this reason, it is critical to aspirate prior to each injection of local anesthetic   If blood is encountered, the needle should be redirected (and re-aspirated) to ensure that intravenous injection is not performed. Recommended infusion volumes vary from 0.1ml – 1.6 ml.  The published maximum recommended total dose of local anesthetics is 2 mg/kg (single agent or combination).  

How Do You Perform Regional Anesthesia?

Here are instructions for performing the three most common local nerve blocks utilized in my practice:

Infraorbital Block

The infraorbital block is effective for the ipsilateral maxilla, teeth, and associated soft tissues.  The infraorbital canal runs rostrally just above the apices of the maxillary fourth premolar and exits the maxilla over the distal root of the third premolar. To approximate the dorso-ventral location, it is helpful to imagine the fourth premolar as being approximately the same size mesio-distal as corono-apical.   Therefore, first measure the width of the tooth and then also that same distance dorsally from the cusp tip.  The infraorbital canal is just apical to this point.  The foramen is easily palpated, especially in cats and large breed dogs.  Advance the needle in a caudal direction, close to the maxillary bone. 

I generally recommend placement to the medial canthus of the eye in mesocephalic and dolichocephalic breeds. This is to ensure efficiency without causing undue damage to the neurovascular structures. 

In brachycephalic dogs and feline patients, the infraorbital canal is VERY short, and this may easily allow for orbital penetration.  In these patients, barely enter the foramen and direct the needle ventrally.  

Mental Block

Properly performing the mental block affects the inferior alveolar nerve, anesthetizing the ipsilateral mandibular first premolar to the central incisor in the dog and canine to central incisor in the cat, including the surrounding bone and associated soft tissue. Needle placement must be slightly within the foramen.    

The middle mental foramen is located rostro-caudally apical to the mesial root of the second premolar in the dog and midway in the diastema between the canine tooth and third premolar in the cat.   Dorsoventrally it is approximately 2/3 of the distance ventrally down from the dorsal border of the mandible.

To perform this nerve block, the mandibular labial frenulum is retracted ventrally, and the needle is inserted at the rostral aspect of the frenulum and advanced at approximately a 45-degree angle along the mandibular bone until it just enters the canal. 

Mandibular (Inferior Alveolar) Block

The inferior alveolar nerve enters the mandibular foramen on the lingual aspect of the caudal mandible. The caudal mandibular block is performed by infiltrating the nerve at this level prior to its entry into the canal. This block can be performed via either an intra or extra-oral approach, but I tend to perform this block intraorally.  

To perform the intraoral approach, the patient is placed in dorsal recumbency, and the mouth opened.  Measure the width of the last molar tooth (M3 in dogs and M1 in cats). The insertion is started the width of the tooth caudally from the caudal edge of the last molar.  The mucosa is entered on the lingual aspect of the mandible and advanced right along the mandible.  Insert the needle on a 45-degree angle caudally, advancing along the bone approximately ½ way ventral from the dorsal aspect of the mandible.

References

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