Project – Morphometric Evaluation of the Foramen Magnum and Its Clinical Significance. A Study of Adult dry skulls from the Department of Anatomy, University of Lagos

CHAPTER ONE

INTRODUCTION

1.1 Background to the Study

The foramen magnum is the largest opening in the human skull and is located in the inferior aspect of the occipital bone, forming an important anatomical component of the craniovertebral junction. It provides a passage between the cranial cavity and vertebral canal for structures including the medulla oblongata, meninges, vertebral arteries, spinal arteries, spinal roots of the accessory nerve and associated ligaments. The anterior and posterior margins of the foramen magnum are represented by important craniometric landmarks, the basion and opisthion, respectively. Because of its relationship with the brainstem, spinal cord and major blood vessels, variations in its dimensions and configuration can have important anatomical and clinical implications (Standring, 2021; D’Souza & Abdelnour, 2025).

Morphometry is concerned with the quantitative assessment of anatomical structures and is particularly useful in establishing normal ranges and identifying variations among populations. In the case of the foramen magnum, important morphometric parameters include the anteroposterior or sagittal diameter, transverse diameter, area, circumference and foramen magnum index. The dimensions of the foramen magnum are not constant among individuals, and differences may occur according to sex, age, ancestry and population. Eboh (2014), in a study of 74 adult dry skulls from a Southern Nigerian population, reported that the anteroposterior dimension of the foramen magnum was significantly greater than the transverse dimension, while oval and round forms were the most frequently observed shapes. Such population-specific anatomical data are important because cranial dimensions may vary between populations and therefore measurements established in one population should not automatically be generalized to another (Eboh, 2014; Fernandes et al., 2024).

The shape of the foramen magnum also represents an important component of its morphological evaluation. Different forms, including oval, round, irregular, pentagonal, hexagonal, egg-shaped and other configurations, have been reported in anatomical studies. The shape can be expressed quantitatively through the foramen magnum index, usually obtained from the relationship between its anteroposterior and transverse dimensions. A study of adult Ethiopian dry skulls demonstrated considerable variation in foramen magnum shape, with round, oval, egg-like, pentagonal, hexagonal, irregular and rectangular forms occurring at different frequencies. Similarly, studies using imaging techniques have demonstrated significant differences in foramen magnum size and shape between individuals and sexes, emphasizing the importance of documenting both metric and non-metric characteristics in anatomical research (Gebremariam et al., 2019; Toneva et al., 2018).

One of the major areas in which foramen magnum morphometry has attracted research attention is forensic anthropology. Determination of sex is an important component of the identification of unknown skeletal remains, particularly when the pelvis is unavailable or when only fragmented cranial remains are recovered. Since the foramen magnum is located deep within the skull base and is relatively well protected from external damage, its dimensions may remain available for examination when other anatomical features have been damaged. Studies have reported that the sagittal and transverse diameters and calculated area of the foramen magnum can show sexual dimorphism, with males generally having larger measurements than females. However, the degree of sexual dimorphism and the accuracy of sex estimation vary among populations, making locally derived reference data particularly important (Uthman et al., 2012; Saini et al., 2012; Fernandes et al., 2024).

Evidence from different populations further demonstrates why population-specific morphometric studies are necessary. In a Saudi Arabian study involving 200 adults, significant sexual dimorphism was observed in several foramen magnum parameters, including length, width and area, although the accuracy of individual measurements for sex discrimination was relatively limited; combining several measurements improved classification accuracy. Likewise, a Turkish computed tomography study involving 600 adults found that all evaluated foramen magnum measurements were significantly greater in males and reported that the calculated foramen magnum area provided the strongest individual discriminatory performance. These findings indicate that although the foramen magnum can contribute to sex estimation, its usefulness depends on the characteristics of the population being examined and on the combination of measurements employed (Kanchan et al., 2017; Gök et al., 2020).

Beyond forensic identification, the morphometry of the foramen magnum has considerable clinical and neurosurgical significance because of the vital structures that pass through or lie immediately around it. Abnormal dimensions or unusual anatomical configurations may influence the relationship between the cranial cavity and the upper cervical spinal canal and may be relevant to conditions involving compression at the craniovertebral junction. The anatomy of the foramen magnum is also important in procedures such as foramen magnum decompression, treatment of lesions around the craniovertebral junction, vascular surgery and approaches to tumors in the region. Variations in the size and shape of the opening and the position of the occipital condyles can influence the available surgical corridor and the amount of bone that may safely be removed during operative procedures (Bozbuğa et al., 1999; Muthukumar et al., 2005; D’Souza & Abdelnour, 2025).

The importance of generating local anatomical data is particularly relevant to African populations, including Nigerians. Existing investigations have shown that morphometric characteristics of the skull may differ between populations, and a recent study of dry skulls from Southwestern Nigeria demonstrated measurable variation in cranial and mandibular dimensions, including foramen magnum measurements. In that study, the average anteroposterior diameter of the foramen magnum was greater than its transverse diameter, further supporting the observation that the foramen magnum generally has an elongated configuration in many populations. Although Nigerian studies have contributed useful information, there remains a need for additional institutional datasets involving adult dry skulls and standardized measurements, particularly in anatomical collections in Lagos. A morphometric evaluation of adult dry skulls available in the Department of Anatomy, University of Lagos, can therefore contribute locally relevant anatomical reference information for teaching, research, forensic anthropology and clinical application (Eboh, 2014; Akinola et al., 2024).

1.2 Statement of the Problem

The foramen magnum is a critical anatomical structure because it forms the transition zone between the cranial cavity and vertebral canal and accommodates the lower brainstem, spinal cord-related structures and important vascular and neural structures. Despite its importance, the dimensions and morphology of the foramen magnum vary considerably among individuals. Such variation can create difficulties when attempting to establish a single standard measurement for all individuals or populations. A lack of adequate morphometric information may limit the ability of anatomists, forensic investigators and clinicians to distinguish normal anatomical variation from clinically significant abnormalities (Standring, 2021; Eboh, 2014).

A further problem concerns the use of foramen magnum measurements in sex estimation and forensic identification. Although several studies have demonstrated sexual dimorphism in foramen magnum dimensions, the level of accuracy differs substantially between populations. For example, studies from Saudi Arabia, India, Turkey and Nepal have reported different levels of discriminatory accuracy, suggesting that measurements developed from one population may not provide equally reliable results in another population. Consequently, reliance on foreign reference standards without sufficient consideration of Nigerian anatomical characteristics may produce inaccurate conclusions in forensic or anthropological applications (Kanchan et al., 2017; Saini et al., 2012; Gök et al., 2020; Atreya et al., 2023).

Another problem is the limited availability of sufficiently detailed institutional data on the morphology and morphometry of the foramen magnum among Nigerian adult dry skulls. Although Eboh (2014) provided valuable information from a Southern Nigerian population and more recent Nigerian research has reported foramen magnum measurements from Southwestern Nigerian skulls, variations in sample characteristics, geographic origin, sex composition, measurement techniques and anatomical collections make it necessary to generate additional datasets. In particular, there is a need to document the dimensions, index, area and morphological patterns of adult dry skulls available in the Department of Anatomy, University of Lagos. Without such data, the usefulness of the available skeletal collection for anatomical teaching, comparative research and population-specific reference development may remain underutilized (Eboh, 2014; Akinola et al., 2024).

Finally, variations in the dimensions and shape of the foramen magnum have practical clinical implications, particularly in procedures involving the craniovertebral junction. The relationship between the foramen magnum, occipital condyles and surrounding structures influences the available surgical space and may affect operative planning. Studies of dry skulls have shown considerable variation in these structures, while surgical anatomy investigations have emphasized the importance of accurate morphometric knowledge for procedures involving the transcondylar region and lesions around the foramen magnum. Therefore, failure to adequately document the morphometric characteristics of adult dry skulls in the University of Lagos anatomical collection represents an important anatomical and clinical knowledge gap that this study seeks to address (Bozbuğa et al., 1999; Muthukumar et al., 2005; Keshavarz et al., 2023).

1.3 Aim of the Study

The aim of this study is to evaluate the morphometric characteristics of the foramen magnum and determine its clinical significance using adult dry skulls from the Department of Anatomy, University of Lagos.

1.4 Objectives of the Study

The specific objectives are to:

1. Determine the anteroposterior and transverse diameters of the foramen magnum in adult dry skulls from the Department of Anatomy, University of Lagos.
2. Determine the area and foramen magnum index of the foramen magnum among the examined adult dry skulls.
3. Identify and classify the different morphological shapes of the foramen magnum observed in the adult dry skulls.
4. Assess the clinical and forensic significance of the morphometric characteristics of the foramen magnum, particularly their relevance to craniovertebral surgical procedures and sex estimation.

1.5 Research Questions

The following research questions will guide the study:

1. What are the anteroposterior and transverse diameters of the foramen magnum in adult dry skulls from the Department of Anatomy, University of Lagos?
2. What are the area and foramen magnum index of the foramen magnum among the examined adult dry skulls?
3. What morphological shapes of the foramen magnum are present among the adult dry skulls?
4. What are the clinical and forensic implications of the observed morphometric characteristics of the foramen magnum?

1.6 Research Hypothesis

Null Hypothesis (H₀): There is no statistically significant relationship between the morphometric characteristics of the foramen magnum and their potential clinical and forensic significance among adult dry skulls from the Department of Anatomy, University of Lagos.

1.7 Significance of the Study

The study will be significant to anatomists and anatomy educators because it will provide additional quantitative information on the morphology and morphometry of the foramen magnum in adult human skulls. The findings can serve as supplementary reference material during practical anatomy teaching, particularly in relation to the occipital bone, cranial base and craniovertebral junction.

The study will also be relevant to forensic anthropologists and forensic investigators. Measurements of the foramen magnum may provide supplementary information during the examination of unidentified skeletal remains, especially when other parts of the skull or skeleton are unavailable. Although the foramen magnum should not be considered a standalone method for sex estimation, population-specific morphometric data may improve the interpretation of skeletal evidence when combined with other established forensic techniques (Fernandes et al., 2024).

The findings may further benefit neurosurgeons, radiologists and other clinicians involved in the management of conditions and procedures affecting the craniovertebral junction. Knowledge of the dimensions and variations of the foramen magnum can contribute to anatomical understanding of the region and provide background information relevant to procedures such as decompression and surgical approaches to lesions around the foramen magnum (Bozbuğa et al., 1999; Muthukumar et al., 2005).

Finally, the study will contribute to the Nigerian anatomical literature by providing additional morphometric information from an anatomical collection in Lagos. The findings may provide baseline data for future comparative studies involving different Nigerian populations, sex groups, age categories or imaging-based investigations.

1.8 Scope of the Study

The study will focus on the morphometric and morphological evaluation of the foramen magnum in adult dry human skulls available in the Department of Anatomy, University of Lagos. The principal morphometric parameters will include the anteroposterior diameter, transverse diameter, calculated area and foramen magnum index. The study will also document the observable shapes of the foramen magnum.

The clinical significance of the findings will be discussed in relation to the anatomical structures surrounding the foramen magnum, craniovertebral junction procedures, forensic anthropology and the potential use of foramen magnum measurements as supplementary parameters in sex estimation. The study will not attempt to establish a definitive forensic sex-estimation standard applicable to the entire Nigerian population.

1.9 Operational Definition of Terms

Foramen Magnum: The large opening in the occipital bone at the base of the skull through which the medulla/spinal cord and associated neural, vascular and ligamentous structures pass.

Morphometry: The quantitative measurement and statistical assessment of anatomical structures.

Morphology: The study and description of the form, shape and structural characteristics of an anatomical structure.

Anteroposterior Diameter: The maximum distance measured from the anterior margin (basion) to the posterior margin (opisthion) of the foramen magnum.

Transverse Diameter: The maximum width of the foramen magnum measured between its lateral margins.

Foramen Magnum Area: The calculated surface area enclosed by the margins of the foramen magnum based on the measurements obtained.

Foramen Magnum Index: A numerical ratio derived from the relationship between the anteroposterior and transverse dimensions of the foramen magnum and used to characterize its shape.

Adult Dry Skull: A human skull that has undergone preparation and drying for anatomical study and is considered to belong to an adult individual.

Clinical Significance: The relevance of the anatomical characteristics or variations of the foramen magnum to diagnosis, treatment, surgical procedures or understanding of disease involving the craniovertebral region.

Sexual Dimorphism: Differences in anatomical characteristics between males and females that may be quantified or observed.

Craniovertebral Junction: The anatomical region where the skull base, particularly the occipital bone, articulates with the upper cervical vertebrae, principally the atlas.

Project – Morphometric Evaluation of the Foramen Magnum and Its Clinical Significance. A Study of Adult dry skulls from the Department of Anatomy, University of Lagos