The main objective in this study was to measure the distances between the anatomical
landmarks of the human brainstem for defining the safe entry zones on the ventrolateral surface of the human brainstem. As discussed here, there are a number of studies on the morphometry of the ventrolateral surface of the human brainstem. In these different measurement parameters, different techniques and study populations are used.
The pontomesencephalic and bulbopontine sulci distance was measured by Kazda and Putz [17] as 28.6 mm (min. 22 - max. 33 mm). In this study, we measured the average pontomesencephalic sulcus - bulbopontine sulcus diameter 26.92 mm (min. 21.77 - max. 34.19 on the right side; min. 21.94 - max. 32.63 mm on the left side). For the basilar sulcus - lateral side of pons distance, Kazda and Putz [17] measured the average length as 18.6 mm on the right side and 19.7 mm on the left side. In the present study, we measured the basilar sulcus - lateral side of pons distance 17.14 mm (min. 14.36 - max. 22.12 mm) on the right side, 17.32 mm (min. 14.15 - max. 20.07 mm) on the left side, and 17.23 mm for both sides.
Akar et al. [1] measured the transverse length of the pyramid as 6.1 mm (min. 5 - max. 7 mm on the right side; min. 4 - max. 8 mm on the left side. For the same parameter, Kazda and Putz [17] measured the length as 5.5 mm (min. 4 - max. 7 mm). In the present study, we measured the transverse length of the pyramid as 5.38 mm (min. 3.82 - max. 86 mm) on the right side and 5.45 mm (min. 4.44 - max. 6.78 mm) on the left side. When both the right and lefts sides were considered, the mean pyramid transverse length was 5.42 mm.
The vertical length of pyramid was measured as 19.7 mm (min. 16 - max. 22 mm) on the right side; 18.5 mm (min. 13 - max. 21 mm) on the left side in study of Akar et al. [1]. In the present study, we measured the vertical length as 21.43 mm (min. 17.48 - max. 23.55 mm) on the right side, and 21.29 mm (min. 17.31 - max. 24.18 mm) on the left side; the mean diameter was 21.36 mm.
The safe entry zone for the lateral mesencephalic sulcus is between the substantia nigra anterolaterally and the medial lemniscus posteriorly [6]. Recalde et al. [25] measured lateral mesencephalic sulcus length as 9.6 mm (min. 7.4 - max. 13.3 mm). In the present study, we measured this length as 12.8 mm (min. 10.4 - max. 14.35 mm) on the right side, and 12.67 mm (min. 9.91 - max. 15.75 mm) on the left side. Yang et al. [29] reported the average length of the safe entry zone, extending from the medial geniculate body to the pontomesencephalic sulcus, as 9.6 mm (min. 7.4 - max. 13.3 mm), with an average depth of 8 mm (min. 4.9 - max. 11.7 mm).
The anterolateral surface of the pons is considered a safe zone for entering the brainstem [26]. Recalde et al. [25] also cut the descending fibers at the level of the trigeminal nerve, resected rostrally, and observed the substantia nigra as well as the intraneural projection of the oculomotor nerve. They measured the distance between the oculomotor nerve where it penetrated the substantia nigra and the lateral mesencephalic sulcus as 8.02 mm (min. 4.9 - max. 11.7 mm). In this study, we measured the distance between the oculomotor nerve where it emerged from the anterior surface of the brainstem and the lateral mesencephalic sulcus. This length was 13.99 mm (min. 11.54 - max. 15.95 mm) on the right side, and 13.71 mm (min. 11.72 - max. 15.35 mm) on the left side; 13.85 mm for both sides.
Recalde et al. [25] also measured the distance between the trigeminal nerve and pyramidal fibers as 4.64 mm (min. 3.1 - max. 5.6 mm). In the present study, we measured the distance between the trigeminal nerve where it emerged from ventrolateral surface of brainstem and the upper tip of pyramid as 17.58 mm; 17.36 mm (min. 13.6 - max. 22.18 mm) on the right side, and 17.81 mm (min. 14.54 - max. 21.68 mm) on the left side.
For the transverse length of the inferior olive, Akar et al. [1] measured the superior and the inferior portions of the inferior olive. However, their measurements for the inferior olive were not at the midpoint line, but more lateral. We measured the transverse length of the inferior olive at the midpoint line as 5.21 mm; 5.24 mm (min. 3.96 - max. 6.77 mm) on the right side and 5.18 mm (min. 4.19 - max. 6.59 mm) on the left side. Akar et al. [1] measured the vertical length of inferior olive as 11.7 mm (min. 9 - max. 15 mm) on the right side, and 10.8 mm (min. 10 - max. 15 mm) on the left side. Our measurements for this length were higher, 14.79 mm (min. 10.91 - max. 17.82 mm) for the right side, 14.75 mm (min. 11.09 - max. 17.03 mm) for the left side.
Sabanciogullari et al. [26] measured the tectum structures on the ventrolateral brainstem on MR images of patients, unlike our measurements on 10% formalin-fixed cadavers. They measured the length as 3.92 mm (min. 2.2- max. 6.1 mm) and 4.79 mm (min. 3 - max. 6.3 mm) for the superior and inferior colliculi, respectively. In the present study on 10% formalin-fixed cadavers, the length was 4.95 mm (min. 3.11 - max. 7.14 mm) on right side, and 3.76 mm (min. 3.39 - max. 4.14 mm) on the left side; and 5.06 mm; 4.83 mm (min. 3.4 - max. 5.95 mm) on the right side, 5.3 mm (min. 4.13 - max. 6.48 mm) on the left side, for the superior and inferior colliculi, respectively. The length of tectum was also measured by Sabanciogullari et al. [26] as 13.57 mm (min. 10.3 - max. 16 mm). Our results were 14.96 mm (min. 12.9 - max. 20.79 mm) on the right side, and 14.04 mm (min. 11.47 - max. 15.84 mm) on the left side. When both left and right sides were considered, our mean tectum length was 14.5 mm.
The interpeduncular fossa length was measured as 11.72 mm (min. 7 - max. 20 mm) on unfixed human brain specimens by Pedroza et al. [23]. In the present study, the interpeduncular fossa length was measured as 11.4 mm (min. 9.67 - max. 13.13 mm) of the right side, and 11.13 mm (min. 9.94 - max. 11.79 mm) on the left side. When both left and right sides were considered, the mean interpeduncular fossa length was 11.26 mm.
In conclusion, this study provides the morphometry of the ventrolateral brainstem structures as a reference for surgical approaches to intrinsic brainstem lesions. Thorough anatomical knowledge and careful analysis of preoperative imaging are essential before surgery of brainstem lesions such as gliomas, cavernomas or abscesses. Presenting many measurement parameters related to safe entry zones to the brainstem, we suggest the findings of this study may be of significant assistance to neurosurgeons operating on this challenging region.