A simple method for locating and calibrating the keyhole for the retrosigmoid approach using CT plain scans

INTRODUCTION/BACKGROUND: The precise positioning of the keyhole is crucial for neurosurgical operations in the retrosigmoid approach. Although recent implementations like 3D Slicer localization, brain 3D-printed model localization, neuro-navigation, and augmented reality technology can perform individualized analysis and precise positioning, they come with high equipment requirements, complex procedures, and increased costs, significantly limiting their widespread use in primary healthcare institutions or primary hospitals in developing countries. It is necessary to introduce a method for using CT plain scans, combined with the relationship between the end point of the digastric groove and the Frankfurt line, to accurately localize and calibrate the keyhole for the retrosigmoid approach. MATERIALS AND METHODS: Using data from CT plain scans combined with surface anatomical landmarks of the skull, based on the relationship between the end point of the digastric groove and the vertical line of the Frankfurt plane, which passes from it, the surgeon can determine preoperatively whether the keyhole position is reliable and optimize the final keyhole drilling position intraoperatively. RESULTS: The 'theoretical keyhole' offers a safe and reliable landmark for locating the cranial projection of the posterior margin of the transverse-sigmoid sinus junction. This target is identified craniometrically via the vertical relationship between the posterior endpoint of the digastric groove and the Frankfurt horizontal plane. This directly determines the intraoperative drilling position. A few anatomic variations require optimized calibration, termed "calibrated keyhole". A small subset of cases exhibit show "calibrated keyhole" being positioned rearward or forward compared to the "theoretical keyhole." DISCUSSION: Preoperative CT imaging data review and systematic evaluation of anatomical variations are prerequisites for individualized keyhole localization in the retrosigmoid approach, which is critical to optimizing surgical precision and minimizing procedural risks. CONCLUSION: Accurate intraoperative analysis of drilling location from CT data can improve safety and surgical efficiency in the retrosigmoid approach, reduce the risk of venous sinus injury, and reduce postoperative complications.