■ OBJECTIVES: The sinus tympani is a cavity in the middle ear where cholesteatoma and retracted epithelium can occur. The surgeon must understand the anatomy within the sinus when removing disease from this part of the middle ear. The dimensions of the sinus have been reported by other authors, but a detailed anatomical description is lacking. Therefore, a study was done to examine the interior configuration of the sinus tympani.

■ METHODS: Fifty temporal bones were examined. Removal of the anterior canal bone, its overlying soft tissues, and the tympanic membrane permitted direct visual and instrumental access into the sinus tympani. An operative microscope enabled detailed study of the sinus. Measurements were made with various right angle picks and a millimeter ruler. A telescopic rigid lens was used to produce DVD recordings.

■ RESULTS: Four parameters were recorded: the configuration of the posterior wall of the sinus, the sinus depth, the opening into the sinus, and the lateral wall structures. In 8 specimens, the posterior wall of the sinus was flat in contrast to 42 ears in which the posterior  wall converged as an apex with a maximum depth of 8 mm. In 6 ears, a narrow opening restricted full exposure of the sinus. Prolapse of the stapedius muscle or facial nerve into the sinus created a ridge along the lateral wall of the sinus tympani in 23 specimens.

■ CONCLUSIONS: This study reveals new anatomical details of the sinus tympani. A small opening can compromise access into the sinus where cholesteatoma and retracted epithelium may accumulate in the deep apex of the posterior wall or behind a ridge formed on the lateral wall. The surgeon must be able to remove disease from these obscure areas within the sinus tympani, and based upon findings from this study, refined surgical techniques can be developed. Detailed photographs will demonstrate the anatomical details.