the practice of transcanal endoscopic mastoid dissection on cadaver

June 26, 2016 Practice of transcanal endoscopic mastoid dissection on cadaver - Dr. Sheikh Shawkat Kamal

The Practice of Transcanal Endoscopic Mastoid Dissection on CadaverAuthor:

Dr. Sheikh Shawkat Kamal, MBBS FCPSConsultant OtolaryngologistSurgiscope Hospital Limited

ChittagongBangladesh

Cell Phone: 880 1711406943E-mail: [email protected]

Abstract:

Transcanal endoscopic mastoid exploration for cholesteatoma surgery is a growing concept. For such approach

surgeon needs to be well oriented with the endoscopic anatomy of various important structures confined within the

mastoid. In this article, the author described a practical guideline for the transcanal endoscopic mastoid dissection on

cadaver. Arrangement of work station, steps for excavation of various important structures such as the mastoid part

of the facial nerve, the semicircular canals, the endolymphatic sac, etc. were depicted sequentially in this guideline.


Introduction:

The transcanal endoscopic exploration of the mastoid

antrum has already been proved to be effective in removing

the antral cholesteatoma1,2. Such endoscopic approach

involves removal of posterior auditory canal wall for making a

wide access from canal to mastoid antrum. In comparison to

other approaches for mastoid antrum, this endoscopic

approach is the least invasive in nature and owns remarkable

aesthetic and financial benefits3.

Necessary skills have to be adopted before attempting such

approach on patient. Not only this, surgeons attempting such

approach require sound knowledge about the endoscopic

anatomy of various vital structures confined within the

mastoid. To gain these necessary skills and knowledge, the

practice of the cadaveric mastoid dissection through transcanal

route purely under endoscopic guidance is an obligation.

The purpose of this article was to make a practical guideline

for the practice of the transcanal endoscopic mastoid

dissection on cadaver. The author’s way of such endoscopic

mastoid dissection has been depicted elaborately below.

Work Station:

Position of the cadaveric specimen: A fresh cadaveric temporal

bone preserved in formalin was placed with the help of

temporal bone holder on a table in between the surgeon and

the video tower. The temporal bone was attached in such a

way that the tip of mastoid was facing the right side of the

surgeon in case of right temporal bone and left side of the

surgeon in case of left temporal bone. The bone was tilted

towards the surgeon so that the external auditory canal and

surgeon’s eye vision were in same axis.

Position of the surgeon: The surgeon was in sitting position

facing the monitor and temporal bone. The endoscope with

camera head was held by the non dominant hand of the

surgeon. Necessary instrumentations were done with the

dominant hand. An assistant was present during dissection at

the right side of the surgeon. The height of the temporal bone

was adjusted at the lower chest level of the surgeon.

Position of the video tower: Video tower comprising camera

control unit, light source and monitor placed in a special trolley

was positioned in front of the surgeon.

Placement of the other instruments: Necessary instruments,

sucker nozzle and hand piece of electric micro drill were placed

near the dominant hand of the surgeon. Gauze piece soaked with

anti-fogging agent was placed near the non dominant hand of the

surgeon.

List of required instruments:

1. 4mm 18cm, 0 0 rigid endoscope.2. 2.7 mm 11cm, 00 rigid endoscope.3. 2.4 mm × 3.5mm Plester’s round cutting flap knife, 16cm.4. 2mm round cutting knife, 450 angled, 16cm.5. Plester’s sickle knife.6. 1.5 + 1.8 mm House double ended curette, 15cm.7. 3.2mm Wullstein’s curved raspatory, 16cm.8. 1 × 5 mm straight alligator forceps.9. 1mm wide elevator, 16cm10. Curved needle,16cm11. 2mm/6 F G Fergusson’s suction tube 18cm.12. Luer cone adaptor.13. 1.5mm, 1mm Luer cone suction needle/cannula.14. 1mm, 2mm, 3mm, Ferris-Smith-Kerrison pituitary punch,

400 and 900 upward cutting, 18cm.15. 1.8mm, 2.7mm, 3.5 mm steel cutting burr, 7cm.16. 1.8mm, 3.5 mm diamond burr, 7cm.17. Straight Intra hand piece (1:1).18. Intra coupling micro motor with cable.19. Electric motor control unit.20. Endoscopic camera head with control unit.21. Fiber optic light source with cable.22. Monitor.23. Temporal bone holder.24. Suction unit.25. Antifogging agent.26. Irrigation saline water.

Dissection:

All the dissection procedures were done purely under 00,

4mm- diameter endoscope. 00, 2.7mm-diameter endoscope was

chosen for narrow canal. The camera head was attached with

endoscope in such a way that the endoscopic video images of

different structures of the ear in the monitor were viewed in

their anatomical position. As for example, while viewing the

tympanum in the monitor, the incus was at the back, eustachian

tube end was at the front, epitympnum was at the top and

hypotympanum at the bottom. This was maintained throughout

the dissection procedures.

Both right and left hands of the surgeon had to be adjusted at

different positions during dissection for the purpose of optimal

endoscopy and instrumentations. Such adjustments were

separately illustrated below for left and right temporal bones

[figure 1 & 2].


Figure2: Chronological pictures A, B, C showed the different adjusted positions of right (R) and left (L) hands of the surgeon during dissection ofvarious parts of the mastoid in the left temporal bone.

Transcanal endoscopic drilling involved short timed drill with

use of intermittent irrigation and suction. Every event of such

drilling’s procedure was done sequentially, one after another

with the dominant hand of the author while holding the

endoscope with his non dominant hand. The steps of the

dissection were described chronologically below. For better

understandings, each step was further explained under two

headings - ‘Tasks’ and ‘Observations’. Tasks denoted the

dissection procedures and observations denoted the notable

features.

Step one: Exposure of posterior bony canal wall.

Tasks:

By using the Plester’s flap knife, a straight deep skin incision

was made in the roof of the canal extending from scutum to

the most lateral part of the external auditory canal. Then with

round cutting knife, a skin incision was made in the posterior

canal wall just few millimeters posterior to tympanic

membrane extending vertically from previous straight incision

of the roof to floor of the canal. Similar type another skin incision

was made in the posterior canal wall about 1 cm behind and

parallel to the previous one. These incisions ultimately produced

an anterior based tympanomeatal flap and a wide inferior based

meatal skin flap [figure 3]. Tympanomeatal flap was elevated by

using round cutting knife and Plester’s flap knife. Whereas for

elevation of meatal skin flap, Wullstein’s curved raspatory was

used in addition to flap and round knife. Elevation of these flaps

eventually exposed the entire posterior bony canal wall.

Observations:

Tightly adherent part of posterior canal skin in the

tympanomastoid suture line sometime helped to

identify suture line while it was appeared faint [figure

4].

Appearance of the tympanomastoid suture line in the

posterior canal wall [figure 5].

Notch of Rivinus- a groove in the scutum lateral to the

neck of malleus in between tympanomastoid and

tympanosquamous suture.

Figure1: Chronological pictures A, B, C showed the different adjusted positions of right (R) and left (L) hands of the surgeon during dissection ofvarious parts of the mastoid in the right temporal bone.


Figure 3: Map of the incisions for anterior based thetympanomeatal flap (TMF) and for the inferior based meatalskin flap (MF).

Figure 4: Tightly adherent part of posterior canal skin (blackcolored arrow) in the tympanomastoid suture line.

Figure 5: Exposed posterior bony canal wall showed fainttympanomastoid suture line(black colored arrowheads) andnotch of Rivinus(red colored arrowheads)

Step Two: Exploratory attico-antrostomy.

Tasks:

This initial bony dissection of the scutum and the posteriorcanal wall was started from the notch of Rivinus. Then it was

progressed posterosuperiorly over the tympanomastoid suture

line and was continued till the exposure of anterior part the

mastoid antrum [figure 6 &10]. The sequential exposure of the

body and short process of incus and lateral semicircular canal in

the pathway of dissection further guided the direction of the

dissection [figure 7 & 8]. For dissection purpose, House’s curette,

pituitary punches and small cutting drill burr were used. House’s

curette was useful for dissecting the scutum only and it was not

the right choice for dissecting the thicker bone of posterior canal

wall [figure 9]. Both pituitary punches and cutting drill burr were

perfect for dissecting the thicker posterior canal wall bone as well

as scutum. However, the placement of pituitary punch was

difficult while dissecting the bony wall just superior to the body

of incus.

Observations:

Direction of the dissection in relation to

tympanomastoid suture line.

Endoscopic anatomy of the short process of the incus,

the lateral semicircular canal, the mastoid antrum.

Thickness of bone of the posterior canal wall.

Figure 6: Picture showed the direction of initial dissection onposterior canal wall (green colored arrowheads) in relation to thetympanomastoid suture line (black colored arrowheads).

Figure 7: Progressive pathway of dissection showed the short


process of the incus (SP) and the body of incus (I).

Figure 8: Picture showed the anatomical relationship amongthe short process of the incus (SP), the lateral semicircular canal(LSC) and the antrum (A).

Figure 9: Thickness of the posterior canal wall was showed inbetween the black arrows.

Figure 10: Endoscopic scenario after completion the exploratoryatticoantorstomy. Black colored arrowheads indicated thetympanomastoid suture line. A= the antrum.LSC= the lateralsemicircular canal, SP= the short process of the incus.

Step Three: Total mastoid antrostomy.

Tasks:

The entire mastoid antrum was exposed following removal of

almost entire posterior bony canal wall [figure 11] by using

cutting drill burr. After completion of this step, no overhang bony

wall was left in the passage between canal and mastoid antrum.

Observations:

Endoscopic anatomy of the Kӧrner’s septum, the lateral

semicircular canal, the mastoid antrum proper.

Bony partition between the mastoid antrum proper and

the tympanum which lodged the mastoid segment of

the facial nerve.

Figure 11: Endoscopic view after total mastoid antrostomy. KS=the Korner’s septum. LSC= the lateral semicircular canal. BP= bonypartition in between antrum proper and the tympanum lodgingthe mastoid part of the facial nerve. PCW= remnant of theposterior canal wall forming lateral boundary of the previouslymentioned bony partition. A= the antrum proper.

Step Four: Exposure of the tegmen mastoideum.

Tasks:

Initially the height of the Kӧrner’s septum was reduced to

watch the medial part of the tegmen clearly [ figure 12&13].

Mastoid cells lateral to the septum was first exenterated till the

thin bluish colored tegmen was seen [figure 14]. Thereafter

residual part of the septum and mastoid cells medial to the

septum were exenterated to expose the entire bluish colored

tegmen mastoideum [figure 15]. Cutting drill burr was used

initially for the dissection of overlying mastoid cells and once the

dissection reached near to the tegmen, rest of the dissection was

carried out with diamond burr. Dissection from lateral to medial

part of the tegmen was done cautiously as the height of the


tegmen gradually reduced medially.

Observations:

Mastoid cells over the tegmen mastoideum.

The Kӧrner’s septum/Petrosquamosal lamina: A

septum in between medial petrosal tegmen and

lateral squamous tegmen.

Lateral to medial down slope of the tegmen as the

lateral portion is higher than the medial portion of it.

Figure 12: Endoscopic view near the tegmen mastoideumshowed the Korner’s septum (KS), the lateral semicircular canal(LSC), the medial portion of the tegmen mastoideum (MTM).

Figure 13: After reduction of the height of the Korner’s septum(KS) the medial portion of the tegmen mastoideum (MTM) wasappeared clearly.

Figure 14: Endoscopic view of the lateral portion (LTM) of

the tegmen mastoideum and medial portion (MTM) of thetegmen mastoideum in relation to base of the Korner’s septum(KS).

Figure 15: Endoscopic view of the tegmen mastoideum aftercomplete exenteration of the mastoid cells along with thekorner’s septum.

Step Five: Exposure of the sino-dural angle and the

sigmoid sinus plate.

Tasks:

Further posterolateral extension of the previous tegmen

dissection towards the junction of the tegmen and lateral wall

was done to expose the sino-dural angle [figure 16]. Exenteration

of mastoids cells present at the junction of lateral and posterior

wall of the antrum eventually exposed the sigmoid sinus plate [

figure 17,18 &19]. Mastoid cells were dissected out by using

cutting burr but area near to sinus plate was skeletonized with

diamond burr.

Observation:

Meeting points of the tegmen mastoideum, the

posterior wall and the lateral wall.

Endoscopic anatomy of the sino-dural angle and the

sigmoid sinus plate.

Figure 16: Endoscopic view of the sinodural angel (SDA) in relationto the tegmen mastoideum (TM).


Figure 17: Endoscopic view showed the boundaries of themastoid antrum. PW= the bony posterior wall over theposterior cranial fossa. LW= the lateral wall comprising themastoid cortex and the sigmoid sinus plate. SDA= the sino-duralangle. TM= the tegmen mastoideum.

Figure 18: Dissection at the junction of the posterior wall (PW)and the lateral wall of the mastoid antrum.

Figure 19: Endoscopic view of the exposed bony plate over thesigmoid sinus(SS). SDA= Sino-dural angle. TM= the tegmenmastoideum.

Step Six: Approach to mastoid tip.

Tasks:

Initially the lower area in between the lateral and posterior

wall of the antrum proper was dissected in inferior direction with

cutting drill burr [figure 20]. This initial dissection later produced

a ditch inferomedial to the canal floor [figure 21]. The bony

partition between the newly formed ditch and the canal floor was

lowered to produce a downward slope from the canal to the

ditch. Mastoid cells at the base of the ditch were further

exenterated till the thin bony plate was exposed [figure 22]. It

was noticed that the most lateral group of mastoid tip cells were

difficult to exenterate with this transcanal endoscopic approach.

Observations:

Cells at the lower area of antrum in between the lateral

and posterior wall of antrum proper.

The ditch produced after initial dissection.

Depth of mastoid tip base from the canal floor level

[figure 23].

Figure 20: Picture showed the direction of the dissection (blackcolored arrows) for the mastoid tip. LW= the lateral wall. PW= theposterior wall.

Figure 21: A ditch (D) was formed after initial dissection towardsthe mastoid tip. PW= the posterior wall. PCW= the remnant ofthe posterior bony canal wall. F= floor of the external auditorycanal.


Figure 22: Thin bony plate at the base of the mastoid tip(BMT)after exenteration of tip cells.

Figure 23: Endoscopic view showed the depth of the mastoid tipbase(BMT) in relation to the canal floor (F).

Step Seven: Exposure of the mastoid segment of facial

nerve and approach to retrofacial cells.

Tasks:

Dissection began just inferior to the short process of incus

on the posterior surface of the bony partition in between the

tympanum and antrum proper, in the plane of the incus [figure

24]. It was carried inferiorly paralleling the posterolateral

surface of the vertical segment of the facial nerve. The

dissection was initially carried out with cutting burr to remove

the superficial retrofacial cells; thereafter diamond burr was

used to skeletonize the area. Such meticulous dissection

eventually exposed the pink colored fallopian canal of facial

nerve [figure 25 & 26]. After removal of the bony shell of the

fallopian canal by House’s curette, the whitish colored nerve

sheath was exposed [figure 27 & 28]. Nerve sheath was incised

by the Plester’s sickle knife to expose the nerve fibers [figure

29 & 30]. The retrofacial cells situated at the posteromedial

side of the mastoid part of the facial nerve was gradually

exenterated by the cutting burr till the thin bony plate over the

posterior cranial fossa was exposed .

Observations:

The plane of dissection at the posterior surface of the

bony partition in between tympanum and antrum

proper.

Endoscopic anatomy of the fallopian canal of mastoid

segment of the facial nerve, nerve sheath, nerve fiber.

Positional relationship of the mastoid segment of the

facial nerve with the surroundings [figure 31].

Endoscopic anatomy of retrofacial cells.[figure 32]

Figure 24: Dissection for mastoid segment of the facial nervebegan at the posterior surface of the bony partition (BP), in theplane of incus and was progressed inferiorly (black arrowheads)paralleling the nerve. A= the mastoid antrum proper.

Figure 25: Black arrow indicated the exposed part of the fallopiancanal of the facial nerve. RFC= retrofacial cells.


Figure 26: Dissection of the overlying bone with diamond drillburr at the area of the mastoid segment of the facial nerve.

Figure 27: Dissection of the thin bony shell of the fallopian canalwith House’s curette.

Figure 28: Whitish colored sheath of the mastoid segment ofthe facial nerve.

Figure 29: Incising the nerve sheath with Plester’s Sickle knife.

Figure 30: Red colored arrow indicated the exposed part of facialnerve fiber. Black colored arrow indicated the whitish colorednerve sheath.

Figure 31: Endoscopic view of the mastoid segment of the facialnerve in relation with surroundings. BT= bony partition. PCW= theremnant of the posterior bony canal wall. Tym= tympanum.LSC= the lateral semicircular canal.


Figure 32: Exenteration of the retrofacial cells with cutting drillburr.

Step Eight: Exposure of the semicircular canals.

Tasks:

The dense bone of the lateral semicircular canal was seen

readily exposed at the floor of the aditus ad antrum. The

posterior and the superior semicircular canal were seen

submerged in the perilabyrinthine cells. The area just lateral to

the mastoid segment of facial nerve and inferior to the lateral

semicircular canal was identified as presumptive area of the

posterior semicircular canal [figure 33]. The perilabyrinthine

cells in that presumptive area were exenterated with cutting

drill burr till the dense bone of the posterior semicircular canal

was visible. Further skeletonization of the posterior semicircular

canal wall was done with diamond burr [figure 34].

The superior semicircular canal was searched in the lateral

wall of the aditus ad antrum by dissecting that area with

diamond burr [figure 35]. Finally a small cutting burr was used

to open up the lumen of these three semicircular bony canals

[figure 36 & 37].

Observations:

Anatomical relationship among the posterior

semicircular canal, the mastoid segment of the facial

nerve and the lateral semicircular canal.

Anatomical relationship among the superior

semicircular canal, the tegmen mastoideum and the

lateral semicircular canal.

Long axis of these three semicircular canals.

Figure 33: Presumed area of the posterior semicircularcanal(colored area) in relation to the mastoid segment of thefacial nerve(FN).

Figure 34: Exposed dense bone of the posterior semicircular canal(PSC). FN= the mastoid segment of the facial nerve. PCW=remnant of the posterior canal wall.

Figure 35: Colored area indicated the presumed area of thesuperior semicircular canal in the lateral wall of the aditus adantrum. LSC= the lateral semicircular canal. TM= the tegmenmastoideum.


Figure 36: Exposed lumen of the superior semicircular bonycanal.TM= the tegmen mastoideum. LSC= the lateralsemicircular canal.

Figure 37: The exposed lumen of the three semicircular bonycanals. SSC= the superior semicircular canal. LSC= the lateralsemicircular canal. PSC= the posterior semicircular canal.FN= the mastoid segment of the facial nerve.

Step Nine: Exposure of the Endolymphatic sac.

Tasks:

The posterior bony wall over the posterior cranial fossa in

between the sigmoid sinus plate and the posterior semicircular

canal was skeletonized with diamond burr [figure 38]. The

vestibular aqueduct lodging the endolymphatic duct was seen

passing posterolaterally from behind the posterior semicircular

canal [ figure 39]. Following this aqueduct laterally, the exact

location of the endolymphatic sac was presumed. The posterior

bony wall at the presumed area of the sac was further thinned

out with diamond burr [figure 40]. Thereafter the bone shell of

that area was removed with House’s curette to expose the

endolymphatic sac [figure 41].

Observations:

Location of the area of dissection for endolymphatic sac

in relation to the sigmoid sinus, the posterior

semicircular canal and the mastoid segment of the

facial nerve.

Endoscopic anatomy of the endolymphatic sac and the

vestibular aqueduct accompanying the endolymphatic

duct.

Figure 38: Area that was needed to be dissected whileapproaching to the endolymphatic sac. SS= the bony plate overthe sigmoid sinus. PW= the posterior bony wall over the posteriorcranial fossa. PSC= the posterior semicircular canal. FN= themastoid segment of the facial nerve.

Figure 39: Black arrowheads indicated the vestibular aqueductlodging the endolymphtic duct passing posterolaterally behind theposterior semicircular canal (PSC). FN= the mastoid segment ofthe facial nerve. PW= the posterior bony wall over the posteriorcranial fossa.


Figure 40: Further thinning of the posterior wall at the locationof the endolymphatic sac.

Figure 41: Exposed endolymphatic sac after dissection.

Conclusions:

This description of the dissection was the first of its kind in the

literature mentioning the practice of transcanal endoscopic

mastoid dissection on cadaver. The guideline mentioned here

has been expected to be useful in order to develop necessary

skills and knowledge for a safe transcanal approach to the

mastoid antrum purely under endoscopic guidance.

.

References:

1. Kamal S S. Transcanal endoscopic mastoid surgery withtympanoplasty for the management of cholesteatoma and itsrelated lesions of mastoid antrum. Researchgate. Sheikh ShawkatKamal. 22 April. 2011. Web. 23 May. 2014.https://www.researchgate.net/publication/262559478_Transcanal_endoscopic_mastoid_surgery_with_tympanoplasty_for_the_management_of_cholesteatoma_and_its_related_lesions_of_mastoid_antrum.

2. Marchioni D, Mattioli F, Villari D et al. Endoscopic Treatment ofCholesteatoma with Antral Extension. In: Presutti L, Marchioni D,editors. Endoscopic Ear Surgery- Principles, Indications, andTechniques. 1st ed. Thieme.2015.p.243-261.

3. Kamal S S. Transcanal endoscopic open cavity mastoidectomy-Revised. Researchgate.Sheikh Shawkat Kamal.15May.2016. Web.21June.2016.https://www.researchgate.net/publication/304207045_Transcanal_Endoscopic_Open_Cavity_Mastoidectomy_-_Revised

the practice of transcanal endoscopic mastoid dissection on cadaver

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