2012, Number 3
Morphological correlation of round window niche by tomography and dissection in the pre-operatory evaluation of the patient susceptible for a cochlear implant
Ortiz HR, Soda MA, Cristerna SL, Biasotti FM
Full text
Language: Spanish
References: 17
Page: 133-148
PDF size: 377.23 Kb.
ABSTRACT
Background: Since development of combined electro-acoustic stimulation (EAS), a great interest to preserve the residual hearing of some patients programmed to cochlear implantation has emerged, because this technique offers better possibilities for developing audio-lingual skills through the synergistic effect between both technologies. One of the basic principles for the conservation of the residual hearing is the use of cochlear implant with small, compact, flexible and curved electrodes in order to avoid any damage to adjacent structures. The direct inclusion of the electrodes through the round window membrane decreases acoustic and surgical traumatism caused by excessive milling; also, reduces the loss of perilymph and the entry of blood and bone dust to the scala tympani as well as the release of toxic metabolites to the apical parts of the cochlea, which has deleterious effects on preservation of residual hearing. An adequate knowledge of the anatomy of the round window niche and normal morphological variants, as well as experience in correlating the findings obtained by computed tomography and dissection are of great importance for the audiologist during cochlear implantation, since they allow to accurately predict potential technical difficulties since the preoperative assessment. This information contributes to the planning of the best surgical approach to achieve a non-traumatic insertion of the electrodes. There are no published studies that provide information on the morphological correlation of the round window niche by computed tomography and dissection.Objective: To determine if there is a morphological correlation of the round window niche between computed tomography and dissection findings.
Results: Morphology of the round window niche found by CT and dissection was conical in eight temporal bones (80%) and cylindrical in two temporal bones (20%). Temporal bone VI was conical, with an operculum diameter greater than the length of the round window membrane. The visibility of the secondary eardrum during the dissection was optimal in all cases: it was complete in six temporal bones (60%) and partial in four temporal bones (40%). In the temporal bones V, VI, VII and IX there was a partial exposure of the membrane of the round window due to the smaller diameter of operculum measured by tomography and dissection. The maximum width of the operculum calculated by computed tomography was 2.5 mm (temporal bone I) and the minimum was 0.96 mm (temporal bone IX), with an average of 1.47 mm. The maximum width of the operculum found during the anatomical dissection was 2.3 mm (temporal bone I), and the minimum of 0.95 mm (temporal bone IX), with an average of 1.44 mm. The depth of the niche and the length of the round window membrane were assessed only by tomography. The depth of the niche was in the range of 0.9 to 1.17 mm, with an average of 0.94 mm. The average length of the membrane was 1.13 mm, with a range of 0.80 mm to 1.33 mm.
Conclusions: There is a statistically significant morphological correlation of the round window niche (conical vs cylindrical), the width of the operculum and the niche entry determined by computed tomography and dissection. We recommend oblique tomographic reconstruction as described in this work to otologist with little experience in cochlear surgery. This allows to obtain fine anatomical details of the round window niche useful to choose the most adequate approach for non- traumatic cochlear insertion, ideally through the secondary tympanic membrane, in order to preserve residual hearing.
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