The orbit harbors narrow channels through which arteries, veins, and nerves pass. These structures and channels are important for both clinical and anatomical studies. Among these, MC and WF have been relatively less studied and characterized. The aim of the current study was to provide data regarding the distances between these rare structures and other anatomical landmarks as well as the incidences of these structures, which could be especially useful for surgeons working in the field of orbital surgery. Majority of these channels are thought to close during the development of the orbit, but there is little information regarding their clinical significance in adult skulls. Also, further studies may be warranted to determine whether age and gender affect the development of these channels. In addition, there is no information on possible pathologies of the nerve or vascular structures that pass through these channels between the anterior and posterior parts of the orbit, which may emerge in the presence or absence of these channels.
MC Incidence. Earlier, Macchi et. al (2016) reported that the MC is a somewhat overlooked anatomical entity found in 8.74% of orbits [23]. Chockalingam and Babu (2020) found MC only in 4 (7.8%) of the 32 dry human skulls; 3 were found on the right orbit and 1 was found on the left [9]. Hizay and Sindel (2019) found MC in 20 of the 92 dry human skulls, unilaterally. The authors observed MC in 11 (11.9%) skulls on the right, and in 9 (9.7%) skulls on the left [17]. Regoli and Bertelli (2017) identified MC in 8% of the orbits they examined [27]. We identified the presence of MC in 10 of 153 (6.5%) dry human skulls: 4 (2.6%) of these were on the right, 5 (3.2%) were on the left, and one (0.06%) was bilateral. In some previous studies, bilateral canals were not found at all, or the majority of unilateral canals were localized on the right side. On the other hand, there was a more balanced distribution of MC in terms of the side in the present study. It remains to be seen whether these differences between published reports are related with the ethnic origin or developmental factors. We also measured the distance between MC and frontozygomatic suture and the supraorbital notch to provide some information for surgical procedures involving this area.
WF incidence. The size and morphological characteristics of WF and adjacent structures play an important role in determining the most appropriate operative techniques to approach skull base or for gender determination in forensic medicine. Preoperative assessment of the WF for determining the operative safe zone needs to include correlations between surgical and morphometric parameters. Also, a detailed knowledge of the bony anatomy of this anatomical area may improve the general safety of the surgery.
Until now, WF has been studied only scarcely, research on its incidence has never been done, and little is known regarding the structures transmitted through it [5, 28]. Furthermore, the morphology and size of this foramen appear to be highly variable. Based on a groove running backwards into the cavernous sinus, Warwick (Warwick, 1951) hypothesized that it could convey an inferior ophthalmic vein [28].
Chockkalingam and Babu (2020) failed to observe any WF in 32 dry skulls studied [9]. Bertelli (2014), in his study of 943 skulls, reported a WF incidence of 0.74%. In that study, WF was never bilateral, with 8 and 6 on the left and right orbits, respectively [3]. Also in another study, 0.6 to 0.7% of the skulls examined by Regoli and Bertelli (2017) were found to have WF [27].
WF incidence was higher (2.17%) than previous reports in the study by Araya et al., (2018) who examined dry skull specimens of Chilean origin. In that study, there was one case with right sided WF, one with left sided WF, and one with bilateral WF [2]. Hizay and Sindel (2019), in their study of 92 skulls, found the frequency of WF to be 13.0%. All WFs were unilateral, with 7 (7.6%) on the right, and 5 (5.4%) on the left [17]. Similarly, in another previous study, WF was bilateral only in one case [5]. In our study, 11 human dry skull bases were examined for the presence of WF. Although the number of bone samples was small, we observed a unilateral left canal in 2 (18.2%). This may have been due to ethnic differences, developmental factors, or just coincidental.
Morphometric Measurements. We also performed morphometric measurements in our study to provide data that can aid in surgical procedures. For this purpose, several anatomical reference points were used, including the frontozygomatic suture, supraorbital notch, foramen rotundum, superior orbital fissure, and anterior and medial clinoid processes. Groups were compared in terms of these measurements and no statistically significant difference was found (p > 0.05).
Metoptic Canal Measurements. The distances between MC and frontozygomatic suture reported in the study by Hizay and Sindel (2019) were 29.51 ± 11.20 mm and 27.98 ± 5.67 mm on the right and left sides, respectively [17]. The corresponding figures reported by Chockalingam and Babu (2020) were 5.1 and 6.5 cm [9]. In the present study, the measured distance was 49.62 ± 4.30 mm on the right, and 52.47 ± 3.05 mm on the left side, similar to the results of Chockalingam and Babu (2020), although they were slightly lower. These differences may reflect ethnic variations.
The distance between MC and supraorbital notch reported by Hizay and Sindel (2019) was 40.62 ± 8.87 mm on the right, and 42.24 ± 12.77 mm on the left vs. 5.3 cm and 6 cm in another study by Chockalingam and Babu (2020) [9, 17]. In the present study, this distance was 50.83 ± 4.38 and 54.02 ± 1.84 mm on the right and left side, respectively, in general agreement with those of Chockalingam and Babu (2020).
Warwick’s Foramen Measurements. In Hizay and Sindel’s study (2019), the measured distance between WF and foramen rotundum was 7.27 ± 2.47 mm on the right and 6.18 ± 1.50 mm on the left side [17]. This distance was only measured on the left side in our study (1.86 ± 0.25 mm). In the above-mentioned study, the distance between WF and superior orbital fissure was 5.44 ± 1.85 mm on the right side and 5.96 ± 2.47 mm on the left side [17]. In our study, only a left-sided measurement was possible (2.37 ± 0.42 mm).
Surgical Importance of the foramens. Serious ocular injury has been shown to occur, but there is conflict about the rate of occurrence and localization [11]. The middle fossa injuries can lead to serious cranial nerve injuries when they involve the foramina through which cranial nerves are transmitted [16, 21]. Attention has already been drawn to the structural variations and differences and relative strength of the walls of the orbit and their relationship with the intracranial structures such as WF. Fractures of the posterior 1/3 of the orbit are uncommon. The posterior orbit contains a number of significant and sensitive structures, including the optic nerve, ophthalmic vessels, and ocular muscles as well as their motor nerves, which may pose significant challenges for the surgeon [18]. The most common surgical procedures used to treat orbital lesions involve the lateral wall approach, trans-ethmoidal approach, trans-maxillary approach from the orbital floor, and transcranial approach, in which the orbital roof is removed [24]. It has been previously reported that injury to arteries passing through the small foramen during orbital surgery prolongs the duration of surgical operations and puts structures passing through the superior orbital fissure at risk. During surgery, deep dissection of the lateral wall of the orbit can often lead to bleeding, which can be severe [1, 8]. Postoperative complications in orbital surgery include visual disturbances, enophthalmos, facial asymmetry, and orbital dystopia. Some of these complications may result from severe ischemia [23]. It may also be important to assess the WF prior to surgical interventions for cranial tumors, herniation due to increased intracranial pressure, and trauma in the orbit or skull base. Also, fractures of the region may occur in isolation or can be independently associated with fractures of the adjacent bone complex such as superior orbital margin, the zygoma, sphenoid or temporal bones. Orbital fractures can be generally divided into four types: zygomatic complex fractures, isolated fractures of the orbital rim, isolated fractures of the orbital walls and complex comminuted fractures. Fractures of the orbit should be approached with caution and proper management is essential to prevent aesthetic and functional complications. The purpose of surgery is to identify and characterize blunt ocular and periocular injuries, and to identify orbital cranial nerve lesions and lesions of neighborhood structures such as WF as well as other associated complications. A consensus-based classification of injuries involving this region may assist in developing surgical reconstruction strategies and may allow meaningful comparisons between surgical outcomes. Consultation with ophthalmic, neurosurgical, plastic and maxillofacial surgeons can be of crucial importance for demonstrating anatomical location and accompanying lesions as well as for proper treatment and follow-up. A good knowledge of the size and morphological characteristics of WF and the adjacent structures is crucial for establishing the safest and most appropriate operative techniques in neurosurgery or for gender determinations in forensic medicine. During the preoperative evaluation of WF for the operative safe zone, the correlation between both surgical and morphometric parameters of WF need to be taken into consideration. Detailed knowledge of the bony anatomy of this region is of great importance in terms of the safety of the surgical procedure.
In this study, the distances between some rarely observed anatomical structures and reference points within the orbit were analyzed. Despite limitations in the sample size and measurement techniques, we believe that the data generated may be of use for future studies. Awareness of the presence of orbital structures such as small channels will have clinical implications and shed light to further studies with more detailed hypotheses and robust contributions to science. Confirmation of the existence of these channels in other regions and ethnical groups requires the examination and analysis of skulls from other geographical regions. Morphological and morphometric characteristics of the MC and WF may be useful for orbital surgeons who are dealing with a variety of procedures such as tumor resection, decompression, or post-traumatic orbital or anterior skull base reconstruction. We believe that the data generated in this study may be valuable for researchers in the field of anatomy, orbital surgery, and neurosurgery.