A morphometric and morphological analysis of the foramen magnum, hypoglossal canal and occipital condyles in a select South African population

Seth Hendricks1, Sundika Ishwarkumar2, Pamela Pillay1

1Department of Clinical Anatomy, School of Laboratory Medicine and Medical Sciences, College of Health Sciences, University of KwaZulu-Natal, Durban, South Africa

2Department of Human Anatomy and Physiology, Faculty of Health Sciences, University of Johannesburg, South Africa

[Received: 25 May 2023; Accepted: 21 July 2023; Early publication date: 9 November 2023]

INTRODUCTION

The foramen magnum, occipital condyles and the first two cervical vertebrae are commonly referred to as craniovertebral junction [10]. The foramen magnum is a large orifice located in the base of the skull, which provides a passageway for the medulla oblongata, spinal cord and neurovascular structures that leave the skull [12]. The foramen magnum is surrounded by different parts of the occipital bone, i.e., two condylar, squamous and occipital parts [10, 12]. Variations in the shape of the foramen magnum exist, this variation may be due to fetal development, as the foramen, magnum is one of the ossification centres [10]. The irregular shape of the foramen magnum may cause developmental anomalies of the bone and soft tissues at the craniovertebral junction [10]. According to literary reports, the most common foramen magnum shapes were round [3, 6]; oval [14], hexagonal [6] and tetragonal [10]. Farid and Fattah [4] found an association between the oval morphology and a mean length and width of 35.00 mm and 29.40 mm.

The occipital condyle is a distinctive skull structure containing superior articular facets. The structure and location of the occipital condyle are developed so that the position is slightly oblique, with the anterior region situated more medially. The medial aspect of the occipital condyle is relatively roughened; this is primarily due to the attachments of ligaments [11]. The occipital condyles connect the vertebral column and the cranium, forming the craniovertebral joint. Associated structures of the occipital condyles include cranial nerves, glossopharyngeal and hypoglossal nerves, spinal nerves C1 and C2 and meninges. Generally, the shape of the occipital condyles is described as oval; however, Bayat et al. [2] described the occipital condyles as kidney-shaped. Farid and Fataah [4] found that the occipital condyles had a mean length and width of the occipital condyles to be 23.05 mm and 14.90 mm in their Egyptian sample, respectively. While in the Indian population, the mean length and width were recorded to be 23.60 mm and 14.70 mm, respectively [7]. Hence, these studies denote that population-specific differences regarding the morphology and morphometry of the occipital condyle exist [4].

The hypoglossal canal is a crucial structure located near the occipital condyles at the anterolateral border of the foramen magnum [5]. Kizilkanat et al. [5], when observing Turkish skulls, found the mean length of the hypoglossal canal to be 9.90 mm. However, Muthukumar et al. [7] recorded the mean length of the hypoglossal canal to be 12.60 mm, while the intracranial and extracranial diameters were 7.20 mm and 7.90 mm, respectively.

Transcondylar surgery is a type of approach that involves the location and resection of lesions in the area of the foramen magnum. This surgery is usually performed through the atlantooccipital joint or the occipital condyles [8]. In addition, the hypoglossal canal may also be linked to the transcondylar approach, as in certain instances, lesions are found within or around the hypoglossal canal [5]. Hence, knowledge of the morphometry of the canal and its surrounding structures may assist during surgical procedures [5]. Therefore, this study aimed to document the morphology and morphometry of the foramen magnum, occipital condyles and hypoglossal canals within a South African population of KwaZulu-Natal.

MATERIALS AND METHODS

Study design and selection criteria

This study was conducted on fifty dry adult cadaveric skulls (n = 50) obtained from the Department of Clinical Anatomy at the University of Kwa-Zulu Natal. The age, ancestry and sex of the skulls were unknown. Ethical approval was obtained from the Biomedical Research Ethics Committee (BREC Number: BE362/19). Any skulls that were damaged or deformed were excluded from this study.

Methodology

Morphology

The morphology of foramen magnum, occipital condyles and hypoglossal canals were observed in the selected sample using the following classification schemes:

1. Foramen magnum:
   • oval-shaped,
   • round-shaped,
2. Occipital condyle:
   • kidney-shaped,
   • oval-shaped,
   • round-shaped,
3. Hypoglossal canal:
   • oddly shaped,
   • oval-shaped,
   • round-shaped.

Morphometry

In this study, each of the following morphometric parameters was measured three times by the first author in accordance with Farid and Fattah [6], using a digital Vernier Calliper set to the precision of 0.01 mm:

• length and width of the foramen magnum,
• bilateral length and width of the occipital condyles,
• bilateral length and width of the hypoglossal canal.

The foramen magnum index was calculated for each skull using the formula: Foramen magnum = foramen magnum length/ foramen magnum width. If the Index is greater or equal to 1.2, then the morphology of the foramen magnum is said to be oval. However, if it is less or equal to 1.2, then the foramen magnum has an asymmetrical shape [4].

RESULTS

Morphology

Foramen magnum

The foramen magnum was predominantly oval-shaped in this study, with an incidence of 58.0%. This was followed by the round shape, with an incidence of 42.0% (Table 1).

Occipital condyles

In the present study, the right occipital condyle was recorded to be oval-shaped in 76.0% of the specimens, while in 10.0% and 14.0% of specimens, it was recorded to be kidney and round-shaped, respectively (Table 2). Similarly, the left occipital condyle was also predominantly oval-shaped (64.0%), while the kidney and round-shaped occipital condyles were documented in 16.0% and 20.0%, respectively (Table 2).

Hypoglossal canal

The right hypoglossal canal was observed to be oval, round and oddly shaped in 54.0%, 40.0% and 6.0% of the specimens in this study, respectively (Table 3). The left hypoglossal canal was found to be oddly shaped (10.0%), oval-shaped (44.0%) and round shaped (46.0%) (Table 3).

Morphometry

Foramen magnum

In this study, the mean length and width of the foramen magnum were 35.19 mm and 27.77 mm, respectively (Table 4). The foramen magnum index was calculated to be 1.3, which denoted that the oval-shaped foramen magnum was most prevalent (Table 4).

Occipital condyles

The mean length of the occipital condyle was 21.75 mm on the right side and 21.72 mm on the left side in the selected South African sample (Table 5). While the mean width of the right and left occipital condyle was 12.32 mm and 13.43 mm, respectively (Table 5).

Hypoglossal canal

In this study, the hypoglossal canal had a mean length of 4.59 mm on the right side and 5.68 mm on the left side (Table 5). The mean width of the hypoglossal canal was 3.79 mm and 3.94 mm on the right and left sides, respectively (Table 5).

DISCUSSION

The surgeons must have a firm grasp of the anatomical landmarks during neuro-surgical procedures [12]. In particular, when performing transcondylar surgery and occipital condyle drilling, the craniovertebral junction is an important landmark [9]. Therefore, it is imperative to have extensive knowledge of the relationship between the foramen magnum, occipital condyles and hypoglossal canal in different population groups [1].

Foramen magnum

In this study, the mean length and width of the foramen magnum were recorded to be 35.19 mm and 27.77 mm, respectively. Similarly, Farid and Faataah [4] reported that the foramen magnum had a mean length of 35.00 mm, and a mean width of 29.40 mm in the Egyptian population. However, Muthukumar et al. [7] documented that the length and width of the foramen magnum in the Indian population was 34.60 mm and 29.00 mm, respectively. In addition, Ulcay et al. [13] reported an observed mean length and width of 35.81 mm and 28.14 mm in the Turkish population. When observing the morphology of the foramen magnum, the oval shape was most frequently observed in this study, which correlated with the study conducted by Ulcay et al. [13]. However, in the Indian and Turkish population groups, the foramen magnum was predominantly rounded in shape. Furthermore, in this study, the foramen magnum index was calculated to be 1.3, which indicated that the foramen magnum was oval-shaped (> 1.2), which correlated with the findings of Muthukumar et al. [7]. In contrast, Farid and Fataah [4] found that the asymmetrically shaped foramen magnum (< 1.2) was most prevalent in 60% of their sample, followed by the oval shape (> 1.2) with a 40% prevalence.

Occipital condyles

The right and left occipital condyles had a mean length of 21.75 mm and 21.72 mm, and a mean width of 12.32 mm and 13.43 mm, respectively. In comparison to the present study, Farid and Fataah [4] reported a slightly larger mean length and width of the occipital condyles in the Egyptian population, i.e., a mean length of 22.90 mm and 23.20 mm and a width of 14.80 mm and 15.00 mm on the right and left sides, respectively. Furthermore, Muthukumar et al. [7] found that the mean length and width of the occipital condyles were 23.60 mm and 14.70 mm, respectively. This indicates that population-speci­fic differences may exist. In this study, the occipital condyles were predominantly oval-shaped; however, Bayat et al. [2] documented that the kidney-shaped occipital condyles were most prevalent in the Iranian population.

Hypoglossal canal

In this study, the right and left hypoglossal canals had mean length of 4.59 mm and 5.68 mm, respectively. While the mean width of the hypoglossal canals is 3.79mm on the right side and 3.94 mm on the left side in this study. However, Muthukumar et al. [7] documented that the mean length of the hypoglossal canal was 12.60 mm in the Indian population group. Kizilkanat [5] reported that the hypoglossal canal had a mean length of 9.90 mm in their study. In addition, the right hypoglossal canal was most frequently oval-shaped, while the left was predominantly round in this study.

Limitations of the study

The limitation of this study was that the demographic data of the specimens was not accessible to the authors, hence further studies should investigate, if any differences exists among the different age groups and sexes in South Africa.

CONCLUSIONS

The findings of this study may contribute to the existing body of anatomical knowledge and may assist in reducing the risk of injury and mortalities during trans-condylar approach procedures.

Article information and declarations

Acknowledgments

“Dr P Pillay is a University of KwaZulu-Natal (UKZN) Developing Research Innovation, Localisation and Leadership in South Africa (DRILL) fellow. DRILL is a NIH D43 grant (D43TW010131) awarded to UKZN in 2015 to support a research training and induction programme for early career academics. The content is solely the responsibility of the authors and does not necessarily represent the official views of DRILL and the National Institutes of Health”. The authors also wish to thank the individuals who donated their bodies for anatomy teaching and research.

Funding

The author would like to acknowledge the generous support of the National Research Foundation of South Africa for providing funding required to conduct this project. The findings of this study is that of the authors and not the National Research Foundation of South Africa.

Conflict of interest: None declared.

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