Assessment of the Distance from Soft and Hard Tissue Nasion to Anterior Cranial Fossa: A CBCT Analysis

Orthognathic surgery is a common procedure performed by oral and maxillofacial surgeons.  During a Lefort I osteotomy, a K-wire is placed into the nasal bones to aid in positioning the maxilla into the proper vertical location during surgery.  The risk of placing the K-wire too deeply is entrance into the anterior cranial fossa (ACF) and creation of a CSF leak which could lead to meningitis.  Only one study in the literature has published data on the distance from anterior cranial fossa to hard tissue nasion, but the data was based on only 27 cadaveric skulls of unknown age and gender.  The purpose of this study is to quantify the average distance from soft and hard tissue nasion to the ACF from CBCT images, and to assess the impact of other confounding variables, such as gender and age.

After IRB approval (IRB: 2014-0185), subjects were identified through the UIC College of Dentistry (COD) electronic medical record  (EMR) system (Axium), using dental procedure codes (CDT) for CBCT imaging.  The EMR was queried for scans performed within a one year period between 01/2012 and 12/2013.  Cases were  reviewed to assess for age and gender of the patient at time of CBCT examination, and the distance from soft and hard tissue nasion to the ACF.  Only subjects aged 15 or older were used for this study.  One individual made the measurements using the CBCT software ruler (iCatVision, V1.9.3.13), and to ensure accuracy of each measurement, the coronal cut was used to identify the crista galli as the midline, and the sagittal cut at the midline was used to measure soft and hard tissue nasion to ACF. 

76 subjects age 15 to 76 years were identified.  The average distance of soft tissue nasion to anterior cranial fossa was 20.28 mm (26.78-14.06 mm). The average distance of hard tissue nasion to anterior cranial fossa was 13.98 mm  (23.14-7.35 mm).   It was noted that the shortest distance from nasion to ACF always occurred when the measurement was made in the superior direction above the true horizontal plane. There was no significant difference between gender or age of subject on the measurements.        

We recommend marking the K-wire at 10 mm (with a surgical marker), and directing it from an anterior-superior to posterior-inferior position to avoid inadvertent entrance into the ACF.

References

Scarbrough, F. et al. Anatomic guidelines for the placement of external references for maxillary repositioning. Journal Oral Surgery, Oral  Medicine, Oral Pathology. Vol. 84, No. 5, Nov. 1997.

 Neelambar, K. et al. Accuracy of Computer Programs in predicting orthognathic surgery hard tissue response. Journal of Oral Surgery. Vol. 67, No 8, Aug. 2009.