Comparative Anatomical Study Of Neuroendoscopic Suboccipital Median Transcerebellomedullary Fissure Keyhole Approach Between Bilateral And Unilateral Incision Of The Choroid And Inferior Medullary Velum

Objective:To investigate the anatomical differences between the bilateral dissection of the choroid and inferior medullary velum and the unilateral dissection of the choroid and inferior medullary velum via the neuroendoscopic suboccipital median transcerebellomedullary fissure keyhole approach,and to provide anatomical observations and quantitative measurements of the structures of the fourth ventricle and the perirhinal structures of the brainstem for clinical application.Methods:Eight adult cadaveric specimens were used to simulate a suboccipital median transcerebellomedullary fissure keyhole approach.A 5 cm median incision was made upwards from the 1 cm below from foramen magnum,separated the muscle,and a 2.5-3cm diameter foramen medullaris window was ground out with the occipital scalene,cut the dura and the occipital pool was sharply opened to reveal the foramen of Magendie.The four natural anatomical landmarks at the floor of the fourth ventricle were set,including the opening of the aqueduct(point A),the obex(point C)and the outer edge of the lateral fissure on both sides(points B and D),measuring the distance from the aqueduct to the obex(long diameter a),the distance between the outer edge of the lateral fissure on both sides(wide diameter b),the length from opening of the aqueduct to the lateral fissure on both sides(AB and AD),and the length from the lateral fissure on both sides to the obex(BC and DC).Then calculated the average revealed area(S)of the fourth ventricular floor for both groups,and the differences in the long diameter a,wide diameter b and revealed area S of the fourth ventricular floor between the two incision methods were statistically analyzed.Results:1.Through the neuroendoscopic suboccipital median transcerebellomedullary fissure keyhole approach,incising the choroid and inferior medullary velum bilaterally or unilaterally,all structures of the fourth ventricle floor could be clearly visualized,including the opening of the aqueduct,median sulcus,facial colliculus,hypoglossal trigone,vagal trigone,area postrema,obex,lateral fissure on both sides,superior,middle and inferior cerebellar peduncles and part structures of the dorsal lateral of the brainstem,such as glossopharyngeal nerve,vagal nerve,accessory nerve,hypoglossal nerve and posterior inferior cerebellar artery.During dissection after incising the choroid and inferior medullary velum bilaterally or unilaterally,surgical instruments can be used to explore the above structures with the neuroendoscope,which can meet the basic surgical needs.2.Measurements:(1)The average distances from the opening of the aqueduct to the lateral fissure on both sides(AB and AD)were 28.49±1.54 mm and 28.53±1.50mm,respectively;the average distances from the outer edge of the lateral fissure on both sides to the obex(BC and DC)were 16.44±0.71 mm and 16.24±0.66mm,respectively.(2)In the bilateral dissection group,the average distance between the opening of aqueduct and the obex(long diameter a)was 36.10±1.22 mm,the average distance between the both outer edges of the lateral fissure(wide diameter b)was 25.13±1.80 mm,and the average revealed area at the fourth ventricle floor(S)was 449.82±43.16 mm~2.(3)In the unilateral dissection group,the average distance between the opening of aqueduct and the obex(long diameter a)was 35.98±1.20mm,and the average distance between the both outer edges of the lateral fissure(wide diameter b)was 22.48±0.69mm.and the average revealed area in the fourth ventricle floor(S)was:405.44±25.55mm~2.3.Statistical analysis:After statistical two independent samples’t-test,the comparison between the bilateral and unilateral groups in terms of long diameter a and exposed area S was not statistically significant(P>0.05)and was statistically significant in the comparison of wide diameter b(P=0.034).Conclusions:1.The neuroendoscopic suboccipital median transcerebellomedullary fissure keyhole approach allows exploration of all the structures of the fourth ventricular floor and the dorsolateral part of the brainstem,which can meet the anatomical exposure needs of the fourth ventricle,the dorsolateral part of the medulla oblongata and the area around the lateral fissure.2.There are no statistically significant differences were found in the length of the fourth ventricle floor and the area exposed by bilateral or unilateral dissection of the inferior medullary sails and choroid.It suggests that unilateral incision of the choroid and inferior medullary velum via the neuroendoscope can basically achieve the same exposure results as bilateral incision,meeting the surgical needs of most occupying lesions within the fourth ventricle,achieving the required extent of exposure and space for surgical manipulation,and avoiding possible nerve damage from bilateral incision.However,there are differences between the two in terms of exposure of the wide diameter of the fourth ventricle floor,with unilateral dissection of the choroid and inferior medullary velum resulting in less-than-optimal exposure of the outer edge of the contralateral lateral fissure.This suggests that bilateral dissection of the choroid and inferior medullary velum may be more advantageous when confronted with an occupying lesion within the fourth ventricle,especially if the lesion involves both lateral fissure margins or a large occupancy.