Introduction:
The occipital condyles (OC) are obliquely oriented kidney shaped facets located on either sides of the foramen magnum at the craniovertebral junction (CVJ). (1) The condyle has a convex articular facet on its inferior aspect which articulates with the concave superior facet on the lateral masses of the first cervical vertebra (atlas) at the atlantooccipital joint (AOJ). The anterior end of the OC lies nearer the midline than the posterior end. (2) The lesions at the CVJ such as trauma, rheumatoid arthritis, infections, intra and extradural tumours, synovial cysts, congenital malformations, and degenerative diseases predispose to occipitocervical instability. (3) These have raised the interest of the CVJ among anatomists, orthopaedic surgeons, neurosurgeons, and radiologists. (2)
The recent advancement of surgical stabilisation to restore the structural integrity of AOJ involves the use of occipital plates, rods, and screws that vary in size due to variant morphology, and morphometry of the OC. The surgical fixation of CVJ instability is prone to failure due to poor screw holding by an inappropriate size of the OC plate. (3) Serious complications following inadvertent injury to the neighbouring vital neurovascular structures such as vertebral artery, medulla oblongata, and its meninges, spinal accessory, and hypoglossal nerves may occur intraoperatively. (4,5) These complications include haemorrhage, dural tear with leakage of the cerebrospinal fluid, and epidural hematoma. (3)
Optimal surgical access to lesions around the foramen magnum, clivus, front of brainstem, and CVJ include anterior or lateral approach, posterolateral approach, supracondylar approach, and far lateral transcondylar approach (TCA). (5,6) TCA is an extension of the basic far lateral approach with additional condylar drilling to increase the surface area for surgical exposure, and provide better access to lower clivus, and premedullary area. These help to reduce the depth of surgical field, improve the angle of exposure, and reduce the amount of brain retraction. (5) Far lateral TCA allows extensive vertebral artery dissection, hypoglossal canal exposure, and OC, and jugular tubercle removal. (7) The posterior aspect of the OC is drilled depending on how much of the OC can be resected without damaging the nearby structures, and causing craniocervical instability. (1,4) Therefore, prior knowledge of the OC morphometry is important in the designing of customised OC implants, planning of the surgical approach, and deciding the extent of condylar drilling with minimal injury to the vital structures. (8,9)
Previous literature reports on the morphometric parameters of the OC measured directly on dry skulls or radiologically on computed tomography (CT) have documented variations in different populations. (1,9,10) The use of advanced radiological technology such as CT, and Magnetic Resonance Imaging (MRI) improves the accuracy in skeletal analysis. (11) To the best of our knowledge, the radiomorphometric analysis of the OC in Delta State Nigeria has not been documented. This study therefore aimed at determining the morphometry of the OC in adult Nigerians using CT.
Materials and Methods
This was a descriptive cross-sectional study that retrospectively evaluated the OC using brain CT scans archived in the Picture Archiving Communications Systems (PACS) unit of a Radiology department of Delta State University Teaching Hospital in Nigeria. We obtained approval from the institution's ethical board (Ref no. EREC/PAN/2020/030/0371) prior to commencement of this study. Brain CT images taken using a 64 slice CT machine (Toshiba Aquilon, Japan) between 1st June 2015 to 30th June 2020 for reasons such as chronic headache, space occupying lesions, stroke, and pulmonary embolism were used. We included CT images with complete demographic data; age (≥20 years), and sex. The study excluded images of patients below 20 years of age, poor quality images with artefacts or rotation of patient, and images with skull base fractures, congenital anomalies or evidence of CVJ surgery. Therefore, 336 brain CT images comprising 199 males, and 137 females were used. In total, 672 OC were evaluated in this study.
Using axial sections, the following measurements of the OC were taken following the descriptions below (11):
Length; the maximum distance between the anterior and posterior poles of the OC (Fig. 1A).
Width; the maximum distance between the medial, and lateral borders of the OC, and perpendicular to the OC length (Fig. 1A).
Minimal medial intercondylar distance (Min MID): minimum transverse distance between the medial edges of the right, and left OC articular surfaces (Fig. 1B).
Maximum medial intercondylar distance (Max MID); maximum transverse distance between the medial edges of the OC articular margins (Fig. 1B).
Maximum bicondylar distance (Max BCD): transverse distance between the most lateral edges of the right, and left OC articular surfaces (Fig. 1B).
On coronal section; the maximum supero-inferior thickness of the OC was recognized as the height of the condyle (Fig 2).
The distances of the most anterior, and posterior tips of the condyles to the basion (Ant-Ba, Post-Ba), and opisthion (Ant-Opisth, Post-Opisth) of the foramen magnum were also measured bilaterally on axial sections (Figs 3 A and B).
The OC index was calculated as a ratio between the condylar length, and width.
Data were analysed using Statistical Package for Social Sciences (SPSS) version 23 (IBM® Armonk, New York). Data were classified according to gender, and 10 years' age groups namely; 20-29, 30-39, 40-49, 50-59, 60-69, 70-79, and 80-89 years. We used descriptive statistics such as means, and standard deviations to summarise the data in tables. Independent t-test was used to determine the differences between these morphometric parameters in males, and females. Paired t-test was used to evaluate for side differences in the continuous variables. Analysis of variance (ANOVA) was used to probe for differences in the various age groups. Pearson's correlation test was used to probe for association between the metric parameters. A p-value of < 0.05 was considered statistically significant. The OC were classified based on length; Type I (Short) measuring less than 2 cm while Type II (moderate), and III (long) measured 2-2.6 cm, and more than 2.6 cm respectively. (2)
Results
We measured the morphometric parameters of the OC using brain CT images of 336 adult patients comprising 199 males (59.2%), and 137 (40.8%) females. The age range of the patients was 20- 99 years while the mean age was 53.29±18.18 years. The number of patients in the 20-29, 30-39, 40-49, 50-59, 60-69, 70-79, 80-89, and 90-99 age-groups was 40 (11.9%), 50 (14.9%), 52 (15.5%), 67 (19.9%), 55 (16.4%), 46 (13.7%), 19 (5.7%), and 7 (2.1%) correspondingly. The average length, width, and height of the OC was 2.381±0.271 cm, 1.366±0.213 cm, and 0.945±0.389 cm respectively. The mean min MID, max MID, and max BCD were 1.335±0.298 cm, 3.249±0.455 cm, 5.000±0.401 cm correspondingly. The average Ant-Ba, and Ant-Opisth distance were 0.782±0.214 cm, and 3.871±0.434 cm respectively while the Post-Ba, and Post-Opisth distance were 2.852±0.327 cm, and 2.718±0.449 cm correspondingly (Table 1).
| Table 1: Gender differences in the morphometric parameters of the OC |
| Morphometric parameters (cm) |
Mean±SD (cm) |
|
|
Males |
Females |
Average |
P-value |
Length |
2.443 ± 0.286 |
2.292 ± 0.220 |
2.381 ± 0.271 |
0.001* |
Width |
1.380 ± 0.214 |
1.345 ± 0.212 |
1.366 ± 0.213 |
0.035* |
Height |
0.981 ± 0.170 |
0.894 ± 0.572 |
0.945 ± 0.389 |
0.004* |
Ant-Ba |
0.790 ± 0.184 |
0.772 ± 0.252 |
0.782 ± 0.214 |
0.299 |
Post-Ba |
2.916 ± 0.283 |
2.758 ± 0.364 |
2.852 ± 0.327 |
0.001* |
Ant-Opisth |
3.915 ± 0.429 |
3.806 ± 0.436 |
3.871 ± 0.434 |
0.001* |
Post-opisth |
2.672 ± 0.508 |
2.784 ± 0.340 |
2.718 ± 0.449 |
0.001* |
OC Index |
1.811 ± 0.344 |
1.749 ± 0.336 |
1.786 ± 0.342 |
0.022* |
Min MID |
1.347 ± 0.317 |
1.317 ± 0.267 |
1.335 ± 0.298 |
0.376 |
Max MID |
3.329 ± 0.500 |
3.132 ± 0.352 |
3.249 ± 0.455 |
0.001* |
Max BCD |
5.087 ± 0.447 |
4.873 ± 0.279 |
5.000 ± 0.401 |
0.001* |
| *P considered significant at <0.05 Ant-Ba= Distance from the anterior tips of the occipital condyles to the basion, Post-Ba= Distance from the posterior tips of the occipital condyles to the basion, Ant-Opisth= Distance from the anterior tips of the occipital condyles to the opisthion, Post-Opisth=Distance from the posterior tips of the occipital condyles to the opisthion, Min MID= Minimal medial intercondylar distance, Max MID= maximum medial intercondylar distance, Max BCD=Maximum bicondylar distance, OC=Occipital condyle |
The average morphometric parameters of the bilateral OC in both males, and females is shown on Table 1. Males had significantly larger OC length, width, height, min MID, max MID, max BCD, Ant-Opisth, and Post-Ba distances than females (P<0.05) (Table 1). The Post-Opisth distance was significantly larger in females (p=0.001). Most of the morphometric parameters of the OC were larger on the right than on the left side with significant side difference observed with the width of the OC, and Ant-Ba, and Ant- Opisth distances only (P=0.001 each) (Table 2). All the measurements showed significant differences in the various age-groups except the OC length (P<0.05) (Table 3).
| Table 2: Side differences in the morphometric parameters of the OC |
| Morphometric parameters (cm) |
Side |
Mean±SD (cm) |
|
|
|
Males |
Females |
Average |
P-value |
Length |
Right |
2.438± 0.306 |
2.272 ± 0.249 |
2.370 ± 0.295 |
0.072 |
|
Left |
2.448± 0.265 |
2.311 ± 0.186 |
2.392 ± 0.245 |
|
Width |
Right |
1.423± 0.179 |
1.397± 0.228 |
1.412 ± 0.200 |
0.001* |
|
Left |
1.338± 0.236 |
1.293± 0.181 |
1.320 ± 0.216 |
|
Height |
Right |
0.992± 0.186 |
0.921 ± 0.798 |
0.963 ± 0.529 |
0.083 |
|
Left |
0.970 ± 0.152 |
0.866 ± 0.135 |
0.927 ± 0.154 |
|
Ant-Ba |
Right |
0.826 ± 0.180 |
0.808 ± 0.256 |
0.819 ± 0.214 |
0.001* |
|
Left |
0.753 ± 0.181 |
0.736 ± 0.243 |
0.746 ± 0.208 |
|
Post-Ba |
Right |
2.921 ± 0.280 |
2.737 ± 0.358 |
2.841 ± 0.325 |
0.098 |
|
Left |
2.921± 0.287 |
2.780 ± 0.370 |
2.863 ± 0.330 |
|
Ant-Opisth |
Right |
4.002 ± 0.434 |
3.827 ± 0.386 |
3.931 ± 0.423 |
0.001* |
|
Left |
3.827 ± 0.407 |
3.786 ± 0.481 |
3.810 ± 0.439 |
|
Post-Opisth |
Right |
2.643± 0.621 |
2.809 ± 0.333 |
2.711 ± 0.529 |
0.102 |
|
Left |
2.702± 0.359 |
2.759 ± 0.346 |
2.725 ± 0.354 |
|
OC Index |
Right |
1.736 ± 0.272 |
1.676 ± 0.358 |
1.712 ± 0.311 |
0.001* |
|
Left |
1.886 ± 0.390 |
1.821 ± 0.296 |
1.860 ± 0.355 |
|
| *P considered significant at <0.05 Ant-Ba= Distance from the anterior tips of the occipital condyles to the basion, Post-Ba= Distance from the posterior tips of the occipital condyles to the basion, Ant-Opisth= Distance from the anterior tips of the occipital condyles to the opisthion, Post-Opisth=Distance from the posterior tips of the occipital condyles to the opisthion, OC=Occipital condyle |
| Table 3: Age-group differences in the morphometric parameters of the OC |
| Morphometric parameters (cm) |
Age groups (years) |
20-29 |
30-39 |
40-49 |
50-59 |
60-69 |
70-79 |
80-89 |
90-99 |
P value |
Length |
2.383 |
2.405 |
2.394 |
2.377 |
2.322 |
2.383 |
2.425 |
2.478 |
0.256 |
Width |
1.316 |
1.260 |
1.373 |
1.375 |
1.372 |
1.454 |
1.457 |
1.403 |
0.001* |
Height |
0.973 |
0.940 |
1.037 |
0.909 |
1.001 |
0.850 |
0.866 |
0.886 |
0.017* |
Ant-Ba |
0.755 |
0.920 |
0.929 |
0.758 |
0.691 |
0.660 |
0.734 |
0.751 |
0.001* |
Post-Ba |
2.689 |
2.875 |
2.975 |
2.889 |
2.775 |
2.849 |
2.850 |
2.981 |
0.001* |
Ant-Opisth |
3.569 |
3.874 |
4.198 |
3.870 |
4.051 |
3.632 |
3.714 |
3.725 |
0.001* |
Post-Opisth |
2.613 |
2.770 |
2.671 |
2.703 |
2.940 |
2.614 |
2.627 |
2.616 |
0.001* |
Min MID |
1.310 |
1.438 |
1.413 |
1.423 |
1.270 |
1.173 |
1.250 |
1.119 |
0.001* |
Max MID |
3.449 |
3.281 |
3.337 |
3.382 |
3.103 |
3.072 |
2.961 |
3.034 |
0.001* |
Max BCD |
4.997 |
5.014 |
4.991 |
4.914 |
5.080 |
5.131 |
4.806 |
4.836 |
0.024* |
OC index |
1.851 |
1.939 |
1.773 |
1.770 |
1.744 |
1.676 |
1.713 |
1.799 |
0.001* |
*P considered significant at <0.05
Ant-Ba= Distance from the anterior tips of the occipital condyles to the basion, Post-Ba= Distance from the posterior tips of the occipital condyles to the basion, Ant-Opisth= Distance from the anterior tips of the occipital condyles to the opisthion, Post-Opisth=Distance from the posterior tips of the occipital condyles to the opisthion, Min MID= Minimal medial intercondylar distance, Max MID= maximum medial intercondylar distance, Max BCD=Maximum bicondylar distance, OC=Occipital condyle |
Significant weak positive correlation was observed between the various morphometric parameters (0<r<0.5) (P<0.05). These include, Ant-Ba/Post-Ba, Ant-Ba/Post-opisth, Ant-Ba/Ant-Opisth, Post-Opisth/Ant-Opisth, Ant-Opisth/Post-Ba, max MID/min MID, max MID/max BCD, length/Post-Ba, width/Post-Ba, height/Ant-Opisth, and Post-Opisth/min MID. A significant weak negative correlation was observed between the following; width/min MID, and length/ Post-Opisth (-0.5<r<0; p< 0.05) (Table 4). The morphometric parameters of the OC on the right showed a significant positive correlation with their corresponding dimension on the left (P<0.05) (Table 5).
| Table 4: Correlation between the morphometric parameters of the OC |
| Morphometric parameters |
|
Length |
Width |
Height |
Ant-Ba |
Post-Ba |
Ant-Opisth |
Post-Opisth |
Min MID |
Max MID |
Max BCD |
Length |
r |
1 |
.070 |
.014 |
-.029 |
.265** |
.073 |
-.260** |
-.017 |
-.077 |
.013 |
|
P value |
|
.070 |
.716 |
.446 |
.000 |
.059 |
.000 |
.751 |
.161 |
.811 |
Width |
r |
.070 |
1 |
.067 |
-.011 |
.112** |
.061 |
-.033 |
-.120 |
.062 |
.067 |
|
P value |
.070 |
|
.082 |
.771 |
.004 |
.115 |
.398 |
.028 |
.260 |
.221 |
Height |
r |
.014 |
.067 |
1 |
.048 |
.059 |
.078* |
.032 |
-.021 |
.027 |
-.021 |
|
P value |
.716 |
.082 |
|
.210 |
.124 |
.043 |
.415 |
.706 |
.625 |
.702 |
Ant-Ba |
r |
-.029 |
-.011 |
.048 |
1 |
.109** |
.148** |
.145** |
.014 |
-.040 |
-.010 |
|
P value |
.446 |
.771 |
.210 |
|
.005 |
.000 |
.000 |
.794 |
.463 |
.855 |
Post-Ba |
r |
.265** |
.112** |
.059 |
.109** |
1 |
.180** |
-.042 |
-.005 |
-.002 |
-.083 |
|
P value |
.000 |
.004 |
.124 |
.005 |
|
.000 |
.278 |
.920 |
.975 |
.131 |
Ant-Opisth |
r |
.073 |
.061 |
.078* |
.148** |
.180** |
1 |
.265** |
-.060 |
.036 |
-.023 |
|
P value |
.059 |
.115 |
.043 |
.000 |
.000 |
|
.000 |
.272 |
.505 |
.671 |
Post-Opisth |
r |
-.260** |
-.033 |
.032 |
.145** |
-.042 |
.265** |
1 |
.137* |
.083 |
.021 |
|
P value |
.000 |
.398 |
.415 |
.000 |
.278 |
.000 |
|
.012 |
.128 |
.702 |
Min MID |
r |
-.017 |
-.120* |
-.021 |
.014 |
-.005 |
-.060 |
.137* |
1 |
.324** |
-.018 |
|
P value |
.751 |
.028 |
.706 |
.794 |
.920 |
.272 |
.012 |
|
.000 |
.748 |
Max MID |
r |
-.077 |
.062 |
.027 |
-.040 |
-.002 |
.036 |
.083 |
.324** |
1 |
.312** |
|
P value |
.161 |
.260 |
.625 |
.463 |
.975 |
.505 |
.128 |
.000 |
|
.000 |
|
N |
336 |
336 |
336 |
336 |
336 |
336 |
336 |
336 |
336 |
336 |
Max BCD |
r |
.013 |
.067 |
-.021 |
-.010 |
-.083 |
-.023 |
.021 |
-.018 |
.312** |
1 |
r- Pearson's correlation coefficient. ** Correlation is significant at the 0.01 level (2-tailed). *. Correlation is significant at the 0.05 level (2-tailed).
Ant-Ba= Distance from the anterior tips of the occipital condyles to the basion, Post-Ba= Distance from the posterior tips of the occipital condyles to the basion, Ant-Opisth= Distance from the anterior tips of the occipital condyles to the opisthion, Post-Opisth=Distance from the posterior tips of the occipital condyles to the opisthion, Min MID= Minimal medial intercondylar distance, Max MID= maximum medial intercondylar distance, Max BCD=Maximum bicondylar distance, OC=Occipital condyle |
| Table 5 Correlation between the right and left corresponding OC parameters |
| Variables |
r |
P-value |
Length |
0.596 |
0.001* |
Width |
0.363 |
0.001* |
Height |
0.234 |
0.001* |
Ant-Ba |
0.733 |
0.001* |
Post-Ba |
0.660 |
0.001* |
Ant-Opisth |
0.847 |
0.001* |
Post-Opisth |
0.206 |
0.001* |
r- Pearson's correlation coefficient.
*. Correlation is significant at the 0.05 level (2-tailed). |
The OC index was 1.786±0.342, and was significantly larger in males besides showing a significant difference in the various age-groups (P<0.05) (Tables 1 and 3). The left OC index was significantly larger than the right (P=0.001) (Table 2). Based on the length of the OC, majority of the condyles were classified as Type I (526, 78.3%). This was followed by Type II (136, 20.2%), and Type III (10, 1.5%). Type II was more common in males while Type I and III were more common in females. Higher frequencies of Types II, and III condyles were observed on the left while Type I was predominantly on the right (Table 6). Table 7 shows the classification of the OC in different populations.
| Table 6: Classification of Occipital condyle based on length |
| Types of OC |
Frequency (%) |
|
Right |
Left |
Male |
Female |
Average |
Type I (<2cm) |
270 (80.4) |
256 (76.2) |
296 (74.4) |
230 (83.9) |
526 (78.3) |
Type II (2-2.6cm) |
62 (18.5) |
74 (22.0) |
99 (24.9) |
37 (13.5) |
136 (20.2) |
Type III (>2.6 cm) |
4 (1.1) |
6 (1.8) |
3 (0.7) |
7 (2.6) |
10 (1.5) |
Total |
336 (100.0) |
336 (100.0) |
398(100.0) |
274 (100.0) |
672 (100.0) |
| Table 7: Classification of OC in different populations. |
| Author |
Country |
N |
Method |
Type 1 (%) |
Type 2 (%) |
Type 3 (%) |
|
|
|
|
Male |
Female |
Av. |
Male |
Female |
Av. |
Male |
Female |
Av. |
Sahoo et al. (2) |
India |
150 |
Dry skull |
|
|
10 |
|
|
87.67 |
|
|
2.33 |
Lyrtzis et al. (7) |
Greece |
141 |
Dry skull |
|
|
27.7 |
|
|
|
|
|
26.2 |
Cheruiyot et al. (4) |
Kenya |
52 |
Dry skull |
5.8 |
32.6 |
38.4 |
45.2 |
15.4 |
60.6 |
1 |
0 |
1 |
Current study |
Nigeria |
336 |
CT |
74.4 |
83.9 |
78.3 |
24.9 |
13.5 |
20.2 |
0.7 |
2.6 |
1.5 |
| Av-Average |
Discussion
The length of the OC was 2.381±0.271 cm, and this was higher than the findings by previous CT studies by Gumussoy and Duman (12), and Abo El-Atta et al. (13) in India, and Egypt respectively. Furthermore, measurements of the OC on dry skulls also revealed smaller lengths compared to our finding. (1,14,15) Consistent with previous literature reports, the males in our study had significantly larger OC lengths than the females. (1,10,13) A CT study by Bello et al. (16) documented smaller OC lengths in the male Nigerians of Sokoto State compared to the Nigerians of Delta State in our current study. Additionally, the females in their study had equal OC length bilaterally. The right OC length among the females in our study was similar to their findings however, the length of the left OC was larger. Consistent with the findings of Aristotle et al. (9), the side difference in the OC length was not statistically significant. This contrasted with Bello et al., (16) and Agarwal et al. (1)
Based on the length of the OC, majority of the condyles were classified as Type I (short) (526,7 8.3%). This was followed by Type II (moderate) (136, 20.2%) while Type III (long) (10, 1.5%) was the least type. In India, and Kenya, the skull measurements of the OC length revealed a preponderance of Type II OC. (2,4) According to these authors, the long OC was the least prevalent in Kenya, and India. Lyrtzis et al. (7) documented a higher prevalence of the long OC among Brazilians compared to the findings of the current study. Type II was more common in males while Type I and Type III were more common in females. On the other hand, in Kenya, Cheruiyot et al. (4) documented higher frequencies of Types II, and III OC in males, and Type I in females (Table 7).
The length of the OC plays a vital role in the occipito-cervical stability. The removal of the same amount of bone stock leads to greater occipitocervical instability in shorter OC compared to the longer ones due to the decreased articular surface area. (14) This causes hypermobility in the atlantooccipital joint which predisposes to neurological complications. (1,4) A long OC therefore allows for widespread resection for optimum visualization of the CVJ. (8) A third to two thirds of OC resection may not cause craniocervical instability however, total condylectomy may do so. (14) Therefore, the preponderance of Type I (short) OC in our study informs the surgeons of the risk of AOJ instability following extensive condylectomy in our population.
The width of the OC measured 1.366±0.213 cm, and this was almost similar to the findings by Priya et al. (8) on Indian dry skulls. Smaller OC widths have been documented in previous studies carried out on dry skulls, and CT images. (3,12,15) The wider OC in our study may be more demanding surgically, and require more extensive bony resection compared to narrow ones documented in the above-mentioned studies. The narrow OC are commonly associated with AOJ instability. (4) Parallel to the reports by Rai et al., (10) and Chovalopoulou, and Bertsatos, (17) the OC width was significantly larger in males than females. The right OC had larger width than the left, and the difference was statistically significant. This contradicted with the findings by Sahoo et al., (2) and Aristotle et al. (9) who did not observe any significant side difference. The width of the OC is vital in assessing the screw placement in occipital condylectomy. (9)
The height of the OC was 0.945±0.389 cm which was slightly higher than the CT findings of Bosco et al., (3) and Gumussoy, and Duman. (12) Lower OC heights have been documented from measurements on dry skulls. (8,15) The males had significantly larger OC heights than the females, and this was parallel to the reports by Cheruiyot et al. (4) Smaller left OC height was observed in the Kenyan skulls studied by Cheruiyot et al. (4) Conforming to the reports of Ilhan et al., (14) the height of the OC did not show any significant association with side in the current study. The awareness of the height of the OC is important to surgeons because a greater height of the OC is associated with successful screw placement. (8)
The mean min MID in the current study was 1.335±0.298 cm. This was lower than the findings in Egypt, and Greece, and higher than the documented reports from Sudan, and India. (10,11,13,17) The max MID measured 3.249±0.455 cm, and this was higher than the measurements on CT carried out in India, and Egypt. (10,13) The max BCD in our study measured 5.000±0.401 cm. This was higher than the findings of a previous Nigerian study by Bello et al. (16) in Sokoto. Furthermore, smaller max BCD has been documented by El-Baranny et al., (11) Rai et al., (10) and de Lucena et al. (18) in Sudan, India, and Brazil respectively. Consistent with previous literature reports, the min MID, max MID, and max BCD were significantly larger in males than in females. (1,10,11,18) This has been accredited to the larger foramen magnum dimensions in males. (1)
The OC converge ventrally hence the min MID is more anteriorly located than the max MID. (7) The intercondylar distances are important in determining the quantity of bone to be removed during the resection of the OC. (5,7) Wider intercondylar distances offer a great advantage in accessing the ventral CVJ lesions which demand less extensive bony resection for both sub occipital, and occipital condylectomy. (4,9) Extensive condylectomy requires the consideration of a narrow intercondylar space. (7) Variations in the intercondylar measurements reported in the different studies may be attributed to the differences in the angulation of the OC relative to the sagittal plane. Greater intercondylar distances are due to larger sagittal intercondylar angles. The varying prevalence of the OC overriding into the foramen magnum may also contribute to the differences in the intercondylar morphometry. (4)
The Ant-Ba distance was 0.782±0.214 cm which was lower than the reports of Ilhan et al., (14) and Anjum et al. (15) who measured this distance directly on the bone. Contrary to the reports by the above-mentioned scholars, this distance was significantly larger on the right than on the left side. Sahoo et al. (2) similarly reported a significant side difference in this distance. We report sexual dimorphism in the Ant-Ba distance. The mean Ant-Opisth distance was 3.871±0.434 cm, almost similar to the findings by Ilhan et al. (14) in Turkey. Similar to the reports of these scholars, we observed that this distance was significantly larger on the right than on the left. However, this contradicted to the findings documented by Priya et al. (8) Furthermore, the distance was significantly larger in males than in females.
The Post-Ba distance averagely measured 2.852±0.327 cm, and this was higher than the findings of Ilhan et al., (14) and Anjum et al. (15) Similar to the documented reports by Mahajan et al., (6) we did not observe a statistically significant association between this distance, and side. The Post-Ba distance showed sexual dimorphism in our study. This parameter is important to surgeons since it represents the angle of exposure in suboccipital craniotomy. (6) The Post-Opisth distance was 2.718±0.449 cm. This was slightly lower than the findings of Ilhan et al., (14) and higher than the findings of Mahajan et al. (6) This distance was significantly larger in females than in males in the current study. However, the side difference was not statistically significant, and this was in congruent with Sahoo et al. (2) This Post-Opisth distance is important during surgery whereby a longer distance provides a free corridor, and wider space for posterolateral approaches, and far lateral TCA. (14) This distance represents the width of surgical exposure in suboccipital craniotomy. (6)
The population differences in the dimensions of the OC, and its relation to the foramen magnum has been attributed to variation in genetics, race, ethnicity, diet, climate, and geographical location. (10) The discrepancies could also be ascribed to differences in the sample size, sex composition of the sample population where males were predominant in various study populations, methodology used in the data collection; anatomic versus radiologic, different reference points for taking measurements, and different methods for statistical analysis. (3,13,14) The significantly larger right OC width, right Ant-Ba, and Ant-Opisth distances compared to the left in this study correspond to the reports by Lyrtzis et al. (7) who linked this asymmetry to genetic factors, epigenetic factors as well as the individual's handedness. According to Cheruiyot et al. (4), the sexual dimorphism in the morphometric parameters of the OC is ascribed to the differences in sex hormones that influence bone growth, and development whereby, androgens cause more periosteal thickening compared to estrogen. Hence, the morphometric parameters in males are larger than in females.
We report a significant positive correlation between the corresponding right, and left OC morphometric parameters. Furthermore, this association was also observed between different dimensions of the OC, and the distance of OC tips to the basion, and opisthion. This perhaps suggests that these distances could be used to estimate each other for correct choice of OC implants, and to estimate a safe surgical corridor with minimal risk of injury to vital structures. This correlation additionally implies that a change in one lead to an alteration in the others towards the same direction. On the other hand, the significant negative correlation between the OC width, and the min MID suggests that larger OC width are associated with smaller min MID hence the possibility of a difficult access to the CVJ lesions surgically. An Egyptian study by Abo El-Atta et al., (13) found a significant correlation between the lengths of the right and left OC; widths of the right and left OC, and between max MID and min MID distances. In Greece, a study on the dry skulls by Lyrtzis et al. (7) reported a strong positive correlation between the right and left OC lengths; right and left OC widths; right and left OC height as well as the right OC length, and posterior intercondylar distance. They suggested that this strong correlation implied strong dependency between the left, and the right OC dimensions.
Consistent with the findings of Sholapurkar et al., (19) the OC index was significantly larger in males than in females bilaterally. We also observed significant side difference in the OC index with the left being larger. All the measurements showed significant differences in the various age-groups except the OC length. This contrasted with the reports by Bosco et al. (3) who documented no significant association of the OC dimensions with age groups.
Conclusion
The morphometry of the OC in the studied population differed from previous literature reports. Therefore, consideration of population specific morphometry is paramount for safe CVJ surgeries and in the design of customised OC implants.
Acknowledgements: We would like to acknowledge Priscilla Ejiroghene, and Jaiyeoba-Ojigho Jennifer Efe who assisted with data collection and data analysis respectively.
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