1. Morphological features
It is seen that no consensus exists between the results reported by researchers regarding the frequency of PF, its distribution between sides, and PF on SS (Table 4). The prevalence of PF varies between 50% (USA) and 86% (Thailand) in different ethnic groups [14, 30]. The frequency of bilateral PF ranged from 24.2% (European) to 74.65% (Turkey), while the frequency of unilateral PF ranged from 20.2% (African) to 66.07 (Chinese) right side and 63.21 (Chinese) left side [8, 12, 27]. Similarly, the frequency of PF on the SS ranged from 0.7% (Japan) to 33.7% (Turkey) [8, 15]. The PF rate in our study (64.3%) appears to be very close to the results of Keskil et al. [11], which was previously conducted on the Turkish population (63%). However, it is not consistent with a recent study on the Turkish population by Guzelad et al. [8]. Since the parietal bones in our study were not bilateral (does not belong to the same individuals), bilateral and unilateral rates could not be given.
Our PF rate on SS is almost the same as the studies in the literature [11, 18, 30]. Knowledge of PF variations is so important to prevent damage to the PEVs, anastomosing arteries, and neural structures that may pass through the PF during neurosurgical procedures [1]. According to our results, the prevalence of PF in the Turkish population is approximately 3/5 and its localization is variable. Therefore, we believe that during invasive procedures involving the PF or its adjacent structures in the Turkish population, the surgeon's awareness of the presence of the PF and its altered localization may be important not only in increasing the success rate of the procedure but also in preventing/minimizing the risk of complications.
In the literature, PF is often reported to be absent, single or double [2, 3, 15, 18, 21, 22, 29]. In some studies, it was noted that this foramen may be 3 or more in number [4, 8, 9, 17, 26, 30]. Liu et al. [12] stated that all the PFs they observed were singles. Murlimanju et al. [17] studied a total of 232 bilateral parietal bones and reported that PF was single in 73 parietal bones (62.9%), double in 8 bones (6.9%), and triple in 2 parietal bones (1.7%). Similarly, Gangmei et al. [6] studied 96 parietal bones bilaterally and found that PF was single on 71 bones, double on 2 right sides, and triple on 1 right side. Fating and Pawar [5] examined 400 parietal bones bilaterally and found that PF was single in 113 right bones (56.5%) and 119 left bones (59.5%) and double in 4 right bones (2%) and 1 left bone (0.5%). In one of the two previous studies conducted in the Turkish population, Guzelad et al. [8] mentioned the foramen feature (single, double, etc.) but did not give any rate. Keskil et al. [11] are that they did not mention the PF’s feature. In our study, 112 of a total of 174 bones had PF. Of the 112 parietal bones, 107 (61.4%) had single PF, 4 had double PF (2.29%), and 1 had triple PF (0.57) (Figure 4). According to the results of our study, the rate of parietal bone containing at least one PF is approximately 65%. When PF is present, valveless PEVs pass through it, connecting extracranial scalp veins to the SSS [30]. Since neurosurgeons often use the PF to access intracranial structures, failure to recognize PF variants can damage the PEVs, resulting in spontaneous hemorrhage and sinus thrombosis [1]. In this context, we believe that not recognizing the PF variants in our study by surgeons may prevent damage to the PEVs thus resulting in spontaneous hemorrhage and sinus thrombosis in neurosurgery practices in the Turkish population.
In terms of the shape of the PF, very few studies have reported PF's features. de Souza Ferreira et al. [4] reported that the vast majority of PFs were round. Mahakkanukrauh et al. [14] observed three different shapes such as circular (94 %), oval (4 %), and irregular (2 %). Similarly, Guzelad et al. [8] observed circular, oval, slit-like, and enlarged PFs in their study. However, they did not report the rate of these shapes. In the current study, circular (94.1%) and oval (5.9%) types of PF were determined (Figure 3a, b).
Berge and Bergman [2] highlighted that the size of the PF and its distance from the sagittal midline affected the shape of the foramen in the relevant region. In our study, we think that the shape of the PF changes due to obtaining different sizes of PF (Table 2) and changing its distance from the sagittal midline (Table 2). Magalhães et al. [13] stated that calcification of the PF may cause an individual to experience migraine. We think that the calcification of the PF may be related to its shape. PF shape may play a role in the development of calcification, especially in oval-type PF. However, we cannot comment on this issue because we do not have any data.
In the present study, most of the PFs were open (70.4%). Al-Shuaili et al. [1] observed incomplete PF without intracranial communication was 2% (9/440). There is a huge difference between these two studies. The possible reason may be different sample sizes and/or different races. Only the study of Liu et al. [12] investigated the direction of PF.
In their observational study of 560 parietal bones bilaterally, they classified 178 bones with single-hole PF according to their directions and stated that 39.70% (46) anteromedial direction, 17.65% (21) anterior direction, 13.24% (15) anterolateral direction, 13.24% (15) anterosuperior direction, 11.76% (14) anteroinferior direction, 2.94 (3) mediosuperior direction, and 1.47%(2) medioinferior direction. This parameter has been evaluated for the first time in studies on the Turkish population. In our study, we assessed all PF’s direction and the most common was posteroanterior direction (61.9%) (Table 1). In normal aging, brain atrophy occurs with increasing age [24, 25]. Therefore, we think that the PEVs that provide extracranial and intracranial communication may become tense and prone to fragility. The direction of the PF may play a role in increasing the fragility and tension of these veins. For example, an EV connected to the SSS with an axis exactly perpendicular will be less stressed than an EV connected to the SSS more inclined. The values we obtained regarding the direction of the PF may be important from this perspective. Especially in invasive procedures involving the PF, taking into account the direction of the PF and the EV passing through it may be important to prevent complications that may develop in the procedures.
2. Morphometric features
The reliability analysis results of morphometric data of PF show that the results of the study are reliable (Table 3). The list of diameter measurements in the studies reported in the literature is given in Table 4. The majority of these studies used external diameter as a reference, while only the studies of Yoshioka et al. [30] measured both internal and external diameter. In our study, unlike previous studies, external diameter was categorized into 3 classifications. If the PF diameter was 0.5 ≤ DPF <0.8, it was described as small size PF, if the PF diameter was 0.8 ≤ DPF < 1.25, it was described as medium size PF, and if the PF diameter was 1.25 ≤ DPF, it was described as large size PF. Accordingly, the most common PF diameter type was medium size (42.3%), followed by large size (33.1%) and then small size (24.6%). We think that EVs passing through medium and large PFs are also large in diameter, and we emphasize that these morphological features can be easily recognized during invasive procedures. However, realizing the PF by the surgeon, which was observed at a rate of approximately ¼ in our study and through which small-diameter EV probably passes, may be important in preventing related possible complications.
In terms of morphometric distances, the researchers measured PF according to different landmarks (sagittal suture, lambda, bregma, inion vs.) (Table 4). In different ethnic groups, the DSS value ranges from about 5 mm (Thailand) to 12 mm (Turkey) [8, 14]. In this study, the mean DSS was 6.81 ± 3.40 mm. Our results are compatible with the results of Fating et al. [5]. However, they are not compatible with the study of Guzelad et al. [8] that was conducted on the Turkish population. The probable reason is the different sample sizes.
In this study, the LSS and DPL were meanly 108.94 ± 6.91 mm and 33.81 ± 12.43 mm, respectively. To explore the topographic location of the PF, we calculated the LSS/DPL ratio (TPL) and it was approximately 3/1. Collipal et al. [3] determined the PF was located at the junction of the SS with the lambda. Tsutsumi et al. [26] reported that the localization of PF is on the SSS. Murlimanju et al. [17], Shantharam and Manjunath [21], and Halagatti and Sagar [9] agreed that similar to the results of our study, PF is located between the middle and posterior 1/3 of the parietal bones. Shmarhalov et al. [22] reported that the most common location of PF was at the side of the sagittal suture, mid of the distance between the vertex and lambda. The size, presence, content, and topography of PFs are variable. In this context, identifying new anatomical features and learning the variable topographic localization of PFs is crucial for invasive procedures involving the PF Keskil et al. [11]. Thus, damage to these structures during invasive procedures and subsequent complications can be prevented [22]. We believe that our morphometric data collectively may be a guide to easily predict the location of the PF and to increase surgeons' awareness in this region in applications for or including the PF.
Limitations of the study: This study has several limitations. The first is the relatively small sample size. The second is that the parietal bones did not belong to the same individual and therefore, statistical analysis by side and gender could not be performed. Despite these limitations, the fact that our results are consistent with the literature strengthens the results of the study. Moreover, the results of the reliability analysis of the data obtained show that the results are very satisfactory.