SEMICIRCULAR CANAL ANATOMY AS SEEN IN MICRODISSECTION

SUMMARY SEMICIRCULAR CANAL ANATOMY AS SEEN IN MICRODISSECTION Nguyen Thanh Vinh*; Tran Ngọc Anh** Nguyen Hoang Vu***; Le Gia Vinh****; Pham Ngoc Chat***** Objectives: To investigate semicircular canal anatomy as seen in microdissection; evaluate osseous semicircular canal, membranous semicircular canal and the relationship of semicircular canal with the adjacent anatomical structures. Subjects and methods: Samples of 9 human corpse heads, 18 ears were selected belonging to Department of Anatomy, University of Medicine and Pharmacy, Hochiminh City. The semicircular canal was explored with transmastoid and cranial fossa approach. Results: 9 human corpse heads, 18 ears were selected with average age 61.5 (53-70). Horizontal, superior osseous semicircular canal and posterior membranous semicircular canal was clearly identified with cranial fossa approach. Horizontal, posterior osseous and membranous semicircular canals were clearly identified with transmastoid approach. Membranous semicircular canal was situated the outer edge of the osseous semicircular canal. Conclusions: All of these approaches can be used to clearly identify the semicircular canal anatomical structure. * Keywords: Osseous semicircular canal; Membranous semicircular canal; Cranial fossa approach; Transmastoid approach. INTRODUCTION The semicircular canals system is a component of the vestibular system, contributing significantly to the body's balance function. Anatomically, there are three semicircular canals: horizontal (lateral), superior and posterior semicircular canals. They are very small structures, in different planes, lied within the earlobe and buried deep in the temporal bones. When studying, researching or teaching, people has to depend on images printed in textbooks or models, which causes a lot of difficulties to understand clearly, especially related specialties such as: anatomy, neurosurgery and ENT. In the world, there are many books have been written on temporal bone surgery, but the presentation of the approach to this system is still unclear and specific. In Vietnam, there have also been reports of semicircular canals, there are images and clinical applications in the diagnosis and treatment of the disease. * ENT Hospital, Hochiminh City ** Vietnam Military Medical University *** Medicine and Pharmacy University **** Vietnam Medical Asociation Corresponding author: Nguyen Thanh Vinh (drvinhent@gmail.com) Date received: 26/04/20181 Date accepted: 29/06/2018 121

In fact, when participating in the temporal bone surgery courses, it is always difficult to study the anatomy of semicircular canals and requires intensive means as well as the experience of surgeons performing surgery. In order to solve this problem, we need to have a specific approach that can help physicians and practitioners to see and understand correctly the anatomy of semicircular canal system. Therefore, we investigate Labyrinth anatomy as seen in microdissection and evaluate osseous labyrinth, membranous labyrinth and the relationship of labyrinth with the adjacent anatomical structures. SUBJECTS AND METHODS 1. Subjects. Vietnamese adults human corpse heads were selected belonging to Department of Anatomy, University of Medicine and Pharmacy, Hochiminh City. * Selection criteria: - Vietnamese adults. - Corpse heads were selected belong to Department of Anatomy, University of Medicine and Pharmacy of Hochiminh City. - Normal temporal bone in anatomy. * Exclusion criteria: - Age < 18. - Having ear problems. - Interventions for ear surgery. - Congenital malformations of the head and neck. - Traumatic in head or temporal region. 2. Methods. Case series report. * Research location: Department of Anatomy, University of Medicine and Pharmacy, Hochiminh City. * Research facilities: - Temporal bone dissection instruments. - Semicircular canal microsurgery instruments. - Electric drilling machine. - Carving drill bits and sharping drill bits, sizes ranging from 4 mm to 0.5 mm. - Aspirator machine, suction, syringe. - Karz Zeiss microscope. - Camera. - Computers to save images. * Microdissection with cranial fossa approach: - Cut the skull forming oval shape, across the edge of the ear on both sides. - Cut the brain stem, revealing the entire base of the skull. - Determine the Arch convex (prominence of lateral semicircular canal). - Three straight lines are perpendicular to the petromastoid bones, 1 through the center of the convex, 1 tangent to the upper edge of the convex and 1 tangent to the lower edge of the convex. - The line is perpendicular to the three lines above, tangent to the outer edge of the Arch convex. - Use electric drill machine, 3 or 4 mm carving drill pit, drill the bone along the outer edge of the tangent line outside the Arch convex, reveal the mucosal layer of posterior atrium ceiling. - Use the microsurgical knife cut the mucosal layer of posterior atrium ceiling. - Identify short process of incus and lateral semicircular canal. - Determine the superior osseous labyrinth from the Arch convex to the front of lateral osseous labyrinth. 122

- Use the 1-2 mm carving drill pit, drill out mastoid cells of surrounding vestibule group of the superior osseous labyrinth, exposing the entire superior osseous labyrinth. - Use the 3 mm carving drill pit, drill the cranial fossa bone surrounding the superior osseous labyrinth. * Microdissection with transmastoid approach: - Make a postaural incision with No.15 or No. 20 scalpel blade, until the temporal bone. - Detach the musculoperiosteal flap posteriorly and anteriorly to the external auditory canal. - The self-retaining retractor is utilized to pull up the flap, expose the mastoid cortex. - Using a large cutting burr (3-4 mm), drilling is started along the temporal line, then along the posterior wall of the external auditory canal. Finally, a third line is drilled perpendicular to the temporal line, through the mastoid tip, to create a triangle. - Continue to drill the mastoid cells to open the antrum. - Drill the tegmental mastoid cells, expose the middle cranial fossa. - Using a small cutting burr (1-2 mm), drill the anterior and posterior signal cells to expose the sinus and the Citelli s angle. - Continue to drill the perifacial cells, expose the third segment of facial nerve. - Drill posteriorly to the tip cells, expose the bone around the digastric muscle. - Drill to open the aditus, until the incus can be identified. The lateral semicircular canal can be seen. - Drill the mastoid cells around the lateral and posterior semicircular canal. - Using a 0.5-1 mm diamond burr, drill the mastoid cells around the semicircular canal, until no mucosa left. - Grind the edge of 3 semicircular canals. - Identify the subarcuate artery, near the center of 3 semicircular canals. - Using a 0.5-1 mm diamond burr, grind the bone surface of the mastoid segment of CN VII, near posterior and lateral semicircular canal, the second genu of CN VII. * Microdissect the semicircular canals: - Through cranial fossa approach, use a 0.5 mm diamond burr to drill along the medial side of the superior semicircular canal, from the conjunction between the superior semicircular canal and the lateral semicircular canal to the conjuction between the superior semicircular canal to the posterior semicircular canal. - Drill the bony semicircular canal until the mucosa of the membranous semicircular canal can be seen; from there, continue to drill the superior semicircular canal to expose totally the membranous superior semicircular canal, from the ampulla to the crus commune. - Use the 0.5-1 mm diamond burr to drill along the medial side of the lateral semicircular canal, from the connection with the superior semicircular canal to the conjunction between the lateral semicircular canal to the posterior semicircular canal. - Drill the bony lateral semicircular canal until the mucosa of the membranous lateral semicircular canal can be seen; 123

continue to drill carefully the bony lateral semicircular canal to expose totally the membranous lateral semicircular canal. - Left the bony part between the bony superior semicircular canal and the lateral semicircular canal, to distinguish the border line between 2 membranous semicircular canals and the ampulla of the superior and lateral semicircular canal. - Use 0.5-1 mm diamond burr, drill the bony posterior semicircular canal, from the crus commune to the opening of the ampulla into the utricle. Continue to drill to expose the mucosa of the membranous posterior semicircular canal, then expose totally the posterior semicircular canal. - Use 0.5 mm diamond burr to drill the bone between the superior semicircular canal and the lateral semicircular canal. - Use 0.5 mm diamond burr, drill the crus commune of the superior semicircular canal and the lateral semicircular canal. RESULT Through observation in 9 human corps (18 ears), including 5 men and 4 women. * Age: The youngest was 53, the oldest was 71, mean age 61.5. * Gender: Males 5 cases (55.5%); females: 4 cases (44.5%). * Mastoid cells around the semicircular canals: Well-developed: 11 cases (61.1%); moderate developed: 6 cases (33.3%); underdeveloped: 1 case (5.6%). * Lateral bony semicircular canal: Very clear: 4 cases (22.2%); clear: 12 cases (66.7%); not clear: 2 cases (11.1%). Table 1: Relative structure. Relative structure Number of ears Ratio Tympanic segment of CN VII 18 100 Mastoid segment of CN VII 18 100 Second genu of CN VII 18 100 Short process of incus 18 100 Table 2: Bony semicircular canal. Bony semicircul ar canal Cranial fossa approach Postauricular approach Superior Very clear Quite clear Lateral Very clear Very clear Posterial Quite clear Very clear 124

Table 3: Membranous semicircular canal. Membranous semicircular canal Cranial fossa approach Postauricular approach Superior Not clear Quite clear Lateral Quite clear Very clear Posterial Very clear Very clear Table 4: Position of the membranous semicircular canal in bony semicircular canal. Semicircular canal Superior Lateral Posterior Anterior wall 0 5 1 Lateral wall 16 12 15 Medial wall 2 1 2 Posterior wall 0 0 0 * Abnormalities: - The lateral membranous semicircular canal is concave downward in 1 case. - Absence of the crus commune in 1 case. DISCUSSION 1. Bony semicircular canal. In terms of morphology, all three bony semicircular canals are in the same position as being described in books, in which the lateral bony semicircular canal had a higher rate to be seen clearly than other bony semicircular canals in both 2 approaches (cranial and postauricular); only two ears were not really visible because of the extensive development of the mastoid air cells, which surrounded the lateral bony semicircular canal, so it was difficult to see. Due to the relatively vertical position, it was more difficult to recognize superior bony semicircular canal by the postauricular approach than cranial approach. On the other hand, on the base of the skull base, it was possible to see the protrusion of the lateral bony semicircular canal, which was easier to define. If mastoid air cell is well-developed, there will be an air cell between the superior semicircular canal and the base of the skull; so it is more easily recognized due to protrusion of the lateral bony semicircular canal. Particularly, because the posterior semicircular canal was on the horizontal position and lower than the superior and lateral semicircular canal, it was buried deep inside the otic capsule and was difficult to recognize. On the other hand, in mastoid bone with a well-developed air cell, many layers of air cell covered the lateral side of the posterior semicircular canal. Thus, it is more difficult to detect posterior semicircular canal. 125

Through the postauricular approach, to see clearly the semicircular canal, we need to extend the way up to the epitympanum through the mastoid antrum to the malleus-incudal joint. If the mastoid air cells are good, mastoid antrum will be large and the distance from the mastoid antrum to the epitympanic roof will be farther; so the approach was much easier as the lateral semicircular canal was easily recognized. If the mastoid air cell was poorly developed, it was more difficult to access the lateral bony semicircular canal. Because semicircular canal are located inside otic capsule, the lateral semicircular canal plays an important role in guiding to identify the remaining semicircular canals. Thus, rely on an air cells, identification of lateral semicircular canal can be easy or difficult. In autopsy bony semicircular canal, we noted the connection of the bony semicircular canals, particularly the perilymph between the superior and lateral semicircular canal in ampullae of membranous semicircular canal position. Therefore, perilymph connect to all 3 semicircular canals. 2. Membranous semicircular canal. Membranous semicircular canal was inside the bony semicircular canal. It had been noted that membranous semicircular canal was in lateral margin of semicircular canal. In fact, we noted that most membranous semicircular canal were located on the outer edge of the bony semicircular canal. It make the surgeons open the bony semicircular canal carefully, otherwise it will damage the membranous semicircular canal. Ampulle of superior membranous semicircular canal is located near ampulle of lateral membranous semicircular canal in position of utricle. 3. Adjacent structures. In the procedure, we noted that there were structures associated with bony semicircular canal, which will help surgeons to identify semicircular canals in difficult cases. - Short limb of incus: The most nearby anatomical landmark, slightly deviated from the posterior branch of the lateral bony semicircular canal. The more visible the lateral semicircular canal is, the shorter this distance is. In case of much mastoid air cells, the air cells inserted between the lateral bony semicircular canal and the short limb of incus, making this distance farther. - Segment 2 of the nerve VII usually position anterio-inferior to lateral bony semicircular canal, bony covered of the second segment can be defective, revealing nerve VII. Due to the location and defection of the bone, this anatomy landmark is less mentioned although it is considered to be related to the semicircular canal system. - Segment 2 of facial nerve: Usually is located under short limb of incus, surrounded by bone, covered above by air cells. Thus, it is more difficult to identify than short limb of incus. If mastoid has less air cells, segment 2 of facial nerve are located near the bony lateral semicircular canal rather than short limb of incus. If mastoid has more air cells, segment 2 will be far away from lateral bony semicircular canal. 126

- Segment 3 of the facial nerve is more related to the posterior bony semicircular canal, in the position of the ampulle poured into the utricle. Normally, ampulle of the lateral bony semicircular canal is located just below - segment 3 of the facial nerve, at the moment of passing of facial nerve. If mastoid air cell is welldeveloped, more mastoid air cells insertion between posterior bony semicircular canal and segment 3 of the facial nerve, widening the distance between ampulle posterior bony semicircular canal and segment 3 of the facial nerve. 4. Autopsy approach. For ENT specialists, the postauricular approach is a common since this way is easy to learn and easy to apply to the surgery. The semicircular canal approach is to drill half of the bony semicircular canal to clearly observe membranous semicircular canal. The postauricular approach helps accessing easily in the following order: Lateral semicircular canal, posterior semicircular canal and lastly superior semicircular canal. Lateral semicircular canal is easy to approach because the direction of the microscope is straight, and the lateral semicircular canal protrudes more distal than other semicircular canal. On the other hand, superior semicircular canal, due to the vertical position, is higher than the lateral semicircular canal and contact with the skull bone; so this semicircular canal is partially hidden, difficult to dislocate. Moreover, the posterior semicircular canal is usually more accessible than superior semicircular canal because it is only covered by well-developed air cell on the surface. Surgeons, after drilling these air cells, can recognize posterior semicircular canal. As membranous semicircular canal, the postauricular approach helps surgeon see anatomy structures clearly in the following order: Posterior membranous semicircular canal, lateral membranous semicircular canal, and finally superior semicircular canal. If only doing surgery to see the anatomy structures, approach type is not a big matter. However, as we do research, our autopsy approach will be more accurate than other approach. Furthermore, for our application in surgery, our autopsy approach is much more applicable in real surgery for disease treatment. Cranial approach is easier to manipulate with the superior semicircular canal, easier to observe the lateral membranous semicircular canal than the postauricular approach. On the other hand, this approach is wider than the postauricular one, which can manipulate on all three semicircular canals. However, this approach is only used in autopsy, not applied in real surgery. CONCLUSION Through a combination of two approaches: The postauricular approach and cranial approach for accessing to bony - membranous semicircular canal, with 9 corpses, 18 ears, we can conclude: * Postauricular approach: - Helping to approach bony semicircular canal well in following order: Lateral, posterior, and finally superior. - For membranous semicircular canal, the order is posterior, lateral, and superior. 127

* Cranial approach: - Helping to approach bony semicircular canal well in following order: Superior, lateral and posterior. - For membranous semicircular canal, the order is lateral, posterior, superior. Therefore, all two autopsy approaches support each other, providing good access to all three bony and membranous semicircular canals. REFERENCES 1. J.V Beek-King. Retrolabyrithine approach. Operative Techniques in Otolaryngology. 2013, Vol 24, pp.169-171. 2. J Bowman. The translabyrinthine approach. Operative Techniques in Otolaryngology. 2013, Vol 24, pp.149-156. 3. C. Cremers, J Mulder. Total labyrinthectomy. Temporal Bone Dissection Manua. Kluger Publication, Amsterdam, Netherlands. 2011, pp.33-36. 4. C. Cremers, J Mulder. Translabyrinthine approach of the internal acoustic canal. Temporal Bone Dissection Manual. Kluger Publication, Amsterdam, Netherlands. 2011, pp.37-40. 5. H.W Francis, J.K Niparko. Laryrinthectomy. Tymporal Bone Dissection Guide. 2 th Ed, Thieme, New York. 2016, pp.49-52. 6. H.W Francis, J.K Niparko. Translabyrinthine exposure of the internal audistory canal. Tymporal Bone Dissection Guide. 2 th Ed, Thieme, New York. 2016, pp.53-54. 7. A.J Gulya. Surgical anatomy of the temporal bone and dissection guide. Surgery of the Ear. 6 th Ed, People Medical Publishing House-USA, Connecticut. 2010, pp.771-790. 8. M Goycoolea. Temporal bone dissection. Atlas of Otologic Surgery and Magic Otology. Jaypee Brothers Medical Publicshers, London. 2012, Vol 1, pp.141-198. 9. M Sanna, T. Khrais. Transmastoid approaches. The Temporal Bone. 1 th Edition, Thieme, New York. 2006, pp.22-54. 10. M Sanna, T. Khrais. Transotic approach. The Temporal Bone. 1 th Edition, Thieme, New York. 2006, pp.98-105. 11. S.A Shanna, M Eid. Dehiscences of the semicircular canals as discrete third window lesions of inner ear. The Egyptian Journal of Radiology and Nuclear Medicine. 2013, Vol 44, pp.15-21. 12. C Wijaya, A Dias. Superior semicircular canal occlussion - transmastoid approach. International Journal of Surgery Case Reports. 2012, Vol 3, pp.42-44. 128