The cervical vertebrae are distinguished from the vertebrae of other regions of the vertebral column by the presence of the foramen transversarium (FT) within their transverse processes. The foramen transversarium is a distinctive anatomical feature formed by the specialised development of the cervical transverse process, which comprises components derived from the true transverse process and a vestigial costal element. This osseous canal provides a protected pathway for important neurovascular structures and therefore represents a structure of considerable anatomical and clinical significance. The principal structure traversing the foramen transversarium is the vertebral artery, accompanied by the vertebral veins and sympathetic nerve plexus. The vertebral arteries generally ascend through the transverse foramina of the upper cervical vertebrae before entering the cranial cavity through the foramen magnum, where they unite to form the basilar artery. The seventh cervical vertebra is an important exception, as its foramen transversarium is usually smaller and commonly transmits the vertebral vein and sympathetic fibres rather than the vertebral artery [1]. Thus, the close anatomical relationship between the FT and the vertebral vascular and neural structures makes even relatively subtle morphological variations potentially significant. Although the foramen transversarium is normally single on each side, it demonstrates considerable morphological variability. Variations may involve its size, shape, symmetry, completeness, number, and even complete absence. One of the more distinctive numerical variations is the presence of an additional or accessory foramen, resulting in a double foramen transversarium, also described in the literature as foramen transversarium bipartite [2]. An accessory foramen is generally smaller than the principal foramen and may be separated from it by a bony or fibrous septum or bridge. Depending on its morphology, the accessory opening may occur unilaterally or bilaterally. The occurrence of a double foramen transversarium is of particular anatomical interest because the formation of the FT is closely related to the development and course of the vertebral vessels. The embryological development of the vertebral artery involves longitudinal anastomoses between cervical intersegmental arteries, accompanied by selective regression and persistence of these vascular channels. Variations in this developmental process, together with alterations in the course or configuration of the vertebral artery, have been proposed as possible factors underlying the formation of accessory or duplicated foramina. However, the precise mechanism responsible for the development of a double FT remains incompletely understood. The morphological relationship between a double FT and the vertebral artery is particularly noteworthy. An accessory foramen may represent a pathway for the vertebral vein, vertebral nerve, sympathetic fibers, or, in some circumstances, a variant vascular channel. The presence of a duplicated or fenestrated vertebral artery has also been discussed in association with double foramina transversaria. Consequently, the presence of an accessory opening should not be regarded merely as an incidental osteological variation; rather, it may reflect underlying variation in the neurovascular anatomy of the cervical region. The clinical relevance of these variations is substantial. Alteration in the size, configuration, or number of foramina transversaria may influence the course and spatial relationship of the vertebral artery and associated structures. Narrowing, distortion, or abnormal bony margins may potentially contribute to neurovascular compression and have been associated in the literature with manifestations such as vertebrobasilar insufficiency, headache, dizziness, fainting episodes, and, in some circumstances, auditory or other neurological symptoms [3]. Recent anatomical and radiological observations have further emphasised that abnormal bony structures around the FT may potentially compromise the vertebral artery, vein, or nerve [4]. Knowledge of double foramina transversaria is therefore particularly important in radiological evaluation and cervical spine surgery. Variations in the FT may alter the expected radiological appearance of the cervical vertebrae and can pose difficulties in the interpretation of plain radiographs and computed tomography images. More importantly, during posterior approaches to the cervical spine, instrumentation, screw placement, decompression, and other operative procedures performed in close proximity to the transverse processes may place the vertebral artery at risk if an unrecognised anatomical variation is present. Recognition of such variations before and during surgical intervention may consequently contribute to improved operative planning and prevention of inadvertent vascular or neural injury. The reported frequency and distribution of double foramina transversaria vary considerably among different osteological studies and populations. Both unilateral and bilateral duplications have been documented, with several studies reporting a predominance of unilateral duplication. Furthermore, accessory foramina have been reported more frequently in the lower cervical vertebrae, although their distribution is not uniform across studies. Such variability highlights the importance of population-specific osteological investigations and systematic documentation of the level, laterality, and morphology of these foramina. Despite the clinical importance of the foramen transversarium, the available literature demonstrates considerable variation in the reported incidence of its duplication, and relatively fewer studies have focused specifically on the morphological characteristics and distribution of double foramina transversaria in different Indian populations. Therefore, the present study was undertaken to investigate the occurrence and anatomical characteristics of double foramina transversaria in cervical vertebrae. The present study was undertaken to determine the incidence and morphological characteristics of accessory foramen transversarium in dried cervical vertebrae to compare the observed incidence with that reported in different populations and ethnic groups worldwide.

Study Design and Setting The present study was a descriptive cross-sectional osteological study conducted in the Department of Anatomy, ESIC Medical College and Hospital, Bihta, Patna, Bihar, India. The study was undertaken to determine the incidence and morphological pattern of accessory/double foramina transversaria in dried cervical vertebrae and to compare the findings with those reported in different populations worldwide. Sample Size Calculation The sample size for the present study was determined using G*Power software (version 3.1.9.7). Considering the descriptive nature of the study and the objective of assessing the occurrence of accessory/double foramen transversarium, a chi-square goodness-of-fit test was selected for sample size estimation. The calculation was performed with an effect size (w) of 0.30, α error probability of 0.05, power (1−β) of 0.80, and 1 degree of freedom. Under these assumptions, the minimum required sample size was calculated to be 88 cervical vertebrae. To provide a margin for possible exclusion of damaged or unsuitable specimens and to facilitate more robust estimation of the incidence and subgroup distribution, the sample size was rounded up to 100 dried cervical vertebrae for the present study. Study Sample Thus, a total of 100 dried human cervical vertebrae available in the osteological collection of the Department of Anatomy, ESIC Medical College and Hospital, Bihta, Patna, Bihar, India, were included in the study. Among these, 72 were typical cervical vertebrae (C3–C6), and 28 were atypical cervical vertebrae (C1, C2 and C7). The specimens were examined macroscopically for the presence and morphological characteristics of accessory/double foramina transversaria. Inclusion Criteria • Dried human cervical vertebrae available in the departmental osteological collection. • Specimens with intact and adequately preserved transverse processes permitting clear identification and evaluation of the foramina transversaria. • Vertebrae were included irrespective of age and sex, as the demographic details of the osteological specimens were not available. Exclusion Criteria • Vertebrae with damaged, fractured, or incomplete transverse processes that prevented accurate assessment of the foramina transversaria. • Poorly preserved or severely eroded specimens in which the morphology or number of foramina transversaria could not be reliably determined. • Specimens showing gross pathological or destructive changes affecting the transverse processes or foramina. Method of Examination Each cervical vertebra was examined macroscopically for the presence, number, and morphological characteristics of the foramina transversaria on both sides. The transverse processes were carefully inspected to identify the normal single foramen transversarium and the presence of an additional or accessory foramen. A double foramen transversarium was defined as the presence of two distinct foramina within the transverse process on one or both sides, with the accessory foramen separated from the principal foramen by a bony or fibrous partition/bridge. The specimens showing duplication were photographed and documented. The vertebrae exhibiting double foramina transversaria were classified according to their laterality as: 1. Unilateral double foramen transversarium: duplication present on only one side of the vertebra. 2. Bilateral double foramina transversaria: duplication present on both sides of the same vertebra. Ethical Consideration The study involved examination of dried skeletal specimens from the departmental osteological collection and did not involve living human participants, patient intervention, or collection of personal data. Therefore, individual informed consent was not applicable. The specimens were examined solely for academic and research purposes. Statistical Analysis All data were entered into Microsoft Excel and analysed using Statistical Package for the Social Sciences (SPSS) software (IBM SPSS Statistics, Version 25.0). Categorical variables were expressed as frequencies and percentages. The Chi-square test or Fisher's exact test was used, as appropriate, to compare the distribution of accessory foramen transversarium between typical and atypical cervical vertebrae and between different patterns of occurrence. A p-value <0.05 was considered statistically significant, with a 95% confidence level.

A total of 100 dried cervical vertebrae were examined for the presence of accessory foramen transversarium. Accessory foramen transversarium was identified in 14 vertebrae (14.0%). Of these, 8 vertebrae (8.0%) showed unilateral duplication on the right side, 3 (3.0%) showed unilateral duplication on the left side, and 3 vertebrae (3.0%) demonstrated bilateral duplication. Thus, unilateral double foramen transversarium was more frequent than bilateral duplication, with a predominance of right-sided occurrence (Table 1). Among the 100 vertebrae examined, 72 (72.0%) were typical cervical vertebrae and 28 (28.0%) were atypical cervical vertebrae. Accessory foramen transversarium was observed in 9 typical cervical vertebrae (12.5%) and 5 atypical cervical vertebrae (17.9%). Although the incidence was relatively higher among atypical cervical vertebrae, the difference was not statistically significant (p = 0.527; Fisher's exact test) (Table 2). Overall, the incidence of accessory foramen transversarium in the present study was 14.0%, with unilateral and right-sided occurrences being the predominant patterns. Table 1: Side-wise distribution of accessory foramen transversarium Type of cervical vertebra U/L Double FT B/L Double FT Total Right Left Typical Cervical Vertebra 06 (6.0%) 01 (1.0%) 02 (2.0%) 09 (9.0%) Atypical Cervical Vertebra 02 (2.0%) 02 (2.0%) 01 (1.0%) 05 (5.0%) Total 08 (8.0%) 03 (3.0%) 03 (3.0%) 14 (14.0%) FT - Foramen Transversarium; U/L - Unilateral; B/L – Bilateral: Fisher's exact test; p = 0.527. Table 2: Comparison of accessory foramen transversarium between typical and atypical cervical vertebrae Type of cervical vertebra Accessory FT Present n (%) Accessory FT Absent n (%) Total n (%) P-Value Typical 09 (12.5%) 63 (87.5%) 72 (100%) 0.527 Atypical 05 (17.9%) 23 (82.1%) 28 (100%) Total 14 (14.0%) 86 (86.0%) 100 (100%) Table 3: Comparative prevalence of double foramen transversarium reported in different populations. Sr. No. Author (Year of Study) Number of Specimens Studied Incidence of Double FT (%) 1. Taitz et al. [4] (1978) 480 7% 2. Das S et al. [5] (2005) 132 1.5% 3. Sharma et al. [6] (2010) 200 8% 4. Kaya et al. [3] (2011) 22 22.72% 5. Murlimanju et al. [7] (2011) 363 1.4% 6. Laxmi C et al. [8] (2013) 210 4.76% 7. Chaudhari et al. [9] (2013) 133 16.54% 8. Rathnakar P et al. [10] (2013) 140 5.7% 9. Karau et al. [12] (2013) 102 3.9% 10. Patil et al. [11] (2014) 175 5.71% 11. Ramachandran K et al. [13] (2014) 120 15.8% 12. Murugan M et al. [14] (2014) 150 12.6% 13. Katikireddi RS et al. [15] (2014) 100 3% 14. Yadav Y et al. [16] (2014) 120 6.67% 15. Akhtar MJ et al. [18] (2015) 174 14.36% 16. Patra A et al. [23] (2015) 150 22% 17. Ulusoy M et al. [24] (2020) 86 17.4% 18. Chakravarthi K K et al. [25] (2024) 182 28.57% 19. Deepa G et al. [26] (2024) 150 10% 20. Present study (2026) 100 14%

The foramen transversarium (FT) is a distinctive anatomical feature of the cervical vertebrae, and variations in its number, morphology and laterality have been documented in different populations. The presence of an additional opening, commonly termed an accessory or double foramen transversarium, represents an important osteological variation. Differences in its reported frequency may reflect population characteristics, sample size, vertebral levels included, and differences in the criteria used to define accessory foramina. In the present study, accessory foramen transversarium was identified in 14 of 100 cervical vertebrae (14.0%). Unilateral duplication was more frequent than bilateral duplication, occurring in 11 vertebrae (11.0%), with a predominance of right-sided involvement (8.0%) over left-sided involvement (3.0%). Bilateral duplication was observed in 3 vertebrae (3.0%). Accessory foramina were identified in 9 of 72 typical cervical vertebrae (12.5%) and 5 of 28 atypical cervical vertebrae (17.9%). Although the proportion was higher among atypical vertebrae, the difference was not statistically significant (p = 0.527). These findings indicate that accessory FT is not uncommon in the studied population, although its distribution between typical and atypical cervical vertebrae does not appear to be statistically different in the present sample. Several previous studies have reported considerable variation in the incidence of double FT. Taitz et al. [4], in their examination of 480 foramina transversaria, reported double foramina in 34 cervical vertebrae (approximately 7%). They noted that the accessory opening was usually smaller than the principal foramen, although equal-sized foramina were observed in a few vertebrae, particularly at C6 and C7. They also described unusual findings, including absence of the FT in several vertebrae and a single case of triple FT. The lower frequency reported by Taitz et al. compared with the present study may be related to differences in the population studied and the composition of the vertebral sample. Das et al. [5] reported a considerably lower incidence, identifying double FT in only 2 of 132 cervical vertebrae (1.5%). Sharma et al. [6] observed accessory FT in 16 of 200 cervical vertebrae (8%), with a relatively greater occurrence in C6. In contrast, Kaya et al. [3], in their study of 22 cervical vertebrae from an ancient Byzantine population, reported double FT in 5 vertebrae (22.72%), including both unilateral and bilateral occurrences. Such marked differences between populations support the possibility of population-related variation in the development and expression of accessory foramina. Murlimanju et al. [7] reported accessory transverse foramina in only 6 of 363 cervical vertebrae (1.6%), including five bilateral and one unilateral occurrence. Laxmi et al. [8], in a series of 210 cervical vertebrae, documented double FT in 10 cases (4.76%), comprising both bilateral and unilateral forms. Chaudhari et al. [9] reported double FT in 22 of 133 cervical vertebrae, although the percentage reported in their study does not correspond to the calculated proportion based on the stated denominator. The incidence observed in the present study (14.0%) is therefore higher than that reported in several of these Indian studies, although it remains within the broad range described in the literature. Rathnakar et al. [10] observed accessory FT in 8 of 140 cervical vertebrae (5.7%). Patil et al. [11], in their study of 175 cervical vertebrae, reported double FT in 10 specimens (5.71%), with unilateral duplication being more frequent than bilateral duplication. They also noted a greater occurrence of accessory foramina in the lower cervical region. Karau and Odula [12], studying 102 cervical vertebrae from a Kenyan population, identified double FT in only 4 vertebrae (3.9%). These findings further demonstrate the considerable interpopulation variation in the prevalence of this anatomical feature. Ramachandran et al. [13] reported double FT in 19 of 120 cervical vertebrae (15.8%), comprising 9 unilateral and 10 bilateral cases. The overall frequency in their study is close to that observed in the present investigation (14.0%), although the pattern of laterality differed, as unilateral duplication predominated in the present series. They also described five morphological types of FT, with the round type being the most frequently encountered. Murugan and Verma [14] reported double FT in 19 of 150 cervical vertebrae (12.6%), a frequency that is also comparable with the 14.0% observed in the present study. Katikireddi and Setty [15] found a much lower frequency, with double FT occurring in 3 of 100 vertebrae (3%), including two unilateral and one bilateral case. Yadav et al. [16] reported 8 cases (6.67%) among 120 cervical vertebrae, with both unilateral and bilateral patterns represented. Mishra et al. [17] further highlighted the morphological diversity of this variation by reporting a rare C6 vertebra with bilateral double FT associated with a non-bifid spinous process. Taken together, the findings of these studies indicate that the prevalence of accessory FT varies substantially, ranging from relatively low frequencies of approximately 1–5% in some series to more than 20% in selected populations. The 14.0% incidence observed in the present study is therefore within the broad spectrum reported previously and is particularly comparable with the findings of Ramachandran et al. [13], Murugan and Verma [14] and Akhtar et al [18]. Differences between studies may be attributable to variation in sample size, racial and geographical background, distribution of typical and atypical cervical vertebrae, vertebral level, preservation of specimens, and the criteria used to classify a foramen as accessory. In addition, differences in the relative contribution of unilateral and bilateral forms may account for some of the variability between reports. The predominance of unilateral duplication in the present study is consistent with several previous observations. An accessory FT may occur on either side or on both sides of a vertebra, and the accessory opening is generally smaller than the principal FT. The developmental basis of this variation is not completely established. Since the cervical transverse process develops from contributions of the true transverse process and the costal element, and the vertebral artery develops through longitudinal anastomoses of cervical intersegmental arteries, alterations in the normal developmental relationship between the developing bone and vascular structures have been proposed as possible mechanisms for the formation of accessory foramina. Variations in the course or configuration of the vertebral artery may therefore influence the morphology of the surrounding osseous structures. However, the exact mechanism responsible for the development of a double FT remains uncertain. The relationship between FT morphology and the vertebral artery is of particular clinical interest. Hadley [19] and Hyyppa et al. [20] suggested that an elongated or tortuous vertebral artery may produce pressure effects and bony remodelling of the cervical vertebrae, potentially influencing the dimensions and configuration of the FT. Such observations provide a possible explanation for some cases of asymmetry or unusual morphology of the FT. Epstein [21] also discussed differences in the calibre of the vertebral arteries, including the tendency for the left vertebral artery to be larger in some individuals, which may contribute to asymmetrical osseous morphology. Nevertheless, the relationship between arterial calibre and FT dimensions should not be considered uniformly predictable, and further radiological and anatomical studies are required to establish a direct association. The presence of an accessory FT may also have implications for the structures passing through or adjacent to it. Depending on its size and anatomical configuration, the accessory opening may transmit venous, neural or sympathetic structures and, in selected cases, may be related to variations in the vertebral artery. Duplication and fenestration of the vertebral artery represent additional vascular variations that may coexist with unusual FT morphology. Sim et al. [22] reviewed cases of extracranial vertebral artery fenestration and highlighted the importance of recognising such vascular variations. Although the present osteological study cannot establish which neurovascular structure traversed the accessory foramina, the possibility of an underlying vascular variation emphasises the clinical relevance of identifying this anatomical feature. From a clinical perspective, recognition of double FT is important during radiological evaluation and operative procedures involving the cervical spine. An accessory opening may alter the expected appearance of the transverse process on radiographs and computed tomography and should not be mistaken for a pathological lytic lesion or other osseous abnormality. More importantly, the close relationship of the FT to the vertebral artery makes knowledge of its anatomical variations relevant during posterior cervical instrumentation, screw placement and other procedures performed near the transverse processes. Preoperative identification of unusual FT morphology may therefore assist in surgical planning and help reduce the risk of inadvertent neurovascular injury. The present study also demonstrated a slightly higher proportion of accessory FT among atypical cervical vertebrae (17.9%) compared with typical cervical vertebrae (12.5%). However, this difference was not statistically significant (p = 0.527). Therefore, the observed difference should be interpreted as a descriptive finding rather than evidence of a true association between vertebral type and the occurrence of accessory FT. Larger studies with adequate representation of individual vertebral levels would be valuable to determine whether a genuine predilection exists for particular cervical vertebrae. Overall, the findings of the present study add to the existing osteological literature by documenting the occurrence and laterality of accessory FT in cervical vertebrae from the Bihar population. The observed 14.0% incidence, predominance of unilateral duplication, and greater occurrence on the right side reinforce the considerable morphological variability of the FT. The comparison with previously published studies also demonstrates that the frequency of this variation differs substantially among populations, underscoring the value of population-specific anatomical studies. Further investigations combining osteological examination with computed tomography or vascular imaging could provide better insight into the relationship between accessory FT morphology and vertebral artery variations and would help clarify its clinical significance. Limitations of the study: The present study has certain limitations. The study was conducted on a relatively small sample of 100 dried cervical vertebrae obtained from the departmental osteological collection, which may limit the generalizability of the findings to the wider population. The age and sex of the specimens were unknown, and therefore, any possible association of accessory foramen transversarium with age or sex could not be assessed. As the study was based on dry bones, the actual neurovascular structures passing through the accessory foramina could not be determined. Furthermore, the study was primarily morphological and osteological in nature; therefore, radiological correlation and assessment of the relationship of the accessory foramen with the vertebral artery were not performed. Larger multicentric studies incorporating radiological and anatomical correlation would be useful to further elucidate the clinical significance of this variation.

The present osteological study demonstrates that accessory foramen transversarium is a relatively frequent anatomical variation of the cervical vertebrae, occurring in 14% of the specimens examined. Unilateral duplication was more common than bilateral duplication, with a clear predominance of right-sided occurrence. Although accessory foramen transversarium was proportionally more frequent among atypical cervical vertebrae than typical cervical vertebrae, the difference was not statistically significant (p = 0.527). Recognition of this variation is important because of the close anatomical relationship of the foramen transversarium with the vertebral artery and other neurovascular structures. A thorough understanding of such variations is therefore relevant to osteological identification, radiological interpretation, and preoperative planning for cervical spine procedures, where unrecognised anatomical variations may increase the risk of neurovascular injury.

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