Evaluation of facial nerve course, posterior tympanotomy width and visibility of round window in patients with cochlear implantation by performing oblique sagittal cut computed tomographic scan temporal bone

During posterior tympanotomy the surgeon can approach the round window niche and promontory, where a cochleostomy is carried out for cochlear implant electrode array insertion [2]. Evaluation of facial nerve course, posterior tympanotomy width and visibility of round window in patients with cochlear implantation by performing oblique sagittal cut computed tomographic scan temporal bone Mohamed S. Hasaballah, Tarek A. Hamdy


Introduction
Posterior tympanotomy is a well-known otologic procedure that allows surgeons access to the middle ear cavity. Th is technique was fi rst described by Jansen in 1958 and is achieved by opening of the facial recess, which is a triangular space defi ned medially by the mastoid segment of the facial nerve, laterally by the chorda tympani nerve and superiorly by the incudal fossa [1].
During posterior tympanotomy the surgeon can approach the round window niche and promontory, where a cochleostomy is carried out for cochlear implant electrode array insertion [2].

Background
Posterior tympanotomy is a well-known otologic procedure that allows surgeons access to the middle ear cavity. During posterior tympanotomy the surgeon can approach the round window niche and promontory, where a cochleostomy is carried out for cochlear implant electrode array insertion. The mastoid segment of the facial nerve and the chorda tympani nerve could be injured in cases of narrow facial recess or inadequate posterior tympanotomy. With the image reconstruction in an oblique sagittal plane and curve reconstructions, the whole tympanic and mastoid segments of the facial nerve can be visualized in just one image. It is necessary to preoperatively estimate both the facial nerve status and the anatomical relationships between the facial recess and the round window, this may reduce the risk of facial nerve injury and in uence the decision on which side to implant, which approach to use and whether to enter the cochlea via cochleostomy or round window membrane route.

Objectives
The aim of our study is to evaluate the facial nerve (course and anomalies), visibility of the round window membrane and the width of posterior tympanotomy before cochlear implantation by using oblique sagittal cuts CT scan temporal bone.

Methods
A prospective study; done on 18 consecutive patients with severe to profound sensorineural hearing loss who are candidates for cochlear implantation in Ain Shams University Hospitals during years 2011 & 2012. We focused on oblique sagittal cut CT scan & its role to evaluate the course of facial nerve, posterior tympanotomy width and visibility of the round window.

Results
We tried to make a statistical correlation between CT scan and intraoperative ndings. Statistically signi cant positive correlation between posterior tympanotomy width and 2nd genu angle, distance from facial bony canal to round window and distance from facial nerve to round window. The mean distance from facial bony canal to round window was longer in operatively viewed round window than non-viewed window (4.7 and 4.4 mm respectively) (P < 0.05). The mean distance from facial nerve to round window was longer in operatively viewed round window membrane than non-viewed window (5.9 and 5.5 mm respectively) (P < 0.05). The mean width of posterior tympanotomy was wider in operatively viewed round window niche than non-viewed window niche (3.1 and 3.0 mm respectively) (P < 0.01).

Conclusion
Oblique sagittal cuts CT scan temporal bone is very helpful preoperative radiological tool for evaluation of the facial nerve course and anatomical factors that may determine the eld of view or the accessibility of the posterior tympanotomy for either cochleostomy or round window membrane approach. Other approaches can be used with more safety when the position of Two sides of this triangle could not be seen from the posterior aspect, and this is a problem during bone drilling. Th e mastoid segment of the facial nerve and the chorda tympani nerve could be injured in cases of narrow facial recess or inadequate posterior tympanotomy [3]. Th is complication generally occurs because of a limited understanding of the anatomy of the facial recess and diff erent mastoid segment anomalies [4]; therefore, radiological evaluation of the anatomy, anomalies of the facial nerve course, and anatomical factors that may determine the fi eld of view or the accessibility of the posterior tympanotomy are very important to minimize the risk for nerve injury.
Facial nerve palsy was reported as a complication of cochlear implant surgery at a rate of 1-3%. Th e knowledge of the anatomical relationship of the facial nerve with various adjacent landmarks in the surgical fi eld may help to avoid this complication [5].
Th e round window is the most important surgical landmark during electrode insertion either through the round window membrane or even in cochleostomy. Visibility of the round window membrane was graded according to the St Th omas' Hospital classifi cation after an 'optimal' posterior tympanotomy had been performed, and any overhang of the bony round window niche was removed without breaching the round window membrane. In type I, full exposure of the round window membrane is achieved; in type IIa, more than 50% of round window membrane is achieved, whereas in type IIb, less than 50% of the round window membrane is achieved. In type III St Th omas' Hospital classifi cation, the round window membrane lies more posteriorly, and therefore closer to the facial nerve, making it much more challenging to perform a round window approach and most patients will need a conventional bony cochleostomy [6].
With image reconstruction in an oblique sagittal plane and curve reconstructions, the whole tympanic and mastoid segments of the facial nerve can be visualized in just one image. Th e use of this reconstruction plane can help in the diagnosis of the nerve anomalies and study the relationships of the facial nerve with the surrounding landmarks [7].
Th e purpose of this study was to visualize the temporal bone structures aff ecting the posterior tympanotomy using the oblique sagittal view computed tomography (CT) of the temporal bone before performing cochlear implant to assess the width of the facial recess. It is helpful to preoperatively estimate both the facial nerve status and the anatomical relationships between the facial recess and the round window; this may infl uence the decision on which side to implant and which approach to use and whether to enter the cochlea by cochleostomy or round window membrane route. Other approaches (transcanal or transattic) can be used with more safety when the position of the facial nerve prevents an adequate posterior tympanotomy, as there is no need to open the posterior tympanotomy in these approaches.

Patients and methods
Th is was a prospective study that included 18 consecutive patients with severe-to-profound sensorineural hearing loss who were candidates for cochlear implantation in Ain Shams University Hospitals during 2011 and 2012. Institutional Review Board approval and patients consent were obtained.
Th ere were 11 male patients and seven female patients. Two of them were adult and 16 were children with a mean age of 8 years (range 3-26 years).
All patients were evaluated preoperatively using pure tone audiometry, auditory brain stem response, otoacoustic emissions, IQ level and multidetector computed tomography, and MRI temporal bone.
Th e anatomy and pathology of the temporal bone involve small structures; hence, resolution is thus highly important. Collimation is of optimal importance to achieve this high resolution. We routinely used a collimator of 0.6 mm.
CT protocol for temporal bones/internal auditory meatus (IAMs):  Intraoperatively, two variants were measured and observed: (1) Width of the posterior tympanotomy in its maximal diameter. (2) Visibility of the round window niche.
We tried to make a statistical correlation between CT scan and intraoperative fi ndings.

Results
A statistical correlation between CT scan and intraoperative fi ndings was performed. Statistically signifi cant positive correlation was observed between posterior tympanotomy width and second genu angle, distance from facial bony canal to round window membrane, and distance from Oblique sagittal view of the facial nerve canal shows the length of the tympanic facial nerve labeled as 1.

Figure 1
Oblique sagittal view of the facial nerve canal shows the distance between bony facial nerve canal and round window membrane labeled as 3 and the distance between facial nerve itself and round window labeled as 4.

Figure 3
Oblique sagittal view of the facial nerve canal shows the second genu angle labeled as 2.

Figure 2
Oblique sagittal view of the facial nerve canal shows the width of the bony facial nerve canal labeled as 5. facial nerve to round window membrane. Th e mean distance from facial bony canal to round window was longer in operatively viewed round window niche than in nonviewed window niche (4.7 and 4.4 mm, respectively) (P < 0.05). Th e mean distance from facial nerve to round window was longer in operatively viewed round window niche than in nonviewed window niche (5.9 and 5.5 mm, respectively) (P < 0.05). Th e mean width of posterior tympanotomy was wider in operatively viewed round window niche than in nonviewed window niche (3.1 and 3.0 mm, respectively) (P < 0.01) (Tables 1-3).
Least angle associated with operatively viewed round window niche was 92° (Table 4).

Discussion
CT scan with oblique sagittal view of the temporal bone can be of great value in predicting possible diffi culties in cochlear implantation and posterior tympanotomy.
We concluded that the two factors determining the posterior tympanotomy width and the visibility of the round window niche intraoperatively were as follows: (1) Th e second genu angle: When it increases this means that the vertical (mastoid) part of the facial nerve is displaced posteriorly in the mastoid away from the chorda tympani and round window, and usually the round window is visible. It was also noticed that, when the second genu angle increases, there is a wider distance between chorda tympani nerve and facial nerve; this means a wide posterior tympanotomy. (2) Th e distance between the facial nerve (bone and trunk) and round window membrane: When this distance increases, this means also that the facial nerve is displaced backwards in the mastoid away from the     This table illustrates the differences in radiological measurements in relation to the operative view of round window niche. The mean distance from facial bony canal to round window membrane was longer in operatively viewed round window niche than non-viewed window niche (4.7 and 4.4 mm respectively) (P < 0.05). The mean distance from facial nerve to round window membrane was longer in operatively viewed round window niche than non-viewed window niche (5.9 and 5.5 mm respectively) (P < 0.05). The mean width of posterior tympanotomy was wider in operatively viewed round window niche than non-viewed window niche (3.1 and 3.0 mm respectively) (P < 0.01) round window membrane and chorda tympani leaving a wider posterior tympanotomy and more visible round window niche intraoperatively.
We suspect that the length of the horizontal part (tympanic) of facial nerve can aff ect the width of posterior tympanotomy, but we observed that this length has no relationship with the posterior tympanotomy. Th is may be explained by the following: (1) Fewer number of patients.
(2) Th e second genu angle is more critical in determining the position of mastoid part of the facial nerve whether anteriorly or posteriorly displaced.
We cannot accurately detect either the anatomical relationship or the true distance between the facial nerve and round window membrane in this sagittal view to expect if the round window niche will be visible intraoperatively or not. We recommend that performing CT scan axial view of the temporal bone is essential, as it is more informative about the visibility of the round window membrane (Fig. 5).

Conclusion
Oblique cut sagittal CT scanning of the temporal bone can be a very helpful preoperative radiological tool for the evaluation of the facial nerve course and anatomical factors that may determine the fi eld of view or the accessibility of the posterior tympanotomy for either cochleostomy or round window membrane approach. Other approaches can be used with more safety when the position of the facial nerve prevents an adequate posterior tympanotomy.