Three-dimensional CT evaluation of anatomical constraints during posterior lip drilling in the retrosigmoid approach to the internal auditory canal.

PURPOSE: To develop a CT-based three-dimensional morphometric framework for evaluating the depth and orientation of posterior lip drilling during the retrosigmoid approach.

METHODS

One hundred normal temporal bones from 50 adults were reconstructed from high-resolution CT. Five surgically relevant landmarks were identified at the endolymphatic sac-duct junction, posterior internal auditory canal (IAC) porus, common crus, lateral IAC fundus, and Bill's bar. Six linear distances and four angular relationships were measured to characterise the drilling corridor relative to the IAC, labyrinth, and endolymphatic sac/duct system. Associations with sex, age, and laterality were explored using a significance threshold of p < 0.01.

RESULTS

The distance from the posterior IAC porus to the fundus-related landmark was 10.4 ± 1.9 mm, and that from the common crus to the fundus-related landmark was 5.8 ± 1.2 mm. Bill's bar lay 0.9 ± 0.9 mm from the reference plane. The drilling-corridor angle relative to the posterior IAC was 46.1 ± 9.0°. The angle between the endolymphatic sac-common crus axis and the lateral semicircular canal plane was 30.7 ± 15.3° and showed the greatest variability. No significant sex-related or left-right differences were identified. Age showed significant negative correlations with two linear measurements.

CONCLUSION

CT-based three-dimensional assessment demonstrated patient-specific variation in drilling orientation as well as depth. This framework may support selective preoperative anatomical planning, but pathological and intraoperative validation is required before these measurements can be considered operative limits.