Microsurgical Anatomy Of Anterior Circulation Of Willis Circle Via Supraorbital Keyhole Approach

ObjectiveTo study the surgical exposure of anterior circulation of Willis circle via supraorbital keyhole approach(SKA),in order to provide anatomical basis of the supraorbital keyhole approach for clipping anterior cerebral circulation aneurysms.Methods1 Skulls measurement:10 adult skulls.With the help of ruler,sliding caliper,angle gauge and conimeter, the distances and angles between the zygomatic process and ipsilateral and contralateral anterior and posterior clinoid process are measured.2 Heads injection:10 adult cadaveric heads formalin fixed without head diseases.Firstly,the cadaveric heads are soaked in 30% hydrogen for 2-4 days,and then are put in 66% ethyl alcohol solution for next dissection.Next,the bilateral common carotid arteries,vertebral arteries,and internal jugular veins are exposed approximately 2 to 3 cm.Subsequently,the great vessels are cannulated,intravenous tubing for vetebral arteries and suction tubing for common carotid arteries and internal jugular veins.The vessels are tightly affixed to the tubes with sutures.The cannulated vessels can be secured to the surrounding muscles.The next step is irrigation to wash the vessels free of clotted blood.During the second irrigation,any arteries or veins which leak are checked and shoud be stopped.The last step is injection of the colored silicone.A little ammonia water is added in the colored silicone,and arteries and veins are respectively injected with the final silicone mixtures through a 50-ml syringe.Vessel clamps are used to clamp all vessels other then one to be injected. The silicone mixture is injected into a vessel using a 50-ml syringe. A considerable amount of force on the plunger is required to push the mixture into the distal vascular tree. At some point during the injection, it becomes very difficult to inject more material. At this juncture, the contralateral partner of the injected vessel is opened. Injection continues until a free flow of mixture escapes from the open vessel. As the mixture is draining out of the open vessel, the clamp is replaced and the injection ends.To remember that red silicone for arteries and blue for veins.The immediate dissection is forbidded.The specimens should be placed in a shady place for 2 to 3 days,and then put in 66% ethyl alcohol solution for the subsequent dissection.3 Heads dissection:The cadaveric head is fixed by a self-made headframe,rotated to the side opposite the planned craniotomy in 20°-40°,and retroflected in an approximate 20°.After precise definition of the frontal anatomic landmarks(e.g.,the orbital rim,supraorbital foramen,zygomatic process,temporal line,zygomatic rim ),the line of skin incision of the craniotomy is marked on the skin.The skin incision runs laterally from the supraorbital foramen and continues following supraorbital arch within the eyebrow into the frontozygomatic process,the lateral projection of the brow. After skin incision, the frontal skin flap should be dissected subcutaneously upward,thus achieving optimal exposure of the orbicularis oculi and the frontal and temporal muscles,being aware of protecting the supraorbital nerve and artery.The frontal muscle and temporal fascia and muscle are incised laterally by turns parallel to the orbital rim.The frontal muscle is retracted upward with strong stitches,and the temporal muscle is stripped from its bony insertion and retracted laterally to expose the frontozygomatic process.The orbicular muscles are gently pushed downward to the orbit,like a flap.A single frontobasal burr hole should be placed posterior to the frontozygomatic process,pointing to the frontal cranial base.Special attention must be given to this procedure for preventing the drilling from penetrating the orbit.A straight-line cut is then made with a high-speed craniotome parallel to the orbital rim from the lateral to the medial position,supraorbital foramen as the medial border.Thereafter,a C-shaped line is sawed from the medial border to the burr hole,thus creating a bone flap with a height of approximately 1.0 to 2.0 cm and a width of approximately 1.5 to 3.0 cm.An important step after removal of the bone flap is the drilling of the inner edge of the croniotomy and osseous process of the anterior cranial base with a high-speed drill.The dura should be opened in a curved fashion,with its base toward the anterior cranial base,and fixed downward with sutures.After durotomy,the operation microscope is placed in a proper position for microdissection.The brain is dissected medially to the ipsilateral anterior clinoid process along the minor wing of sphenoid.After cutting off the arachnoid trabeculas and some bridging veins between the subface of the frontal lobe and dure,the frontal lobe will natrally sink to form a enough space for surgery as a result of gravity.After opening the ipsilateral carotid cistern,the exposure and relative relationships of supraclinoid segment of interal carotid artery,oculomotor nerve,posterior communicating artery and anterior choroidal artery are studied through the second and third diastema.After opening the sylvian fissure near the carotid cistern,the exposure of middle cerebral artery and its branches is studied.After opening the chiasmatic cistern, the exposure of diaphragma sellae,stalk hypophysial and contralateral interal carotid artery is studied through the first diastema.After opening the lamina terminalis cistern,the exposure of A1 and A2 segment of anterior cerebral artery,anterior communicating artery and recurrent artery of Heubner is studied through the fourth diastema.The surgical exposure of the anterior circulation of Willis circle in the SKA at three directions of lateral,medial and superior is studied and recorded.The distances between frontozygomatic process and bilateral internal carotid artery bifurcations are measured.The exposure of branches of A1 segmeng of anterior cerebral artery and anterior communicating artery is studied,and the origin and numbers of the branches are recorded.The variences of the recurrent artery of Heubner are studied.Results1 Osseous anatomy data:The distances from zygomatic process to ipsilateral anterior and posterior clinoid process, 5.92±0.25cm,6.93±0.29cm.The distances from zygomatic process to contralateral anterior and posterior clinoid process, 7.41±0.29cm,7.49±0.29cm.The angles between zygomatic process and ipsilateral anterior and posterior clinoid process, 34.65±2.53°,33.87±2.53°. The angles between zygomatic process and contralateral anterior and posterior clinoid process, 50.69±2.63°,43.99±2.57°。2 Basalis microanatomy:The surgical exposure of the anterior circulation of Willis circle in the SKA:With the help of operating microscope,the bilateral supraclinoid segment of ICA,PCoA,AChA,A1 and A2 of ACA,ACoA,recurrent artery of Heubner,and M1 and M2 of ipsilateral MCA could be viewed.In addition,we also could see subface of the frontal lobe,medial part of the sylvian fissure,anterior clinoid process,olfactory groove,olfactory tracts,bilateral optic nerves,optic chiasma, stalk hypophysial, oculomotor nerve,diaphragma sellae,dorsum sellae and posterior clinoid process.The maximum exposing length of the ipsilateral MCA and the contralateral ICA and ACoA complex were 1.88±0.26cm,0.77±0.15cm,0.66±0.17cm,respectively. The distances between frontozygomatic process and bilateral ICA bifurcations were 6.40±0.24cm ipsilaterally and 7.70±0.24cm contralaterally.The exposure of the branches of anterior circulation of Willis circle:With the help of operating microscope,all the branches could be seen clearly.The numbers of branches of A1 were 6.35±2.18(3-11), 71.7% of them arising from the lateral wall of A1 and 28.3% of them from the medial wall of A1.The numbers of branches of ACoA were 3.10±1.20 (1-5),most of them arising from the posterior and inferior wall of ACoA and without one branch that we saw on the anterior wall.The numbers of the recurrent artery of Heubner were 1.20±0.62(0-3),and most of them arose from the lateral wall of ACA, 45.8% from the level of ACoA, 20.8% from A1,and 33.3% from A2.During the dissection,we found only one case which had three recurrent artery of Heubner arising from A1 in one half of the brain and zero in the other.We still found one case which had two recurrent artery of Heubner in each half of the brain,all of them arising from the level of ACoA.Conclusion1 Supraorbital foramen,frontozygomatic process,anterior clinoid process,optic nerve and supraclinoid segment of ICA are the anatomic landmarks in SKA.The anatomic data is valuable for clinical operation of chosing surgical instruments. 2 The SKA provides a good vision of the anterior circulation of Willis circle and its perforating branches,and is appropriate for clipping aneurysms arising from these areas.