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Pain, what a Pain! (Part 2) – Practical Tips On How To Perform Dental Nerve Blocks In Companion Animal Practice

In Part 1 we discussed locoregional anaesthesia (LRA) and how the application of these techniques can improve patient welfare as well as having a positive impact on the overall safety of the anaesthetic process.

In this article, we focus on the LRA techniques associated with dental and oral surgery in small animal patients. By doing so, we aim to provide practical tips on the more common dental nerve blocks and highlight safety concerns that the veterinary surgeon should be aware of prior to performing them.

Why use Dental Nerve Blocks?

In a recent study, Aguiar et al. 2015 identified that in cats undergoing dental extractions, those that had received an appropriate nerve block prior to surgery had lower isoflurane requirements during the procedure as well as lower post-operative pain scores post-operatively. These findings confirm, as is also seen with other LRA techniques, that the use of dental blocks can reduce the degree of nociceptive processing during the surgical procedure and provide ongoing analgesia during the patient’s anaesthetic recovery.

Which drug(s) should be used?

Lidocaine Hydrochloride and Bupivacaine Hydrochloride appear to be the most commonly used local anaesthetics in small animal practice. Table 1. (also available in Part 1.) summarises the use of these agents for the purpose of LRA.

Drug Lidocaine hydrochloride
Time to onset 2-5 minutes
Duration of action 1-1.5 hours
Dose 3-4mg/kg
Maximum safe dose 10mg/kg (Dogs), 6mg/kg (Cats)
Comments Licenced formulations available. Combined with epinephrine which can prolong the anaesthetic effect.
Drug Bupivacaine hydrochloride
Time to onset 5-10 minutes
Duration of action 4-8 hours
Dose 1-2mg/kg
Maximum safe dose 2mg/kg (Dogs and Cats)
Comments No current UK licence for use in veterinary species.


Table 1.
(Adapted from Dugdale, 2001 and Duke-Novakovski, 2016).

It is often the case that multiple dental blocks are required to be performed on the same patient. It is vitally important in these instances that attention is paid to the total dose of agent being used to avoid systemic toxicity. It is recommended that for the selected drug, the maximum safe dose is calculated and not exceeded. This calculated dose should then be divided by the number of sites for injection. If the volume of drug is insufficient to prove effective at all sites (see ‘volume of drug required’ outlined for each block below), then the drug can be diluted with saline to achieve the required amount (Milena and Gurney, 2016). If the block has been performed successfully, the patient should not react to surgical stimulus. If reaction to such stimulus is observed, and the patient has received the maximal safe dose of local anaesthetic (therefore preventing repeating the nerve block), then appropriate additional analgesia should be provided through systemic means.

When considering a patient’s maximum safe dose, the clinician must also be aware of whether they have used local anaesthetic at any other stage of the anaesthetic. For example, if lidocaine has been sprayed onto the larynx in cats to facilitate intubation this must be factored into that patient’s total dose of that drug.

A Review of the Relevant Anatomy

The trigeminal nerve, the largest of all cranial nerves, provides sensory innervation of the face and deeper structures (i.e. oral cavity) as well as motor innervation to the masticatory muscles. As its name suggests it is made up of three main branches, these include:

  1. Ophthalmic Branch: sensory information from the eye, as well as associated areas of the nasal cavity and skin (not discussed any further in this article).
  2. Maxillary Branch: sensory information from oral cavity of the maxilla, as well as associated areas of the nasal cavity and skin.
  3. Mandibular Branch: sensory information from the mandibular oral cavity and associated areas of the skin. Motor activity to masticatory muscles. 

Sensory innervation to the upper dental arcades is derived entirely from the maxillary branch of the trigeminal nerve (Figure 1.). The maxillary nerve exits the round foramen of the skull and courses along the pterygopalatine fossa, before it enters the infraorbital canal. The nerve courses through the canal, where it now becomes known as the infraorbital nerve, and exits at the infraorbital foramen.

Prior to entering the infraorbital canal, the maxillary nerve divides into the pterygopalatine nerve which later gives rise to the lesser and greater palatine nerves. At the level that the greater palatine nerve exits its foramen it provides sensory innervation to the anatomy detailed under ‘Palatine nerve block’.

The mandibular nerve provides sensory innervation of the lower dental arcades. This nerve exits the skull at the oval foramen. One division of the mandibular nerve, known as the inferior alveolar nerve, enters the mandible through the mandibular foramen and courses through the mandibular canal before exiting the middle mental foramen as the mental nerve. 

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Figure 1.Simplified distribution pattern of the trigeminal nerve in a dog

Common Dental Nerve Blocks

1. Maxillary (Caudal Maxillary) Nerve Block

Anatomy Desensitised

Indications

Volume of drug required

Ipsilateral to the block:
• Maxilla
• All maxillary teeth
• Oral mucosa
• Buccal Soft tissues
• Upper lip
• Maxillary tooth extraction
• Mass excision
• Palate Surgery
• Maxillectomy
Approximately 1-2ml depending on the size of the patient (dilution may be required to achieve appropriate volume without exceeding maximum safe dose of drug).

Intra or extraoral approaches for this block are reported. The extraoral approach is preferred and this technique has been associated with increased accuracy with no evidence of macroscopic damage to associated anatomy (Viscasillas et al. 2013).

  • Extraoral Approach (Figure 2): Local anaesthetic is deposited around the maxillary nerve as it courses across the pterygopalatine fossa. The needle is inserted below the cranioventral border of the zygomatic arch, between the caudal border of the maxilla and the cranial border of the mandibular ramus (Milena and Gurney, 2016). The needle is advanced in a direction parallel to the plane of the hard palate and 1cm caudal to the lateral canthus of the eye (Milena and Gurney, 2016). The needle is advanced until bone is felt, the syringe should be aspirated prior to injection of any local anaesthetic given the high chance of venepuncture in this location.
  • Intraoral Approach (Figure 3): the needle is inserted palatal to the last molar and advanced in a rostral direction towards the maxillary foramen.

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Figure 2. Maxillary Block. Extraoral approach in the dog.

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Figure 3. Maxillary Nerve Block. Intraoral approach in the cat.

2. Infraorbital (Rostral Maxillary) Nerve Block (Figure 4.)

Anatomy Desensitised

Indications

Volume of drug required

Ipsilateral to the block:
• Upper lip
• Nose
• Roof of Nasal Cavity
• Maxillary teeth as far caudal as the infraorbital foramen
• Skin as far caudal as the infraorbital foramen
• Tooth extraction
• Mass excision
• Nostril surgery
Maximum volume to inject is approximately 0.5-1ml (dilution may be required to achieve appropriate volume without exceeding maximum safe dose of drug).

Block of this nerve can be performed extraoral but given the ease to palpate landmarks, the intraoral approach is usually employed. Local anaesthetic is deposited around the infraorbital nerve as it exits the infraorbital foramen. This can be palpated in a location dorsal to the third premolar, approximately halfway between the zygomatic arch and the root of the canine (Milena and Gurney, 2016).

A technique where by the needle is advanced into the infraorbital canal is also commonly reported. In cats and brachycephalics, the reduced dimensions of the infraorbital canal mean that advancing the needle too far caudally could result in globe penetration. A small volume of agent is also recommended should this approach be used to avoid pressure injury to surrounding anatomy and neuropraxia.

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Figure 4. Infraorbital Nerve Block. Approach in dog (left) and cat (right)

Anatomy Desensitised

Indications

Volume of drug required

Ipsilateral to the block:
• Mucosa of the hard palate
• Palate surgery Maximum volume to inject is approximately 0.1-0.3ml (dilution may be required to achieve appropriate volume without exceeding maximum safe dose of drug)..

Palpation of landmarks is difficult. Injection of local anaesthetic should be made at the level of the fourth pre-molar at a location halfway between the dental arcade and the midline of the palate.

There is risk of palatine artery puncture and haematoma formation with needle placement in this location.

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Figure 5. Palatine Nerve Block. Approach in dog (left) and cat (right)

4. Mandibular (Inferior Alveolar) Nerve Block

Anatomy Desensitised

Indications

Volume of drug required

Ipsilateral to the block:
• Mandible
• All mandibular teeth
• All other hard and soft tissues (including rostral 2/3 of the tongue)
• Mandibular tooth extractions
• Mass Excision • Mandibulectomy
Maximum volume to inject is approximately 0.5-1ml (dilution may be required to achieve appropriate volume without exceeding maximum safe dose of drug).

This block can be performed intra and extraoral. With both approaches the mandibular foramen is palpated lingually to the mandible and both ventral and caudal to the last molar (at a depth half way along an imaginary line between the mandibular process and last molar). If direct palpation of the foramen is not possible then the neurovascular bundle is usually easily located in this region and acts as a useful landmark for needle placement.

  • Intraoral approach: this approach is more technically challenging than the extraoral approach given the limed space in this location. The needle is advanced caudally to the position of the neurovascular bundle and the appropriate volume of agent injected.
  • Extraoral approach (Figure 6.): the needle is inserted, in the region of the neurovascular bundle, perpendicular to the skin and medial to the body of the mandible. In dogs a useful landmark for this is the mandibular notch, a depression on the caudoventral aspect of the mandible. 

Blockade of this nerve may result in inadvertent anaesthesia of its lingual branch and therefore a loss of sensation to the patient’s tongue. This could put the tongue at risk of damage through traumatisation or chewing, especially during the recovery phase.


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Figure 6. Mandibular Nerve Block. Extraoral approach in dog (left) and cat (right).


5. Mental Nerve Block (Figure 7.)

Anatomy Desensitised

Indications

Volume of drug required

Ipsilateral to the block:
• Lower lip rostral to mental foramen
• Buccal soft tissues rostral to mental foramen
• Lower lip/buccal surgery Maximum volume to inject is approximately 0.2-0.5ml (dilution may be required to achieve appropriate volume without exceeding maximum safe dose of drug).

The mental foramen is palpated through the frenulum of the lower lip and rostrally to the second premolar. Injection of the agent is recommended at the opening of the foramen. Introduction of the needle into the foramen is also reported but is not only practically difficult in small patients but also associated with risks such as neuropraxia as described for the Intraocular Nerve Block.

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Figure 7. Mental Nerve Block. Approach in dog (left) and cat (right)


6. Splash Block

A suitable volume of 2% Lidocaine or 0.5% Bupivacaine applied topically to a wound or surgical site can be used to provide either anaesthesia for surgical closure or for post-operative analgesia. To increase the success of a splash block, it is recommended that the local anaesthetic used has a tissue contact time of two minutes so that adequate penetration of localised nerve endings can occur (Duke-Novakovski, 2016).

Please note that not all the drugs discussed in this article are licenced for use in small animals despite their frequent use in veterinary practice. Their use is the responsibility of the administering veterinary surgeon following an appropriate risk/benefit analysis in each case and in line with the cascade.

Article by
Dr. Dan Cripwell
BSc (Hons) BVSc CertAVP (EM) PgCert (VPS) MRCVS

Veterinary Technical Advisor UK
RCVS Recognised Advanced Veterinary Practitioner

Originally published: Thursday, 14th June 2018

References

Aguiar, J., Chebroux, A., Martinez-Taboada, F. and Leece, E.A., 2015. Analgesic effects of maxillary and inferior alveolar nerve blocks in cats undergoing dental extractions. Journal of feline medicine and surgery. 17(2): 110-116.

Dugdale, A., 2011. Veterinary anaesthesia: principles to practice. John Wiley & Sons.

Duke-Novakovski, T. 2016. Pain management II: local and regional anaesthetic techniques. In: BSAVA manual of canine and feline anaesthesia and analgesia (Third Edition). John Wiley & Sons.

Milella L., Gurney M. 2016. Pain management II: local and regional anaesthetic techniques. In: BSAVA manual of canine and feline anaesthesia and analgesia (Third Edition). John Wiley & Sons.

Perry, R., Moore, D. and Scurrell, E., 2015. Globe penetration in a cat following maxillary nerve block for dental surgery. Journal of feline medicine and surgery. 17(1): 66-72.

Viscasillas, J., Seymour, C.J. and Brodbelt, D.C., 2013. A cadaver study comparing two approaches for performing maxillary nerve block in dogs. Veterinary anaesthesia and analgesia. 40(2): 212-219.

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