The infraorbital nerve (ION) derives from the maxillary nerve and emerges from infraorbital foramen (IOF) on the maxilla bone [1]. This nerve provides mid-facial sensation around lower eyelids, lateral side of nose, nasal septum, and upper lips [2, 3]. The ION block is used for regional anesthesia in many procedures including maxillofacial, eye, nose, and dental surgeries [4-7]. Moreover, it is beneficial in treating intractable infraorbital neuralgia [8, 9] and reducing postoperative pain [5, 10, 11]. Localization of IOF is essential to perform the ION block. In addition to IOF, knowledge of accessory infraorbital foramen (AIOF), a variation of IOF, is also important because neurovascular structures emerging from AIOF, accessory infraorbital nerve (AION), and accessory infraorbital artery (AIOA), were associated with structures from IOF [12]. The infraorbital margin was widely used to predict the location of IOF. The location of IOF was described to be 8–10 mm below the infraorbital margin in the mid-pupillary line [13]. However, this distance varied between studies and could range from 3.2–13.2 mm [14, 15]. There was a failure rate of 17% when performing the ION block by this method of IOF identification [16]. Previous studies examined various bony and soft tissue landmarks to identify the location of IOF and AIOF, for example, the piriform aperture, zygomaticomaxillary suture, ala nasi, lateral palpebral commissure and lacrimal caruncle [17-23]. These studies observed landmarks which can be examined either on skulls which cannot be palpated on the face of patients or soft tissue landmarks that cannot identified on skulls and some of them were not applicable in practice.

Therefore, in this study, we aimed to determine the location of IOF and AIOF using bony landmarks that can be easily palpated both in skulls and soft tissue which are the anterior nasal spine (ANS) and the lowest point of the zygomaticomaxillary junction (Z). Prevalence of AIOF, anatomical relationship between IOF and AIOF, and accuracy of the proposed predictive method in cadavers were assessed.

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A total of 216 human dry skulls and 15 embalmed cadaveric heads were obtained from the Department of Anatomy, Faculty of Medicine, Chulalongkorn University, Thailand. Samples with facial distortions were excluded. Skulls of undetermined age and sex were obtained, the general size and architecture, supraorbital ridges, mastoid process, and occipital protuberance were used for sex determination of skulls. Male skulls are larger in size, have more prominent supraorbital ridges and occipital protuberance with a long and broad mastoid process [24-26]. To avoid bias from one observer, two observers examined all skull separately and a consensus was made if there was any conflict. Therefore, with this process, 153 male and 63 female skulls were determined. This study was approved by the Faculty of Medicine, Chulalongkorn University IRB committee (IRB no.698/64). Data collection was performed cautiously with a structured procedure. This study had two major steps: analysis of skulls and cadaveric dissection.

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Localization of IOF is essential to achieve anesthesia of the mid-facial region when performing the ION block. In addition to IOF, understanding the AIOF is also important because the AION emerges from this foramen and it is associated with the ION [12]. Although numerous studies examined various landmarks to identify the location of IOF and AIOF, the results varied between studies. We aimed to determine anatomical landmarks for more precise localization of IOF and AIOF by using easily palpable landmarks both in bone and soft tissue. Therefore, the line between ANS and Z was taken into consideration. Moreover, we introduced a direct identification of the bony landmarks and measurement by imaging software under the stereoscopic microscope. The location of IOF was predicted by using the ratio of distance from ANS to the intersecting point of the vertical line from IOF to line A (distance D) and distance A (ratio D:A) as the predicted horizontal distance, and the predicted vertical distance from the middle-upper edge of IOF to line A (distance B). We found that the mean ratio D:A was 63.35%±3.90% which was about two-thirds of distance A and predicted distance B was 15.14±1.99 mm. For localizing the AIOF, we used the ratio of the distance from ANS to the intersecting point of the vertical line from AIOF to line A and the distance A (ratio E:A) which was 51.8%±5.9% (about half of the distance A) and the predicted vertical distance from the middle-upper edge of AIOF to line A (distance C) which was 19.34±3.36 mm. There were no statistically significant differences between sex and sides, therefore, we can use the same value in both sex and sides. We analyzed the accuracy of our predictive method by confirming locations of IOF and AIOF in cadavers. The result showed that the prediction of IOF was 50% accurate in cadavers. Some of the predicted IOFs were found to be inferior and lateral to the exact IOF. In cadaveric dissection, only two AIOFs were identified. Therefore, there might be too little information to draw a conclusion. However, the mean distance error between predicted and exact foramina ranged from 1–2 mm which is small and might not be affected by palpation with fingertips in clinical situations. It can be assumed that this predictive method is accurate and clinically applicable for locating IOF and AIOF.

This study provides detailed information about the AIOF. Prevalence of AIOF was 19.91% in dry skulls and 6.67% in cadavers which was consistent with the systematic review describing that the prevalence of AIOF in skulls and cadavers ranged from 0.8% to 27.3% [27]. On the other hand, later studies found higher prevalence of 32.1% [28] and 35% in dry skulls [29]. These cadaveric results were also inconsistent with later studies which found the frequency of AIOF to be 36.7% [12]. We found that AIOF was commonly a single foramen. There were double AIOFs in 2.4% of skulls and no triple foramen was found. Previous studies reported that AIOF was mostly a single foramen [27, 29, 30] and more common on the left side [20, 27-29, 31-33] similar to our findings. There were 50% of skulls that had bilateral AIOFs in the present study which was higher than previous reports of AIOF occurring bilaterally ranging from 9.1% to 25% [27, 28, 34, 35]. These differences can be due to differences in population, race, and sample size.

The observations made in this study were consistent with previous reports regarding that most of the AIOFs located superomedial to IOF [27, 32, 33]. However, there were three AIOFs that located in the inferolateral position which was different from previous studies. According to Rai et al. [36] the mean distance from AIOF to IOF was 9.79 mm, whereas, Tezer et al. [33] found that the mean distance between AIOF and IOF was 3.95±1.6 mm. In this study the mean distance from IOF to AIOF was 6.10±2.20 mm in horizontal and 4.10±2.06 mm in vertical direction. These discrepancies could be attributed to the method of measurement. The mean distances were measured in two directions separately in our study which differed from previous studies that measured the shortest distance between the IOF and AIOF regarding the direction of the distance.

Some previous studies measured the distance between IOF and ANS, but the method was different from this study. Agthong et al. [20] measured the shortest distance from ANS to the center of IOF (right, 34.1±0.2 and left, 34.3±0.2 mm) while Chrcanovic et al. [18] measured the shortest distance from the medial-inferior wall of IOF to ANS (32.38±2.61 mm). In this study, we measured the distance in two directions, the perpendicular distance from the middle-upper edge of IOF to line A and the horizontal distance from ANS to the intersecting point of the vertical line to line A. This predictive method is more accurate and convenient in practice than measuring the shortest distance from one landmark in which the angle must be considered.

Our approach provides an easy and accurate way for localizing the IOF and AIOF. Clinicians can use this method as an approximation for performing ION block by palpating bony prominences of ANS at the uppermost part of the philtrum at the level of the nostrils of the patient and the lowest bony point of the patient’s cheek, then drawing a line between these points (Fig. 4A). Marking at the medial two-thirds of that line (Fig. 4B) and drawing perpendicular with and above the line approximately 15 mm (Fig. 4C), to identify IOF (Fig. 4D). Like IOF, clinicians can mark the point in the middle of the line between the ANS and the lowest bony point of the patient’s cheek (Fig. 5B) then measure above that point at approximately 19 mm (Fig. 5C) where AIOF is suspected to be (Fig. 5D).

This study has limitations that should be recognized. Though we identified the sex of each skull by using sex identification criteria, there is no available record of ethnicity, sex and age of skulls which may affect the evaluated results. Moreover, we could not compare our method with the standard method for identifying the IOF in cadavers since an original pupil position in cadavers had changed and could not be identified accurately. For further consideration, investigating the accuracy of this method in patients who underwent the ION block and comparing the results with the standard method should be performed.

In conclusions, we identified the location of IOF and AIOF using ANS and the lowest point of the Z as reference points. The results of this study provide detailed information of foramina, their relationship and generate the predictive method for localization of IOF and AIOF which is 50% accurate with a mean distance error of 1–2 mm. The proposed method could be an alternative approach and particularly useful in clinical settings.

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