Equine Dental Local Anesthesia
Effective dental nerve blocks that are well tolerated by the horse are necessary when performing advanced dental procedures. Less sedation is required which increases the stability of the horse and safety for both horse and practitioner. Local anesthesia and sedation allow the oral practitioner to perform most advanced procedures on a standing horse. This eliminates complications from general anesthesia and extends the time needed for many procedures. Nerve block techniques for dentistry in the horse are easy to comprehend and do not require specialized equipment. Equine dentistry has evolved to a higher level in equine practices, and proper restraint and anesthe
sia are necessary to progress safely and proficiently as advanced procedures are initiated.
Innervation of the dental arcades and their immediate surrounding structures is supplied by the tri
geminal nerve, which is the fifth cranial nerve. This nerve exits the cranium below the ear and progresses cranially, dividing into the maxillary and mandibular nerves (Figure 1). This illustration is a simplification of the branching of the trigeminal nerve to the dental arcades. The maxillary nerve enters the maxilla through the maxillary foramen which is distal to the eye. After entrance into the infraorbital canal, the nerve proceeds cranially branching off to each individual cheek tooth and then exits from the infraorbital foramen. The mandibular nerve advances rostrally on the medial side of the mandible. The inferior alveolar nerve, which is a branch of the mandibular nerve, enters the mandible at the mandibular foramen and proceeds through the mandible, supplying innervation for the lower cheek teeth. This nerve exits through the mental foramen and continues to the incisors. A thorough understanding of the location of these foramina is necessary when performing dental nerve blocks.
Anatomical View of Dental Arcade Innervation
It is helpful to have a prepared skull available with which to familiarize yourself as to the location and depth of these foramina. The maxillary foramen is approximately 2.25 inches in depth and distal to the border of the zygomatic bone. Notice that there is a space or window between the zygomatic bone and the mandible which gives access to the maxillary foramen.
The mandibular foramen is approximately 3.5 inches rostral to the distal border of the mandible. Remember that this foramen is on the medial side of the mandible, and the medial surface of the mandible in this area is concave. Also, the mandibular foramen is at the level of the occlusal arcades of the cheek teeth. It is not possible to palpate this foramen due to depth and location. However, these nerves are large and consistent in their locations. It is not difficult to place a spinal needle in the area of the nerve prior to entering the foramen, and needle placement is tolerated well by the horse.
Technique: Maxillary Nerve Block
The maxillary foramen is accessed between the zygomatic bone and the mandible at the level of the posterior one-third of the eye. It is helpful to examine a skull before performing the block to aid in identifying this area and the exact location of the maxillary foramen. A 22 gauge spinal needle is inserted just below the zygomatic bone at the level of the posterior third of the eye at a 90 degree angle. A local skin bleb of local anesthesia may be used prior to insertion of the spinal needle, but normally this procedure is tolerated well by the horse. The needle is advanced until bone is contacted, which is approximately 2 to 2.5 inches in depth and then slightly withdrawn. Inject 15 ml of local anesthesia or less into this area. Onset of anesthesia is normally 15 minutes with 2% lidocaine and will last for 2 to 3 hours. Of all the dental nerve blocks,
this one is the least difficult to perform. If there is a problem with placement of the spinal needle, it is usually because the veterinarian is too far rostral. If this is the situation, the needle will not advance more than one inch because of the contour of the maxilla bone. Reposition the needle slightly caudally to correct this problem.
The entire upper arcade on the side of the nerve
block, from the first incisor to the last molar, will be blocked. As previously stated, this block includes the teeth and the immediate surrounding structures. The central incisors tend to have some crossover of nerves from the opposite side, thus preventing a complete block on one side.
Branches of the maxillary nerve outside the infraorbital canal supply sensory innervation to soft tis
sue areas adjacent to the dental arcades. Infiltration of a local anesthetic into the soft tissue area around the tooth is beneficial and eliminates sensitivity of the soft tissue when manipulated by the practitioner.
Technique: Inferior Alveolar Block
The mandibular foramen is located by drawing a line along the buccal edge of the upper dental arcade, which is easily palpated, and extending this to the back of the mandible (Figure 5). The foramen is 3.5 inches from the back edge of the mandible along this line in an average size horse. Mark this spot or have
an assistant place a finger on this location when directing the needle. The mandibular foramen is located on the medial surface of the mandible. It is helpful to examine a skull before performing this nerve block to familiarize yourself as to the location of this foramen. To aid needle insertion, the head is extended to gain access to the back of the mandible. A 22 gauge spinal needle is used for this block, and it is directed rostrally from the back edge of the mandible to the area predetermined to be the location of the mandibular foramen. The medial surface of the mandible in this area is concave, so it is important to keep the direction of the needle insertion close to the medial surface of the mandible. It is not necessary to penetrate the foramen, and it should be avoided whenever possible. The ideal area to block this nerve is just before the inferior alveolar nerve enters the mandibular foramen. Error on the high side of the foramen is better than below the foramen. This nerve block is also well tolerated by the horse. It requires approximately 20 ml of local anesthesia and the onset is 15 to 30 minutes depending on the accuracy of the needle placement. This block will effectively desensitize the entire lower dental arcade and the immediate surrounding tissue.
The mandibular foramen block supplies anesthesia to all teeth in the lower dental arcade.
Technique: Infraorbital Nerve Block
The infraorbital foramen is located between the anterior edge of the facial crest and the commissure
of the nasal bone. This foramen is easy to find and palpate under the levator labii superioris proprius and levator nasolabialis muscles. The levator labii superioris proprius muscle should be manipulated away from the foramen prior to needle insertion. A needle or catheter can be placed into this foramen and threaded posteriorly while infusing local anesthesia ahead of the needle. The most difficult part of this block is starting the needle into the foramen. The horse does not tolerate this procedure well and tends to move his head as the nerve is touched by the needle. Lidocaine injected slowly as the needle advances helps accomplish this block. The author favors the maxillary nerve block over the infraorbital nerve block because it is easier to administer and the horse does not object to needle placement. Both of these nerve blocks are effective, but the maxillary foramen nerve block is faster and easier to perform and is preferred by most practitioners. The length of the infraorbital canal is approximately 4.5 inches. It is not necessary to insert the needle this entire distance for a complete block, because the local anesthetic will progress caudally in the canal with pressure as it is injected.
The infraorbital nerve must be blocked caudally to the cheek tooth desired for anesthesia. For example, local anesthesia for the last molar will require a depth of 3.5 inches into the infraorbital canal.
Technique: Mental Nerve Block
The mental foramen is located ventral to the commissure of the lips on the lateral side of the mandible. The depressor labii inferioris muscle covers the foramen and must be moved dorsally to palpate
and inject anesthetic. It is very difficult to enter this foramen with a needle, and it is not well tolerated by the horse. Normally, this nerve block is only used for lower incisor procedures, however this block does not work well unless local anesthetic is injected into the foramen. Bilateral nerve blocks are advised when working on central incisors due to the crossover of innervation at the rostral portion of the premaxilla and mandible.
The mental foramen is difficult to palpate, and threading a needle into the canal caudally can be a problem. This block is not well tolerated by the horse.
Infiltration of Local Anesthesia
Infiltration of local anesthetic around a tooth is beneficial for elevation of periodontal tissue and is
commonly used for wolf teeth and incisor cap extractions. The needle should be placed in the loose mucous membranes where it is easy to inject the local anesthetic rather than the tight gingival tissue that is adhered to bone. These blocks are quick and easy and should be used whenever elevation of tissue is required for incisors or wolf teeth.
Infiltration of local anesthetic is also helpful when
extracting cheek teeth. The maxillary and inferior alveolar nerves, which enter the infraorbital and mandibular canals, supply sensory innervation for the tooth and immediate surrounding structures, but does not innervate much of the soft tissue surrounding the tooth. Branches of the maxillary and mandibular nerves outside of the infraorbital and mandibular canals supply sensory innervation to most of the soft tissue areas adjacent to the teeth. A needle is placed from the outside of the cheek caudally and slightly above the gum line of the tooth that is to be extracted for the maxillary cheek teeth and slightly below the gum line in mandibular cheek teeth. 10-15cc of local anesthetic is adequate for most extractions. Infiltration of local anesthetic on the medial side of the upper cheek teeth is not necessary because the sphenopalatine nerve which branches off ventrally from the maxillary nerve is also blocked when performing a maxillary nerve block. Extraction of cheek teeth usually takes a considerable amount of time, elevation, and manipulation with large instruments. This infiltration technique, along with the trigeminal nerve blocks, allows most advanced procedures to be performed on a standing horse with mild sedation.
The maxillary nerve block and the inferior alveolar nerve block are effective, economical, and easy to perform with proper orientation. These nerve blocks are tolerated well by the horse, and they reduce the amount of sedation required to perform advanced procedures. Also, these nerve blocks can be used before surgery that requires general anesthesia to reduce stimulation caused by the surgeon and amount of anesthesia required.
Complications from the maxillary and inferior alveolar nerve blocks are rare, but can occur and include hematoma, excessive swelling, temporary loss of palpebral function, and infection. The needle technique and disinfecting the area are always important in any procedure where injection is required. The hypodermic needle has a cutting edge, and the head is highly vascular, so try to minimize in and out redirecting of needle placement, which can lead to excessive hemorrhage and infection.
Local anesthetic should be injected in both the palatal and labial sides of the incisor for complete anesthesia.
When blocking wolf teeth, the needle should be placed at the level of the root and directed caudally to insure adequate anesthesia for elevation and extraction.
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