The Study Of Microsurgical Anatomy For Far Lateral Transcondylar Approach

Objective: To study the microsurgical anatomy for two of the most concentrative issues of far lateral transcondylar approach which are how to protect vertebral artery and venousplex around it, and how to drill off occipital condylar safely and effectively. So we can provide more anatomical basis for clinic and decrease unnecessary injuries during operations.Method: Ten(20 sides) cadaveric adult heads with connected necks whose arteries and veins had been injected with colored latex were micro-anatomized according to the procedure of far lateral transcondylar approach. We measured the distance between inner margin of V3h and midline, the distance between inner margin of the suboccipital cavernous sinus(SCS) venousplex around V3h and midline for emphasis. We watched the orientation and variations of vertebral artery, the modality of the SCS, the locational relation between vertebral artery and occipital condylar, and the modality of occipital condylar (stability of atlantoccipital joint), the orientation and inclusion of hypoglossal canal. How to drill off occipital condylar effectively is also the matter how much it could be drilled off. The occipital condylar obstructs the field of view in petroclival direction during far lateral approach. In order to know the effect occipital condylar made on exposure scope along clivus direction during operations, we took angle between the sagittal plane and the vertical line which was set from the posterior border of the occipital condylar to the midline of the clivus as an index. Compare this angle when the condylar was intact with when it was drilled off to the endostoma of hypoglossal canal by t-test. Take p≤0.05 for distinct difference. Check if they were different.Result: 1 Protection of vertebral artery and venousplex around it1.1 Observation and measurement of vertebral artery: the sect of vertebral artery between axis transverse foramen and duramater(V3) differed a lot. The bend degree and direction of artery loop of V3v were different. The V3h of vertebral artery sometimes went through the osteal canal which was formed by ossified posterior atlantoocipital membrane and groove for vertebral artery on atlas posterior arch. The left distance between the inner margin of V3h and midline was (14.46±2.69)mm, and the right was (16.23±2.06)mm. The relative position between vertebral artery and occipital condylar changed a lot. The occipital condylar could be exposed without moving vertebral artery. We met a sample whose occipital condylar was in anterior and superior direction of vertebral artery and it was unnecessary to move vertebral artery when dealing with occipital condylar. 1.2 Observation and measurement of the SCS: the modality of SCS is irregular. The middle and outer parts of venousplex around V3h were more developed than the inner part. The veins of middle part linked with venousplex between subsurface muscles backwards. The venousplex around V3v was also developed and linked with venousplex between subsurface muscles, but it was sparse near the axis level, and sent out less embranchments. The left distance between the inner margin of the venousplex around V3h and midline was (12.19±2.29)mm, and the right was (12.60±3.09)mm. The venousplex around V3h differed a lot individually. The venousplex was always undeveloped when the V3h of vertebral artery went through the osteal canal. Contrarily it was developed.2 drilling off the occipital condylar2.1 The modality of occipital condylar and stability of atlantoccipital joint: the occipital condylar lay on the 1/3 anterior and lateral part of foramen magnum. The surface of atlantoccipital joint was irregular and flexural which transited from oblique coronal plane to oblique horizontal plane. This structure guaranteed the stability of atlantoccipital joint in fore-and-aft direction. The surface of joint was large enough to stabilize center of gravity of head.2.2 Observation and measurement of hypoglossal canal and condylar canal: the locational relation between hypoglossal canal and occipital condylar was always fixed. The hypoglossal nerve went through the occipital condylar from inboard and back to outboard and front. We would meet endostoma of hypoglossal canal first when drilling off occipital condylar, and the hypoglossal nerve would not be hurt at this time. Emissary went through the condylar canal which was in superior and lateral direction of hypoglossal nerve. Its diameter was wide and it would bleed a lot when was hurt.2.3 The exposure scope change during operation brought by drilling off occipital condylar: the angle between the sagittal plane and the vertical line which was set from the posterior border of the occipital condylar to the midline of the clivus was 71.3±5.8 degree when the condylar was intact, and was 83.9±3.9 degree when the condylar was drilled off to the endostoma of hypoglossal canal. t=18.96, p<0.05. They were different.Conclusion: 1 We should take care of these things when deal with V3 of vertebral artery during far lateral transcondylar approach operations:①Identify the vertebral artery carefully by the ambient structures. Be careful of the places where it could appear variations. Try to avoid unnecessary injuries.②The vertebral artery would not be injured if operations were confined not far than 14mm away from the midline.③Decide if separate or move the vertebral artery individually.2 We should take care of these things when deal with SCS:①Start with inner part of V3h and inferior part of V3v as long as possible when separate the vertebral artery. Try to avoid dealing with developed venousplex as beginning.②Be careful in the area 12mm away from the midline, for the venousplex around vertebral artery may appear.3 The stability of alantoccipital joint will become worse and worse as the occipital condylar was drilled off more and more. We should try to drill off as less occipital condylar as we can on the base of exposing proper scope.4 We should take care of these things when drilling off occipital condylar:①The hypoglossal canal will appear when the cancellous bone turn to cortical bone. The hypoglossal nerve would not be hurt when the occipital condylar was drilled off to the endostoma of hypoglossal canal. The hypoglossal nerve goes from back and inside and underside to front and outside and upside. There are developed venousplex around it. Unnecessary injuries will decrease when we know orientation and inclusion of hypoglossal canal.②The condylar canal lies on the posterior and lateral part of occipital condylar. There is wide emissary in it which would be easily hurt during drilling off the occipital condylar. We should take care of it.③There are vertebral artery and venousplex around it behind the occipital condylar. The entering duramater part of vertebral artery gets close to the inner side of occipital condylar. It will be easily hurt when we drill off occipital condylar. So we should be gentle when operating, and try to protect these structures.5 The exposure scope in clivus direction would be increased when the condylar is drilled off to the endostoma of hypoglossal canal. The occipital condylar is an obstructor in clivus direction during this operation. We should decide whether to drill off the occipital condylar by tumor's character and position in clinic.