Evaluation of Parameters Relevant to Paraclinoid Microsurgery: Morphometric Analysis of the Anterior, Middle, and Posterior Clinoid Processes

Background/Objectives: The paraclinoid region—bounded by the anterior clinoid process (ACP), middle clinoid process (MCP), posterior clinoid process (PCP), optic strut (OS), and sella turcica—is critical in skull base anatomy owing to its proximity to the optic nerve, internal carotid artery (ICA), and cavernous sinus. Because the bony anatomy of this region varies considerably between individuals, detailed morphometric characterization is a prerequisite for anticipating the working corridor before anterior clinoidectomy and related approaches. This study aimed (1) to quantify the morphometric parameters of the optic canal (CO) and the ACP, MCP, and PCP using computed tomography angiography (CTA), (2) to test right–left and sex-based differences and topographic correlations among these parameters and cranial dimensions, and (3) to derive exploratory morphometric classifications of ACP shape and the ACP–OS relationship. Methods: High-resolution CTA images from 102 patients (204 sides) were retrospectively analyzed. Bilateral morphometric measurements were obtained for the CO and the ACP, MCP, and PCP, together with bilateral and ipsilateral interclinoid distances and cranial (biparietal, fronto-occipital) diameters. Data were analyzed with descriptive statistics, right–left and sex-stratified comparisons, and Pearson correlations. ACP pneumatization was classified into four types, and three exploratory, data-derived classifications—the Anteroclinoid Morphometric Index (AMI), ACP Size Score (ASS), and Paraclinoid Access Index (PAI)—were constructed from the linear measurements. Results: ACP basal width, length, ACP–OS distance, and CO-level basal width were symmetric and sex-independent, whereas the ACP–PCP and ACP–MCP distances were significantly greater on the left (p = 0.015 and p < 0.001), a pattern more pronounced in men. ACP pneumatization was most commonly absent (Type 0; 79.4% right, 82.4% left) and did not differ by side or sex. Bilateral interclinoid distances correlated with one another and, more strongly in men, with biparietal diameter. The ASS and, more weakly, the PAI correlated with the ipsilateral ACP–MCP distance (right ASS–ACP-MCP r = 0.382, p < 0.001; left r = 0.312, p = 0.001) and with CO transverse diameter, chiefly on the left (ASS r = 0.232, p = 0.019; PAI r = 0.352, p < 0.001), indicating that these indices track broader topographic variation in the paraclinoid complex rather than the ACP alone. The predominant bilateral categories were AMI-2 (39.2%), ASS-2 (31.4%), and PAI-2 (61.8%). Conclusions: ACP, MCP, and PCP morphometry in this cohort was largely symmetric, with a consistent left-sided predominance restricted to the ACP–PCP and ACP–MCP distances. The AMI, ASS, and PAI are exploratory, data-derived classifications—not validated surgical indices—that provide a quantitative framework for describing ACP morphology and its relationship to the optic strut; whether they carry independent value for predicting surgical corridor width or complication risk has not been tested against operative outcomes and requires prospective validation.