Nerve Blocks Take the Bite Out of Small Animal Oral Surgery
By John Lewis, VMD
For Veterinary Practice News
Posted: Sept. 21, 2012, 4:45 p.m. EDT
Aside from an uncomplicated professional cleaning, nearly every dental procedure may result in intraoperative or postoperative discomfort. The most effective approach to pain relief is pre-emptive analgesia.
This prevents the windup phenomenon, a perceived increase in pain intensity due to repeated stimulus of nerve fibers. Local anesthetics work by blocking sodium channels in neuron cell membranes, preventing development of an action potential.
In addition to the benefit of preventing windup pain, nerve blocks decrease the amount of inhalant anesthetic gas needed, potentially improving intraoperative blood pressures and allowing for easier maintenance of necessary anesthetic plane. Depending on the type of local anesthetic used, nerve blocks may provide up to 10 hours of postoperative analgesia.
Although they are often called local anesthetics, we use them more commonly in oral surgery to provide regional anesthesia of specific nerves to block an entire region of the body (e.g. the right maxilla).
Some veterinary dentists prefer to use a dental aspirating syringe—like what human dentists use—that allows for aspiration and injection with one hand, theoretically causing less movement of the needle tip when aspirating.
At the University of Pennsylvania, our doctors use standard 3cc or 1cc syringes, with a 27-gauge needle, ranging in length from 0.5 to 1.5 inches, depending on which block will be performed. The most common anesthetic used is 0.25 to 0.5 percent bupivacaine.
Local anesthetics can be purchased with or without epinephrine, which causes vasoconstriction at the site of injection, resulting in longer lasting blocks and possibly less bleeding at the surgical site. Use epinephrine with caution in cardiac, hyperthyroid and asthmatic patients.
We tend to use bupivacaine without epinephrine, which takes effect in four to 20 minutes, and the duration of effect may be from four to eight hours. The total maximum safe dose of bupivacaine we abide by is 1.5 mg/kg in cats and 2 mg/kg in dogs. This maximum safe dose may be particularly important to keep in mind with emaciated cats or micro-dogs where all four quadrants require blocks.
Lidocaine also may be used. It takes effect in three to five minutes, and its duration of effect tends to be one to two hours. Lidocaine may be a good choice in cases where the onset of oral surgery will be rapid after induction.
Having access to a dog and cat skull is very helpful when you first begin to perform nerve blocks—use them as a frequent reference for learning locations of foramina and anatomic landmarks.
Common Regional Blocks
1. Infraorbital nerve block. Palpate the infraorbital foramen below the orbit and dorsal to the maxillary third premolar tooth. Insert the needle into the foramen at the level of the mesial root of P3, and gently redirect the needle if the tip does not advance easily. Never force the needle if it is difficult to advance.
Sometimes, flexing the needle by moving the needle hub medially helps to lift the needle tip off the bone to advance it. Aspirate before injecting to make sure the needle is not in a blood vessel. If you aspirate blood, reposition and aspirate again, or pull out and use a new syringe and needle if you cannot tell whether you are continuing to get a flashback of blood.
A caudal infraorbital nerve block is performed in a dog using a 3 cc syringe and a 1.25 inch, 27-gauge needle. With the needle inserted nearly to the hub in this midsized dog, the entire maxillary quadrant can be blocked.Photo by Dr. John Lewis
Use digital pressure over site after needle is removed to increase effect and decrease hematoma formation. The region blocked depends on how far the needle is inserted in the foramen; assess how far the needle needs to be inserted by laying it over the area to see if the needle will reach the tooth of interest. The foramen is large enough that a 27-gauge needle can be inserted with minimal risk of causing damage to the nerve.
2. Middle mental nerve block. Palpate the foramen caudal to labial frenulum of the lower jaw. Insert needle in region of the foramen and infuse with anesthetic after aspirating. This foramen narrows down rapidly and, therefore, I do not feel comfortable inserting the needle deep into the foramen due to resistance and concerns about damage to the neurovascular structures.
Since this block as described does not reliably provide generalized desensitization to tissues of the incisive and rostral regions of the mandible, I rarely use this block, except for the occasional lateral rostral lip mass removal.
3. Inferior alveolar nerve block. Palpate the foramen intraorally, distal to the mandibular third molar (dog) or the mandibular first molar (cat), and ventral, about 1cm dorsal to the ventral cortex of the caudal mandible where the ramus meets the body. This block anesthetizes the entire mandibular quadrant.
Intraoral approach: Insert needle into region of foramen (do not attempt to enter foramen) at a 20-degree angle to the long axis of the mandible approximately 1 cm caudal to the mandibular third molar (dog), or 0.5 cm caudal to mandibular first molar (cat). Stay close to the bone and avoid placing the anesthetic too medially.
Extraoral approach: Palpate the facial vascular notch on the ventral cortex of the mandible. Insert the needle through the skin at this region, staying as close to the medial surface of the mandible.
Use the index finger of your opposite hand to feel for the foramen and assess whether the tip of the needle is in the correct position. For the novice, the extraoral approach is helpful since the index finger placed intraorally ensures that the anesthetic is deposited in the correct position.
Caution: This nerve block has the potential to inadvertently block the lingual nerve, which is medial to the inferior alveolar nerve. Blocking the lingual nerve results in lack of sensation of the tongue and self-trauma upon recovery from anesthesia.
This is not a common problem, but anesthetists should be aware of this as a potential complication. Avoid this by staying close to the medial surface of the mandible and use a small volume of anesthetic so it doesn’t diffuse medially to block the lingual nerve.
4. Maxillary nerve block. This blocks the entire maxillary quadrant on the buccal and palatal sides of the teeth (same effect as a caudal infraorbital nerve block).
Intraoral approach: Use a half-inch needle. Bend the needle approximately 1 cm from the tip. Identify where the hard palate meets the soft palate just caudal to the maxillary second molar (dog) or maxillary first molar (cat). Insert the needle perpendicular to the palate 0.5 cm deep in the cat and 1 cm deep in dogs. Be careful not to insert the needle too far since the needle is directly ventral to the eye.
These techniques are ideally learned in a wet lab. Go to VeterinaryDentalForum.com for a list of nerve block CE opportunities.
Dr. Lewis, FAVD, Dipl. AVDC, is assistant professor of dentistry and oral surgery at the University of Pennsylvania School of Veterinary Medicine.
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Nerve Blocks Take the Bite Out of Small Animal Oral Surgery
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