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Computed tomography determination of the cephalic index of the igbo tribe

Introduction

Cephalic index (CI) is an objective and highly useful parameter for determining the skull shape (1), racial variations (2), and for assessing the pre- and post-operative correction of skull deformations (1, 3). It is also helpful in forensic medicine, plastic and reconstructive surgery, orthodontics and clinical diagnosis (4).

The concept of cephalic index (CI) was introduced as percentage of width to length in any skull and categorized as brachycephalic (≤ 75 %), mesocephalic (75 – 79.9 %) and dolichocephalic (≥ 80) (1). Caucasoids were characterized by a doliocephalic shape with receded zygomas, large browridge and a narrow nasal aperture. Secondly, Negroids were characterized by a mesocephalic head shape, with receded zygomas and wide nasal aperture. Third, Mongoloids were characterized as a brachycephalic head shape, absent browridges, small nasal aperture and projecting zygomas.

Additionally, Australoids whose craniofacial type fell between Negroids and Caucasoids was added. India was considered to fall in this group’s craniofacial measurements. African crania are broader (mesocephalic). European skulls tend to be narrow (dolicochephalic). Europeans and American Indians share many cranial similarities. Asian skulls are typically wide (brachycephalic). Australian aborigines are often characterized by narrow skulls (dolichocephaly) (5).

The cephalic index may be determined applying anthropometric methods, dry skull measurements and radiological methods. The sliding caliper is a popular tool for anthropometric determination of cranial length. But the procedure can become complicated by scaphocephalic head shape itself, as bossing of the forehead region superior to the glabella may distort the glabello-opisthocranial axis so that it no longer represents the maximum cranial length (6). Imaging methods like computed tomography is an excellent tool and it is widely accepted as a standard protocol for the assessment of the cephalic index (7, 8, 9).

The cephalic index of the Igbo people of Nigeria using calipers has been severally documented. However, literature on CI using computed tomography are scanty. This study aimed to generate baseline data of cephalic index using this advanced imaging technique, which could come in handy in research and in forensic investigation (10).

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Materials and methods:

A retrospective study carried out in October, 2015. It involved analyzes of digital computed tomography images of the head generated between July, 2014 – August, 2015. Ethical approval was got from the institutional research ethics committee. A General Electric (GE) Brightspeed Excel computed tomography scanner, manufactured in 2007 and installed in 2012, with 4-slice per rotation capacity was also available at the centre. It had both axial and helical modes and all abdominal cases were scanned supine, at an azimuth of 180 degrees and in helical mode. The scanner had a self-calibrating software which was activated daily. The population of head CT image was subjected to convenience sampling to arrive at an appropriate sample. Confidentiality was guaranteed by the activation of partial image anonymity feature which masked subject’s names, examination number and clinical diagnosis. Files included were those of subjects aged 18 years and above, with two scanograms that were scanned supine, at an azimuth of 90 and 180 degrees respectively, and with no evidence of bandages, scalp oedema or distortion of bony skull tables or facial bones. Files excluded had non-Igbo surnames, a rotated postero-anterior scanogram as well as incomplete data on age and gender.

The files were analyzed by the researchers in the computed tomography suite. On-screen linear measurement cursor was activated. On the postero-anterior (PA) scanogram, the most elongated lateral bony prominences which represent the biparietal (BPD) diameters were identified. A horizontal line was then drawn from one parietal bone to the other, at an azimuth of 90 degrees. This line has a distance of about 4 – 5.5cm superior to the supraorbital margin. An azimuthal variation from 90 degrees is evidence of rotation of PA scanogram and the image qualified for exclusion. On the lateral scanogram, occipito-frontal diameter (OFD) was measured using a straight line which bisected the hypodense frontal sinus (glabella) at an azimuth of 77-90 degrees. The cursor was then extended to the most protuberant part of the occipital bone (inion) posteriorly (images I and II).

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The percentage of the BPD to the OFD was then calculated and represented the cephalic index (CI) for that subject. The digital measurements were recorded but not saved in order to allow the images revert to their original state for clinical reasons. In all, only one hundred and fifty four (154) files met the inclusion criteria out of a total of two hundred and twenty-eight (228) head images. Statistical package for the social sciences, version 20.0 (SPSS Incorporated, Chicago, Illinois, USA) was used to analyze the data.

Results

The subjects were aged 18 -91 with males, female and combined population showing a cephalic index of 75.14 ± 5.7, 75.89 ± 5.3 and 75.37 ± 5.5 respectively (Table 1). The population had an increasing frequency amongst the cephalic index types of brachycephalic (17.5%), mesocephalic (33.1%) and dolicocephalic (49.4%) respectively (Table 2). The Igbo are generally mesocephalic. This finding was compared with previous works (Table 3).

Discussion

Of the methods of measuring skulls for the purpose of determining topographical relations, cranial indices, or the cephalic indices in case of measurements taken on the living human is the most important. The broader or rounder a head is, the higher is its cephalic index, and vice versa (8). The cephalic index in this present study was neither measured in the living nor the dead but in the images of the living. The self-calibrating ability of the scanner makes it reliable for such studies.

The cephalic index of the digital cranium in the present study was 75.14 ± 5.70 and 75.89 ± 5.30 for male and female populations respectively. This represents the mesocephalic head type. This is in agreement with two previous works done amongst the Igbo six to ten years ago (13″,14). Mesocephalic head types were also the findings of previous works amongst Hausa, Ilorin Yoruba [5], and female folks of Yoruba, Ijaw and Ogoni extraction (10, 12, 14, 15) (Table 3). However, the findings from this work are at variance with some works also done amongst the Igbo three years ago where it was noted that the tribe was brachycephalic (11″,12).

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The reason for this variation may hypothetically, be due to environmental cause as the mesocephalic index from this work came from Anambra State whereas the brachycephalic indices are from the more distant Abia and Imo States. Delta Igbo who appear to be in closer geographical proximity to the Anambra Igbo are similarly mesocephalic.

Adults from the Itsekhiri, Okpe (15) and Yoruba from Ibadan (8) were consistently dolicocephalic, a head type with a predominant frequency (49.4%; Table 2) amongst the Igbo but which is alien is to the combined, male and female Igbo population from this work (Table 3). While the sharp differences may be evidence of distinct ancestry, the similarities appear to be evidence of environmental and geographical interference in cephalization.

Conclusion:

Although there are several works on cephalic index using calipers, this work contributes to the topic using computed tomography. This will be of significance to future researchers and in forensics.

Limitation of study: It was not possible to ascertain Igbo ancestry from the digital folders beyond the second generation.

Recommendations: More works on cephalic index using computed tomography is recommended to ascertain if there are statistically significant differences between this modality and calipers.

Declaration of conflicting interests: The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Acknowledgements: The authors would like to thank the Radiologists and Radiographers at the centre, particularly Ikegwuonu, NC for her technical support.

Funding: nil

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