Introduction
Dental nerve blocks are commonly performed for dogs undergoing various dental procedures, including deep teeth cleaning, tooth extraction, root canal treatment, and correction of bony malformations or traumatic injuries of the maxilla and mandible. These blocks have been shown to decrease the intraoperative anesthetic requirement1 and postoperative need for additional analgesia.2 A maxillary nerve block desensitizes the soft tissue and bony structures of the upper dental arcade, including the sensory innervation to various maxillary teeth, and has been advocated as the preferred nerve block to reduce responsiveness during posterior rhinoscopy.2,3
In dogs, the maxillary nerve provides sensory neurons to the upper eyelid, part of the nasal cavity including the nasal mucosa, maxillary teeth, and upper lip. It leaves the cranial vault through the round foramen, courses through the alar canal, exits through the rostral alar foramen, and then trifurcates into the zygomatic, pterygopalatine, and infraorbital nerves in the pterygopalatine fossa. The pterygopalatine nerve branches from the maxillary nerve over the dorsal surface of the medial pterygoid muscle as the maxillary nerve continues its course toward the maxillary foramen. The minor and major palatine nerves, the latter just distal to the minor nerve, arise from the pterygopalatine nerve and provide sensory neurons to the soft palate and most of the mucosa of the hard palate, respectively. After the major palatine nerve branches from the pterygopalatine nerve, the pterygopalatine nerve continues as the caudal nasal nerve and supplies sensory neurons to the nasal mucosa of the ventral part of the nasal cavity. The ethmoidal nerve, a branch of the ophthalmic nerve, also provides sensory innervation to the mucosa of the nasal cavity (lateral walls) plus the nasal conchae and nasal septum.4
On the ventral aspect of the pterygopalatine fossa, the caudal superior alveolar nerve branches that supply sensory neurons to the caudal maxillary premolar and molar teeth arise from the infraorbital nerve. Then, the infraorbital nerve enters the infraorbital canal through the maxillary foramen where the nerve sends out middle superior alveolar nerve branches from its ventral surface to innervate the maxillary cheek teeth. Just before the infraorbital nerve exits the infraorbital canal through the infraorbital foramen, it sends out rostral superior alveolar nerve branches from its ventral surface to innervate the maxillary canine and incisor teeth.4
Because of the extensive distribution of the maxillary nerve to various upper dental arcade structures and the location for and route of administration of a local anesthetic, the clinical efficacy of a maxillary nerve block may vary greatly. Although various approaches to perform a maxillary nerve block in dogs have been described, including intraoral,5 lateral percutaneous,6,7 infraorbital canal,8 and transorbital,9 details of their clinical efficacy are lacking. A lateral percutaneous maxillary approach is well established in veterinary medicine,10 but a recent cadaveric dog study11 showed that inexperienced anesthetists may be more successful at blocking the maxillary and pterygopalatine nerves with a modified infraorbital approach.
The objective of the study reported here was to compare the efficacy of a lateral percutaneous maxillary approach with that of a modified infraorbital approach at desensitizing the maxillary nerve. We hypothesized that the lateral percutaneous approach would result in more consistent blockade.
Materials and Methods
Dogs
Six approximately 2-year-old sexually intact mesaticephalic female hound-type dogs (mean ± SD body weight, 21.3 ± 2.0 kg) were used. Dogs were judged to be healthy on the basis of a physical examination, CBC, and serum biochemical analysis. The stage of estrus was not determined. The study was approved by the Institutional Animal Care and Use Committee of the University of California-Davis (protocol No. 19039).
Anesthesia and instrumentation
Food was withheld from dogs for 12 hours, and water was available ad libitum until they were transported to the research laboratory on the study day. On that day, a 20-gauge, 4.8-cm IV cathetera was placed aseptically in a cephalic vein. An induction dose of propofolb was titrated (mean ± SD dose, 4.0 ± 0.7 mg/kg) to achieve orotracheal intubation. Anesthesia was then maintained with isofluranec in oxygen with a partial rebreathing anesthetic circuit. By use of a multiparameter monitor,d heart rate and rhythm were monitored with lead II ECG, arterial blood pressure with oscillometric technology, and end-tidal carbon dioxide concentration and end-tidal isoflurane concentration with an infrared gas analyzer. Intermittent positive-pressure ventilatione was initiated to maintain end-tidal carbon dioxide concentration between 21 and 41 mm Hg. An end-tidal isoflurane concentration of 1.68 ± 0.16% (mean ± SD) was needed to maintain an anesthetic plane that prevented the dogs' movement caused by electric stimulation of the gingiva or hard palate. Dogs were placed on a warm water blanket in left or right lateral recumbency, and a forced-air warming systemf was used to maintain an esophageal temperature between 36.6°C and 40.0°C. A fluid pumpg was used to administer lactated Ringer solutiong at 5 mL/kg/h for the duration of anesthesia.
Procedures
For this crossover study, dogs were randomizedh for side (left or right) and maxillary nerve block approach (lateral percutaneous or infraorbital). A washout period of at least 2 weeks was allowed between approaches. The local anesthetic used consisted of a 2-mL mixture (50:50 [vol/vol]) of 2% lidocainei and 0.5% bupivacainei solutions.
For the infraorbital approach, a 22-gauge, 4.5-cm-long IV catheterj was used. The upper lip on the treatment side was retracted dorsally, and the infraorbital foramen was palpated through the oral mucosa. After the catheter was inserted at the rostral portion of the infraorbital canal, the metal stylet was retracted 2 to 3 mm inside the catheter before the catheter was further advanced into the infraorbital canal. Then, the catheter was inserted to its full length, parallel to the maxillary dental arcade, and the stylet was removed. A syringe with the analgesic solution was attached to the free end of the catheter, aspiration was performed, and if no blood was aspirated, 1 mL of the solution was injected over 15 seconds at this site. The catheter was then retracted 2 cm, and aspiration was again performed before injecting the remainder (1 mL) of the solution over 15 seconds. After injection, the catheter was removed and pressure was applied at the catheter entry site for approximately 20 to 30 seconds.
For the lateral percutaneous approach, the skin was swabbed with alcohol and then a 22-gauge, 4.5-cm-long shielded stimulating needlek was inserted at the caudoventral border of the zygomatic arch and directed slightly proximally. The needle was withdrawn slightly after it contacted bone; contact with bone helped to ensure the needle tip was near the maxillary nerve and lateral to the pterygopalatine muscle. An electric pulse of 1 mA was delivered for 0.5 milliseconds at 1 Hz.l If an REMP waveform from the digastricus muscle was not obtained, the needle was slowly advanced or withdrawn until a waveform was obtained. Current intensity was reduced to 0.7 mA and then to 0.3 mA in conjunction with careful manipulation of the needle so that an REMP was obtained at 0.7 mA but not at 0.3 mA. Once the needle was positioned, a syringe with the analgesic solution was attached to the free end of the needle, aspiration was performed, and if no blood was aspirated, 2 mL of the solution was injected at this site. If resistance to injection was encountered, the needle was slightly repositioned to prevent intraneural injection.
REMP
At each stimulation site (oral structure), 2 shielded unipolar stimulating-needle electrodesm were inserted approximately 5 mm apart and as close as possible to the dental-gingival margin. Two additional shielded unipolar stimulating-needle electrodes were inserted SC over the digastricus muscle ipsilateral to the treatment side to record the REMP associated with each stimulation site. Another electrode was inserted SC over the dorsal cervical region to serve as ground. The REMP was measured with an evoked potential measurement system that used a proprietary software program.l
To determine the maximal stimulus at each site for each data aquisition time, an electric stimulus was applied to the site, and the intensity of current was varied to obtain maximal amplitude of the first REMP waveform with minimal stimulus artifact. A 0.5-millisecond pulse width at a frequency of 1 Hz for 20 seconds was used, and the machine determined the average of the resulting waveforms. The intensity of current varied between 20 and 90 mA.
The REMP was measured at the vestibular aspect of MC, MPM4, and MM2 and the hard palate mucosa (approx at the level of the first molar tooth) ipsilateral to the treatment side and at the vestibular aspect of MC contralateral to the treatment side. Three baseline values at 10-minute intervals were obtained for each stimulation site. The REMP was measured before (baseline) and at 5, 10, 15, 30, 45, and 60 minutes after injection (time of injection denoted as time 0) and then every 20 minutes thereafter until the stimulation sites were no longer desensitized or for up to 6.7 hours. Each REMP measurement cycle (ie, measurement at all 5 sites) took approximately 2 to 3 minutes to complete.
After the last REMP measurement, each dog received a dose of carprofen (2 mg/kg, IV). Isoflurane was then discontinued, and each dog was allowed to recover from anesthesia.
Statistical analysis
For each data acquisition time, the REMP for the untreated contralateral MC was used as a control to normalize the REMP measurements obtained from the 4 ipsilateral stimulation sites. The normalized area under the first REMP waveform was the primary outcome of interest, but REMP amplitude (height of the waveform), latency (time from application of the stimulus to start of the waveform), and duration (time elapsed between initiation of the waveform and return to same voltage) were also assessed.
The following equation was used to normalize the area under the first REMP waveform for all ipsilateral measurements:
where C is the mean of 3 baseline areas under the first REMP waveform for the untreated contralateral MC, TC is the subsequent area under the first REMP waveform of the untreated contralateral MC at time t, TT is the subsequent area under the first REMP waveform for a specific treated ipsilateral stimulation site (MC, MPM4, MM2, or hard palate) at time t, and TRC is the mean of 3 baseline areas under the first REMP waveform for that treated ipsilateral stimulation site. A stimulation site was considered desensitized (blocked) if an REMP was not observed (flat line) or the normalized area under the first REMP waveform was < 15% after treatment. A site was considered recovered (ie, no longer desensitized) when the area under the first REMP waveform was approximately 15% of the control value.
Descriptive statistics were generated. Data for onset and duration of desensitization were summarized as median and range. Mixed multilevel logistic regression by QR decomposition was used to evaluate the effect of nerve block approach (infraorbital vs lateral percutaneous) and stimulation site (MC, MPM4, MM2, and hard palate) on whether a nerve block was successful. Mixed linear regression was used to evaluate the effect of approach and stimulation site on the onset and duration of the nerve block. All models included an identity covariance matrix structure to control for individual dog as a random effect. Wald tests with the Bonferroni method were used when post hoc multiple comparisons were performed to examine the effect of treatment approach on the dependent variables. Statistical softwaren was used to perform analyses. Values of P < 0.05 were considered significant.
Results
The procedure was abandoned for 2 dogs after the lateral percutaneous maxillary block because of a lack of effect by 2 hours after the injection for one dog and oronasal hemorrhage for the other dog. Both dogs successfully completed the experiment at a later date. One dog inadvertently had the maxillary nerve blocked via the infraorbital approach twice, but the data for that dog were still included in the analysis. The proportion of dogs with successful nerve blockade did not significantly (P = 0.904) differ between the infraorbital and lateral percutaneous approaches after adjusting for individual dog and stimulation site (Table 1). The proportion of dogs with successful nerve blockade did not significantly (P = 0.359) differ among the 4 stimulation sites after adjusting for individual dog and approach.
The number of healthy adult hound-type dogs in which various oral structures were desensitized (blocked) and the median (range) time to onset and duration of desensitization following injection of 2 mL of a 2% lidocaine–0.5% bupivacaine mixture (50:50 [vol/vol]) via infraorbital (n = 7) and lateral percutaneous (6) approaches.
| Approach | Oral structure* | No. of dogs in which treatment resulted in desensitization of the oral structure | Time to onset of desensitization (min) | Duration of desensitization (min) |
|---|---|---|---|---|
| Infraorbital | ||||
| MC | 7 | 15 (10–30) | 190 (130–370) | |
| MPM4 | 7 | 5 (5–30) | 350 (195–375) | |
| MM2 | 4 | 10 (5–45) | 70 (55–235) | |
| Hard palate | 2 | 12.5 (10–15) | 97.5 (90–105) | |
| Lateral percutaneous | ||||
| MC | 3 | 15 (10–45) | 105 (90–155) | |
| MPM4 | 6 | 17.5 (5–120) | 267.5 (20–375) | |
| MM2 | 4 | 10 (5–60) | 20 (5–230) | |
| Hard palate | 4 | 5 (5–10) | 22.5 (5–75) | |
Each dog was randomized for side (left or right) and nerve block approach (infraorbital or lateral percutaneous) with at least a 2-week washout period between treatments. An oral structure was considered desensitized if a flat line was achieved during REMP measurement or the normalized area under the first REMP waveform was < 15% after treatment and was considered recovered (ie, no longer desensitized) when the area under the first REMP waveform was > 15% of the control value. The contralateral MC was used as the control.
Ipsilateral to the side in which the treatment was injected.
For instances of successful nerve blockade, the time to onset of desensitization did not significantly differ by approach (P = 0.111) or stimulation site (P = 0.461) after adjusting for individual dog. However, blockade duration was significantly (P = 0.002) longer with the infraorbital approach, compared with the lateral percutaneous approach, after adjusting for individual dog and site. Blockade duration also significantly (P < 0.001) differed by stimulation site, with blockade for MPM4 significantly longer than for MC (P = 0.002), MM2 (P < 0.001), and hard palate (P < 0.001) and with blockade for MC significantly longer than for MM2 (P = 0.025) and hard palate (P = 0.012). Blockade duration did not significantly (P = 0.638) differ between MM2 and hard palate.
Discussion
Results of the present study indicated that both lateral percutaneous and modified infraorbital approaches used for the injection of a mixture of 2% lidocaine and 0.5% bupivacaine (50:50 [vol/vol]) successfully desensitized the 4 oral structures of interest (MC, MPM4, MM2, and hard palate), but not with 100% efficacy. The proportion of dogs with successful blockade and the time to onset of desensitization did not significantly differ between approaches. However, duration of desensitization was significantly longer with the modified infraorbital approach versus the lateral percutaneous approach.
Desensitization of maxillary and pterygopalatine nerves is imperative to provide locoregional anesthesia of the nose, upper lip, palate, and maxilla,10 especially with procedures involving the upper dental arcade. The volume of local anesthetic should be sufficient to cover 6 mm of the targeted portion of the nerve to ensure blockade of 70% of the transmembrane sodium channels over 3 consecutive nodes of Ranvier.12,13 A study11 involving cadaveric dogs indicated that an injection of 0.5 mL (0.034 mL/kg) of methylene blue with an infraorbital approach resulted in > 6 mm of staining of all branches (zygomatic, pterygopalatine, greater palatine, lesser palatine, caudal nasal, infraorbital, and caudal superior alveolar nerves) within the pterygopalatine fossa for nearly 65% of the dogs versus only approximately 22% with a lateral percutaneous approach. Plus, pterygopalatine nerve staining was 70% with the modified infraorbital approach, compared with 21% with the lateral percutaneous approach. The volume of the lidocaine-bupivacaine solution used in the present study was 2 mL (0.094 mL/kg), and the infraorbital approach yielded successful desensitization of MC and MPM4 for all dogs, suggesting that at least 6 mm of the targeted nerve (maxillary nerve) was covered by the mixture. Additionally, Fizzano et al14 reported that a maxillary nerve blockade (0.1 mL/kg of 0.5% bupivacaine) with the modified infraorbital approach resulted in successful desensitization of the nasal cavity for 6 of 8 dogs such that rhinoscopy could be adequately performed and nasal biopsy specimens could be obtained. On the basis of these results, the modified infraorbital approach may be best, especially for inexperienced anesthetists.11
Injection of local anesthetic within the infraorbital canal at variable depths has resulted in inconsistent blockade of the molar teeth of dogs.3,8,11 A previous study15 revealed that an infraorbital approach resulted in desensitization at MM2 in only 1 of 6 dogs of similar size to those in the present study. In that study,15 a total of only 1 mL of 2% lidocaine or a mixture of 2% lidocaine and 0.5% bupivacaine (50:50 [vol/vol]) was injected at approximately two-thirds the length of the infraorbital canal. However, in the present study and another recent study,16 1 mL of the same mixture of 2% lidocaine and 0.5% bupivacaine (50:50 [vol/vol]) was injected at the full insertion length of the catheter, as in the previous study15; however, unlike the previous study,15 another 1 mL was injected after withdrawing the catheter about 2 cm. Blockade of MM2 was achieved for 4 of 6 dogs (4 of 7 attempts) in the present study and 5 of 6 dogs in the recent study16 with this modified infraorbital approach. Hence, these results suggest that for blockade of MM2, injecting 2 mL of a mixture of 2% lidocaine and 0.5% bupivacaine (50:50 [vol/vol]) at 2 locations beyond the infraorbital canal (infraorbital approach), compared with at 1 location, may be optimal. This modified infraorbital approach may have been superior through successful blockade of the caudal superior alveolar nerves that arise from the infraorbital nerve before it enters the maxillary foramen. Yet a single injection of the entire 2 mL of the analgesic mixture via a lateral percutaneous approach also successfully desensitized MM2 for 4 of 6 dogs in the present study. Both approaches likely permitted deposition of the mixture in the pterygopalatine fossa, resulting in the blockade of caudal superior alveolar nerves. Similar-sized catheters and dogs were successfully used for the modified infraorbital approach in the present study and in the Fizzano et al14 cadaveric dog study; therefore, in the present study, the tip of the catheter was expected to be situated in the pterygopalatine fossa after the catheter was inserted to its full length through the infraorbital canal.
Although overall proportions of dogs with successful desensitization did not significantly differ between approaches, a larger proportion of dogs was noted to have had desensitization of the hard palate with the lateral percutaneous approach (4/6) versus the infraorbital approach (2/6 [2/7 attempts]). This difference may be because of the larger volume of local anesthetic (2 mL vs 1 mL) injected in the pterygopalatine fossa with the lateral percutaneous approach. The volume used with the lateral percutaneous approach in the present study was similar to that Fizzano et al14 recommended and may have resulted in better blockade of the minor and major palatine nerves (arising from the pterygopalatine nerve) and therefore desensitization of the hard palate.
The method used to stimulate the nerve (electrolocation) for assessment of the lateral percutaneous approach in the present study was different from the approach used clinically (strictly on the basis of anatomic landmarks). In veterinary medicine, electrolocation is achieved through stimulation of a nerve to obtain a visible muscle twitch. Human patients may also report sensory stimulation, usually as a paresthesia,17 but this is not possible in an anesthetized animal. The stimulation method used in the present study relied on a current from the stimulating needle to sufficiently depolarize the afferent nerve to induce an efferent digastricus muscle reflex after signal processing in the brain. Our expectation with this stimulation method was that a positive result at 0.7 mA (REMP observed) indicated that the stimulating needle was near the nerve and a negative result at 0.3 mA (no REMP observed) indicated that the needle was not in the nerve. In a study18 of people, a current of 0.5 mA induced a muscle twitch for only 75%, even when the stimulating needle contacted the nerve (median, radial, musculocutaneous, or ulnar nerve), as determined by means of ultrasonography; the remaining patients required a current of up to 1 mA. In another study,19 the distance from the stimulating-needle tip to the radial or ulnar nerve was measured when a muscle twitch was obtained with different currents; the needle was touching the nerve at 0.5 mA, but at 1 mA, the distance ranged from 0 to 3.2 mm from the nerve with 1 approach. These data suggest that a positive reflex (muscle twitch) at 0.7 mA requires the stimulating-needle tip to be within a few millimeters of the nerve. If a needle penetrates a nerve, resistance to injection of analgesic solution (local anesthetic) increases dramatically; therefore, we planned to stop injecting and then reposition the needle if great resistance was encountered. However, we did not encounter resistance to injection for any dog. With the lateral percutaneous approach, the needle is directed toward the pterygopalatine fossa and is therefore close to the thin palatine bone. Despite careful placement of the needle, we had to abandon the testing for 1 dog because of needle penetration of the palatine bone.
The duration of desensitization was significantly longer with the modified infraorbital approach, compared with the lateral percutaneous approach. The difference in duration may be partly because of the volume of the lidocaine-bupivacaine mixture injected in the infraorbital canal. In the modified infraorbital approach, 1 mL of the mixture was injected in the area of the pterygopalatine fossa and 1 mL was injected in the infraorbital canal, whereas in the lateral percutaneous approach, the entire 2 mL of the mixture was injected in the area of the pterygopalatine fossa. One milliliter of the mixture injected in the infraorbital canal likely resulted in its deposition near the infraorbital nerve, middle superior alveolar nerves, and cranial superior alveolar nerves. Additionally, delayed systemic absorption secondary to pressure generated by the mixture in the noncompliant infraorbital canal may have resulted in longer duration of the block. Retrograde movement of 1 mL of the mixture from the infraorbital canal to the pterygopalatine fossa was also possible, thus leading to longer desensitization of the caudal superior alveolar and palatine nerves. This may have yielded longer duration of blockade for MM2 and hard palate with the infraorbital approach versus the lateral percutaneous approach. However, between the 2 approaches, no significant difference in the proportion of dogs with desensitization of the evaluated oral structures was noted; yet a difference may have been noted if more dogs were included.
The metal stylet was withdrawn into the sheath of the catheter before the catheter was advanced through the infraorbital canal, thereby reducing the likelihood of nerve damage.11,14 Stylet withdrawal also reduced the possibility of an intraneural injection.20 The catheter was made from flexible material (fluorinated ethylene propylene polyester) that has a low coefficient of friction, so frictional damage to the structures in the infraorbital canal was minimal.21 A few published studies3,11,14 indicate that a modified infraorbital approach has minimal complications.
The lateral percutaneous approach is technically more challenging and has been associated with some adverse effects. The present study revealed that the modified infraorbital approach achieved maxillary nerve blockade for the evaluated oral structures as effectively as the lateral percutaneous approach, but the duration of desensitization was longer with the infraorbital approach. This observation from the present study coupled with the recommendation from the authors of a previous study11 suggested that inexperienced anesthetists may have more success with the modified infraorbital approach. Additionally, electrolocation and ultrasonography may not be available to aid with needle placement for the lateral percutaneous approach in a clinical setting. However, if the infraorbital approach is contraindicated (eg, tumor or abscess), the lateral percutaneous approach may provide a similar block but of shorter duration.
Acknowledgments
This study was funded by the Center for Companion Animal Health.
The authors declare that there were no conflicts of interest.
Presented in abstract form at the Association of Veterinary Anaesthetists Meeting, Berlin, Germany, November 2017.
The authors thank Dr. Michelle Giuffrida for assistance with the statistical analysis.
Abbreviations
| MC | Maxillary canine tooth |
| MM2 | Maxillary second molar tooth |
| MPM4 | Maxillary fourth premolar tooth |
| REMP | Reflex-evoked muscle potential |
Footnotes
Insyte, Becton Dickinson, Sandy, Utah.
Abbott Animal Health, Abbott Park, Ill.
Piramal Enterprises Ltd, Mumbai, India.
Carescape B650, GE Healthcare, Waukesha, Wis.
Mark 4, Bird Corp, Palm Springs, Calif.
3M Bair Hugger, Arizant Healthcare Inc, Eden Prairie, Minn.
Baxter Healthcare Corp, Deerfield, Ill.
True random number generator, Random.org, Randomness and Integrity Services Ltd, Dublin, Ireland. Available at: www.random.org.
Hospira Inc, Lake Forest, Ill.
B. Braun Melsungen AG, Melsungen, Germany.
EchoStim, Hakko Co Ltd, Chikuma-shi, Nagano-ken, Japan.
Viking IVD, Nihon Kohden America Inc, Irvine, Calif.
10-mm stimulating electrodes, Grass Products, Warwick, RI.
Stata Corp LP, College Station, Tex.
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