The feasibility of US assistance was evaluated in 2 patients (3 greater palatine canal blocks) in whom the location of the GPF was marked on the mucosa using US. The dentist performing needle insertion entered the canal at the premarked site after which 0.5 mL of 2% lidocaine with 1:100,000 epinephrine was injected into the canal. The blocks were tested similar to GPN blocks.
We evaluated 6 female and 10 male hemisectioned skulls making a total of 16 specimens of which 7 were edentulous and the other 9 had normal dentition (Table 1). The hard palate was visible under US as a hyperechoic white line on the US scan, and the GPF was identified as an interruption in this hyperechoic line near the region of the maxillary molars in all 16 specimens (100%; Fig. 1). In 7 of the 16 hemisectioned cadaveric specimens (n = 7/16), mild to moderate traces of black India ink were found within the greater palatine canal and pterygopalatine fossa (Fig. 4). In the other 9 specimens (n = 9/16), although dye injection was in the region of the GPF on US, dye was deposited in the mucosal tissue of the hard palate lying anterior to the GPF. In 2 of these specimens, traces of dye were found in the tissue of the soft palate. The number of attempts required to inject dye was variable because tracking of the needle on the US needed frequent adjustments. Dye was injected in the first attempt in 5 cases (n = 5/16), whereas 3 or more attempts were required for the other cases (n = 11/16). With detailed side-to-side scans, we also observed the direction of the GPF opening which was downward in 15 specimens and anterior in 1 specimen showing a lower margin of bone at the GPF (Fig. 5). We could not differentiate the vessels and the nerves from the soft tissue under US imaging in the cadaveric tissue.
In the clinical evaluation of the technique, 6 of the 8 attempted blocks (n = 6/8) in 5 patients were successful. The block procedure was abandoned in 1 patient because we were unable to position the probe on the palate. This patient differed had sleep apnea and had a large tongue. In another patient receiving bilateral GPN blocks, the artery was not well visualized on 1 side and therefore the block on that side was abandoned. Patient and block characteristics are summarized in Table 2. The indications for block placement were molar and wisdom teeth extractions. In 1 patient who had previous palatal surgery, >1 arterial pulsation was seen on the palate but only 1 artery was noted to enter the GPF. We experienced difficulty in aligning the probe with the artery and maintaining the position during needle insertion in the initial 2 blocks. The US-guided blocks were successfully performed on first attempt in 2 of the 8 attempted blocks (n = 2/8). In 2 additional patients (n = 2/2) in whom US assistance was evaluated for greater palatine canal injections, a single-needle insertion was required to enter the GPF and all the blocks were successful. One patient found the probe to be bulky and painful.
Our study shows that US is reliable for identifying the GPF in cadaveric specimens. It is evident from anatomical studies regarding the variable location of the GPF that methods to consistently identify the foramen are sparse. US is one such modality that is used extensively for the performance of nerve blocks due to its ability to consistently identify anatomical structures. US can reliably identify the GPF as a defect in the hard palate as was evident from our cadaveric study. We used hemisectioned cadaveric heads to simulate live human heads. US probe insertion and block technique were performed in a manner making the technique applicable to live human subjects.
The GPN enters the oral cavity through the GPF, accompanied by the palatine artery and vein. Greater palatine nerve block is commonly performed by the dentist/surgeon using the second or third molars as the bony landmark against which the GPF is thought to be located. The local anesthetic is commonly injected into a bony depression adjoining the third molar. Alternately, the surgeon or dentist may infiltrate the entire mucosa of hard palate with the local anesthetic, in order to cover all the branches of the greater palatine nerve. Because the mucosa of the hard palate (except for the region around the GPF) is densely textured, these infiltrations may be painful, whereas injections close to the foramen may be less painful due to the presence of loose connective tissue.4
A possible explanation for the high failure rate in the cadaveric component of our study is probably related to the embalming of the cadavers. The loss of natural suppleness of the mucosal tissue in all of the scanned specimens probably contributed to higher failure rates causing difficulty in needle maneuverability within the hardened mucosal tissue around the GPF. US scanning of the foramen also identified anatomical variations in the direction of the opening, such as a bony ledge partially covering the opening of the foramen (Fig. 5). Human anatomical studies have shown the incidence of anterior opening of the foramen to be approximately 9% to 18%,5,6 which is higher than the finding of 1 in 16 cases in our study. This may be due to the lower number of specimens studied. US can identify the direction of the GPF, thereby identifying those cases where a greater palatine canal approach for greater palatine nerve block or maxillary anesthesia may be difficult to perform.
From the clinical component of the study, certain pertinent points need to be stressed. It is challenging to optimally insert and position even the smallest available US probe in the mouth. Probe insertion may be impossible if the oral configuration does not permit it as noted in one of our patients. The limiting factors may include a high arched palate, large tongue, limited mouth opening, and morbid obesity. Inability to see the artery may lead to difficulty in confirming the GPF as noted in one of our blocks where on identifying the GPF as a bony defect, arterial pulsations could not be visualized and hence the block was abandoned. The average block performance time in our study was 5 minutes with an average number of attempts being approximately 2, whereas a trained dentist may accomplish a successful block in <2 minutes.
The promise with US in this area might be in facilitating greater palatine canal blocks and sphenopalatine blocks with preprocedural scanning as noted in the 3 greater palatine canal injections in our study. US assistance for these blocks may result in fewer punctures in a sensitive area. The success rate of the technique might improve with increased experience and the availability of high-frequency probes with suitable curvature to apply over the curvature of the palate.
One of the limitations of our study was that it was not randomized. We needed to evaluate the feasibility before launching prospective randomized studies. The numbers of patients in this pilot study are too small to be analyzed statistically. Although not all clinically performed blocks were successful (6/8 attempted injections were successful), the success rate of GPN block can be increased with an increase in anesthetic volume because flooding the vicinity of the GPF with LA can also result in anesthesia without the needle entering the canal. Because the needle tip could not be visualized once it entered the canal due to the bony margins and the oblique approach of needle insertion, we could not be sure of the exact injection point within the canal, but injection outside of the GPF can be excluded if there is no visualization of the injectate using US. To our surprise, because of the patients’ IV sedation, gagging was not a major problem with probe insertion and movement. These technical issues related to real-time US guided injections suggest that US assisted blocks with pre-procedural localization of GPF may be better suited for clinical application. US may have other applications such as identification of occult submucous cleft palate in children7 and identification and grading of submucosal fibrosis with concomitant use of Doppler8 and may also help to prevent iatrogenic injuries after maxillofacial surgeries. GPF canal injections with US assistance could also be used to reduce bleeding during sinus surgery.9
To conclude, US can identify the GPF reliably in human cadavers and in live subjects and may be useful in guiding the performance of GPN blocks.
Name: Najmus Sahar Hafeez, MD.
Contribution: This author helped in the anatomical component of the study as a master’s degree student.
Name: Rakesh V. Sondekoppam, MD.
Contribution: This author helped in design and conduct of the anatomical and clinical conduct of the study, data analysis, and manuscript preparation.
Name: Sugantha Ganapathy, FRCPC, FRCA.
Contribution: This author is the senior investigator for the study and was crucial for study design, generation of the technique, conduct of the anatomical and clinical component of the study, data collection, data analysis, and manuscript preparation.
Attestation: Sugantha Ganapathy approved the final manuscript and is the archival author for the clinical component.
Name: Jerrold E. Armstrong, BSc, DDS, MSc, FRCD(C).
Contribution: This author helped identify patients and verify the onset of block and performed US-assisted blocks in 2 patients.
Name: Michael Shimiz, DDS, PhD.
Contribution: This author helped identify patients and verify the onset of block.
Name: Marjorie Johnson, PhD.
Contribution: This author helped in study design and facilitated the anatomic component of the study.
Name: Peter Merrifield, PhD.
Contribution: This author was involved in mentoring the master’s student.
Name: Khadry A. Galil, DDS, PhD.
Contribution: This author helped in generation of the idea and facilitated the conduct of the anatomic component of the study.
Attestation: Khadry A. Galil is the archival author.
This manuscript was handled by: Terese T. Horlocker, MD.
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© 2014 International Anesthesia Research Society
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