Postnatal morphometric development and variability of the posterior canaliculus: an analysis in a Japanese population using photon-counting detector computed tomography.

Objective: The posterior canaliculus (PC) is a critical landmark in otologic surgery, but a quantitative in vivo characterization of its postnatal development is lacking. This study aimed to utilize the ultra-high spatial resolution of photon-counting detector computed tomography (PCD-CT) to conduct a definitive in vivo morphometric analysis of the PC's length and its spatial relationship to the mastoid segment of the facial nerve (FN) across a wide age range in a Japanese population, characterizing its developmental trajectory and variability.

Methods: This retrospective, cross-sectional study analyzed temporal bone PCD-CT scans from 128 consecutive Japanese patients (265 sides), aged 0 to 59 years. Using multiplanar reconstructions, the length of the PC (PC length) and the distance between the PC and the mastoid segment of the FN (PC-FN distance) at the level of the tympanic entry were measured. The relationships between these parameters and age were analyzed using locally estimated scatterplot smoothing (LOESS).

Results: The mean PC length was 8.31 ± 2.50 mm (range: 2.59-14.58 mm), and the mean PC-FN distance was 2.80 ± 0.74 mm (range: 1.01-6.18 mm). LOESS analysis revealed a distinct developmental trajectory: PC length increased progressively from birth, plateauing around age 20, with the most rapid growth occurring in the first year of life. Conversely, PC-FN distance decreased from birth, stabilizing around age 9. Both parameters exhibited significant inter-individual variability, particularly in the pediatric cohort. Notably, the PC was unidentifiable in 14 temporal bones, all from patients under 10 years old (mean age: 2.3 years).

Conclusion: This study provides the first in vivo quantitative evidence of the PC's postnatal morphogenesis, demonstrating significant elongation and a concurrent narrowing of PC-FN distance during childhood. This dynamic anatomical development, coupled with high individual variability, defines a "high-risk window" in pediatric otologic surgery. The findings underscore the necessity of preoperative high-resolution imaging for patient-specific surgical planning and highlight the transformative capability of PCD-CT in micro-anatomical research.