The Mastoid Foramen and Mastoid Emissary Vein Canal as Surgical Landmarks for Mastoid Pneumatization During Retrosigmoid Approach

BACKGROUND AND OBJECTIVES: The retrosigmoid approach (RSA) is a widely used technique for accessing the posterolateral skull base. Knowledge of anatomic landmarks remains crucial, particularly concerning the variability of temporal bone pneumatization. This study explores the anatomic, radiological, and intraoperative correlations between the mastoid foramen (MF), mastoid emissary vein canal (MEVC), and mastoid pneumatization during the RSA. METHODS: An anatomic analysis was performed on 2 dry temporal bones and 2 formalin-fixed specimens. The radiological study analyzed 100 patients' high-resolution computed tomography scans. Temporal bone pneumatization was graded using the Han classification. The MF was classified as “posterior” or “anterior” to the most posterior mastoid air cell (MAC); the MEVC was noted as “posterior to” or “within” the MACs. The mastoid foramen-mastoid air cell (MF-MAC) distance and minimal mastoid emissary vein canal-mastoid air cell (mMEVC-MAC) distance were defined as the gaps between the MF and MEVC and the most posterior MAC, respectively. The surgical study reviewed 54 patients undergoing RSA, for which we described the surgical technique and examined intraoperative and postoperative complications. RESULTS: Pneumatization grade inversely correlated with age ( P < .001). The MF and MEVC were posterior to MACs in 99.4% and 88.0% of cases, respectively. Higher pneumatization grades were significantly associated with shorter MF-MAC and mMEVC-MAC distances ( P < .001). A direct correlation was found between MF-MAC and mMEVC-MAC distances (r = 0.741, P < .001). Intraoperative MAC opening occurred in 5.6% of cases, all presenting Han grade 4 pneumatization and MEVC “within” MACs. No major intraoperative complications occurred, with only one case (1.9%) of cerebrospinal fluid wound leakage. CONCLUSION: The MF and MEVC serve as consistent and practical landmarks for predicting mastoid pneumatization and guiding RSA craniectomy. Preoperative radiological assessment enables targeted surgical planning to manage intraoperative opening of MACs.