Microsurgical Anatomy Of Jugular Foramen Region Via Far-lateral Suboccipital Approach

Jugular foramen, through which the numerous nerves and venous channels pass,is indeed the most complex of the foramina. Lesions originating from theneurovascular structures in the jugular foramen constitute one of the major challengesof skull base surgery. With the application of skull base approaches, such aspostauricular transtemporal approach, retrosigmoid approach and far-lateral approach,tumors in this area can be removed, surpassing the difficulties posed by deep locationand surrounding critical structures.Although the subject of many studies, the jugular foramen remains poorlyunderstand. Undoubtedly, knowledge of anatomy of jugular foramen is crucial inperforming tumor resections in this region. So, the microsurgical anatomy of thejugular foramen via mimics three extensions of far-lateral approach were carried outin this study, in order to select the optimal method to allow adequate exposure of thejugular foramen while preserving the blood vessels and cranial nerves in this region.PartⅠ.Microsurgical anatomy of osseous jugular foramen and its adjacentstructures via far-lateral suboccipital approachObjective: To investigate the microsurgical anatomy of the osseous jugularforamen (JF) and the adjacent structures via far-lateral approach, in order to explorethe feasible landmark and exposure method of jugular foramen.Methods: The morphology of the osseous jugular foramina and the adjacentstructures of 10 adult dry skulls and atlas were observed under 10×magnification, the relative data of osseous jugular foramina and the adjacent structures ,especially thedistances between the slected structures and intracranial or extracranial orifices ofjugular foramina were measured and analyzed with statistics software.Results:①The jugular foramen was located between the temporal bone and theoccipital bone with the long axis directly from posterolateral to anteromedial. Theanterolateral margin formed by the temporal bone and a posteromedial margin formedby the occipital bone. The jugular foramen was divided into the petrosal and sigmoidportion. The junction of the sigmoid and petrosal parts was bony prominence on theopposing surface of the temporal and occipital bones, called the intrajugular processes.The temporal processes were observed in all of spcimens , while the occipitalprocesses were occurred in 40% specimens. The bony bridge was only found in oneside, which occupied 5 percent. The jugular foramina were varied in size andshape,the right jugular formina were larger than left ones in major specimens. Thejugular fossa was located between the intracrinal orifice and extracranial orifice of thejugular foramen, the depth of the jugular fossa were 13.75±2.30mm at left side and13.62±2.63mm at right side with no significant difference.②The inferior petrosalsinus was anteromedial to the jugular foramen, and sigmoid sinus was locatedposterolateral to jugular foramen.The mastoid foramina were found in 16 of 20 sidesspceimens which possessed 80 percentage. The jugular tubercle located at thejunction of the basal and condylar part of the occipital bone and situated medial to themedial edge of the jugular foramen.The average thickness of the jugular tubercle was7.55 mm at left side and 7.48 mm at right side. No signifcant difference was found inthe thickness of the jugular tubercle. The internal ocoustic meatus was locatedanterolateral to the anteromedial part of jugular foramen. From the intracrinal view,the internal ocoustic meatus, the anteromedial part of jugular foramen and theintracrinal orifice of the hypoglossal canal were almost located on a beeline. ③The occipital condyle was located along the lateral margin of the anterior half ofthe foramen magnum in the area below and medial to the jugular foramen. Therewere no significant differencese were found in both sides of occipital condyle,including the length of long axis , maximum width and the distance between themidpoint of posterior edge of foramina magnum and posterior edge of occipitalcondyle. The condular fossa and canal were posteromedial to the jugular foramen.The occurrence of the condular canal was 18 sides, which occupied 90 percent.Thehypoglossal canals, which pass through the condylar parat of the occipital bone in theeara above the occipital condyles, were located medial to the jugular foramina. In 10specimen, the compartment in one side was found in 2 specimens, and thecompartment in two side was found in 1 specimen. The compartments were limited inthe intracrinal orifice of hypoglossal canal. On the extracranial side, the styloid andmastoid processes were laterally to JF. The jugular process of the occipital bone wasposterior to JF. The tympanomastoid notch was laterally to the stylomastoid foramen.The styloid process ,mostoid process and the stylomastoid foramen were locatedlateral to the outer orifice of the jugular foramen, with the styloid process beinglocated slightly anteromedial to the stylomastoid foramen. The grooves of vertebralartery of two in 20 sides were tubelized completely and other 6 sides were tubelizedpartially .The appearance percentages were 10% and 30% respectively.③Beside ofthe minimum distance between the internal margin of extracranial orifice of jugularforamina and occipital condyle, which were 6.39 mm at left and 5.70 mm at right side,and the distance between the stylomastoid foramina and the extracranial orifices ofjugular foramina ,which were 6.2hnm at left and 4.81ram at right side, the othermorphological data about relationship of adjacent structures with intracranial orextracranial orifices of jugular foramina were invariable without significantdifference in both side of same specimen. Conclusions: The complex relationship exists between the osseous jugularforamen and it's adjacent structures. The root of styloid process, mostoid process andthe surface project of SS can be used as landmark to identify jugular foramen.Thegrinding of jugular process of the occipital bone, mastoid process, jugular tubercleand transverse process of atlas, drilling of condyle are beneficial to exposure thejugular foramen. The removal of the jugular tubercle should be performed accuratelyto avoid the defect of the crainial nervesPartⅡMicrosurgical anatomy of extracranial soft tissues in jugularforamen region via far-lateral suboccipital approachObjective: To study the extracranial soft tissue in jugular foramen and providean anatomic basis for selection of incision, formation of bone window, and treatmentof vertebral artery and jugular bulb in performing tumor resection via far-lateralsuboccipital approach.Methods: 10 adult cadaveric head specimens fixed in formalin were used todissect via mimics three extensions of far-lateral suboccipital approach. Theprocedures were described as follow : The skin was incised in a inverse U shape, andthe muscles of the suboccipital region were dissected from superficial to deep toexpose the vertebral artery, venous plexus and C2 nerve. To expose the JF areasufficently, the anterior arch of the atlas, transverse process of the atlas, the jugulartubercle and the jugular process were removed in part or in whole. The occiptialcondyle was maitained .During this operation, the soft tissues of extracarnial part injugular foramen, including muscles, blood vessels and nerves were observed under10×magnification. The relative data were obtained. And then, the advantage anddisadvantage of the the three extensions of far-lateral suboccipital approach to exposure the jugular foramen region and surronding structures were analyzed.③④Results:①The rectus capitis lateralis muscle was located anteriorly to posteriorgroup of cranial nerves , sympathetic trunk, internal carotid artery and vein, andanteriorly to vertebral artery and surrounding venous plexus.②The vertebral arteryrun through the transverse foramen of atlas and encased in the suboccipital triangleand covered posteriorly behind the lateral mass of atlas in the vertebral groove andsurrounded by a periosteal sheath enclosing the perivertebral venous plexus. Thearteries of the nerve roots were origined invariably from the posteromedial wall ofverical part of the third segment of vertebral artery, whereas, the muscular brancheswere variable. In 9 samples, the right and left vertebral arteries were in similardiameter. And, anther one specimen showed a severe asymmetry in diameter of rightand left vertebral artery, with a fine right vertebral artery. Tubelization of vertebralgroove were only found in left side of one specimen. The average distance betweenthe site where the vertebral artery enter atlantooccipital fascia and the median linewere 15.64mm at left and 15.68mm at right side. The jugular bulb and adjacent partof the internal jugular vein receive drainage from both intracranial and extracranialsources, which include the sigmoid and inferior petrosal sinuses, the vertebral venousplexus, the venous plexus of the hypoglossal canal, the posterior condylar emissaryvein. Individual differences were observed in the posterior condylar emissary vein. 16posterior condylar emissary veins poured into posterior margin of the end of sigmoidsinus, 4 poured into the posterosuperior wall of jugular bulb. The diameter of twoposterior condylar emissary veins were less than 2mm, which possessed 10 percent.The diameter ranged from 2 to 5mm were found in 17 veins (85%) and more than5ram in 1 side(5%).In 7 of 10 specimens, the peak of the jugular bulb was higher thanthe level of inferior wall of the internal acoustic meatus with 5 at right and 2 at left.③The glossopharyngeal, vagus and accessory nerves passed through the jugular foramina together and apart in extracranial side . After exited from the jugularforamen, the glossopharyngeal nerve turned forward, crossing the lateral surface ofthe internal carotid artery deep to the styloid process;the vagus nerve run downward;the accessory nerve descended obliquely laterally between the internal cartoid arteryand internal jugular vein and then backward across the lateral surface of the vein toreach its muscles. The hypoglossal nerve exited the inferolateral part of thehypoglossal canal and passed adjacent to the vagus nerve, descended between theinternal carotid artery and the internal jugular vein to the level of the transverseprocess of the atlas.Conclusions: Removal of the occipital condyle has a limited help to expose thejugular foramen. In most cases,with drilling the jugular process, jugular tubercle andremoving arch of posterior atlas and transverse foramen of atlas without disturbingthe condyle, the approach through transverse process of atlas combined withsupracondylar and paracondylar approach provides wide and sufficient exposure ofthe structures in the jugular foramen region. The jugular process can be located basedon its relationship with the rectus capitis lateralis muscle and shoul be removed viaposterior margin of the foramen magnum. The extradural removal of the jugulartubercle should be performed with caution because of the risk of injuring theglossopharyngeal, vagus, and accessory nerves. The vertebral artery and venousplexus should be protect while the atlas was removed.PartⅢMicrosurgical anatomy of intracranial soft tissues in jugular foramenregion via far-lateral suboccipital approachObjective: To explore the morphological characteristics of the intracranial softtissue in jugular foramen region, and provide an anatomic basis for incision of dura,intracranial operation in performing tumor resection via far-lateral suboccipital approach.Methods: 10 adult cadaveric head specimens fixed in formalin were used todissect via mimics three extensions of far-lateral suboccipital approach. The softtissues of intracarnial part in jugular foramen, including dura architecture and divisionof the jugular foramina, intracranial blood vessels and nerves were observed under10×magnification. The relative data were obtained. And then, the advantage anddisadvantage of the the three extensions of far-lateral suboccipital approach toexposure the jugular foramen region and surronding structures were analyzed.Results:①At the intracranial orifice, the jugular foramen was divided into threecompartments by dura mater. The petrosal compartment situated anteromedially, thesigmoid compartment situated posterolaterlly , and neural compartment situatedbetween the petrosal and sigmoid parts at the site of the intrajugular process of thetemporal and occipital bones, the intrajugular septum, and the glossopharyngeal,vagus and accessory nerves. The dura mater over the neural part of the foramen hadtwo characteristic perforation, a glossopharyngeal neatus, through which theglossopharyngeal nerve passed, and a vagal meatus, through which the vagus andaccessory nerves passed. 18 sides (90%) were separated by dura mater with differentwidth ranging from 0.7 to 5.2mm. The septum were formed by bony bridge covereddura mater in 2 sides and fibous tissue in 16 sides. No septum was found in 2 sides.,.②The intracranial segment of vertebral artery were run anteriorly to theglossopharyngeal, vagus and accessory nerves, and varied in the relationship with thehypoglossal nerve. 15(75%) vertebral arteries passed ventrally to theglossopharyngeal nerve roots; 4 (20%)vertebral arteries acossed through theglossopharyngeal nerve roots; one vertebral artery run dorsally to this nerve.Theposterior inferior cerebellar artery origined from the vertebral artery was found in 18sides with 90% occurance percentage, the origins of the posterior inferior cerebellar arteries were laterally to the site where the vertebral passed through the dura materwith a distance ranging from 9.45 to 26.57mm Another 2 posterior inferiorcerebellar arteries were origined from the initial point of the basilar artery withavarage diameter 1.65mm at left and 1.59mm at right. Complex relationship of theposterior inferior cerebellar artery with glossopharyngeal, vagus and accessorynerves were observed.12 posterior inferior cerebellar arteries were passed through the vagus rootlets; 4posterior inferior cerebellar arteries run between the accessory nerve and vagus nerve,which occupied 20 percent. The posterior inferior cerebellar artery rolled inferiorly tothe accessory nerve and to upward were found in 2 specimens. Anther one was rolledsuperiorly to the vagus nerve and to downward. The anterior inferior cerebellararteries were found in all specimens. 19 of 20 specimens, the anterior inferiorcerbellar arteries were origined from the basilar arteries . One anterior inferiorcerebelllar artery was derived from the vertebral artery. Sigmoid sinus was larger onright side in 8 spceimens. The mastoidal emissary veins were found in 12 sides,which possessed 60 percent.③The glogssopharyngeal nerve coursed through thejugular foramen along the medial side of the intrajugular ridge.The vagus nervecoursed anterior and inferior as it acossed below the midportion of the intrajugularprocess of the temporal bone. In the area immediately below the dura at the level ofthe intrajugular processes, there were no fibrous bands between the glossopharyngealnerve and the vagal ganglion. The accessory nerve was composed of the cranial andspinal portions. In this study, 70% of the carinal and spinal portions were united. 6accessory nerves were banded in vagal sheath and separated with vagus nerves byarachnoid septum. The hypoglossal nerves entered the hypoglossal canal after runthrough the archnoid mater and dura mater.Conclusions: The opening of the dura mater can be performed posteriorly to the vertebral artery without rerouting, and can provide sufficient exposure and operationspace of the vetral structures of brain stem down to the C2 cervical nerve, theforamen magnum region. It is essential to maste the microanatomic characteristic tosurgeon for treating the tumors in jugular foramen area.SUMMARY TO THIS STUDYThe advantages of the approach descrided in previous study ,which was combinatedwith the approach through transverse process of atlas , supracondylar andparacondylar approach, could be summized briefully as follow:1. The integrality of the atlantooccipital joint was maintained, because the occipitalcondyle was not removed.2. The better exposure of the intracranial orifice of jugular foramen could beprovided by removal of the jugular tubercle. Furthermore, the posterior wall ofjugular foramen can be exposed sufficently by removing the jugular process.3. The usage of the inverse U shape cision is helpful to separate the muscles.4. This appoach is helpful to protect the blood vessels and nerves.5. The procedure of operation is simplified.6. Based on this approach, the tumor in the jugular foramen region with intracranialand/or extracranial extension could be removed completely at one stage.