Previous studies have reported on FT morphology, including variations in size and shape. Clinically, variations of FT could cause vertebral compression, leading to neurological symptoms. Additionally, these variations are important for surgeons and radiologists to consider prior to surgery on the posterior upper cervical spine [
13]. The morphometric alterations of FT diameters have been documented in many populations including Chilean [
6], Kenyan [
8], Greek [
4], Indian [
7,
14], American [
15], Turkish [
16], Japanese [
17], Chinese [
17], and Korean [
18]. We herein observed the shape and size of the FT in Thais for the first time. Using Digital Vernier Caliper (Mitutoyo), our results showed shorter FT diameters in both sides of the upper cervical spine compared with Chilean and Kenyan populations [
6,
8]. The transverse diameter of the male C1 observed in one recent study was significantly greater than that of females, unlike what was found for Kenyans [
8], which showed AP diameters in males that were significantly greater than in females. Similar to Asian populations [
17], transverse diameters in Thais were significantly greater in males than in females. However, the transverse diameter in Kenyans was not different between sexes, whereas its diameter was greater in Thai males. In South Africans, no difference in FT diameters of the upper spine vertebrae have been found [
5]. In general, the transverse diameter of the left side is greater than that of the right side in some nationalities including the Thai population [
5–
7], as summarized in
Table 5. In the literature, the shape of the FT can be classified into 5, 12, and 13 types [
5–
8,
17]. The FTs in this study were classified into five types as shown in
Fig. 4. We found that the type 2 FT in C1 is mostly investigated (male, 69.81%; female, 79.63%) which is similar to that of the Kenyan [
8], South African [
5], Japanese, Chinese, and Indian populations [
17]. In contrast, mostly only type 4 in Israeli and types 4 and 5 in Chilean and South African populations were found, with equal incidence [
5–
7]. However, Kimura et al. [
17] in 1985 reported that mostly type 5 was observed in the Japanese, Chinese, and Indian population. For C2, the most common type of FT found in Thailand was type 1, which is similar to that reported in many countries [
5–
7,
17]. In addition, the accessory FT (also called retrotransverse foramen) of C1 most commonly found in this study was BFT (25.23%), which was higher than that found in the Romanian (1.41%) [
19], South African–Spanish (1.5%) [
20], Greek (1.61%) [
21], Spanish (2.27%) [
22], Indian (4%) [
23], and Indian populations (4.08%) [
24]. The incidence of UFT was approximately 9.38%, which was lower than that found in the Indian population (18%) [
23]. In contrast, this incidence was higher in South Africa–Spanish (5.8%) [
20]. Furthermore, the presence of TFT and HFT in C1 from our study was 1.86%, which has not been reported previously. The finding of HFT in both C1 and C2 (1.86%) agreed with that found in C6 (1.1%) of the Italian population [
25]. Moreover, the incidence of FA, sulcus or retroarticular canal for the vertebral artery, was found to be highest in the IOB subtype (9.35%). However, the highest incidences of UCFA were reported in the northern Greek (24.43%) [
26] and Turkish populations (15.38% and 10%, respectively) [
16,
27]. The incidence of the FA subtype (BCFA) observed in the Thai population was 0.93%, which was low and comparable with that of the northern Greek (1.13%) [
26] and Turkish (1.6%) populations [
27]. In contrast, Cirpan et al. [
16] in 2017 reported that BCFA had not been investigated in the Turkish population.