A refined radiological classification of anterior clinoid process pneumatization

The anterior clinoid process (ACP) is a critical anatomical landmark during skull base surgery. However, ACP pneumatization poses several risks during anterior clinoidectomy, including cerebrospinal fluid (CSF) leakage and optic nerve injury. Existing classification systems inadequately address clinically significant variations such as those involving the optic strut or planum sphenoidale. Therefore, this study aimed to determine the prevalence and morphological patterns of ACP pneumatization in a Thai population and propose a refined radiological classification system based on the route and extent of pneumatization.

A retrospective computed tomography (CT)-based study was conducted on 400 ACPs from 200 patients aged ≥10 years. Pneumatization patterns were categorized into eight subtypes based on the pneumatization route (optic strut, planum sphenoidale, or both) and the degree of ACP involvement (≤50% or >50%). ACP morphometric data and associated bone variations were also assessed.

ACP pneumatization was observed in 30.8% of ACPs, with bilateral involvement in 5% of cases. The most frequent subtype was isolated optic strut pneumatization (subtype 1, 16%), followed by limited ACP involvement via the optic strut (subtype 2a, 6%). Planum-based and combined subtypes (3a and 4b) were uncommon (<4%). Male patients demonstrated significantly greater ACP base width (9.09 ± 1.61 mm vs. 8.54 ± 1.39 mm; p = 0.015) and length (13.23 ± 1.72 mm vs. 12.61 ± 1.64 mm; p = 0.010) than females. Middle clinoid processes and interclinoid calcifications were present in 5.8% and 8.8% of patients, respectively.

ACP pneumatization, particularly via the optic strut, is a common anatomical variation. The proposed eight-subtype classification provides a nuanced framework for preoperative imaging description and communication. Although prior classifications were largely discussed in the context of transcranial approaches, the observed pneumatization patterns may also be relevant to endoscopic endonasal anatomy, particularly regarding optic canal exposure and potential sinonasal communication. Prospective surgical correlation studies are warranted to determine concordance with intraoperative findings and to clarify clinical relevance.