Impact of oral cavity geometry on micro-sized aerosol deposition in the upper airway during oral inhalation

Accurate representation of oral airway geometry during inhalation is critical for optimizing drug delivery, yet the shape of the oral cavity and oropharynx varies significantly with breathing posture. This pilot study compares airflow dynamics and particle deposition between two CT-derived airway models from a single healthy subject: one with an artificially opened mouth during nasal breathing, and another with a real oral inhalation during active oral inhalation using a 2 cm mouthpiece. Computational fluid dynamics simulations were conducted at inhalation rates of 15, 30, and 60 L/min using spherical particles. The real-oral-inhalation model showed an enlarged oral cavity, smoother and more uniform airflow, peak pharyngeal velocities of 5–6 m/s, and an anteriorly directed laryngeal jet. This airway geometry eliminated oral cavity deposition and consistently shifted particle deposition deeper into the airway, resulting in 17–19.6% deposition in the larynx across all flow rates. In contrast, the artificially opened model produced higher peak velocities ($\approx$ 7.5 m/s), jet-like flow impinging on the posterior pharyngeal wall, and persistent oral cavity deposition that increased with flow rate. Tracheal deposition remained minimal in both models. Differences in tongue and soft palate positioning, likely contributed to the observed aerodynamic and deposition patterns. These results highlight the role of imaging protocols that capture true inhalation posture and soft tissue configuration. Future studies that incorporate the realistic airway geometry during physiologically realistic breathing conditions may provide new inhalation drug delivery strategies and improve clinical relevance of CFD-based inhalation models.