What is the most common cause of orbital cellulitis?

The most common cause of orbital cellulitis is the extension of infection from the nasal sinuses. This condition often arises due to a sinus infection, particularly from the ethmoid or maxillary sinuses, which can spread to the orbit. The anatomical proximity of the sinuses to the orbital contents allows for this transmission of bacterial infection, especially in cases of sinusitis, where inflammation and infection can compromise barriers that normally protect the orbit.

In the context of orbital cellulitis, other causes such as systemic viral infections, allergic reactions, or direct trauma to the eye are less common. Systemic viral infections may contribute to inflammation but do not typically lead to the localized pus accumulation seen in orbital cellulitis. Allergic reactions don't cause the infection characteristic of cellulitis, while direct trauma can lead to infection but is not a typical precursor. Therefore, the pathway from sinus infection to orbital involvement through extension is the most prevalent mechanism resulting in orbital cellulitis.