Prevalence of anatomic variations of the atlas vertebra

Anatomical variations of the atlas arches: prevalence assessment, systematic review and proposition for an updated classification system

Objective and background

This study focuses on the atlas, a pivotal component of the craniovertebral junction, bridging the cranium and spinal column. Notably, variations in its arches are documented globally, necessitating a thorough assessment and categorization due to their significant implications in clinical, diagnostic, functional, and therapeutic contexts. The primary objective is to ascertain the frequency of these anatomical deviations in the atlas arches among a Colombian cohort using cone-beam computed tomography (CBCT).

Methodology

Employing a descriptive, cross-sectional approach, this research scrutinizes the structural intricacies of the atlas arches in CBCT scans. Analytical parameters included sex distribution and the nature of anatomical deviations as per Currarino's classification. Statistical analyses were conducted to identify significant differences, including descriptive statistics and Chi-square tests. A systematic review of the literature was conducted in order to enhance the current Currarino's classification.

Results

The study examined 839 CBCT images, with a nearly equal sex distribution (49.7% female, 50.3% male). Anatomical variations were identified in 26 instances (3%), displaying a higher incidence in females (X2 [(1, N = 839) = 4.0933, p = 0.0430]). The most prevalent variation was Type A (2.5%), followed by Type B (0.4%), and Type G (0.2%) without documenting any other variation. The systematic review yielded 7 studies. A novel classification system for these variations is proposed, considering global prevalence data in the cervical region.

Conclusion

The study highlights a statistically significant predominance of Type A variations in the female subset. Given the critical nature of the craniovertebral junction and supporting evidence, it recommends an amendment to Currarino's classification to better reflect these clinical observations. A thorough study of anatomical variations of the upper cervical spine is relevant as they can impact important functional aspects such as mobility as well as stability. Considering the intricate anatomy of this area and the pivotal function of the atlas, accurately categorizing the variations of its arches is crucial for clinical practice. This classification aids in diagnosis, surgical planning, preventing iatrogenic incidents, and designing rehabilitation strategies.

Systematic review and meta-analysis of the prevalence of the retrotransverse foramen of the atlas

The retrotransverse foramen (RTF) is a nonmetric variant of the atlas vertebra that can accommodate an anastomotic vertebral vein and occipital nerve. An understanding of this variation and its occurrence is crucial, as it could aid in explanation of the unidentified cause of the high prevalence of variability in this region. The aim of this meta-analysis was to obtain data on the prevalence of the RTF and its variations according to anatomy, sex, and ethnicity. A large-scale search was conducted through the major online databases to establish and determine the pool of studies reporting data relevant to the RTF. No date or language restrictions were applied. The data collection was categorized by prevalence, type (incomplete/complete), side, sex, ethnicity, laterality, and diameter. A total of 17 studies (n = 1979 subjects) were incorporated into our analysis. The overall pooled prevalence for a complete RTF was 11.4% and the overall pooled prevalence of an incomplete (partial) RTF was 9.6%. A complete RTF was most prevalent in Africa (Sub-Saharan population) (12.1%), followed by Europe (11.8%) and Asia (9.7%). As this variant occurs in a significant number of patients in all of the aforementioned populations, recognition, and awareness, especially with thorough investigation with computer tomography angiography (CTA) should be implemented, as it is the only possible way to visualize the possible contents of RTF.

Radiological Study of Atlas Arch Defects with Meta-Analysis and a Proposed New Classification

This study consists of a retrospective cohort study, a systematic review, and a meta-analysis which were separately conducted. This study aimed to investigate the prevalence of atlas arch defects, generate an evidence-based synthesis, and propose a common classification system for the anterior and combined atlas arch defects. Atlas arch defects are well-corticated gaps in the anterior or posterior arch of the atlas. When both arches are involved, it is known as a combined arch defect. Awareness of these defects is essential for avoiding complications during surgical procedures on the upper spine. The prevalence of arch defects was investigated in an open-access OPC-Radiomics (Radiomic Biomarkers in Oropharyngeal Carcinoma) dataset comprising 606 head and neck computed tomography scans from oropharyngeal cancer patients. A systematic review and meta-analysis were performed to generate prevalence estimates of atlas arch defects and propose a classification system for the anterior and combined atlas arch defects. The posterior arch defect was found in 20 patients (3.3%) out of the 606 patients investigated. The anterior arch defect was not observed in any patient, while a combined arch defect was observed in one patient (0.2%). A meta-analysis of 13,539 participants from 14 studies, including the present study, yielded a pooled-posterior arch defect prevalence of 2.07% (95% confidence interval [CI], 1.22%-2.92%). The prevalences of anterior and combined arch defects were 0.00% (95% CI, 0.00%-0.10%) and 0.14% (95% CI, 0.04%-0.25%), respectively. The anterior and combined arch defects were classified into five subtypes based on their morphology and frequency. The present study showed that atlas arch defects were present in approximately 2% of the general population. For future studies, larger sample sizes should be used for studying arch defects to avoid the small-study effect and to predict the prevalence accurately.

Proposing novel dorsal Proatlas-manifestations: Description and classification of three rare phenomena in humans

The craniocervical junction (CCJ) of humans and other vertebrates is a developmental restless region. Due to complex phylogenetic and ontogenetic processes, many anatomical variations can be found in that transitional area. Therefore, newly described variants must be registered, named, and classified into existing concepts explaining their genesis. This study aimed to describe and classify anatomical peculiarities that have not or rarely been reported on before in the literature. This study is based on the observation, analysis, classification, and documentation of three rare phenomena of three different human skull bases and upper cervical vertebrae, which come from the body donor program of the RWTH Aachen. As a result, three osseous phenomena (accessory ossicles, spurs, and bridges) at the CCJ of three different body donors could have been documented, measured, and interpreted. Due to extensive collecting efforts, careful maceration, and accurate observation, it is still possible to add new phenomena to the long list of Proatlas-manifestations. Further on, it could have been shown again that these manifestations can cause damage to the elements of the CCJ due to altered biomechanic conditions. Finally, we have succeeded in showing that phenomena can exist that can imitate the presence of a Proatlas-manifestation. Here, a precise differentiation between Proatlas-based supernumerary structures and the results of fibroostotic processes is necessary.

The computed tomographic evaluation of bony bridge of C1 as bleeding risk factor at the screw placement

PURPOSE

To investigate the prevalence and morphological characteristics of ponticulus posticus (PP) and ponticulus lateralis (PL) using computed tomography (CT) images on a large study sample of the Anatolian population. The presence of the PP and PL bridges can limit gap available for placement procedure through the bony elements of C1. Routine screw techniques are contraindicated because of high risk of fatal bleeding of vertebral artery (VA).

METHODS

The CT images of 1000 subjects (500 males, 500 females) were examined for the morphological characteristics and presence of PP and PL. The anteroposterior diameter, superoposterior (transverse) diameter, surface area, and central thickness of the bony bridge of the PP, PL, and transverse foramina (TF).

RESULTS

The prevalence of PP was 14.8%, and bilateral complete PP was the most common PP type at 6.8%. The prevalence of PL was 4.1% and left-side complete PL was the most common PL type at 1.2%. The prevalence of both PP and PL was more common in males and bilateral complete PP were more predominant in males (p = 0.004, p = 0.038, and p = 0.010, respectively). The surface area of PP and PL were determined to be smaller than the surface area of the ipsilateral TF (p < 0.001 and p = 0.042, respectively).

CONCLUSION

PP is not an uncommon anatomic anomaly and PL is even less frequently encountered. The prevalence of PP and PL was more common in males and bilateral complete PP was more predominant in males. Detailed information about the prevalence and morphometry of the PP and PL obtained in the present study could guide the clinicians dealing with neurosurgery, physical medicine and rehabilitation, and radiology in their practice.

Ontogeny of morphological variations in the vertebral column: Prevalence and bony variability in young Spanish children

Pre- and postnatal development and variability in discrete vertebral traits have been poorly described in embryonic studies. Numerous authors have reported that these variations are observable only from adolescence; scientific publications on the vertebrae of fetuses and infants are scarce. Thus, the aims of this study were to (1) describe the ontogeny and variability of anatomical variations in the vertebral column of a Spanish infant population and (2) analyze the frequency and relationship between sex, age, and intertrait variables. A total of 4728 vertebrae from 197 skeletons were studied. The age at death ranged from 22 intrauterine weeks to 8 years. Twenty morphological traits related to vertebral column development were analyzed. A descriptive statistical analysis was performed, and the chi-square test was used to measure the relationship between sex, age, and intertrait variables. We observed that 88.32% of skeletons expressed discrete traits along the spine. In fetuses, the double transverse foramen and unclosed transverse process of the axis were the most prevalent traits. In infants older than one year, the appearance of the L5 cleft neural arch, unclosed transverse process of the atlas, and craniocaudal shifts were frequent. A significant result was found between sex and the unclosed transverse process in the axis. The intertrait relationship was significant for all traits that shared the same embryonic structure. Morphological variations became visible following the appearance of ossification centers during the pre- and postnatal periods, and their etiology was associated with embryonic development.

Arterial Vascular Structures Running Through Retrotransverse Foramen and Retrotransverse Groove of the Atlas Vertebrae

OBJECTIVE

Retrotransverse foramen (RTF) and retrotransverse groove (RTG) are anatomic variations of the atlas (C1) vertebrae. RTF contains an anastomotic vein connecting atlanto-occipital and atlanto-axodian venous sinuses. The purpose of this study was to analyze the arterial vascular structures running though the RTF and RTG.

METHODS

Three-dimensional volume rendered computed tomography angiography (3D VR CTA) images of 427 patients (264 men, 163 women; age 17-87 years) were reviewed and evaluated using the RadiAnt DICOM Viewer (version 5.0.2; Medixant, Poznan, Poland). The incidence of RTF or RTG, the incidence of the V3 segment of vertebral artery variants, and the artery vascular structures inside the RTF and RTG anatomic variation of C1 were analyzed.

RESULTS

Fifty (11.7%) atlases presented RTF anatomical variant; 113 (26.5%) atlases presented RTG anatomical variants. The incidence of the V3 segment of vertebral artery variants was 0.94% (4 of 427). Three (0.7%) were persistent first intersegmental artery and 1 (0.2%) was the fenestration of the vertebral artery on left side. In 4 cases of C1 vertebral artery V3 segmental variants, there were no RTF and RTG. No artery vascular structure was found in RTF or RTG.

CONCLUSIONS

The RTF or RTG of C1 was a common anatomical variant. No arterial vascular structure runs though the RTF or RTG. The presence of C1 RTF and RTG variants had no effect on the V3 segmental course of the vertebral artery. Preoperative understanding of these variations using 3D CTA are helpful for the safe execution of the upper cervical posterior approach surgeries.

Retrotransverse Foramen and Retrotransverse Groove Anatomic Variations of the Atlas Vertebra in the Chinese Population

OBJECTIVE

To analyze the prevalence of retrotransverse foramen (RTF) or retrotransverse groove (RTG) anatomic variations in Chinese atlas vertebra (C1).

METHODS

Three-dimensional volume-rendered computed tomography angiography images of 427 subjects (264 males, 163 females; 17-87 years old) were reviewed and evaluated using dedicated software. The prevalence of RTF and RTG anatomic variation of C1 was analyzed.

RESULTS

RTF anatomic variants were present in 50 (11.7%) atlases. Bilateral RTF, unilateral left RTF, and unilateral right RTF were present in 16 (3.8%), 20 (4.9%), and 14 (3.3%) vertebrae. Comparison between males and females revealed differences in bilateral RTF (P = 0.010) and unilateral left RTF (P = 0.008). RTG anatomic variants were present in 113 (26.5%) atlases. Bilateral RTG, unilateral left RTG, and unilateral right RTG were present in 39 (9.1%), 30 (7.0%), and 44 (10.3%) vertebrae. Comparison between males and females revealed differences in RTG (P = 0.000), bilateral RTG (P = 0.006), and unilateral left RTG (P = 0.034). RTF was detected in 36 cases on the left and 30 cases on the right. RTG was detected in 69 cases on the left and 79 cases on the right. There were no side differences in the prevalence of RTF and RTG.

CONCLUSIONS

The incidence of RTG is higher than the incidence of RTF. Incidence of bilateral RTF, bilateral RTG, unilateral left RTF, unilateral left RTG, and RTG differed between males and females. Preoperative understanding of these variations using three-dimensional computed tomography angiography is helpful for safe execution of upper cervical posterior approach surgery.

Incidental findings of posterior arch defects of the atlas in orthodontic patients: A case series

INTRODUCTION

Congenital defects of the posterior arch of atlas are uncommon anomalies that may go unnoticed. Radiographs and cone-beam computed tomography scans made for orthodontic purposes often show the atlas; therefore, these rare defects might first be noted as incidental findings on orthodontic records.

METHODS

A series of 7 cases of posterior arch defects of atlas (C1) that were detected during routine radiographic examination are presented here.

RESULTS

The distribution of the defects was as follows: type A, 2; type B, 1; type C, none; type D, 1; and type E, 3.

CONCLUSION

As the potential complications associated with these defects may be quite concerning, a thorough analysis of the radiographs for these defects is imperative.

Krapina atlases suggest a high prevalence of anatomical variations in the first cervical vertebra of Neanderthals

The first cervical vertebra, atlas, and its anatomical variants have been widely studied in Homo sapiens. However, in Neanderthals, the presence of anatomical variants of the atlas has been very little studied until very recently. Only the Neanderthal group from the El Sidrón site (Spain) has been analysed with regard to the anatomical variants of the atlas. A high prevalence of anatomical variants has been described in this sample, which points to low genetic diversity in this Neanderthal group. Even so, the high prevalence of anatomical variations detected in El Sidrón Neanderthal atlases needs to be confirmed by analysing more Neanderthal remains. In this context, we analysed the possible presence of anatomical variants in the three Neanderthal atlases recovered from the Krapina site (Croatia) within the Neanderthal lineage. Two of the three Krapina atlases presented anatomical variations. One atlas (Krapina 98) had an unclosed transverse foramen and the other (Krapina 99) presented a non-fused anterior atlas arch. Moreover, an extended review of the bibliography also showed these anatomical variations in other Middle and Upper Pleistocene hominins, leading us to hypothesise that anatomical variations of the atlas had a higher prevalence in extinct hominins than in modern humans.