The AEF and PEF are two foramina located intra-orbitally on the medial orbital wall along the frontoethmoidal suture (Fig. 1) [9]. These openings allow vessels and nerves from the orbit to the ethmoid bone [10]. The AEF transmits the anterior ethmoidal artery (AEA), the anterior ethmoidal nerve (AEN), and occasionally the anterior ethmoidal vein (Fig. 2) [9]. Similarly, the PEF transmits the posterior ethmoidal artery (PEA) to the posterior ethmoidal nerve (PEN), and the posterior ethmoidal vein [9]. The anterior and posterior ethmoidal arteries emerge from the ophthalmic artery in the orbit, which originates in the cranial cavity as a branch of the internal carotid artery [10]. The AEA exits through the AEF into the cranial cavity, giving off its anterior meningeal branch and continuing into the nasal cavity, branching to supply the medial and lateral walls [10]. Then, it passes between the nasal bone and lateral nasal cartilage, terminating as the external nasal branch, where it supplies the skin on the external nose and neighboring tissues [10]. The PEA, which tends to be thinner than the AEA, exits through the PEF, branching to supply the upper aspects of the medial and lateral nasal cavity walls and the ethmoidal cells within the nasal cavity [4, 10]. The AEA is the more constant artery, whereas the PEA is absent approximately 20% of the time [11]. The anterior and posterior ethmoidal nerves originate from the nasociliary nerve, which is a branch of the ophthalmic nerve (V1) (Fig. 2). The AEN travels with the AEA exiting through the AEF to supply the anterior cranial fossa and into the nasal cavity, where it branches to supply the medial and lateral walls. It continues to the external nose, terminates as the external nasal nerve, and supplies the skin on the inferior half of the nose [10]. Similarly, the PEN travels with the PEA exiting through the PEF [10]. It supplies the posterior ethmoidal cells and the sphenoidal sinus [10].

The EF serve as key topographical landmarks for surgeries near or along the medial orbital wall. Surgeons depend on anatomical ratios to guide their operations around this region’s delicate and important structures. The surgical guideline used to navigate this area is known as the “24-12-6” rule. This guideline is based on the approximate distance in millimeters between the anterior lacrimal crest (ALC) to the AEF, from the AEF to the PEF, and from the PEF to the optic canal (OC), respectively [1, 12, 13]. Although useful, this surgical guideline is limited in effectiveness due to the significant anatomical variations in this area. Its accuracy has been investigated in various studies. Damage to the vessels and nerves that pass through the EF could result in severe consequences such as blindness, cerebrospinal infections, extraocular muscle paralysis, orbital hematoma, and intense bleeding [2, 13]. Detailed knowledge of the medial orbital plate, EF, and surrounding vessels and nerves is critical to avoid injury to this area’s fragile and important structures during surgery. A review of reports from the literature describing the variations in EF and surrounding structures will be discussed below.

In addition to the number of EF, the anatomical relationships and geometry of the EF and the medial orbital wall have prompted considerable interest among researchers. Numerous studies have assessed the accuracy of the distances specified in the “24-12-6 mm” surgical guideline. Recent studies from various populations defined ratios of “22-12-9 mm”, “24-10-7 mm”, “25-11-6 mm”, “26-14-12 mm”, “23-10-4 mm”, “24.7-13.8-7.5 mm”, “25.1-14.8-8.3 mm”, and “22-14-10 mm” [1, 5, 13, 15, 16, 18, 19]. The variations in these ratios may be partly attributed to genetic differences among studied populations and partly to variations in the criteria used for classifying anatomical landmarks [1]. A study by Hester et al. [13] investigated the applicability of the current surgical guideline among Hispanic and Caucasian orbits. They analyzed 79 orbits, 70 of which were European and nine which were Hispanic, from 52 cadavers [13]. In an analysis using all cadavers from the study, they found a significant difference between the measured ALC-AEF, AEF-PEF, and PEF-OC distances compared to their respective distance defined by the surgical guideline [13]. Furthermore, they assessed if the applicability of the surgical guideline depended on ethnicity. They found that there were significant differences between the measured ALC-AEF, AEF-PEF, and PEF-OC distances for both the European and Hispanic samples compared to the distance defined by the surgical guideline [13]. Thus, the surgical guideline was not an ideal navigational tool for either ethnic cadaver population.

The surgical ratio and its distances have also been compared between different ethnicities and races. Hester et al. [13] found no significant differences in the ALC-AEF, AEF-PEF, and PEF-OC measurements between the Hispanic and European samples. Similarly, McQueen et al. [19] found no significant differences in these measurements between black and white cadavers. Another study found no significant differences in EF distances between African American and Caucasian orbits [20]. In contrast, one study by Piagkou et al. [1] found a substantial difference in these measurements between different ethnicities.

Most studies found no significant differences in the surgical ratio distances between male and female samples, with a few exceptions [1, 13, 17, 19, 20]. In the study by McQueen et al. [19], the only measurement that was significantly different between sexes was the PEF-OC. The PEF was considerably closer to the OC in females compared to males [19]. Mehta and Perry [20] and Piagkou et al. [1] found no significant difference in these distances between male and female orbits except for the ALC-AEF distance, which was significantly larger in males than females. Hester et al. [13] found a significant difference between sexes for the AEF-PEF measurement. The symmetry of these distances between the left and right side orbits was analyzed, yielding varying results. Some studies reported no significant difference in these measurements between the left and right sides of the face [13, 17, 19]. However, Piagkou et al. [1] noted significant bilateral asymmetry in the ALC-AEF distance. Likewise, Cheng et al. [5] reported significant right-left asymmetry in the PEF-OC measurement.

In addition to investigating the influence of various factors on the distances in the surgical guideline ratio, research has been done to discover any correlations between these measurements. This is valuable information because any correlation that could help surgeons accurately estimate these distances could significantly improve surgical outcomes in this area. One study’s analysis illustrated a strong negative correlation between the AEF-PEF and PEF-OC distance [13]. This suggests that patients with a greater distance between their anterior and posterior EF may have shorter distances between their PEF and OC. This would have clinical implications, bringing the optic nerve closer to the surgical area than expected [13]. Overall, the diversity of results across different studies underscores the need for further research. More studies must be done to establish a more comprehensive understanding of the EF region.

Understanding the complex geometry between anatomical landmarks along the medial orbital wall is fundamental to protecting this area’s important structures. Along with the distances and structures described by the surgical guideline, other anatomical landmarks in the medial orbital wall region play a valuable role in guiding surgeons through this area. Understanding whether these anatomical relationships are influenced by sex, ethnicity, and sidedness may help reduce operative risks in this area. Various studies analyzed this region’s anatomical landmarks’ left and right symmetry. Cheng et al. [5] reported numerous measurements that had significant right-left asymmetry, including ALC-PLC, the frontal notch to the tip of the superior orbital fissure, the frontozygomatic suture to the origin of the infraorbital groove, and from the inferior orbital rim above the infraorbital foramen to the OC, to the end of the inferior orbital fissure, from the infraorbital foramen itself to the entry of infraorbital canal, and the end of the inferior orbital fissure. Conversely, other studies found no significant difference in distances between anatomical landmarks between the left and right sides of the face [4, 15].

Regoli et al. [4] analyzed the position of the EF in relation to the frontoethmoidal suture in 1,413 orbits. Of the 2,925 EFs counted, 61.3% were located at the suture, 38.15% were above the suture, and 0.54% of cases were below the suture. Likewise, Cheng et al. [5] evaluated the relative position of the AEF and the frontoethmoidal suture. Cheng et al. [5] had similar findings, reporting that in 29% of orbits, the AEF was located above the frontoethmoidal suture. The relationship of the EF to the frontoethmoidal suture described by Regoli et al. [4] and Cheng et al. [5] was echoed in other studies [19, 21].

Due to the diverse populations studied, there have been varying findings regarding the potential relationships between the anatomical geometry in this region and factors such as ethnicity, race, and sex. Interestingly, the study by Mehta and Perry [20] found no significant difference in EF distance between African American and Caucasian orbits but did find an increased incidence of the frontoethmoidal groove in African American and Inuit orbits compared to Caucasian orbits. Mueller and Bleier [14] analyzed differences in anatomical relationships between ethnicities and sexes. They found that the orbital floor length of Asian females was significantly shorter than that of males [14]. Additionally, their data suggests that the orbital length is considerably larger in African, Hispanic, and Middle Eastern skulls than in Caucasian and Asian skulls [14]. In a related fashion, Cheng et al. [5] found that orbital measurements were significantly more prominent in males than in females. It is important to note that only 15% of their orbits were female, limiting their findings.

Adult skulls have eight bones; each bone undergoes intramembranous ossification or endochondral ossification during development. Their borders contact each other by fibrous tissue called sutures, which are primarily intramembranous in origin. However, the frontoethmoidal suture is unique because it combines intramembranous and endochondral ossification [22].

The frontoethmoidal suture is formed by joining the frontal bone and ethmoid bone in the orbit. During the development, the frontal bone ossifies intramembranously from two primary and two secondary centers. The primary centers are located around frontal tuberosity in the 8th week of gestation, one on each side of the midline. The secondary centers for the nasal spine are in the 10th week [23]. Ossification extends superiorly to form half of the main part of the bone, posteriorly to form the orbital part, and inferiorly to form the nasal parts [24, 25]. On the other hand, ethmoid bone ossifies in the cartilaginous nasal capsule from three centers: one in the perpendicular plate and one in the bilateral orbital plates. The two orbital plates appear between the fourth and fifth months in utero and extend into the ethmoidal conchae [23].

The anatomy of EF and ethmoidal artery have been studied in cross-sectional images due to their clinical importance, especially in paranasal sinus surgery and epistaxis. McDonald et al. [26] investigated how the AEA and AEF could be reliably identified on routine coronal sinus computed tomography (CT). In the study, two observers identified at least one AEF in 95% of cases bilaterally and in the remaining 5% unilaterally. The AEF was at the skull base level in 72% of the sides studied and below the skull base level, with pneumatized and opacified cells above the remainder. The AEA was visualized in 33% of the CTs [26].

Cankal et al. [27] examined the anterior ethmoidal canals (AECs) and posterior ethmoidal canals (PECs) in CTs of 150 patients and measured the length and angulation of the canals. The study found that AECs were identified as independent canals in 84% of patients and extended in ethmoidal roofs in 16%. Conversely, PECs were identified as separate foramen in 8% and extended in the ethmoidal roof in 92%. In the axial images, they measured distances from AEFs and PEFs to nasion and optic foramen. The mean distances of nasion-AEF and nasion-PEF were 29.8 mm and 46.3 mm. The mean distances of optic foramen-AEF and optic foramen-PEF were 19.7 mm and 6.7 mm. The average lengths of these canals were 4–12 mm (mean 8.2 mm) for the AEC and 2–13 mm (mean 7.6 mm) for the PEC. Additionally, they observed that the AEC left the orbit at an angle of 12°–50° directed anteriorly. The angle was directed superiorly at 0°–45° and inferiorly at 0°–20° on coronal images. The angulation of the PEC was reported to be less than the AEC and was identified as 0°–18° anteriorly, 0°–28° posteriorly in transverse sections, and 0°–22° superiorly and 0°–12° inferiorly in coronal sections. Interestingly, the study reported the frequency of AEC and PEC identification in axial and coronal sections according to the section thickness. PEC had the lowest frequencies to be identified in axial and coronal 3 mm thickness, 56% and 52%, respectively [27].

Mason et al. [28] studied the presence of accessory/middle EF in 50 CT angiographies. In the 50 CT samples, 38% had evidence of right, left, or bilateral middle ethmoidal vessels or foramina. Overall, 26% of samples showed middle ethmoidal arteries (MEA) out of the 100 sides. Unilateral MEAs were more common than bilateral, and right sided were more common than left. They reported no evidence of multiple MEAs on a single side. The average distances from the MEA to the AEA and PEA were measured in the study. For the 9 MEAs on the left side, the average distance of MEA to AEA and MEA to AEA was reported as 11.77±2.09 mm and 11.40±2.34 mm, respectively. For the 17 MEA on the right side, the distance between the MEA and AEA was 10.80±3.13 mm; the MEA and PEA distance was 11.21±3.57 mm [28].