NS02 - Geometric Morphometric Analysis of Clival Anatomy with Application in Approaches to the Posterior Cranial Fossa

Introduction/Purpose: The posterior cranial fossa is an area of intense neurosurgical interest. Chordoma resection and safe navigation around the cerebellopontine angle are particularly relevant. The natural anatomical variability of the basicranial skeleton has been well described using dry skeletal collections; however, geometric analyses of soft tissues, including intracranial cranial nerve locations, have not been thoroughly reported. The primary aims of this research are to quantify and characterize the three-dimensional relationships of the abducens (CN VI) and hypoglossal (CN XII) nerves in relationship to salient bony landmarks.

Methods or Case Description: Typical dissection techniques were used to reveal the posterior cranial fossa on 34 formalin-fixed whole-body donors (13 Male and 21 Female; mean age=84 years). A Microscribe® i+ 3D Digitizer Portable Coordinate Measuring Machine was used to register the location of eight 3D landmarks. Landmarks included four bony basicranial landmarks and four dural meatuses marking the penetration of CN VI and CN XII. Data were analyzed with MorphoJ v1.07a. For the final analysis, 29 donors were retained after filtering for extreme outliers, missing data, or erroneous captures. Generalized Procrustes analysis was used to remove nuisance factors and isolate shape formation. Principal component (PC) analysis was used to explore variability in landmark configurations. Wireframe diagrams and lollipop graphs were used to visualize the results.

Outcomes: PC analysis revealed four loadings capturing a total of 79.08% of the total shape variation in the sample. The first PC (PC1) accounted for 31.55% of the shape variation and indicates a strong inferolateral displacement of the CN VI dural meatus, concomitant with superomedial displacement of the jugular tubercle. The CN XII’s dural exit showed limited variation along PC1. The second PC (PC2) captured 25.61% of the total shape variation and demonstrated lateral displacement of all clival landmarks with inferior recession of the dorsum sella.

Conclusion: The lollipop diagram for PC1 shows substantial variability of CN VI’s penetration of the dura mater. This result is seen accompanied by a more superior and medial jugular tubercle. Interestingly, CN XII’s relative trajectory was very consistent along the first principal component. Graphical results of PC2 suggest a more platybasic morphology, with widened clival features and inferiorly recessed parasellar landmarks. In summary, CN VI is highly variable in its exit from the posterior cranial fossa, particularly when compared to CN XII egression. These findings should be considered when drilling the jugular tubercle. Future directions may include investigating the geometric situation of Dorello’s canal.