Anatomical and Radiological study of the posterior cranial base in relationship to occipital condyles and Foramen magnum

Detailed morphometric analysis is required for various surgical approaches in the craniovertebraljunction. High mortality and morbidity are anticipated for the surgical procedures when undertaken without in-depth anatomical knowledge. With so much clinical importance in this area, our study will present a thorough understanding in terms of skull and CT values.The main aim of this study is to give the morphometric details of occipital condyles and foramenmagnum in cadaveric skulls andCT scans.Seventy dried human skulls and 70 CT images on the three-dimensional volume-rendered reconstruction of the skull base was used for this study.The length and width of the occipital condyle of right and the left side was 22.21 ±5.20 mm; 22.05±4.83 mm; 12.57 ± 2.50 and 12.68 ± 2.92mm respectively in cadaver skull. The length and width of occipital condyles in CT scans for right and left side was 21.61± 3.09 mm; 21.58± 3.50 mm; 13.04± 1.58 mm and 13.13± 2.54 mm respectively. The Anteroposterior and transverse diameters of the foramenmagnum in cadaveric skulls and CT images was 33.17 ± 7.23; 29.22 ± 6.17; 34.15 ± 2.88 and 28.14± 2.43 mm respectively. Each surgical approach and the radiological diagnostic procedures have their limitations. Moreover, analysis of cranial base dimensions of occipital condyles and foramen magnum can be considered as a reliable method for sex determination. Hence this studywill a useful guide for surgeons, radiologists, anthropologists and forensic experts.


INTRODUCTION
The base of the skull is such a complex region with numerous foramina through which several vital structures have access to the closed cranium. Awareness about the various foramina and the anatomical landmarks is essential while planning multiple surgical approaches for reaching the middle and posterior cranial base (Çiçekcibaşi et al., 2004). However, the study of the foramina becomes an integral part of the anthropologist for the study of signi icant change in size and shape during the evolution (Bruner, 2007).
Tumours of the foramen magnum and other Craniovertebral junction surgeries can be approached both ventrally and dorsally, the safest of both these is the dorsal approach which is highly preferred at the craniovertebral junction. The maximum extent of the condylar resection is unclear, ranging from suboccipital craniotomy to total occipital condyle removal. Hence dimensional anatomy of occipital condyle has a vital role in minimizing the complications of the surgical procedures (Kalthur et al., 2014).
The dimensions are not only crucial in establishing safe operational techniques but also to evaluate and analyze the age, gender, stature, and ethnicity. Discrimination functional analysis of foramen magnum is of great help in anthropologic and forensic techniques (Vinutha et al., 2018). This study aims to evaluate the morphometric analysis of occipital condyles and the Foramen magnum in the posterior skull base and its surgical importance.

MATERIALS AND METHODS
The study was carried out after getting Institutional Ethical committee clearance.
Reference no:1162/IEC/2017. Adult cadaveric skulls 70 and 70 CT scans of patients who underwent an examination of the skull region for various reasons were used for these studies. Measurements were taken for both sides. All the cadaveric skulls were collected from the department of anatomy and the department of Forensic medicine. CT scan studies were obtained from the Department of Radiology SRM Medical college hospital and research centre Chennai. All the skulls selected were of south Indian origin, and the damaged skulls in the base were excluded for this study. CT skull images studied were of high resolution 0.6 mm slice thickness. Patients with a history of trauma, surgery, pathological lesions with poor image quality were excluded from this study.
Morphometric measurements of the occipital condyles were recorded for its length, width, Foramen magnum dimensions were measured for its anteroposterior diameter and transverse diameter. For skulls, measurements were taken using the digital image analysis program known as Digimizer Software version 4 .3.0. The pictures captured were calibrated before measuring the parameters under the same position with a regular grid sheet. Figure 1. CT skull images obtained were of 0.6mm-thin sections. The raw data collected were reformatted in the axial, coronal, and sagittal plane, and then three-dimensional images were created using the Radiant software version5.5.0.   formed to analyze between the right and left condylar parameters and comparison between the CT and the Skull values was also done. Signi icance was considered if p<0.05.

RESULTS AND DISCUSSION
The outcome of our study is summarized in the following tables (Table 1 and Table 2).
The right and left side measurements were tested for signi icance, and no signi icant differences were noted. Similarly, there is no signi icant difference between the skull and CT measurements (p>0.05).
The Trans condylar approach is a complex skull base approach that is used to reach the craniocervical junction (CVJ), the foramen magnum and the brainstem. CVJ is in close relationship with vital structures, and therefore, surgeries involved in this area include potential complications (Agnihotri et al., 2014). Detailed knowledge of condylar and foramen magnum dimensions helps the surgeon in making important decisions for deciding the type of approach and the extent of condylar drilling to be carried out to minimize the injury of neural structures. The condylar width plays an important factor in assessing the screw placement in occipital condylectomy.
In There are not many variations observed in the width of occipital condyles in our study, and by the authors mentioned above. The main focus for extensive investigation of the morphometry of occipital condyles was for the placement of occipital condylar screws (Le et al., 2011).
The anteroposterior and transverse diameter of the foramen magnum for the cadaveric skull in our study was noted as 33.17 ± 7.23 and 29.22 ± 6. 1mm. All the metric variables assessed in the present study for foramen magnum were found to have similar values with the previous studies as reported by Muthukumar et al. (2005)  Similarly, the CT scan results of our study for anteroposterior and transverse diameters were 34.15 ± 2.88 mm and 28.14 ± 2.43mm, respectively. Notably, these values were even higher than the mean anteroposterior and transverse diameters as observed by Vinutha et al. (2018)  The difference in morphometric measurements of different studies could contribute to the different angulations of the occipital condyles relative to the sagittal plane. The larger the sagittal intercondylar angles, the more considerable intercondylar distances were noted (Verma et al., 2016). Moreover, the difference could also be associated with the varying prevalence of Occipital condyle overriding into the foramen magnum. The presence of wider intercondylar distances is advantageous in accessing ventral craniovertebral junction lesions,which demands less extensive bony resection for both suboccipital and occipital condylectomy.

CONCLUSION
Analysis of the various parameters from this study concludes that there is no signi icant difference between the cadaveric skulls and CT skull measurements. Besides, the data obtained from anatomical and radiological studies did not differ statistically in the sides also.Understanding the knowledge of anatomy simpli ies the understanding of underlying pathological lesions. Moreover, the dimensions of the foramen magnum and occipital condyle canbe usedfor performingtranscondylar approaches for reaching the middle and posterior cranial base surgeries. Hence, this study concludes that preoperative morphometric assessment is mandatory forknowingthe feasibility of occipital condyle in screw ixation.To add up these dimensions will also throw light for the sex determination, thus contributing to new research in the ield of anthropology and forensic sciences.

ACKNOWLEDGEMENT
The author thanks the SRM institute of science and technology for providing adequate support for the study. Besides, the faculties of Radiology who guided a lot in this study.

Source of Funding Support
Nil

Con lict of Interest
None declared.