Ultrasound (US) has become a precise tool for performing trigeminal nerve blocks [1]. Numerous US-guided injection techniques targeting the pterygopalatine fossa—to block the maxillary nerve and the sphenopalatine ganglion—have been detailed [2]. This procedure is widely used for treating orofacial pain and providing local anesthesia for orofacial surgeries (e.g., cleft palate repair) [3].

In 2012, Sola et al. [4] reported a supra-zygomatic out-of-plane technique whereby the US transducer is placed caudal to the zygomatic process of maxilla. The needle is inserted perpendicular to the skin at the junction of the frontal and zygomatic bones above the zygomatic arch, and is then advanced until it touches the greater wing of the sphenoid bone. In 2013, Nader et al. [5] outlined a posterior-to-anterior infra-zygomatic in-plane technique for patients with facial pain. In this method, the US transducer is placed below the zygomatic process of the maxilla, just anterior to the condylar process of the mandible. The needle is guided in a posterior-to-anterior and lateral-to-medial direction, advanced in-plane with the transducer through the lateral pterygoid muscle, and then docked just above the lateral pterygoid plate adjacent to the pterygopalatine fossa. In a 2018 cadaveric study, Kampitak et al. [6] described an anterior-to-posterior infra-zygomatic in-plane approach. The needle is advanced in an anterior-to-posterior and lateral-to-medial direction through the lateral pterygoid muscle until it reaches the top of the lateral pterygoid plate. The needle is then withdrawn and redirected medially to hit the pterygopalatine fossa.

The previously described US-guided approaches have their limitations. For in-plane techniques, the needle trajectory is too steep, making it difficult to clearly capture the entire needle shaft (even with a curvilinear transducer). Additionally, when redirecting the needle cranially to target the pterygopalatine fossa, the needle can be obstructed by the acoustic shadow of the zygomatic process of maxilla. In the out-of-plane method, the needle is introduced from the supra-zygomatic region while images are obtained from the infra-zygomatic area. Since the needle entry point and the transducer’s footprint are separated by the zygomatic process, it remains challenging to track the needle path under US imaging.

To address the aforementioned limitations, we propose a novel US-guided injection technique - validated on a male cadaveric model. This involves an intra-oral route to target the pterygopalatine fossa and a (60° bent) 25-gauge needle to facilitate cranial advancement (Fig. 1). During the injection, it is crucial that the patient’s mouth remains open, either voluntarily or with the assistance of a mouth opener, to prevent the coronoid process of the mandible from blocking the view of the needle trajectory. The needle is inserted posterior to the third molar teeth (or second teeth depending on the presence of the third molar teeth), to puncture the oral mucosa, and is then advanced cranially towards the zygomatic process of maxilla.

Figure 1. The illustration demonstrates the use of a bent needle to perform an ultrasound-guided transoral pterygopalatine fossa block (left subgraph: lateral view; right subgraph; frontal intra-oral view). Colored rectangles indicate the transducer position for Fig. 2 and 4.

A curvilinear US transducer is placed in the horizontal plane caudal to the zygomatic process of maxilla [7]. The needle appears as a dot inside the anterior edge of the lateral pterygoid muscle (Figs. 2, 3). If the majority of the needle shaft is visible, it indicates that the needle is pointing posteriorly instead of cranially. This orientation should be adjusted to make the needle perpendicular to the transducer footprint, similar to an out-of-plane approach. By moving the dot to the center of the screen and rotating the transducer to the coronal plane to capture the entire needle shaft, the injection transitions to an in-plane approach. The needle can then be adjusted to advance between the outer surface of the sphenoid bone and the deep fascia of the lateral pterygoid muscle, substantially reducing the intra-muscular spread of local anesthetic (Figs. 4, 5). Before administering the injectate, it is essential to activate the power Doppler mode to verify the location of the maxillary artery. In our cadaveric model, 5 mL of methylene blue was injected using the described method whereby the pterygopalatine fossa was successfully stained [8] (Fig. 6).

Figure 2. With the transducer placed horizontally and caudal to the zygomatic process, the needle appears as a dot (black arrowhead) within the lateral pterygoid muscle (LPM). The transducer position corresponds to the blue rectangle to Fig. 1. MAX: maxilla, LPP: lateral pterygoid plate, MAS: masseter muscle, TEM: temporalis muscle.

Figure 3. The cadaveric horizontal cross-section resembles the scanning plane when the transducer is placed horizontally and caudal to the zygomatic process. Star represents the out-of-plane view of a needle. Images adapted from cadaveric images obtained via the Visible Human Project^® of the National Library of Medicine. Excerpts from these images, featured in the VH Dissector, are used with permission from Touch of Life Technologies Inc. TEM: temporalis muscle, LPP: lateral pterygoid plate, LPM: lateral pterygoid muscle.

Figure 4. When the transducer is rotated to the coronal plane, the entire needle trajectory (black arrowheads) can be visualized near the sphenoid bone (SPE) within the lateral pterygoid muscle (LPM). The transducer position corresponds to the red rectangle to Fig. 1. TEM: temporalis muscle.

Figure 5. The cadaveric coronal cross-section resembles the scanning plane when the transducer is placed perpendicular to the zygomatic process. Arrow represents the in-plane view of a needle. Images adapted from cadaveric images obtained via the Visible Human Project^® of the National Library of Medicine. Excerpts from these images, featured in the VH Dissector, are used with permission from Touch of Life Technologies Inc. TEM: temporalis muscle, LPM: lateral pterygoid muscle, SPE: sphenoid bone.

Figure 6. In the cadaveric model, the zygomatic process was removed. The methylene blue dye (black arrowheads) is seen to stain the pterygopalatine fossa. SPE: sphenoid bone, MAX: maxilla.

Compared to the existing US-guided techniques [2], our approach has a notable advantage. As the needle advances from caudal to cranial along the outer surface of the sphenoid bone, the needle shaft remains parallel to the transducer footprint in the coronal plane. This orientation ensures that the needle is clearly visible, thereby preventing collateral neurovascular damage. The potential disadvantages of our proposed method are the patient's uneasiness during the procedure and the difficulty in visualizing the initial needle entry point due to the intra-oral path. Future cadaveric studies are needed to verify that the injectate can spread as predictably as in our pilot cadaveric model. Additionally, human studies are necessary to determine if this novel technique is clinically more effective than the former US-guided injection methods targeting the pterygopalatine fossa.

The pictures of the cadaveric injection model were created using donated bodies with the approval of the Department of Anatomy, Faculty of Medicine, Chung Shan Medical University, Taichung, Taiwan. The authors sincerely thank those who donated their bodies to science so that anatomical research could be performed. The results of such research have the potential to increase mankind's overall knowledge, which could improve patient care. Therefore, these donors and their families deserve our highest gratitude.

Data sharing is not applicable to this article as no datasets were generated or analyzed for this paper.

No potential conflict of interest relevant to this article was reported.

This study was funded by the National Taiwan University Hospital, Bei-Hu Branch; Ministry of Science and Technology, Taiwan (MOST 106-2314-B-002-180-MY3 and MOST 109-2314-B-002-114-MY3) and National Science and Technology, Taiwan (NSTC 112-2314-B-002-134, NSTC 113-2314-B-002-208-MY2 and NSTC 113-2314-B-002-209-MY2).

Ke-Vin Chang: Writing/manuscript preparation; Jui-An Lin: Writing/manuscript preparation; To-Jung Tseng: Writing/manuscript preparation; Cheng-Wei Hsu: Writing/manuscript preparation, Investigation; Tzu-Ruei Liao: Investigation; Wei-Ting Wu: Investigation; Levent Özçakar: Supervision.

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