PF is a recognizable anatomical landmark that can help pinpoint nearby eloquent structures, both in vivo and in MRI studies15–17. This can cut back surgery time as it increases accuracy, especially during emergency surgeries and in hospitals that lack access to a reliable neuronavigation equipment18–20. It is also obvious that misplacement of the craniotomy window over the emissary vein can result in iatrogenic morbidity, either as consequence of SSS air embolism or as a pathways by which infections are carried into the cranial cavity3,4. So, surgeons should be aware of the PF position and consider this anatomic variation.
The present study has revealed the topography of the PF in Brazilian individuals. The PF was found in 69.7% of cases, always anteriorly to the lambda, and laterally to the sagittal suture. One strength of our study is that—in order to make accurate assessments—we used contrast-enhanced MRI imaging instead of dry skulls and confirmed that vascular tethers penetrate the foramen. Little holes without emissary veins were not considered foramina in this study4. Also, we were able to compare different external landmarks to find the most stable guide to the PF.
Inion, bregma and lambda were used to determine PF location (Fig. 1 and 2) within a 20 mm radius from each parameter’s mean value, a different PF scatter pattern was generated. 92.6%, 91.8%, 90.1% of the PFs were inside that radius when measured from the (#1) lambda (mean 35.3mm), (#5) inion (mean 102.8 mm) and (#4) bregma (mean 94.9mm), respectively. This could be due the different cranial trajectories implied in each measurement. An opposite relation was found when analyzing the coefficient of variance (Table 1). Bregma (8.8%) had the smallest variation by a substantial margin, followed by inion (13.4%) and lambda (19.6%). Additionally, lambda is a landmark that can hardly be palpated on a clinical setting. This gives an advantage to both bregma and inion, as they are easily visible and palpable.
A possible reason for lambda and inion to present a bigger coefficient of variance than the more stable bregma is a large variance in the squamous part of the occipital bone’s area. whose mean was 10,477mm2 ± 1,792mm2 with a coefficient of variance 17.1%, a value intermediate to inions (13.4%) and lambdas (19.6%). Such variation affects lambda more than the inion, as the lambda is at the peak of the occipital bone, influenced by the whole bone height, whilst the inion is in the occipital bone’s middle third. Additionally, the occasional presence of Wormian bones inside lambda can decrease this points accuracy to find the PF21.
On the other hand, bruner at al. has shown that the distance between both the paracentral lobule and bregma, and the parietal lobe and lambda are actively modified by variations in precuneal area size. Hence, the relative cranial position to the cerebral landmarks do vary as a function of the parietal area size22. These findings mean that the specific position of the cerebral parenchymal structures is sensitive to multiple factors, without necessarily a correspondence with the bone landmarks. This eventual absence of correspondence between brain and skull boundaries suggests caution when making inferences about the brain areas from the position of the cranial sutures23.
Amalgamation of this data suggests that bregma can successfully be palpated and used to find PF in a clinical environment, as it is mostly found inside the 20 mm radius. Additionally, when the PF is in fact inside the radius, its consistency is increased, as illustrated by the coefficient of variation. Thus, studies and procedures using bregma as reference should be prioritized.
One particularly interesting finding was that the PF and emissary veins may provide a useful landmark of the SSS, which lies immediately below. Contrast-enhanced MR imaging is useful for delineating the vascular components of the PF4,5,8. The relationship between PF and SSS was measured and compared to previous literature findings of the SSS–sagittal suture and SSS- midline relationships, at the posterior third of the suture24,25. The correlation between the left PF and the SSS was mostly constant, especially in the left hemisphere. 88% of the 62 left PF’s were situated within 1cm, laterally to the left margin of the SSS. The right PF’s distribution was more spread. 61.7% were situated within 1.4 cm laterally from the right margin of the SSS, while 38.3% were directly above the SSS (figure 3).
In a 30-specimen cadaveric study, the SSS was deviated to the right of the sagittal suture in 63%, laid under the suture in 20% and deviated to the left in 17%24. It appears that the relation between SSS and the sagittal suture tends to be less constant in this area of the crania when compared to PF-SSS. Another study concluded that the SSS was consistently displaced on either side of midline. Thus, the midline is not reliable for identifying the SSS25. This could mean that the PF is a more reliable landmark for predicting the SSS position and navigating this region than the sagittal suture and midline, with highlight to the left PF, especially useful to help surgeons avoid iatrogenic injuries in emergency surgeries.
Computerized tomography technique help physician to identify both the underlying emissary vein and SSS in individual patients and so also reduce the risk of direct puncturing the vascular bundle. However, considering the possibility of a lack of reliable equipment or the presence of emergency surgeries, we suggest a simple method that would provide a convenient guideline and aide the clinician in avoid damaging the vessels and potentially fatal hemorrhage. We propose that incisions 1 cm laterality to PF should avoid injuring vascular structures. As even in the cases where the PF was directly above the SSS (11% on the left and 39% on the right), its respective margins distanced no more than 4 mm, to the left PF and 8.5 mm to the right PF.
Parameter #8 small coefficient of variance (9.6%) suggest a vascular relationship in which the confluence of sinuses or adjunct venous structures, may influences PFs positioning, possibly through the formation of the emissary vein. This valveless vein plays an important role in regulating intracranial pressure and brain temperature. its ability to modify the direction of blood from the intracranial space to the extracranial one or vice versa. In the scalp the temperature is, most of the time, lower than in the intracranial compartment and the exchange of blood may regulate the intradural temperature. Furthermore, in patients with parasagittal meningiomas, the emissary vein can supply the tumor. The emissary vein can be used as access to embolize dural vessels26. So, Accurate localization of the PF and emissary vein on radiological examination followed by careful obstruction of the EV can lessen the intraoperative blood loss or SSS risk of injure. Thus, awareness of the PF can also lead to development of new therapeutic strategies.
The angular gyrus is a functionally asymmetric cortex, consistently activated in a variety of tasks. Damage to the left gyri has a devastating effect on word processing, while stimulation on the right, can trigger out-of-body experience27,28. Hence, careful maneuver around such area is necessary. Additionally, anatomical laterality asymmetry is also present. When measured from the obelion to angular gyrus’s reference point upon sagittal suture (parameter #3) the right angular gyrus (34.9±10.6mm) is more anterior than the left (25.6±10.1mm). Alternatively, the lateral distance between the Angular Gyrus’s Reference point upon Sagittal Suture and the angular gyrus averaged on 75.1±12.1mm18,19,29,30.
This study had some limitations. Namely, the number of specimens was insufficient to determine the frequency of the presence of the PF in the brazilian population. Second, our studies were performed only in Brazilian patients. Considering the anatomical variations in cranial bone amongst races, the results could differ in other specimens. Lastly, because of bony structural changes with age31, the topography of the PF as mainly obtained in elderly subjects (age mean 58.9 years) in our study could be different in young subjects, especially pediatric patients. Thus, future studies with larger samples including different age groups should investigate the frequency in regions of the world as well as racial differences.
In conclusion, considering our findings and previous reports, the high prevalence of the PF means that a surgeon should be aware of this anatomic variation when performing any surgical or invasive procedures in the parietal region. Especially in procedures that need to maneuver around the SSS, which the PF appears to be a reliable landmark. The bregma is the most stable and accurate landmark to locate the PF. The standardization of a method on localizing the PF can assist clinicians performing parietal craniotomies and thereby prevent iatrogenic SSS damage, and aide further studies on PF’s anatomical correlations.