ANATOMICAL STUDY

Subtemporal-Anterior Transtentoral Approach to Middle Cranial Fossa Microsurgical Anatomy Zhiming Xu, MM,*† Weimin Wang, MM,† Jingjing Zhang, MD,‡ Wei Liu, MD,† Yugong Feng, MD,§ and Gang Li, MD*|| Abstract: This study aimed to describe the topography of inferior and external dura mater of the middle cranial fossa through subtemporal–anterior transpetrosal approach and discuss the feasibility of improving the approach. Eight formalin-fixed adult cadaveric heads were studied, with the bones milled away in the lateral triangle region of the petrous bone, Kawase rhombus region, and inner triangle region of the petrous apex. The distances between the targets in these regions, as well as the angles after the dissection of zygomatic arch, were measured, and then the exposed petroclival and retrochiasmatic areas were observed under the microscope. There were significant variations in the distances between targets in the 3 milled regions among the specimens. After the dissection of zygomatic arch, the surgical view got an average increase of 12 degrees. The subtemporal anterior transpetrosal approach, as an improved subtemporal approach, can expose the lesions optimally, causing no injury to the hearing and reducing injuries to temporal lobe. On the other hand, the lateral bone of the petrous parts of the temporal bone is removed so as to improve the view to the retrochiasmatic area and expand the operative field. Key Words: Subtemporal–anterior transpetrosal approach, anatomy, regional topography, bony landmarks (J Craniofac Surg 2014;25: 2220–2222)

T

he petroclival region is the most deep-seated zone in the base of skull, having complex anatomic relationship with the adjacent structures, such as brainstem.1,2 Posterior cavernous sinus region is also extremely difficult to be exposed because of its complexity in anatomy. Meningioma and cholesteatoma are the common lesions diagnosed in these regions.3,4 Treatments for the lesions usually involve many vessels and nerves, including internal carotid artery and cranial nerves. Thus, the surgery is difficult and is frequently accompanied by traumas and complications.5 It is of great importance to give sufficient exposure of the lesions in such surgeries.

From the *School of Medicine, Shandong University, Jinan; †Department of Neurosurgery, Qingdao Municipal Hospital, Qingdao; ‡Department of Gynecology, Affiliated Hospital of Qingdao University, Qingdao; §Department of Neurosurgery, Affiliated Hospital of Qingdao University, Qingdao; and ||Department of Neurosurgery, Qilu Hospital of Shandong University, Jinan, Shandong, People’s Republic of China. Received February 26, 2014. Accepted for publication April 24, 2014. Address correspondence and reprint requests to Gang Li, MD, School of Medicine, Shandong University, 44 Wenhua Xi Road, Jinan 250012, Shandong, People’s Republic of China; E-mail: [email protected] The authors report no conflicts of interest. Copyright © 2014 by Mutaz B. Habal, MD ISSN: 1049-2275 DOI: 10.1097/SCS.0000000000001073

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The lateral subtemporal approach is short and straight from the petroclival region, but with unsatisfactory exposure for being blocked by the petrous part of the temporal bone.6 Subtemporalanterior transtentoral approach could provide a direct sight of the upside of the petroclival region, petrous apex, ventral brainstem, and the postzone of retrochiasmatic opticum by drilling bones in the petrous part of the temporal bone, without hearing damage and the pulling of the temporal lobe. This approach deals with the fundus of the tumors in this region and could clarify the status of jagged vessels, making it possible to remove the petroclival tumors and basilar artery aneurysm.7–9 In the current study, to obtain a broader view of the interior and external dura mater of the middle cranial fossa, the lateral triangle region of the petrous bone was further milled, as well as the bones in the Kawase rhombus region and the inner triangle of the petrous apex.

MATERIALS AND METHODS Surgical Procedure Eight adult cadaver heads were selected for microsurgical dissection. After fixation with formalin (10%), the arteries of each cadaver head were injected with red latex. Then, the subtemporalanterior transtentoral approach was stimulated.7 First, the specimens were positioned in a lateral position with the vertex 10 degrees toward the floor, to facilitate the natural sagging of the temporal lobe. Then, a question-mark incision near the ear was made, the bottom of which was 2 cm beneath the zygomatic arch and 1 cm ahead of antilobium; the inner side was the crossing point of the midline and hairline, and the backside reached the trailing edge of the ear. Next, both ends of the zygomatic arch were isolated from the periosteum and then dissected. The temporalis muscle and dissected zygomatic arch was turned to the base of the skull, then the squamous temporal bone was drilled down to the floor of the middle fossa. Second, the epidural was separated under the operating microscope. Briefly, the middle meningeal artery was dissected, and then the arcuate eminence was separated, after which marked point, sinus lift was performed from the shallow trench in the petrosal crest. Sequentially, to improve the approach and expand the operative field, the bones in the lateral triangle region of the petrous bone were first milled away (Fig. 1I). After finding of its course was finished, the vertex of the internal auditory meatus was drilled open to expose the dural over the internal auditory nerve. Then, the wedge bone located among the internal auditory foramen, genu of facial nerve, and superior semicircular canal was drilled away.8 Next, the bones in the Kawase rhombus region (Fig. 1II) were drilled with the same depth of inferior petrosal sinus. Afterward, the dural was cut open; meanwhile, the temporal lobe was lift to expose the tentorium of cerebellum and to cut the Meckel cavity and the lateral walls of the cavernous sinus. For the exposure of the inner triangle of the petrous apex (Fig. 1III), the tentorium cerebelli section of the trochlear nerve was exposed first; second, the tentorium cerebelli

The Journal of Craniofacial Surgery • Volume 25, Number 6, November 2014

Copyright © 2014 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.

The Journal of Craniofacial Surgery • Volume 25, Number 6, November 2014

Subtemporal-Anterior Transtentoral Approach

TABLE 1. The Most Lateral Triangle Side Length of Temporal Bone Line

Distance, mm

A–B

31.90 (2.14) (21.32–37.08) 27.46 (1.67) (21.32–32.08) 20.24 (1.12) (15.24–24.60)

A–C B–C

FIGURE 1. Microsurgical anatomic vision. I: The lateral triangle region of the petrous bone. A, The migrated zone of sigmoid sinus and transverse sinus; B, the outermost tip of lateral semicircular canal; C, crossing point between the squamous part of the temporal bone superior to the external acoustic foramen and petrous bone. II: Kawase rhombus region. A, The posterior edge of the trigeminal nerve impression in petrosal crest; B, crossing point between the shaft extension of the arcuate eminence and petrosal crest; C, crossing point between the shaft extension of the arcuate eminence and shaft extension of the greater superficial petrosal nerve; D, crossing point between the greater superficial petrosal nerve and mandibular nerve. III: Inner triangle region of the petrous apex. A, Crossing point between projection of the trigeminal nerve lateral border and inferior petrosal sinus; B, crossing point between the projection of the trigeminal nerve lateral border and greater superficial petrosal nerve; C, lateral edge of the Dorello duct. IV: Anterior lateral view of the midbrain. A, Tentorium cerebelli; B, trochlear nerve; C, oculomotor nerve; D, posterior cerebral artery.

was opened for the inward movement of the gasserian ganglion; and finally, the inside triangle was milled away.

Data Collection and Analysis At the end of the surgery, the cranium was removed to observe the back zone of cavernous sinus and bifurcation and branches of the basilar artery (Fig. 1IV). The distances between the vertexes of the removed 3 regions and the angles after the dissection of zygomatic arch were also determined using bow compasses (Deli 8601, Zhejiang, China) and Vernier caliper (SANTO 8012, Shanghai, China), separately. During the measurement, the anatomical position was affirmed via 2 associate chief physicians in neurosurgery, and the measurement was conducted by another 2 well-trained physicians. The average values were recognized as the final data. As shown in Figure 2, the angle was calculated as follows: sin α= a / c (a is distance between the outer edge and middle cranial fossa; c, longest distance of the exposed bottom of the temporal lobe). The data are expressed as mean (SD).

RESULTS In this study, the petrous temporal bone was divided into lateral triangle region, Kawase rhombus region, and inner triangle of the petrous apex, and the distances between the vertexes in each region are shown in Tables 1, 2, and 3, respectively. Obviously, the distances among specimens varied considerably. After the dissection of zygomatic arch, the surgical view got an average increase of 12 degrees (Table 4).

A, the migrated zone of sigmoid sinus and transverse sinus; B, the outermost tip of lateral semicircular canal; C, crossing point between the squamous part of the temporal bone superior to the external acoustic foramen and petrous bone.

even for skilled and experienced surgeons. The disadvantage of subtemporal approach is obvious: intraoperative cortical damage will cause epilepsy; intraoperative Labbé vein damage will cause aphasia; limited spaces can be exposed.10,11 In order to obtain a larger exposure of the petroclival region, the resection ranges were expanded out to include the zygomatic arch and temporal-mandibular joint and inward to drill the inner triangle of the petrous apex in combination with the extradural approach, and the improved approach was successful in many cases.14–16 The subtemporal approach in combination with further dissection of zygomatic arch could reduce the blocking of temporalis, with a 10- to 20-degree increase of the view point,12 and the exposed area of tentorium of cerebellum increases 17%.13 In this study, there was an average viewpoint increase of 12 degrees; however, the data varied significantly among the specimens. Considering that 2 specimens with better elasticity showed larger view field, we inferred that the bad elasticity and blocked traction of the formalin-fixed specimens were the main reason of differences. Besides zygomatic arch dissection, we took other solutions to loosen the brain tissues for a better operative field in this study, including overventilation, infusion of mannitol, and external lumbar drainage of the cerebrospinal fluid. In addition, the adipose capsule of superficial temporal fascia and flap should be lifted together during the dissection of zygomatic arch to avoid the injury to the frontotemporal branches of the facial nerve. To avoid the threshold effect, the removal of the squamous temporal bone should be low enough. As there are no important tissues, bones (cancellous bone and cellulae mastoideae) in the lateral triangle region of the petrous temporal bone can be milled away in an inside-out and front-back order. During the drilling, attention should be paid on the changes of bone mass, and the end point was the compact bone. The injury to blue line of semicircular ducts will increase the injury to the labyrinth.14 Overdrilling of the lateral triangle region usually results in the damage of facial nerve, and the safe depth detected in our research was 2.26 cm. The drilling could provide increased exposure of the petrous apex region and the back zone of retrochiasmatic, without the incision of the temporal-mandibular joint. Basilar bifurcation aneurysm mostly occurs in young people, accompanied with high

DISCUSSION The petrous part of the temporal bone is compact and encloses the cochlea.9 Surgical approaches to the region are challenging,

TABLE 2. The Range of Kawase Rhombus Line

Distance, mm

A–B

20.42 (2.76) (13.68–31.20) 18.65 (1.42) (12.46–22.34) 17.14 (2.15) (10.26–24.04) 14.34 (0.74) (9.08–16.82)

B–C C–D A–D

FIGURE 2. Schematic diagram of zygomatic arch before and after the dissection. A, Distance between the outer edge and middle cranial fossa; b, the longest distance of exposed middle cranial fossa; c, longest distance of the exposed bottom of temporal lobe.

A, The posterior edge of the trigeminal nerve impression in petrosal crest; B, crossing point between shaft extension of arcuate eminence and petrosal crest; C, crossing point between the shaft extension of arcuate eminence and shaft extension of the greater superficial petrosal nerve; D, crossing point between the greater superficial petrosal nerve and mandibular nerve.

© 2014 Mutaz B. Habal, MD

Copyright © 2014 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.

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The Journal of Craniofacial Surgery • Volume 25, Number 6, November 2014

Xu et al

occurrence rate of giant aneurysm. Al-Mefty reported that only 14% to 19% of branches of basilar artery was lower than the posterior clinoid process.15 Correspondingly, the removal of the lateral triangle region in the subtemporal–anterior transpetrosal approach will raise the exposure of the posterior clinoid process, thus making it possible to detect the relationship between aneurysm and the ipsilateral posterior communicating artery, and lobes of the Liliequist membrane. In managing the basilar artery, the improved approach in this study could expose a 1- to 1.5-cm-long trunk end of the artery and the homolateral forward P2 segment, and the removal of the lateral triangle region ameliorates the viewpoint among the posterior clinoid process. The key in drilling the bones in the Kawase rhombus region is the accurate location of the cochlea, and the protection of the cochlea is also the premise of subtemporal–anterior transpetrosal approach. We detected that the distance between cochlea and geniculate ganglion and knee of the internal carotid was 3.53 (SD, 0.37) mm, and 2.76 (SD, 0.54) mm, respectively. They are very close to each other. The methods and orders in drilling are very important especially when the cancellous bone boundaries are not obvious. The superior wall of the external auditory canal was first drilled (the bottom is about 2 mm); the location of Bill’s bar in the bottom was detected, and then the location of cochlea was found. Finally, the bones in the Kawase rhombus region were milled away. In the drilling process of the inner triangle of the petrous apex, the correct order was from outward to front, inside, and backward. Petrosphenoidal ligament is above the Dorello tube, so it was used as the landmarks of the abducens nerve. Only the loose bones in this region were drilled to avoid the injury to trigeminal nerves and inferior petrosal sinus, and the thin bones around the nerves and venous sinus were bitted away using the needle holder.16,17 The improved subtemporal–anterior transpetrosal approach could provide abundant exposure of the back zone of cavernous sinus, Meckel cavity, superior petroclival region, internal auditory meatus, and some of the homolateral brainstem. Cranial nerves III–VIII and the contralateral abducens were also in the viewpoint. The major advantage of this approach is concluded as follows: (1) could obtain a passage to the superior petroclival and cavernous sinus regions without damage to hearing; (2) less pulling to the temporal lobes; (3) could expand the exposure of the outside middle cranial fossa; and (4) make the distance between outside bases of skull and lateral sellar. There were also some disadvantages: (1) the limited pulling to temporal lobe could cause damage of the temporal cortex and Labbé veins; (2) there is possibility of hearing damage because of the injury to cochlea and semicircular canal; (3) the pulling to facial nerves may induce postoperative facial paralysis; (4) poor exposure to the inferior petroclival region; (5) poor exposure to lesions with long distance to the posterior clinoid process.

CONCLUSIONS The improved subtemporal–anterior transpetrosal approach could provide an operative space including posterior clinoid process, internal auditory meatus, and the Dorello tube with abundant exposure, as well as an ameliorative viewpoint of the retrochiasmatic TABLE 3. The Side Length of the Inner Petrous Apex Triangle Line

Distance, mm

A–B

17.36 (1.74) (14.86–21.04) 18.76 (0.68) (16.46–20.34) 15.78 (1.47) (14.02–18.64)

B–C A–C

A, Crossing point between the projection of the trigeminal nerve lateral border and inferior petrosal sinus; B, crossing point between the projection of the trigeminal nerve lateral border and greater superficial petrosal nerve; C, lateral edge of Dorello duct.

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TABLE 4. Angles After the Dissection of the Zygomatic Arch Specimen 1 2 3 4 5 6 7 8

α (Left), Degrees

α (Right), Degrees

13.7 15.7 16.1 13.5 10.5 11.3 11.9 8.7

13.5 16.6 15.5 13.7 11.0 10.6 10.7 10.0

area. Considering the complexity of the anatomical structure in this approach, physicians should apply it with familiar knowledge of the structures.

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© 2014 Mutaz B. Habal, MD

Copyright © 2014 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.

Subtemporal-anterior transtentoral approach to middle cranial fossa microsurgical anatomy.

This study aimed to describe the topography of inferior and external dura mater of the middle cranial fossa through subtemporal-anterior transpetrosal...
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