Morphological Classification of the Medial Frontal Cortex Based on Cadaver Dissections: A Guide for Interhemispheric Approach

Abstract

The medial frontal cortex (MFC) is a part of the medial surface of the frontal lobe situated in the rostral portion of the corpus callosum (CC). In a surgical interhemispheric approach (IHA), the MFC covers the anterior communicating artery (Aco) complex until the final stage of dissection. To clarify the anatomical relationship between the MFC and the Aco complex, and to facilitate orientation in IHA, we analyzed the morphological features of the MFC in number, size, and pattern of gyri from the medial surface of the hemisphere in the subcallosal portion using 53 adult cadaveric hemispheres. The mean width of the MFC excluding cingulate gyrus (MFCexcg) was 20.6 ± as mm in the subcallosal portion. MFCexcg consisting of 2, 3, 4, or 5 gyri were observed in 7.5%, 56.6%, 32.1%, or 3.8% of the hemispheres, respectively. Bilateral MFCexcg consisting of >2 gyri were observed in approximately 85% of the hemispheres. Therefore, in many cases, the dissection performed at 2 cm upward from the base of the straight gyrus (SG) or 3-4 gyri of the MFC is sufficient to safely reach the upper portion of the cistern of lamina terminalis located distal to the Aco complex in IHA. The MFC is a good landmark for intraoperative orientation in IHA.

Keywords: basal interhemispheric approach; cingulate gyrus; interhemispheric approach; interhemispheric fissure; medial frontal cortex.

Fig. 1

(A) Nomenclature of the medial surface of the anterior hemisphere and the relationship between the MFC and BA. Ventral MFC is an area below the black wavy line which is an imaginary line drawn horizontally from the genu toward the frontal pole. (B) Measurement of the width of each gyrus of the MFC. The width of each gyrus of the MFC was measured at Line A. Line A is a line drawn perpendicularly to the intercommissural line from the anterior margin of the callosal genu on the medial surface of the hemispheres. MFC excluding CG (MFCexcg) consists of 2–5 small gyri at the subcallosal portion and each gyrus is defined as MFC1, MFC2, MFC3, MFC4, and MFC5 in order from the outermost layer. (C) Anatomical view of the MFC. This image shows the medial view of the MFC in the sagittal plane and an upside down coronal view in which the superior side is the cranial bottom. This coronal view corresponds to the actual operative findings in IHA. The eye-mark shows the actual intraoperative direction in IHA. APaOS: anterior paraolfactory sulcus, CG: cingulate gyrus, CS: cingulate sulcus, IHA: interhemispheric approach, imSFG: inferior medial segment of SFG, IRS: inferior rostral sulcus, MFC: medial frontal cortex, PaOG: paraolfactory gyrus, PCG: paracingulate gyrus, PCS: paracingulate sulcus, SG: straight gyrus, smSFG: superior medial segment of SFG, SRS: superior rostral sulcus.

Fig. 2

(A) Schema shows the results of the pattern of the MFC in the sagittal view. (B) The patterns of numbers of the bilateral components of MFCexcg in the subcallosal portion are demonstrated in an upside down coronal view. MFCexcg, Medial frontal cortex excluding cingulate gyrus.

Fig. 3

Three-step procedure of the dissection in an IH approach. (A) Anterior IHA. (B, C) Variations of dissection in a basal IHA. The orange arrow shows the first step of dissection. The yellow arrows show the second steps of dissection. The green arrows show the third step of dissection. The area of light blue shows the range of dissection. ACA: anterior cerebral artery, A-com: anterior communicating artery, IHA: interhemispheric approach.

Fig. 4

The actual operative view of the MFCexcg in IHA. The yellow two-way arrow shows the outermost 3 gyri of the MFC in the subcallosal portion. The light blue two-way arrow shows the outermost 4 gyri of the MFC in the subcallosal portion. MFC: medial frontal cortex.