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. Author manuscript; available in PMC: 2017 Jan 1.
Published in final edited form as: J Craniofac Surg. 2016 Jan;27(1):234–237. doi: 10.1097/SCS.0000000000002332

Orientation of the foramen ovale: an anatomical study with neurosurgical considerations

Matthew J Zdilla 1,2,*, Scott A Hatfield 1, Kennedy A McLean 1, Jillian M Laslo 1, Leah M Cyrus 1, H Wayne Lambert 3
PMCID: PMC4706813  NIHMSID: NIHMS733714  PMID: 26703059

Abstract

Unsuccessful cannulation of the foramen ovale (FO) continues to occur with both fluoroscopic technique and technique utilizing CT paired with navigational technology. Despite advances in stereotactic neurosurgical imaging and technique, anatomical variation of the FO occasionally prevents successful cannulation. Morphometric study of the FO has been limited to length, width, and area parameters; therefore, this report analyzed the orientation of the FO. One hundred thirty-nine crania (235 foramina ovalae) were photographed and assessed digitally by ImageJ software (NIH). Foramina were fit with a best fit ellipse. For orientation, the midsagittal plane was located by bisecting the basilar process of the occiput, the coronal plane was identified as perpendicular to the midsagittal plane. The angles between the major axis of the best fit ellipse of the FO and the midsagittal and coronal planes were measured. The angle formed between the major axis of the best fit ellipse of the FO and the coronal plane averaged 35.43° ± 9.74° (Mean ± SD) on the left and 36.47° ± 7.60° on the right. The angle formed between the major axis of the best fit ellipse of the FO and the sagittal plane averaged 54.57° ± 9.74° on the left and 53.53° ± 7.60° on the right. No significant difference was found between FO orientation among the sexes. Understanding the orientation of the FO may aid in stereotactic neurosurgical planning and successful cannulation of the FO.

Keywords: anatomic variation, cannulation, skull base, stereotactic surgery, trigeminal neuralgia

Introduction

The foramen ovale (FO) of the sphenoid bone is located anteromedial to the foramen spinosum (FS) and posterolateral to the foramen rotundum.1 The morphology of the FO has been described by ambiguous terms such as “almond”, “D shape”, “elongated oval”, “oval”, “round”, semicircular”, “slit”, “pear”, and “truly oval”.1–8 The border of the FO may be irregular and bony spurs, spines, and tubercles have been documented to project into the FO.2–4,7,9 Also, little morphometric data has been reported aside from length, width, and area of the FO.

Differences in the morphology of the FO have been reported to contribute to difficulties in the cannulation of the foramen.10,11 The cannulation of the FO has been performed for electroencephalographic analysis of the temporal lobe among patients undergoing selective amygdalohippocampectomy,12 percutaneous biopsy of parasellar lesions,13–15 and, more commonly, for the treatment of trigeminal neuralgia.16–19

Percutaneous ballon compression, radiofrequency rhizotomy, and glycerol rhizotomy, performed via transovale cannulation for the treatment of trigeminal neuralgia, have been facilitated by a variety of localization modalities. Fluoroscopy is a particularly common method for transovale cannulation. However, the procedure has been reported to have a mean radiation dose per patient of 1137.18 mGy cm2, ranging from 639.6 mGy cm2 to as much as 1738 mGy cm2.20 Therefore, the radiation required to perform this operation is significant for the surgeon in addition to the patient. Likewise, a number of reports utilizing fluoroscopic guidance have noted complications due to improper cannulation of the FO.21–27 Moreover, fluoroscopy provides poorer visualization of the FO when compared to CT.11 Therefore, a number of studies have more recently utilized CT in conjunction with navigation systems for the cannulation of the FO.11,28–32 However, despite improved resolution and visualization of trajectory, even CT paired with navigation technology has proved unsuccessful in cannulating the FO in 5.17% of patients (9 of 174) because of suspected variation in FO morphology.11

The FO is important in numerous neurosurgical procedures; however, surgical confusion with regard to the cannulation of the FO is still encountered despite advances in methodology. Therefore, there is a need for more morphometric information regarding the FO, aside from that which has already been described (i.e., length, width, area). The aim of this study is to describe the angle at which the major axis of the FO’s best fit ellipse intersects with the midsagittal and coronal planes.

Materials and Methods

The study utilized direct observation of foramina from 139 dry adult human crania of undetermined age-at-death, sex, and race held in the anatomical collections of West Liberty University, West Virginia University School of Medicine, Franciscan University of Steubenville, Ohio University – Eastern, Bethany College, John Marshall High School, California University of Pennsylvania, and Washington & Jefferson College. FO which were confluent with the FS were excluded from the study.

Photography of the middle cranial fossae was performed with a digital camera (Canon PowerShot SX50 HS, 12.1 Megapixel). Photographs were then examined via the built-in functions of ImageJ software (NIH). First a line traveling through the midline of the basilar process of the occiput was identified. The angle between the line and the bottom of the photograph was used as a reference to re-orient the photograph by rotating the photograph either clockwise or counterclockwise depending on the orientation of the photograph.

Each individual FO was then identified and selected to be fit with a best fit ellipse. The built-in functions of ImageJ identified the major axis of the best fit ellipse and also measured the angle made by the major axis and a line parallel to the bottom of the photograph (i.e. the coronal plane) (Figure 1). Using geometry, the angle between the major axis of the best fit ellipse and the midsagittal line was determined and recorded.

Figure 1.

Figure 1

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Cranial view of the left middle cranial fossa in a dry human skull. The sagittal and coronal planes are marked by black lines. The midsagittal plane, used to orient the photograph, was identified by bisecting the basilar process of the occiput (located at the junction of the two black lines). The coronal plane was identified by drawing a line perpendicular to the midsagittal plane. The red oval indicates the best fit ellipse to the corresponding foramen ovale. The blue line is continuous with the major axis of the best fit ellipse. A: The angle formed between the major axis of the best fit ellipse of the foramen ovale and the coronal plane. The average angle calculated from the 121 foramina on the left side was 35.43° ± 9.74° (Mean ± SD). The corresponding angle on the right side of the skull (mirror image) was determined to be 36.47° ± 7.60°. B: The angle formed between the major axis of the best fit ellipse of the foramen ovale and the sagittal plane. The average angle calculated from the 121 foramina on the left side was 54.57° ± 9.74° (Mean ± SD). The corresponding angle on the right side of the skull (mirror image) was determined to be 53.53° ± 7.60°.

Because no demographic information existed for the crania, sexes were estimated using the morphometry of the foramen magnum from the photographs. The descriminant function: D=−15.109+0.158 × the perimeter of the foramen magnum, that has been applied with 67% accuracy was utilized in estimating the sexes of the skulls in order to estimate influence of sexual dimorphism on ovale orientation angles.33 Because the confidence in sexing a cranium as male is higher when the value of D is much higher than the decision value, and the confidence in female diagnosis is higher when the value of calculated D is much lower is much lower than the decision value, samples of males and females, determined by the descriminant scores with the most deviation from the D value, were compared.33

Recorded data was then assessed using GraphPad Prism statistical software, version 6.00 (GraphPad Software, La Jolla, CA, USA). Statistical methods utilized included the paired and independent samples t-tests. Graphical representation used in this report was also produced using GraphPad Prism software.

Results

A total of 235 FO were measured (121 left-sided foramina and 114 right-sided foramina). Both the left- and right-sided FO angles between the major axis of the best fit ellipse and the sagittal and coronal planes were normally distributed. The descriptive statistics regarding the angles formed between the planes are summarized in Table 1. Frequency distribution of the angles, illustrating the normal distribution, is shown in the histograms found in Figure 2 and Figure 3. A paired t-test, performed on the angles of the 113 paired foramina, did not reveal a statistically significant difference between the angles on the left side and the angles on the right side (t=1.298(112); p=0.197; 95% CI: −0.6898 to 3.311). Among the skulls most confidently estimated to be male or female, an independent samples t-test revealed no statistically significant difference between male and female foramen ovale orientation (t=0.02603(81); p=0.9793; 95% CI: −3.669 to 3.766) (Figure 4).

Table 1.

Angles formed between the major axis of a best fit ellipse of the foramen ovale and both the coronal and sagittal planes

Angle Side(s) N Mean (°) SD (°) SEM (°) Min (°) Max (°)
Major axis - coronal plane ∠ L 121 35.43 9.74 0.89 0.68 66.66
R 114 36.47 7.60 0.71 15.64 58.15
L and R 235 35.93 8.76 0.57 0.68 66.66
Major axis - sagittal plane ∠ L 121 54.57 9.74 0.89 23.34 89.33
R 114 53.53 7.60 0.71 31.85 74.36
L and R 235 54.06 8.76 0.57 23.34 89.33
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Figure 2.

Figure 2

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Histogram illustrating the normal distribution of foramen ovale orientation as indicated by the angles between the major axis of a best fit ellipse and the coronal plane. The majority of left-sided foramina were oriented at angle bin centers of 35° and 40° (23.14% and 23.97%, respectively). Most of the right-sided foramina were oriented at an angle bin center of 35° (29.82%).

Figure 3.

Figure 3

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Histogram, mirroring that of Figure 1, illustrating the normal distribution of foramen ovale orientation as indicated by the angles between the major axis of a best fit ellipse and the sagittal plane. The majority of left-sided foramina were oriented at angle bin centers of 50° and 55° (23.14% and 23.97%, respectively). Most of the right-sided foramina were oriented at an angle bin center of 55° (29.82%).

Figure 4.

Figure 4

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Sexual dimorphism comparison between foramen ovale orientation among the crania most confidently estimated to be male (n=41) and female (n=42). The independent samples t-test revealed no statistically significant differences in the orientations of male and female foramina (t=0.02603(81); p= 0.9793; 95% CI: −3.669 to 3.766).

Discussion

Most reports that document morphometric measurements of the FO are limited to length, width, and area contained within the foramen. While length, width, and area are important in understanding the structure of the FO, they offer no information regarding the orientation of the FO. This report offers novel information regarding the orientation of the FO by use of the angles between major axis of a best fit ellipse and the sagittal and coronal planes.

Some reports have documented the angles at which the foramen ovale is approached for cannulation.34–38 In 2013, Huo et al.36 described that “[t]he angle of introducing the cannula ranged from 15.17°–35.48° rotation to the midline with an average of 26.24° and 38.47°–51.89° angulation to the Reid line with an average of 46.09°.” In 2014, Huo et al.37 noted that “[t]he angle of introducing the cannula ranged from 15.32° to 35.48° rotation to the midline (average 25.18°) and 38.47°–51.89° angulation to the Reid line (average 46.17°).” Zhu et al.38 noted that “[t]he horizontal component of the angle between the needle axis and y axis should be more than 22 degrees in women and 20 degrees in men to avoid the injury of arteria meningea media.” Yao et al.35 noted that the angle formed by a line connecting the center of the foramen ovale with the medial edge of the trigeminal impression and the sagittal plane was an average of 6.62°. They noted that the angle formed by a line connecting the center of the foramen ovale with the lateral edge of the trigeminal impression was an average of 50.74°.35 Additionally, Yao et al.35 documented that the angle formed by a line connecting the center of the foramen ovale with the medial edge of the trigeminal impression and a line connecting the center of the foramen ovale with the lateral edge of the trigeminal impression averaged 44.12°. Pang et al.34 noted that the angle between the trajectory and coronal plane is 40.27 degrees for men and 37.31 degrees for women, the angle between the trajectory and the horizontal plane is 49.37 degrees for men and 52.26 degrees for women, and the angle between the trajectory and the sagittal plane is 3.78 degrees. Likewise, shape descriptors such as circularity and solidity have been utilized to objectively demonstrate the shape of the FO.39 Using data such as the aforementioned trajectory angles of cannulation and objective shape descriptor data, in conjunction with the data presented within this report, may help aid in stereotactic neurosurgical planning and transovale cannulation.

Acknowledgments

Funding: The work was supported by two West Liberty University Faculty Development Grants in addition to grant funding from the WV Research Challenge Fund [HEPC.dsr.14.13], West Virginia IDeA Network for Biomedical Research Excellence [P20GM103434], and NIH-NIAID [5K22AI087703].

The authors would like to thank West Liberty University, West Virginia University School of Medicine, Franciscan University of Steubenville, Ohio University – Eastern, Bethany College, John Marshall High School, California University of Pennsylvania, and Washington & Jefferson College for access to their anatomical collections, without which, the study would not have been possible.

Footnotes

Conflict of Interest: None

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