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To investigate the anatomic limitations of the endoscopic transcanal approach to sinus tympani (ST), in particular, how this approach might allow a complete visualization of the ST recesses and to propose a morphologic classification of the ST using computed tomography and to examine if this could be useful for the surgeon in preoperative planning to decide the type of approach to the ST.Case series.Forty patients affected by middle ear disease with ST involvement were included in our study. All patients underwent a preoperative computed tomographic scan of the temporal bone. All patients underwent tympanomastoid surgery with intraoperative use of the endoscope. We assessed the variation in depth of the ST area, analyzing the posterior and medial extension of the medial boundary of the ST, and performing a radiomorphologic classification of the ST.Of the 40 patients, it has been possible to describe the detailed morphology of the shape of the ST in 38. We classified the depth of the ST into 3 types based on radiologic findings: Type A (a limited ST), Type B (a deep ST), and Type C (a deep ST with a posterior extension). Of 35 ST of Types A and B, there was good exposure of the medial boundary of the sinus using a transcanal endoscopic approach in 33 (94.2%), whereas in 3 (60%) of 5 patients of Type C, it was not possible to explore the complete depth of the sinus, especially the ST portion under and posterior to the facial nerve.The transcanal endoscopic approach to the ST is indicated in ST of Types A and B. In ST of Type C, it is not always possible to have good control of the ST; in these cases, a posterior retrofacial approach is suggested.