Objectives: To determine the limits of visualization during transcanal endoscopic ear surgery (EES) by correlating the relationship between radiologic and endoscopic anatomy using angled optics.
Methods: Radiology and endoscopic visualization of tensor fold, protympanum, facial sinus (FS), sinus tympani (ST), subtympanic sinus (STS), hypotympanum, and aditus ad antrum were analyzed using a transcanal approach in 30 human temporal bones specimens with different angled endoscopes (0 degree, 45 degrees, 70 degrees) to check for the full visualization of these regions. High-resolution computed tomography (CT) was performed prior to dissection to classify retrotympanic anatomy. According to previously published descriptions, FS, ST, and STS were classified into types A, B, and C depending on their morphology relative to the mastoid segment of the facial nerve. These radiologic findings were compared to endoscopic visualization of these same structures using a Chi-squared test.
Results: Visualization of the posterior wall of three different retrotympanic areas was significantly associated (FS p < 0.01; ST p < 0.01; STS p = 0.02) with the radiologic classification and endoscopic optical angle. Angled endoscopy improved visual access to the other subsites, especially the aditus ad antrum and the tensor fold (>70% with 45 degrees and 70 degrees).
Conclusion: Complete visual access to the hidden recesses of the middle ear can be achieved using angled endoscopes (45 degrees and 70 degrees). We observed a statistically significant association of endoscopic visualization to radiologic description of the retrotympanum on CT and the optical angle used. The prediction of the endoscopic exposure of the retrotympanum from the preoperative CT is possible. Even with the use of 70 degrees lens, retrotympanum is not fully visualized on transcanal endoscopy if a type C retrotympanic recesses (posterior and medial to the facial nerve) is present. This represents a technical limit of exclusive transcanal EES.