Statistical shape analysis of the incus

The incus is one of the three auditory ossicles involved in the conductive process of audition. The incus is often a target in otologic procedures, which have high revision and failure rates. Foundational knowledge of the incus' morphological parameters is generally limited to linear dimensions due to its small size and complex articulations. Our aim is to utilize a landmark-based approach to capture and quantify the bone's shape.

Incus bones were harvested from 53 anatomical body donors and scanned in a Bruker 1276 Skyscan micro-CT with 100 μm resolution. 3D Slicer was used for geometric morphometric landmarking and analyses. Warps corresponding to the major axes of variation determined by the PCA were created to visualize shape changes. Wilcoxon rank-sum tests were used to compare PC scores between sexes and sides (α = 0.05). Centroid size was calculated and correlated with shape scores.

The PCA revealed 53 PCs with the first ten PCs accounting for 83.5% of the variation in the sample. The first PC (18.0%) corresponded with inverse crural lengths, while PC2 accounted for 16.9% of the variation and was associated with angulation of the incudomallear joint. There were no significant differences in shape between sexes or left and right sides. Centroid size had a significant relationship with PC1 scores (R² = 0.1638, p = 0.0026).

Crural form - specifically lengthening and anterior-curving long crura - may represent morphological adaptations associated with an increase in size. Incudomallear joint angle variation was notable, which should be considered in predisposition to pathological processes.