Microanatomic Studies On The Posterior Fossa Surgery Via A Keyhole Approach

Part I: Microanatomic Study on Retrosigmoid Keyhole ApproachObjective: To modify the traditional retrosigmoid approach according to the principle of minimal invasive meurosurgery, further assess the keyhole approach quantitatively under neuro-navigation, explore its feasibility and indications, and validate the modified keyhole approach in clinical cases.Methods: Six cadaveric heads (twelve sides) fixed by formalin and injected with colored latex were used for microanatomic studies on the retrosigmoid keyhole approach, compared with traditional retrosigmoid craniotomy. Microanatomic structures were observed. Then exposure areas of petroclivus and brainstem were measured and calculated under stryker frameless stereotactic navigation. Angles of attack for six different target points, which included the Meckle cave, trigeminal nerve root, internal auditory meatal, facial nerve root, jugular foramen and glossopharyngeal nerve root, were measured. After obtaining the anatomic data of the retrosigmoid keyhole approach, the traditional approach was performed on the same sides. The same parameters were detected with the same method. Exposure areas and angles of attack between the two approaches were compared and analyzed by student's t test. In clinical cases, the retrosigmoid keyhole approach were performed on 15 patients, who suffered from lesions located in cerebellopontine angle, petrovlival region and tentorium.Results: The retrosigmoid keyhole approach exposes nearly the same anatomic architecture as that of conventional approach, namely, it can expose the area superior to the anteriolateral margin of the tentorium, inferior to the foramen Magnum, medial to the anteriolateral of pons and medulla.The anatomic structure deeply seated in the ispilateral petroclivus can also be observed clearly through the cranial nerve intervals. Exposure areas at the petroclivus and brainstem under the retrosigmoid keyhole approach was 304.73±28.93mm2,143.9±31.87mm2 respectively. The counterpart of the traditional retrosigmoid approach was 346.43±42.80mm2,136.05±9.05mm2 respectively, no statistical difference lies in the exposure area (P>0.05). The vertical and horizontal attack angles under the traditional approach were wider than those of the keyhole approach at the selected six target points(P<0.05). In a total of 15 clinical cases, 13 tumors were completely resected, the remain 2 were subtotally resected. 4 patients suffered from postoperative temporary facial nerve paralysis, and 7 patients lost hearing completely who have already lost hearing partially before operations, and they didn't have any other new cranial nerve complications.Conclusion: The retrosigmoid keyhole approach can provide similar exposure range cimpared with that of the traditional retrosigmoid approach. It can be used to resect small and middle-size tumors located in cerebellopontine angle, the lateral two thirds of the ispilateral petroclivus, anteriolateral of midbrain and pons, as well as large and giant tumors mainly located in these regions. It's a safe, succinct, and minimally invasive way with high feasibility to resect lesions of posterior fossa.Part II: Microanatomical Study on the Suboccipital Midline Transcerebellomedullary Fissure Keyhole ApproachObjective: To design a new suboccipital midline transcerebellomedullary keyhole approach based on minimally invasive keyhole principle, further assess it quantitatively under neuro-navigation, explore its feasibility and indications, and validate the new keyhole approach in clinical cases, and provide support for further practice.Methods: Six formaldehyde-fixed adult cadaveric heads injected with colored latex were applied for microanatomic study on the suboccipital midline transcerebellomedullary fissure keyhole approach. First the suboccipital midline transcerebellomedullary keyhole approach was performed, microscopic anatomical structures were observed, the area of exposure of the floor for theⅣventricle, and the angles of approach to the aqueduct, the point where the line between the foramen of Luschka cross with the medial sulcus, the obex were measured under stryker stereotactic navigation. Then the C1 posterior arch was removed for the purpose of evaluating whether this process can further widen the observing angles and increasing the exposure extent, corresponding parameters were measured. After, the C1 arch was fixed with titanium plates and screws, then the conventional approach were performed without C1 arch removal and with C1 removal respectively, to observe any changes in observing angle and exposure areas.These data were analyzed with student's t test. In clinical cases, 14 tumors were removed through the suboccipital midline transcerebellomedullary keyhole approach, which including 5 lesions located in the inferior vermis (3 medulloblastomas,1 glioma,1 metastatic carcinoma ),6 lesions located in thr fourth ventricle (1 choroid plexus papilloma, 4 ependymomas, 1 arachnoid cyst), 2 lesions located in the dorsal of brain stem (1 cavernomas,1 gliosis ), and 1 organized hamatoma dorsal to the brain stem.Results: By means of adjusting the head position and the projection angle of the microscope, as the the tela choroidea and inferior medullary velum were dissected gradually, structures of the floor, lateral recess ofⅣventricle and vermian were exposed. There were no obvious difference between the keyhole approach and conventional approach in the area of exposure to the floor of the fourth ventricle (P=0.06), and the C1 arch removal couldn't increase the exposure extent (P=0.84). The conventional approach had the advantage of wider observing angles than the keyhole approach (P<0.01), and the C1 arch removal could increase the vertical angles to all the points (P<0.05), but not in horizontal angle (P>0.05). Wider angles under traditional approach can increase the surgical freedom and ensure the capability to perform multidirectional work, but the exposure extent under the traditional approach can't be enlarged. Narrower attack angles under the keyhole approach result in less surgical freedom, but have no effect on the exposure of the target point and surgical manipilation because of the keyhole prinple.In 14 clinical casrs, total resection were achieved under the keyhole microsurgery in all patients. One patient died from serious pulmonary infection after operation, the symptom of the others relieved at discharge. There were no serious complications relating to surgical injuries of the brain stem and cranial nerves, no"cerebellar mutism"occurred that might result from the transvermian approach.Conclusion: Suboccipital midline transcerebellomedullary keyhole approach can expose similar anatomic architectures as those of the conventional approach, which can be used to remove the tumors located in the fourth ventricle, the dorsal of pons and medullary, vermis of cerebellum, without drilling the posterior arch of atlas. As a safe, succinct and minimally invasive approach, the suboccipital midline transcerebellomedullary keyhole approach is proper to resect lesions of posterior fossa.